WELDING OF TOOL STEEL

Transcription

1 WELDING OF TOOL STEEL 1

2 This information is based on our present state of knowledge and is intended to provide general notes on our products and their uses. It should not therefore be construed as a warranty of specific properties of the products described or a warranty for fitness for a particular purpose. Classified according to EU Directive 1999/45/EC For further information see our Material Safety Data Sheets. Edition 6, The latest revised edition of this brochure is the English version, which is always published on our web site SS-EN ISO 9001 SS-EN ISO 14001

3 WELDING OF TOOL STEEL CONTENTS General information on welding of tool steel... 4 Welding methods for tool steel... 4 The welding bay... 6 Filler material... 7 Hydrogen in tool steel... 8 Elevated working temperature... 9 Welding procedure Het treatment after welding Guidelines for welding in hot work tool steel cold work tool steel plastic mould steel

4 WELDING OF TOOL STEEL General information on welding of tool steel Tool steel contain up to 2.5% carbon as well as alloying elements such as manganese, chromium, molybdenum, tungsten, vanadium and nickel. The main problem in welding tool steel stems from its high hardenability. Welds cool quickly once the heat source is removed and the weld metal and part of the heat-affected zone will harden. This transformation generates stresses because the weld is normally highly constrained, with a concomitant risk for cracking unless great care is exercised. In what follows, a description is given of the welding equipment, welding technique and weld consumables that are required in order to weld tool steel successfully. Of course, the skill and experience of the welder is also a vital ingredient in obtaining satisfactory results. With sufficient care, it is possible to achieve weld repairs or adjustments which, in terms of tooling performance, are hardly inferior to that of the base steel. Welding of tooling may be required for anyone of the following reasons: refurbishment and repair of cracked or worn tooling renovation of chipped or worn cutting edges, e.g. on blanking tools adjustment of machining errors in tool making design changes Welding methods for tool steel Shielded metal-arc welding (SMAW or MMA) PRINCIPLE An electric arc generated by a DC or AC power source is struck between a coated, rod-like electrode and the work-piece (Fig.. The electrodes consist of a central wire core, which is usually lowcarbon steel, covered with a coating of pressed powder (flux). The constitution of this coating is complex and consists of iron powder, powdered ferro-alloys, slag formers and a suitable binder. The electrode is consumed under the action of the arc during welding and drops of molten metal are transferred to the workpiece. Contamination by air during the transfer of molten drops from electrode to workpiece and during solidification and cooling of the weld deposit is inhibited partly by slag formed from constituents in the electrode coating and partly by gases created during melting of the electrode. The composition of the deposited weld metal is controlled via the constitution of the electrode coating. POWER SOURCE For MMA welding, it is possible to use either an AC or DC power source. However, whichever is used, the source must provide a voltage and current which is compatible with the electrode. Normal arc voltages are: normal recovery electrodes: V high recovery electrodes: V Uddeholm welding consumables are of normal recovery type. A suitable power source for these is a DC unit with an open voltage of 70 V and which is capable of delivering 250A/ 30V at 35% intermittence. 4

5 WELDING OF TOOL STEEL Gas tungsten-arc welding (GTAW or TIG) PRINCIPLE In MMA welding, the electrode from which the arc is struck is consumed during welding. The electrode in TIG welding is made of tungsten or tungsten alloy which has a very high melting point (about 3300 C/6000 F) and is therefore not consumed during the process (Fig. 2). The arc is initially struck by subjecting the electrode-workpiece gas to a high-frequency voltage. The resulting ionization permits striking without the necessity for contact between electrode and workpiece. The tungsten electrode is always connected to the negative terminal of a DC power source because this minimizes heat generation and thereby any risk of melting the electrode. Current is conducted to the electrode via a contact inside the TIGgun. Any consumables which are required during TIG-welding are fed obliquely into the arc in the form of rod or wire. Oxidation of the weld pool is prevented by an inert-gas shroud which streams from the TIG gun over the electrode and weld. POWER SOURCE TIG welding can be performed with a regular MMA power source provided this is complemented with a TIG control unit. The gun should be water cooled and be capable of handling a minimum current of 250 A at 100% intermittence. A gas lens is also a desirable feature in order that the inert gas protection is as efficient as possible. Welding is facilitated if the current can be increased steplessly from zero to the optimum level. Laser Welding PRINCIPLE High power laser light is generated and focused through a lens to the welding spot. As filler material a thin wire with a diameter between mm is primarily used. The welder guides the wire to the area to be welded. The laser beam melt the wire and the base material. The molten material solidifies leaving behind a small raised area. The welder continues spot by spot and line by line. Argon gas shields the process from oxidation (Fig.. Electrode holder Core wire Electrode holder + Pole Power source Pole Coating Cooling water Slag Weld Melt pool Filler material Protective gas Tungsten electrode Pole Power source + Pole Fig. 1 Shielded Metal-Arc Welding SMAW (MMA) Fig. 2 Gas Tungsten Arc Welding GTAW (TIG) Protective gas Protective glass Laser beam Fusion zone Deposited material Filler wire Workpiece Fig. 3 Laser Welding 5

6 WELDING OF TOOL STEEL POWER SOURCE For deposition welding normally a pulsed solid state laser of Nd: YAG type is used. Typical performance: Nominal output W Max pulse output kw Pulse time ms Frequence Hz Spot diameter mm ( mm) The welding bay In order to be able to effect satisfactory welding work on tool steel, the following items of equipment are to be regarded as minimum requirements. Dry cabinet The coated electrodes used for MMA welding are strongly hygroscopic and should not be allowed to come into contact with anything other than dry air. Otherwise, the weld will be contaminated with hydrogen (see later). Hence, the welding bay should be equipped with a dry cabinet for stor- age of electrodes. This should be thermostatically controlled in the range C ( F). The electrodes should be removed from their containers and lie loose on racks. For welding of tooling outside the welding bay, it will also be found useful to have a portable heated container in which the electrodes can be carried. Workbench It is particularly important during critical welding operations, of the type performed with tool steel, that the welder enjoys a comfortable working position. Hence, the workbench should be stable, of the correct height a sufficiently level that the Electrical elements for an insulated preheating box. work can be positioned securely and accurately. It is advantageous if the workbench is rotatable and adjustable vertically, since both these features facilitate the welding operation. Preheating equipment Tool steel cannot be welded at room temperature without considerable risk for cracking and it is generally necessary to pre-heat the mould or die before any welding can be attempted (see later). While it is certainly possible to weld tools successfully by preheating in a furnace, the chances are that the temperature will fall excessively prior to completion of the work. Hence, it is recommended that the tool be maintained at the correct temperature using an electrical heating box supplied from a current-regulated DC source. This equipment also enables the tool to be heated at a uniform and controlled rate. To place the tool on a heated table or plate could sometimes be sufficient to maintain the temperature. For minor repairs and adjustments, it is acceptable that the tool is preheated using a propane torch. Hence, liquid propane cylinders should be available in the welding bay. Preheating in an insulated box. Grinding machines The following should be available: disc grinder with minimum 180 Ø x 6 mm wheel (7 Ø x 0,25 ) for preparing the joint and grinding out of any defects which may occur during welding flat grinder capable of rpm for grinding of minor defects and of the finished weld if a welded mould is subsequently to be polished or photo-etched, it may be necessary to have a grinder capable of giving a sufficiently fine finish small rotating metal files in different shapes and sizes 6

7 WELDING OF TOOL STEEL Filler material The chemical composition of a weld deposit is determined by the composition of the consumable (filler metal), the base steel composition and the extent to which the base material is melted during welding. The consumable electrode or wire should mix easily with the molten base steel giving a deposit with: uniform composition, hardness and response to heat-treatment freedom from non-metallic inclusions, porosity or cracks suitable properties for the tooling application in question Since tool steel welds have high hardness, they are particularly susceptible to cracking which may originate at slag particles or pores. Hence, the consumable used should be capable of producing a high-quality weld. In a similar vein, it is necessary that the consumables are produced with very tight analysis control in order that the hardness as welded and the response to heat treatment is reproducible from batch to batch. Highquality filler metals are also essential if a mould is to be polished or photoetched after welding. Uddeholm welding consumables meet these requirements. Filler rods are normally produced from electro-slag remelted stock. The coated electrodes are of basic type, which are far superior to rutile electrodes as regards weld cleanliness. Another advantage with basic coated electrodes over those of rutile type is that the former give a much lower hydrogen content in the weld metal. In general, the consumable used for welding tool steel should be similar in composition to the base material. When welding in the annealed condition, e.g. if a mould or die has to be adjusted while in the process of manufacture, it is vital that the filler metal has the same heat treatment characteristics as the base steel, otherwise the welded area in the finished tool will have different hardness. Large compositional differences are also associated with an increased cracking risk in connection with hardening. Uddeholm welding consumable are designed to be compatible with the corresponding tool steel grades irrespective of whether welding is carried out on annealed or hardenedand tempered base material. Obviously, the weld metal of welded tools will require different properties for different applications. For the three main application segments for tool steel (cold work, hot work and plastic moulding), the important weld-metal properties are: COLD WORK Hardness Toughness Wear resistance HOT WORK Hardness Temper resistance Toughness Wear resistance Heat checking resistance PLASTIC MOULDING Hardness Wear resistance Polishability Photoetchability Uddeholm welding consumables COATED ELECTRODES Impax Weld QRO 90 Weld Calmax/Carmo Weld Caldie Weld TIG-RODS Impax TIG-Weld Stavax TIG-Weld Corrax TIG-Weld Nimax TIG-Weld Unimax TIG-Weld QRO 90 TIG-Weld Dievar TIG-Weld Calmax/Carmo TIG-Weld Caldie TIG-Weld LASER RODS Stavax Laser Weld Nimax Laser Weld Laser welding consumables from Uddeholm. 7

8 WELDING OF TOOL STEEL Hydrogen in tool steel Welds in tool steel have high hardness and are, therefore, especially susceptible to cold cracking derived from hydrogen ingress during welding. In many cases, hydrogen is generated as a result of water vapour being adsorbed in the hygro-scopic coating of MMA electrodes. The susceptibility of a weld to hydrogen cracking depends on: the microstructure of the weld metal (different microstructures have different hydrogen sensitivities) the hardness of the steel (the greater the hardness, the higher the susceptibility) the stress level the amount of diffusible hydrogen introduced in welding Microstructure/hardness The characteristic microstructures giving high hardness in the heataffected zone and weld metal, i.e. martensite and bainite, are particularly sensitive to embrittlement by hydrogen. This susceptibility is, albeit only marginally, alleviated by tempering. of runs). However, no measures to reduce stress will help if the weld is seriously contaminated by hydrogen. Content of diffusible hydrogen As regards the susceptibility of welds to cold cracking, this is the factor that it is easiest to do something about. By adhering to a number of simple precautions, the amount of hydrogen introduced during welding can be reduced appreciably. Always store coated electrodes in a heated storage cabinet or heated container once the pack has been opened (see earlier). Contamination on the surfaces of the joint of the surrounding tool surface, e.g. oil, rust or paint, is a source of hydrogen. Hence, the surfaces of the joint and of the tool in the vicinity of the joint should be ground to bare metal immediately prior to starting to weld. If preheating is performed with a propane burner, it should be remembered that this can cause moisture to form on the tool surfaces not directly impinged by the flame. Stress level Stresses in welds arise from three sources: contraction during solidification of the molten pool temperature differences between weld, heat-affected zone and base steel transformation stresses when the weld and heat-affected zone harden during cooling In general, the stress level in the vicinity of the weld will reach the magnitude of the yield stress, which for hardened tool steel is very high indeed. It is very difficult to do anything about this but the situation can be improved somewhat via proper weld design, (bead location and sequence Dry cabinet for storage of electrodes. 8

9 WELDING OF TOOL STEEL Elevated working temperature The basic reason for welding tool steel at elevated temperature derives from the high hardenability and therefore crack sensitivity of tool steel welds and heat-affected zones. Welding of a cold tool will cause rapid cooling of the weld metal and heat-affected zone between passes with resulting transformation to brittle martensite and risk of cracking. Cracks formed in the weld could well propagate through the entire tool. Hence, the mould or die should, during welding, be maintained at C ( F) above the M s - temperature (martensite-start temperature) for the steel in question. The critical temperature is the M s of the weld metal, which may not be the same as that of the base metal. In some instances, it may be that the base steel is fully hardened and has been tempered at a temperature below the M s -temperature. Hence, pre-heating the tool for welding will cause a drop in hardness. For example, most low-temperature tempered cold-work steel will have to be preheated to a temperature in excess of the tempering temperature, which is usually ca. 200 C (400 F). The hardness drop must be accepted in order to perform a proper preheating and mitigate the risk of cracking during welding. During multi-run welding of a properly pre-heated tool, most of the weld will remain austenitic under the entire welding operation and will transform slowly as the tool cools down. This ensures a uniform hardness and microstructure over the whole weld in comparison with the situation where each run transforms to martensite in between passes. It will be clear from this discussion that the entire welding operation should be completed while the tool is hot. Partially welding, letting the tool cool down and then preheating later on to finish the job, is not to be recommended because there is considerable risk that the tool will crack. While it is feasible to pre-heat tools in a furnace, there is the possibility that the temperature is uneven (creates stresses) and that it will drop excessively before welding is completed (especially if the tool is small). The best method, of preheating and maintaining the tool at the requested temperature during welding, is to use an insulated box with electrical elements in the walls (see page 6). Uddeholm Stavax Weld/TIG-Weld and Uddeholm Impax Weld/TIG-Weld match their corresponding tool steel grades exactly and give perfect results after polishing or texturing of a welded mould. 9

10 WELDING OF TOOL STEEL Welding procedure Joint preparation The importance of careful preparation can not be over-emphasized. Cracks should be ground out so that the groove angle will be 60 if possible. The width of the bottom should be at least 1 mm greater than the maximum electrode diameter which will be used. Erosion or heat-checking damage on hot work tools should be ground down to sound steel. The tool surfaces in the immediate vicinity of the intended weld and the surfaces of the groove itself must all be ground down to clean metal. Prior to starting welding, the ground areas should be checked with penetrant to make sure all defects have been removed. The tool should be welded as soon as the preparation is finished, otherwise there is risk of contamination of the surfaces with dust, dirt or moisture. Building up the weld To avoid undercut in the border line, between the weld and the base material, start with fine sink runs. The initial layer should be made with a small diameter MMA electrode, 2,5 mm, or via TIG welding (max. current 120 A). The second layer is made with the same electrode diameter and current as the first in order to minimize the heat-affected zone. The remaining of the groove can be welded with a higher current and electrodes with larger diameter. The final runs should be built up well above the surface of the tool. Even small welds should comprise a minimum of two runs. Grind off the last runs. During MMA welding, the arc should be short and the beads deposited in distinct runs. The electrode should be angled at 90 to the joint sides so as to minimize undercut. In addition, the electrode should be held at an angle of C to the direction of forward movement. The arc should be struck in the joint and not on any tool surfaces which are not being welded. The sore form striking the arc is likely location for crack initiation. In order to avoid pores, the starting sore should be melted up completely at the beginning of welding. If a restart is made with a partly-used MMA electrode, the tip should be cleaned free from slag. For repair or adjustment of expensive tooling, e.g. plastic mould with a polished or textured cavity, it is essential that there is good contact between the return cable and the tool. Poor contact gives problems with secondary arcing and the expensive surface can be damaged by arcing sores. Such tools should be placed on a copper plate which provides for the best possible contact. The copper plate must be preheated along with the tool. The completed weld(s) should be carefully cleaned and inspected prior to allowing the tool to cool down. Any defect, such as arcing sores or undercut, should be dealt with immediately. Before the tool has cooled, the surface of the weld should be ground down almost to the level of the surrounding tool before any further processing. Moulds where welded areas have to be polished or photo-etched should have the final runs made using TIG-welding, which is less likely to give pores or inclusions in the weld metal. BUILD UP SEQUENCE Undercut Sink run Undercut Sink run GROOVE PREPARATION Crack risk OK Remove cracks Clean surface 10

11 WELDING OF TOOL STEEL Heat treatment after welding Depending on the initial condition of the tool, the following heat treatments may be performed after welding: tempering soft annealing, then hardening and tempering as usual stress relieving Tempering Fully-hardened tools which are repair welded should if possible be tempered after welding. Tempering improves the toughness of the weld metal and the heat affected zone (HAZ). The tempering temperature should be chosen so that the hardness of the weld metal and base steel are compatible. An exception to this rule is when the weld metal exhibits appreciably improved temper resistance over the base material (e.g. Uddeholm Orvar Supreme welded with Uddeholm QRO 90 Weld); in this case, the weld should be tempered at the highest possible temperature concomitant with the base steel retaining its hardness (typically 20 C/ 40 F under the previous tempering temperature). Product brochures for Uddeholm welding consumables and tool steel give tempering curves from which the tempering conditions for welded tools can be ascertained. Very small repairs may not need to be tempered after welding; however, this should be done if at all possible. Soft annealing Tools which are welded to accommodate design changes or machining errors during toolmaking, and which are in soft-annealed condition, will need to be heat treated after welding. Since the weld metal and HAZ will have hardened during cooling, it is highly desirable to soft anneal the weld prior to hardening and tempering of the tool. The soft annealing cycle used is that recommended for the base steel. The welded area can then be machined and the tool may be finished and heat treated as usual. However, even if the tool can be finished by merely grinding the weld, soft annealing is first recommended in order to mitigate cracking during heat treatment. Stress relieving Stress relieving is sometimes carried out after welding in order to reduce residual stresses. For very large or highly-constrained welds, this is an important precaution. If the weld is to be tempered or soft annealed, then stress relieving is not normally necessary. However, pre-hardened tool steel should be stress relieved after welding since no other heat treatment is normally performed. The stress relieving temperature must be chosen such that neither the base steel nor the welded area soften extensively during the operation. Very small weld repairs or adjustments will normally not require a stress relieving treatment. Further information Information concerning heat treatment of the tool subsequent to welding can be obtained from the brochures for the welding consumable and/or the tool steel in question. Heat treatment of a die-casting die after welding. 11

12 WELDING OF TOOL STEEL 12

13 WELDING OF TOOL STEEL Guidelines for welding in Uddeholm tool steel The tables, on following pages, give details concerning weld repair or adjustment of tooling made from Uddeholm steel grades for hot work, cold work and plastic moulding applications. WELDING IN HOT WORK TOOL STEEL MMA (SMAW) WELDING PREHEATING HARDNESS POST STEEL GRADE CONDITION METHOD CONSUMABLES TEMPERATURE AS WELDED TREATMENT REMARKS VIDAR SUPERIOR VIDAR 1 Soft annealed MMA QRO 90 WELD Min. Soft annealing VIDAR 1 ESR Hardened (SMAW) UTP C (620 F) HRC Tempering ORVAR SUPREME ORVAR SUPERIOR ORVAR 2 Soft annealed MMA QRO 90 WELD Min HRC Soft annealing MICRODIZED Hardened (SMAW) UTP C (620 F) HRC Tempering Soft annealed MMA Min. Soft annealing DIEVAR Hardened (SMAW) QRO 90 WELD 325 C (620 F) HRC Tempering QRO 90 SUPREME Soft annealed MMA Soft annealing HOTVAR Hardened (SMAW) QRO 90 WELD 325 C (620 F) HRC Tempering Soft annealing, see product brochure Temper hardened material C (20 40 F) below last tempering temperature ALVAR MMA UTP A 73 G C Stress relieve large ALVAR 14 Prehardened (SMAW) ESAB OK ( F) HB None repairs WELDING IN HOT WORK TOOL STEEL TIG (GTAW) WELDING PREHEATING HARDNESS POST STEEL GRADE CONDITION METHOD CONSUMABLES TEMPERATURE AS WELDED TREATMENT REMARKS VIDAR SUPERIOR VIDAR 1 Soft annealed TIG QRO 90 TIG WELD Min. Soft annealing VIDAR 1 ESR Hardened (GTAW) DIEVAR TIG WELD 325 C (620 F) HRC Tempering ORVAR SUPREME ORVAR SUPERIOR ORVAR 2 Soft annealed TIG QRO 90 TIG WELD Min. Soft annealing MICRODIZED Hardned (GTAW) DIEVAR TIG WELD 325 C (620 F) HRC Tempering Soft annealed TIG DIEVAR TIG WELD Min. Soft annealing DIEVAR Hardened (GTAW) QRO 90 TIG WELD 325 C (620 F) HRC Tempering QRO 90 SUPREME Soft annealed TIG Soft annealing HOTVAR Hardened (GTAW) QRO 90 TIG WELD 325 C (620 F) HRC Tempering Soft annealing, see product brochure Temper hardened material C (20 40 F) below last tempering temperature UTP A 73 G4 ALVAR TIG ESAB OK TIG ROD C Stress relieve large ALVAR 14 Prehardened (GTAW) ( F) HB None repairs 13

14 WELDING OF TOOL STEEL GUIDELINES FOR WELDING IN COLD WORK TOOL STEEL MMA (SMAW) WELDING PREHEATING HARDNESS POST STEEL GRADE CONDITION METHOD CONSUMABLES TEMPERATURE AS WELDED TREATMENT REMARKS Tempering ARNE Type AWS E HB C RIGOR ESAB OK HRC (20 40 F) VIKING Hardened MMA UTP 67S C HRC below last FERMO* Prehardened (SMAW) UTP 73 G2 ( F) HRC tempering temp. Initial layers with soft weld metal MMA C Tempering CALDIE* Hardened (SMAW) CALDIE WELD ( F) HRC 510 C (950 F) Tempering C (20 40 F) MMA CALDIE WELD HRC below last SLEIPNER Hardened (SMAW) UTP C (480 F) HRC tempering temp. Tempering Type Inconel HB C UTP 73 G HRC (20 40 F) SVERKER 21 MMA UTP 67S HRC below last SVERKER 3 Hardened (SMAW) UTP C (480 F) HRC tempering temp. Initial layers with soft weld metal MMA CALMAX/CARMO CARMO* Prehardened (SMAW) WELD C ( F) HRC Tempering MMA CALMAX (SMAW) See Welding guidelines for plastic mould steel Tempering 200 C (390 F) or 505 C Type Inconel HRC (940 F) depend- VANADIS 4 MMA UTP 73 G2 200 C HRC ing on the last EXTRA** Hardened (SMAW) UTP 690 (390 F) HRC used temp. temp. * Minor welding operations in Uddeholm Fermo, Uddeholm Caldie and Uddeholm Carmo can be done at ambient temperature. ** Welding in Uddeholm Vanadis 4 Extra should generally be avoided due to the risk of cracking. Initial layers with soft weld metal 14

15 WELDING OF TOOL STEEL GUIDELINES FOR WELDING IN COLD WORK TOOL STEEL TIG (GTAW) WELDING PREHEATING HARDNESS POST STEEL GRADE CONDITION METHOD CONSUMABLES TEMPERATURE AS WELDED TREATMENT REMARKS ARNE Tempering RIGOR Type AWS ER HB C VIKING Hardened TIG UTP ADUR C HRC below last FERMO* Prehardened (GTAW) UTP A 73 G2 ( F) HRC tempering temp. Initial layers with soft weld metal TIG C Tempering CALDIE* Hardened (GTAW) CALDIE TIG-WELD ( F) HRC 510 C (950 F) Tempering C (20 40 F) TIG CALDIE TIG-WELD HRC below last SLEIPNER Hardened (GTAW) UTP A C (480 F) HRC tempering temp. Tempering Type Inconel HB C UTP A 73 G HRC (20 40 F) SVERKER 21 TIG UTP ADUR HRC below last SVERKER 3 Hardened (GTAW) UTP A C (480 F) HRC tempering temp. Initial layers with soft weld metal TIG CALMAX/CARMO CARMO* Prehardened (GTAW) TIG WELD C ( F) HRC Tempering TIG CALMAX (GTAW) See Welding guidelines for plastic mould steel Tempering 200 C (390 F) or 505 C Type Inconel HRC (940 F) depend- VANADIS 4 TIG UTP A 73 G2 200 C HRC ing on the last EXTRA** Hardened (GTAW) UTP 696 (390 F) HRC used temp. temp. * Minor welding operations in Uddeholm Fermo, Uddeholm Caldie and Uddeholm Carmo can be done at ambient temperature. ** Welding in Uddeholm Vanadis 4 Extra should generally be avoided due to the risk of cracking. Initial layers with soft weld metal 15

16 WELDING OF TOOL STEEL EXAMPLE OF LASER WELDS 16

17 WELDING OF TOOL STEEL GUIDELINES FOR WELDING IN PLASTIC MOULD STEEL MMA (SMAW) WELDING PREHEATING HARDNESS POST STEEL GRADE CONDITION METHOD CONSUMABLES TEMPERATURE AS WELDED TREATMENT REMARKS Stress relieve IMPAX MMA C large repairs SUPREME* Prehardened (SMAW) IMPAX WELD ( F) HB 550 C (1020 F) Heat treatment Soft annealed Soft annealing see product brochure MMA UTP 73 G C Tempering UNIMAX Hardened (SMAW) UTP 67 S ( F) HRC 510 C (950 F) RAMAX LH* Austenitic stainless RAMAX HH* Prehardened MMA steel C (SMAW) Type AWS E312 ( F) HRC Tempering C Soft annealed ( F) Soft annealing MMA CALMAX/CARMO C Heat treatment CALMAX Hardened (SMAW) WELD ( F) HRC Tempering see product brochure Stress relieve MMA C large repairs HOLDAX* Prehardened (SMAW) IMPAX WELD ( F) HB 550 C (1020 F) ORVAR Soft annealed Soft annealing SUPREME MMA Min. VIDAR 1 ESR Hardened (SMAW) UTP C (620 F) HRC Tempering MMA Type Inconel C 280 HB Tempering ELMAX** Hardened (SMAW) UTP 701 ( F) HRC 200 C (390 F) * Minor welding operations can be done at ambient temperature. ** Welding should generally be avoided due to the risk of cracking. Soft annealing, see product brochure. Temper hardened material C (20 40 F) below last tempering temperature 17

18 WELDING OF TOOL STEEL GUIDELINES FOR WELDING IN PLASTIC MOULD STEEL TIG (GTAW) AND LASER WELDING PREHEATING HARDNESS POST STEEL GRADE CONDITION METHOD CONSUMABLES TEMPERATURE AS WELDED TREATMENT REMARKS TIG STAVAX C (GTAW) TIG-WELD ( F) HRC Soft annealing STAVAX LASER Heat treatment Soft annealed LASER WELD None HRC None see product brochure Tempering TIG STAVAX C C (GTAW) TIG-WELD ( F) HRC ( F) STAVAX ESR STAVAX LASER POLMAX Hardened LASER WELD None HRC None Annealing C (1290- Sot annealed 1380 F) 5h Tempering C (20 40 F) TIG STAVAX C below last MIRRAX ESR Hardened (GTAW) TIG-WELD ( F) HRC tempering temp. Stress relieve IMPAX TIG C large repairs SUPREME* Prehardened (GTAW) IMPAX TIG-WELD ( F) HB 550 C (1020 F) TIG (GTAW) NIMAX TIG-WELD NIMAX NIMAX Prehardened LASER LASER WELD None HB None Stress relieve large repairs 550 C (1020 F) Heat treatment Soft annealed UNIMAX HRC Soft annealing see product brochure TIG-WELD TIG UTP A 73 G C Tempering UNIMAX Hardened (GTAW) UTP ADUR 600 ( F) HRC 510 C (950 F) Austenitic stainless steel. RAMAX LH* TIG Type AWS ER C HRC Heat treatment RAMAX HH* Prehardened (GTAW) STAVAX TIG-WELD ( F) HRC Tempering see product brochure Solution treated TIG CORRAX See data sheet for CORRAX Aged (GTAW) TIG-WELD None HRC Ageing Corrax TIG-Weld C Soft annealed ( F) Soft annealing TIG CALMAX/CARMO C Heat treatment CALMAX Hardened (GTAW) TIG-WELD ( F) HRC Tempering see product brochure Stress relieve TIG C large repairs HOLDAX* Prehardened (GTAW) IMPAX TIG-WELD ( F) HB 550 C (1020 F) ORVAR Soft annealed Soft annealing SUPREME TIG DIEVAR TIG WELD Min HRC VIDAR 1 ESR Hardned (GTAW) UTP A C (620 F) HRC Tempering Soft annealing, see product brochure Temper hardened material C (20 40 F) below last tempering temperature TIG C Tempering ELMAX** Hardened (GTAW) UTP A 701 ( F) HRC 200 C (390 F) * Minor welding operations can be done at ambient temperature. ** Welding should generally be avoided due to the risk of cracking. 18

19 Network of excellence is present on every continent. This ensures you high-quality Swedish tool steel and local support wherever you are. ASSAB is our wholly-owned subsidiary and exclusive sales channel, representing Uddeholm in various parts of the world. Together we secure our position as the world s leading supplier of tooling materials.

20 / TRYCKERI KNAPPEN, KARLSTAD is the world s leading supplier of tooling materials. This is a position we have reached by improving our customers everyday business. Long tradition combined with research and product development equips Uddeholm to solve any tooling problem that may arise. It is a challenging process, but the goal is clear to be your number one partner and tool steel provider. Our presence on every continent guarantees you the same high quality wherever you are. ASSAB is our wholly-owned subsidiary and exclusive sales channel, representing Uddeholm in various parts of the world. Together we secure our position as the world s leading supplier of tooling materials. We act worldwide, so there is always an Uddeholm or ASSAB representative close at hand to give local advice and support. For us it is all a matter of trust in long-term partnerships as well as in developing new products. Trust is something you earn, every day. For more information, please visit or your local website.

21 POLISHING MOULD STEEL

22 Contents Why strive for a high surface finish?... 3 Judging surface finish... 3 Factors which affect polishability... 3 Grinding and stoning of moulds... 4 Polishing of moulds... 5 Typical polishing sequences... 6 Different surface conditions prior to polishing... 8 Surface roughness after different heat treatment methods... 8 Polishing problems can be solved... 8 This information is based on our present state of knowledge and is intended to provide general notes on our products and their uses. It should not therefore be construed as a warranty of specific properties of the products described or a warranty for fitness for a particular purpose. Classified according to EU Directive 1999/45/EC For further information see our Material Safety Data Sheets. Edition 4, The latest revised edition of this brochure is the English version, which is always published on our web site SS-EN ISO 9001 SS-EN ISO 14001

23 POLISHING MOULD STEEL Why strive for a high surface finish? The increased use of plastic products has created a higher demand for mirror finish of moulding tools. The highest demands for surface finish are in the optical lens mould where an extreme requirement on polishability is desired. However, in general there are other advantages with high surface finish, including: Easier ejection of the plastic parts from the moulding tool (applies to most plastics) Reduced risk of local corrosion Reduced risk of fracture or cracking due to temporary over loading or pure fatigue. This brochure reviews the factors that affect the polishability of mould steels and gives recommendations on how to economically obtain the required finish on the main steel grades used. In making these recommendations, it is recognized that the skill, experience and technique of the polisher plays an extremely important role in achieving the desired surface finish. Judging surface finish Two things are important when judging the surface of the mould. The surface must first have a geometrically correct shape without any long macro waves. This macro shape is mostly an inheritance from earlier grinding and stoning steps. Secondly, the mirror finish of the mould surface must be free from scratches, pores, orange peel, pitting (pin-holes) etc. The surface finish is normally judged by the naked eye. There are certain difficulties involved in such a visual evaluation. A flat surface can look perfect despite the fact that it is not geometrically completely flat. Thus, the eye can be fooled. In more sophisticated cases, the finish can be judged by instrumental methods, such as optical interference techniques. Factors which affect polishability The surface smoothness which can be achieved by polishing steel depends on factors such as: Tool steel quality Heat treatment Polishing technique. In general, it can be stated that polishing technique is the most important factor. If a suitable polishing tech-nique is used it is almost always possible to achieve acceptable results, providing a correctly heat treated, good quality tool steel is used. If however, an unsuitable technique is used, even the best steels can be ruined. THE TOOL STEEL QUALITY Particles or areas in the steel surface which deviate from the matrix in terms of hardness and other properties can cause problems during polishing. Slag inclusions of various types and porosities are examples of such undesirable constituents. To improve the polishing properties, Uddeholm uses vacuum degassing, electro-slag refining (ESR) and vacuum arc remelting (VAR) techniques in the production of its mould steel grades. Vacuum degassing reduces the risk of large slag inclusions and hydrogen embrittlement and also produces a more homogeneous material. ESR/VAR treatment greatly improves properties from the viewpoint of polishability, even better than those achieved by vacuum degassing. ESR/ VAR treatment reduces the amount of slag inclusions in the steel and ensures that the remaining slag inclusions which cannot be avoided will be small and evenly distributed throughout the matrix, as shown in figure 1. Uddeholm Stavax ESR, Uddeholm Mirrax ESR and Uddeholm Polmax stainless mould steels, produced by the ESR and/or the VAR technique, have proved particularly suitable for moulds with the highest surface finish requirements, e.g. optical lenses. Conventional 70 x Figure 1. A typical inclusion picture in conventional and ESR-material. (An inclusion picture is made up from 70 superimposed photographs at high magnification.) Lens mould with extreme demand on polishability. The material choice was Uddeholm Stavax ESR. ESR 3

24 POLISHING MOULD STEEL HEAT TREATMENT Heat treatment can affect polishability in many ways. A case-hardening steel which has been overcarburized is likely to have an unsuitable structure for polishing. This is caused by the creation of small oxide particles under the steel surface, leading to polishing problems. Decarburization or recarburization of the surface during heat treatment can produce variations in hardness, resulting in polishing difficulties. POLISHING TECHNIQUE Different steel grades effect on polishing techniques Most Uddeholm mould steels, when used at the same hardness levels, take similar polishing times when using standard polishing techniques. Exceptions to this are Uddeholm Stavax ESR, Uddeholm Mirrax ESR and Uddeholm Polmax stainless mould steels. These grades are capable of producing the very best surface quality, but many mouldmakers use a slightly different polishing technique to achieve it. The important thing is to grind to as fine a surface finish as possible before starting the polishing operation. Great importance is placed on stopping the polishing operation immediately the last scratch from the former grain size has been removed. Different hardnesses effect on polishing technique Higher hardness levels make the mould steel more difficult to grind but give higher surface smoothness after polishing. However, harder mould steels require a slightly longer polishing time to achieve higher surface finishes. With higher hardness levels, over-polishing is less likely to be a problem. Grinding and stoning of moulds PRACTICAL HINTS Normally, a mould cavity is produced by means of milling, EDM ing or hobbing. If a very smooth surface is desired, the following sequences should be followed: After milling: rough grinding, fine grinding and polishing. After EDM ing: fine grinding and polishing. After hobbing: a single polishing operation after heat treatment. It should be emphasized that the grinding operation forms the basis for a rapid and successful polishing job. In grinding, the marks left by the rough-machining operation are removed and a metallically pure and geometrically correct surface is obtained. Certain rules should be followed to facilitate the work and ensure good results. This applies to both mechanical grinding and manual stoning. The grinding operation must not generate so much heat and pressure that the structure and hardness of the material are affected. Use plenty of coolant. Use only clean and free-cutting grinding tools with soft stones for hard surfaces. Between each change of grain size, the workpiece and hands should be cleaned to prevent coarse abrasive particles and dust being carried over to the next stage with a finer grain size. Grindability and polishability Polishability (surface smoothness) The finer the grain size used, the more important is the cleaning operation between each change of grain size. When changing to next next-finer grain size, grind in a direction at about 45 to the previous grinding direction until the surface only shows scratches from the present grinding step. After scratches from the previous step have disappeared continue for about 25% longer time before changing to the next grain size (except for Uddeholm Stavax ESR, Uddeholm Mirrax ESR and Uddeholm Polmax). This is to remove the deformed surface layer caused by mechanical stresses induced during previous grinding operations. Changing grinding direction is also important to avoid the formation of irregularities and relief patterns. When grinding large, flat mould surfaces, avoid hand-operated grinding discs. The use of a stone reduces the risk of obtaining large shape irregularities. Figure 2. The relationship between increasing hardness levels, grindability and polishability Soft annealed Hardened Grindability Increasing hardness 4

25 POLISHING MOULD STEEL Polishing of moulds PRACTICAL HINTS Diamond paste is the most common abrasive agent used in polishing. Optimum performance is obtained with the right paste, on the right polishing tool. The most common polishing tools are sticks, pads and blocks for manual use and bobs, brushes and discs for machines. Polishing tools are available in materials of different hardnesses from metals through different types of fibre (e.g. wood, synthetic fibre) to soft felt. The hardness of the polishing tool affects the exposure of the diamond grains and the removal rate. The following figure illustrates this: Soft Medium Hard Felt Wood Steel Hardened steel Time-consuming and expensive polishing can be cut by observing certain rules. Above all, cleanliness in every step of the polishing operation is of such great importance that it cannot be overemphasized. Polishing should be carried out in dust- and draughtfree places. Hard dust particles can easily contaminate the abrasive and ruin an almost finished surface. Each polishing tool should be used for only one paste grade and kept i n dust-proof containers. The polishing tools gradually become impregnated and improve with use. Hands and workpiece should be cleaned carefully between each change of paste grade, the workpiece with a grease solvent and the hands with soap. Paste should be applied to the polishing tool in manual polishing, while in machine polishing, the paste should be applied to the workpiece. Polishing pressure should be adjusted to the hardness of the polishing tool and the grade of the paste. For the finest grain sizes, the pressure should only be the weight of the polishing tool. Heavy material removal requires hard polishing tools and coarse paste. Finish polishing of plastic moulds should be carried out in the release directional. Polishing should start in the corners, edges and fillets or other difficult parts of the mould. Be careful with sharp corners and edges, so they are not rounded off. Preferably use hard polishing tools. Polishing a plastic mould. 5

26 POLISHING MOULD STEEL Typical polishing sequences The choice of grinding and polishing sequences is determined by the experience of the operator and the equipment he has at his disposal. The properties of the material can also affect the sequence. In polishing there are two methods used. In the first method, a paste with a certain grain size is selected and a hard polishing tool is used initially, after which softer and softer polishing tools are used. In the second method, a medium-hard polishing tool is selected and coarse paste is used initially. Then the grain size of the paste is gradually reduced towards finer and finer pastes. A combination of these two methods can be recommended. Example of sequences: Start with a hard polishing tool and a coarse paste. Then change to a softer polishing tool with the same paste. Then use a medium-hard polishing tool and a medium-coarse paste. Change to a soft polishing tool with the same paste. Finally, use a soft polishing tool and a fine paste. Examples of how to combine polishing tool and grain size of the abrasive. Cloth Hardness Cloth material Abrasive Micron Very hard Steel Nylon reinforced Diamond 45, 15, 6, 3 Hard Coated nylon Diamond 9, 6, 3 Hard Silk Diamond 15, 6, 3, 1 Alumina Hard Paper Diamond 15, 6, 3 Alumina Soft Wool Diamond 6, 3, 1 Soft Dense nylon velvet Diamond 3 Very soft Velvet Diamond 1 and smaller Alumina MgO OP-S Milling Turning EDM ing Rough grinding Rough Grain number Fine Fine grinding Rough Grain number 320 FEPA D-series Polishing with diamond paste Rough Micron size 45 µm Fine Fine This diagram shows example of how the polishing sequence can be selected. 6

27 POLISHING MOULD STEEL Grain size conversion table Grain sizes Commercial FEPA µm grain number grain number (22) (40) F-series 200 D-series No. µm 220 No. µm ,0 ± ,3 ± ,5 ± ,5 ±1, ,2 ± ,4 ±1, ,2 ±1, ,2 ±1, ,5 ±1, ,0 ±1, ,0 ±1, ,2 ±1, ,2 ±1, ,3 ±1, ,75±1, ,6 ±1, ,8 ±1, ,8 ±0, ,3 ±1, ,6 ±0, ,2 ±1, Surface roughness after grinding. Magnification x 300 Grain size µm µm µm Arithmetic Average micro inch 8 2,8 1,2 Surface roughness after using diamond paste on nylon cloth. Magnification x 300 Grain size 30 µm 7 µm 1 µm Arithmetic Average micro inch 2,4 0,4 0,24 7

28 POLISHING MOULD STEEL Different surface conditions prior to polishing EDM d surfaces are more difficult to grind than conventionally machined or heat treated surfaces. An EDM-operation should be finished with a fine sparking stage. If the fine sparking stage is performed correctly, there will be no problems. If not, a thin rehardened layer will remain on the surface. This layer is considerably harder than the matrix and must be removed. A nitrided or case hardened surface is more difficult to grind than base material but takes a good surface finish after polishing. However, small defects produced in the surface layer do not always allow the extremely high surface finishes to be obtained. A mould that has been flamehardened or repair welded often shows a soft zone between the treated part and the base material. To avoid a ditch formation along the soft zone use a broad stone. Polishing problems can be solved The predominant problem in polishing is so-called overpolishing. Overpolishing is the term used when a polished surface gets worse the longer you polish it. There are basically two phenomena which appear when a surface is overpolished: Orange peel and Pitting (pin holes). It should be pointed out that overpolishing often occurs in connection with machine polishing. ORANGE PEEL The appearance of an irregular, rough surface, which is normally referred to as orange peel, may depend on a number of different causes. The most common is overheating or overcarburization from heat treatment in combination with high pressure and prolonged polishing. A harder material can better withstand a high polishing pressure, softer steels overpolish more easily. Studies have shown that the overpolishing effect occurs at different polishing times for different hardnesses. Either of the following alternatives can be adopted to restore the surface. Alt 1 Remove the defective surface layer by grinding the surface using the next-to-last grinding step prior to polishing. Start again at the final grinding stage. Use a lower pressure during polishing than before. Alt 2 Stress-relieve at a temperature about 25 C (45 F) below the last tempering temperature. Regrind using the final grinding step prior to polishing until a satisfactory surface has been obtained. Start polishing again, but at a lower polishing pressure than before. If the result is still not good, the hardness must be raised. This can be done in a number of different ways: Increase the surface hardness of the steel by means of nitriding or nitrocarburizing treatment. Heat treat the tool to a higher hardness. Surface roughness after different heat treatment methods Many toolmakers ask the question: How far should I go in grinding steps before heat treatment? It should be borne in mind that during heat treatment some dimensional changes are likely to take place, possibly requiring a final finishing operation. Furthermore, the surface finish of the mould may be affected by the heat treatment medium. There is no point, therefore, in polishing a mould to a very high finish before heat treatment if size/shape changes and/or surface deterioration make further finishing operations necessary. Surface roughness Ra µm 0,06 0,05 0,04 0,03 0,02 0,01 IMPAX SUPREME 300 HB The normal reaction of a person who sees that a surface has deteriorated is to increase the polishing pressure and continue polishing. Such a course of action will inevitably result in further surface deterioration. RIGOR 60 HRC Polishing time, minutes 8

29 POLISHING MOULD STEEL PITTING The very small pits which can occur in a polished surface generally result from slag (non-metallic) inclusions in the form of hard, brittle oxides which have been torn out from the surface by the polishing process. The causal factors which are of im-portance in this connection are: Polishing time and pressure. Purity of the steel, especially with regard to hard slag inclusions. The polishing tool. The abrasive. One of the reasons why pitting can occur is the difference in hardness between the matrix and the slag inclusion. During polishing, the matrix will be removed at a more rapid rate than the hard slag particles. Polishing will gradually undermine the slag particle until the particle is torn out of the material by further polishing. This leaves a pit. The problem is most often encountered in the case of paste grain size less than 10 µm and soft polishing tools (e.g. felt). One way to minimise the risk of pitting is to select high-purity mould steels that have been subjected to vacuum-degassing, electro-slag refining (ESR) or vacuum arc remelting (VAR) during manufacture. If pitting still occurs the following measures should be taken: Regrind the surface carefully using the next-to-last grinding step prior to polishing. Use a soft free-cutting stone. Then start with the final grinding step and then polish. When using grain sizes 10 µm and smaller, the softest polishing tools should be avoided. Polish for the shortest possible time and under lowest possible pressure. 9

30 HEAT TREATMENT Europe Austria Representative office Albstraße 10 DE Neuhausen Telephone: Belgium Europark Oost 7 B-9100 Sint-Niklaas Telephone: Croatia BÖHLER Zagreb d.o.o za trgovinu Zitnjak b.b Zagreb Telephone: Telefax: Czech Republic BÖHLER CZ s.r.o. Division Uddeholm U Silnice Praha 6, Ruzyne Telephone: ,8 Denmark A/S Kokmose 8, Bramdrupdam DK-6000 Kolding Telephone: Estonia TOOLING AB Silikatsiidi 7 EE Tallinn Telephone: Finland OY AB Ritakuja 1, PL 57 FI VANTAA Telephone: France Z.I. de Mitry-Compans, 12 rue Mercier, FR Mitry Mory Cedex Telephone: +33 (0) Branch offices S.A. 77bis, rue de Vesoul La Nef aux Métiers FR Besançon Telephone: +33 (0) LE POINT ACIERS - Aciers à outils Z.I. du Recou, Avenue de Champlevert FR GRIGNY Telephone: +33 (0) LE POINT ACIERS - Aciers à outils Z.I. Nord 27, rue François Rochaix FR OYONNAX Telephone: +33 (0) Germany Hansaallee 321 DE Düsseldorf Telephone: Branch offices Falkenstraße 21 DE Bad Soden/TS Telephone: Albstraße 10 DE Neuhausen Telephone: Friederikenstraße 14b DE Harzgerode Telephone: Great Britain DIVISION BOHLER- (UK) LIMITED European Business Park Taylors Lane, Oldbury GB-West Midlands B69 2BN Telephone: Telefax: Greece STASSINOPOULOS- STEEL TRADING S.A. 20, Athinon Street GR-Piraeus Telephone: SKLERO S.A. Heat Treatment and Trading of Steel Uddeholm Tool Steels Industrial Area of Thessaloniki P.O. Box 1123 GR Sindos, Thessaloniki Telephone: Hungary TOOLING/BOK Dunaharaszti, Jedlik Ányos út 25 HU-2331 Dunaharaszti 1. Pf. 110 Telephone/fax: Ireland : DIVISION BOHLER- (UK) LIMITED European Business Park Taylors Lane, Oldbury UK-West Midlands B69 2BN Telephone: Telefax: Dublin: Telephone: Italy Divisione della Bohler Uddeholm Italia S.p.A. Via Palizzi, 90 IT Milano Telephone: Latvia TOOLING LATVIA SIA Piedrujas Street 7 LV-1035 Riga Telephone: latvia@assab.com Lithuania TOOLING AB BE PLIENAS IR METALAI T. Masiulio 18B LT Kaunas Telephone: , The Netherlands Isolatorweg 30 NL-1014 AS Amsterdam Telephone: Norway A/S Jernkroken 18 Postboks 85, Kalbakken NO-0902 Oslo Telephone: Poland BOHLER POLSKA Sp. z.o.o./co. Ltd. ul. Kolejowa 291, Dziekanów Polski, PL Lomianki Telephone: , -203, Portugal F RAMADA Aços e Industrias S.A. P.O. Box 10 PT-3881 Ovar Codex Telephone: Romania BÖHLER- Romania SRL Atomistilor Str. No com. Magurele, Jud. Ilfov. Telephone: Telefax: Russia TOOLING CIS 9A, Lipovaya Alleya, Office 509 RU Saint Petersburg Telephone: Slovakia Bohler-Uddeholm Slovakia s.r.o. divizia Csl.Armády ˇ 5622/5 SK Martin Telephone: +421 (0) Slovenia Representative office Divisione della Bohler Uddeholm Italia S.p.A. Via Palizzi, 90 IT Milano Telephone: Spain Guifré ES Badalona, Barcelona Telephone: Branch office Barrio San Martín de Arteaga,132 Pol.Ind. Torrelarragoiti ES Zamudio (Bizkaia) Telephone: Sweden TOOLING SVENSKA AB Aminogatan 25 SE Mölndal Telephone: Branch offices TOOLING SVENSKA AB Box 45 SE Anderstorp Telephone: TOOLING SVENSKA AB Box 148 SE Eskilstuna Telephone: TOOLING SVENSKA AB Aminogatan 25 SE Mölndal Telephone: TOOLING SVENSKA AB Nya Tanneforsvägen 96 SE Linköping Telephone: TOOLING SVENSKA AB Derbyvägen 22 SE Malmö Telephone: TOOLING SVENSKA AB Honnörsgatan 24 SE Växjö Telephone: Switzerland HERTSCH & CIE AG General Wille Strasse 19 CH-8027 Zürich Telephone: Turkey ASSAB Korkmaz Celik A.S. Organize Sanayi Bölgesi 2. Cadde No: 26 Y. Dudullu Umraniye TR-Istanbul Telephone:

31 HEAT TREATMENT America Argentina ACEROS BOEHLER S.A Mozart Centro Industrial Garin Garin-Prov. AR-Buenos Aires Telephone: Brazil AÇOS BOHLER- DO BRASIL LTDA DIV. Estrada Yae Massumoto, 353 CEP BR-Sao Bernardo do Campo - SP Brazil Telephone: , Canada Head Office & Warehouse BOHLER- LIMITED 2595 Meadowvale Blvd. Mississauga, ON L5N 7Y3 Telephone: Branch Warehouses BOHLER- LIMITED 3521 Rue Ashby St. Laurent, QC H4R 2K3 Telephone: BOHLER- LIMITED 730 Eaton Way - Unit #10 New Westminister, BC V3M 6J9 Telephone: Heat Treating BOHLER- THERMO-TECH 2645 Meadowvale Blvd. Mississauga, ON L5N 7Y4 Telephone: Colombia AXXECOL S.A. Carrera 35 No Apartado Aereo CO-Bogota 6 Telephone: ASTECO S.A. Carrera 54 No Apartado Aereo 663 CO-Medellin Telephone: Dominican Republic RAMCA, C. POR A. P-2289 P.O. Box Miami, Fl Telephone: domrep@assab.com Ecuador IVAN BOHMAN C.A. Apartado 1317 Km 6 1/2 Via a Daule Guayaquil Telephone: IVAN BOHMAN C.A. Casilla Postal Quito Telephone: El Salvador ACAVISA DE C.V. 25 Ave. Sur, no 763 Zona 1 SV-San Salvador Telephone: Guatemala IMPORTADORA ESCANDINAVA Apartado postal 11C GT-Guatemala City Telephone: guatemala@assab.com Honduras ACAVISA DE C.V. 25 Ave. Sur, no 763 Zona 1 SV-San Salvador Telephone: Mexico ACEROS BOHLER S.A. de C.V. Calle Ocho No 2, Letra C Fraccionamiento Industrial Alce Blanco C.P Naucalpan de Juarez MX-Estado de Mexico Telephone: Branch office BOHLER- MONTERREY, NUEVO LEON Lerdo de Tejada No.542 Colonia Las Villas MX San Nicolas de Los Garza, N.L. Telephone: Peru C.I.P.E.S.A Av. Oscar R. Benavides (ante Colonial) No PE-Lima 1 Telephone: peru@assab.com U.S.A. and Warehouse BOHLER- CORPORATION 2505 Millennium Drive Elgin IL Telephone: or Sales phone: Region East Warehouse BOHLER- CORPORATION 220 Cherry Street Shrewsbury MA Region Central Warehouse BOHLER- CORPORATION 548 Clayton Ct. Wood Dale IL Region West Warehouse BOHLER- CORPORATION 9331 Santa Fe Springs Road Santa Fe Springs, CA Venezuela PRODUCTOS HUMAR C.A. Av. Bolivar, Zona Industrial La Trinidad Edificio. Distribuidora Agrofor, C.A. Piso 3, VE-Caracas 1080 Telephone: or humar@assab.com Other Countries in America ASSAB INTERNATIONAL AB Box 42 SE Solna, Sweden Telephone: Asia & Pacific Australia BOHLER Australia McCredie Road Guildford NSW 2161 Private Bag 14 AU-Sydney Telephone: Bangladesh ASSAB INTERNATIONAL AB P.O. Box Jebel Ali AE-Dubai Telephone: North China ASSAB Tooling (Beijing) Co Ltd No.10A Rong Jing Dong Jie Beijing Economic Development Area Beijing , China Telephone: Branch offices ASSAB Tooling (Beijing) Ltd Dalian Branch 8 Huanghai Street, Haerbin Road Economic & Technical Develop. District Dalian , China Telephone: ASSAB Qingdao Office Room 2521, Kexin Mansion No. 228 Liaoning Road, Shibei District Qingdao , China Telephone: ASSAB Tianjin Office No.12 Puwangli Wanda Xincheng Xinyibai Road, Beichen District Tianjin , China Telephone: Central China ASSAB Tooling Technology (Shanghai) Co Ltd No Humin Road Xinzhuang Industrial Zone Shanghai , China Telephone: Branch offices ASSAB Tooling Technology (Ningbo) Co Ltd No. 218 Longjiaoshan Road Vehicle Part Industrial Park Ningbo Economic & Technical Dev. Zone Ningbo , China Telephone: ASSAB Tooling Technology (Chongqing) Co Ltd Plant C, Automotive Industrial lpark Chongqing Economic & Technological Development Zone Chongqing , China Telephone: South China ASSAB Steels (HK) Ltd Room Tower 2 Grand Central Plaza 138 Shatin Rural Committee Road Shatin NT - Hong Kong Telephone: Branch offices ASSAB Tooling (Dongguan) Co Ltd Northern District Song Shan Lake Science & Technology Industrial Park Dongguan , China Telephone: ASSAB Tooling (Xiamen) Co Ltd First Floor Universal Workshop No. 30 Huli Zone Xiamen , China Telephone: Hong Kong ASSAB Steels (HK) Ltd Room Grand Central Plaza, Tower Shatin Rural Committee Road Shatin NT, Hong Kong Telephone: India ASSAB Sripad Steels LTD T 303 D.A.V. Complex Mayur Vihar Ph I Extension IN-Delhi Telephone: ASSAB Sripad Steels LTD 709, Swastik Chambers Sion-Trombay Road Chembur IN-Mumbai Telephone: , ASSAB Sripad Steels LTD Padmalaya Towers Janaki Avenue M.R.C. Nagar IN-Chennai Telephone: ASSAB Sripad Steels LTD 19X, D. P. P. Road Naktola Post Office IN-Kolkata Telephone: +91 ( ASSAB Sripad Steels LTD Ground floor, Plot No Opp IDPL Factory Out Gate Balanagar IN-Hyderabad Telephone: +91 (40) Indonesia PT ASSAB Steels Indonesia Jl. Rawagelam III No. 5 Kawasan Industri Pulogadung Jakarta 13930, Indonesia Telephone:

32 HEAT TREATMENT Branch offices SURABAYA BRANCH Jl. Berbek Industri 1/23 Surabaya Industrial Estate, Rungkut Surabaya 60293, East Java, Indonesia Telephone: MEDAN BRANCH Komplek Griya Riatur Indah Blok A No.138 Jl. T. Amir Hamzah Halvetia Timur, Medan Telephone: /6 BANDUNG BRANCH Komp. Ruko Bumi Kencana Jl. Titian Kencana Blok E No.5 Bandung Telephone: TANGERANG BRANCH Pusat Niaga Cibodas Blok C No. 7 Tangerang Telephone: , SEMARANG BRANCH Jl. Imam Bonjol No.155 R.208 Semarang Telephone: Iran ASSAB INTERNATIONAL AB P.O. Box IR-1517 TEHRAN Telephone: Israel PACKER YADPAZ QUALITY STEELS Ltd P.O. Box 686 Ha-Yarkon St. 7, Industrial Zone IL YAVNE Telephone: Japan KK Atago East Building Nishi Shinbashi Minato-ku, Tokyo , Japan Telephone: Jordan ENGINEERING WAY Est. P.O. Box 874 Abu Alanda JO-AMMAN Telephone: engineeringway@assab.com Malaysia ASSAB Steels (Malaysia) Sdn Bhd Lot 19, Jalan Perusahaan 2 Batu Caves Industrial Estate Batu Caves Selangor Malaysia Telephone: Branch offices BUTTERWORTH BRANCH Plot 146a Jalan Perindustrial Bukit Minyak 7 Kawasan Perindustrial Bukit Minyak Bukit Mertajam, SPT Penang Telephone: JOHOR BRANCH No. 8, Jalan Persiaran Teknologi Taman Teknologi Senai Johor DT, Malaysia Telephone: New Zealand VIKING STEELS 25 Beach Road, Otahuhu P.O. Box , Onehunga NZ-Auckland Telephone: Pakistan ASSAB International AB P.O. Box Jebel Ali AE-Dubai Telephone: Philippines ASSOCIATED SWEDISH STEELS PHILS Inc. No. 3 E. Rodriguez Jr., Avenue Bagong Ilog, Pasig City Philippines Telephone: /2048 Republic of Korea ASSAB Steels (Korea) Co Ltd 116B-8L, 687-8, Kojan-dong Namdong-ku Incheon , Korea Telephone: Branch offices BUSAN BRANCH 14B-5L, , Songjeong-dong Kangseo-ku, Busan , Korea Telephone: DAEGU BRANCH Room 27, 7-Dong2 F Industry Materials Bldg.1629 Sangyeog-Dong, Buk-Ku Korea-Daegu Telephone: Lebanon WARDE STEEL & METALS SARL MET Charles Helou Av, Warde Bldg P.O. Box LB-Beirut Telephone: lebanon@assab.com Saudi Arabia ASSAB INTERNATIONAL AB P.O. Box SA-Riyadh Telephone: assab@emirates.net.ae Singapore Pacific ASSAB Pacific Pte Ltd 171, Chin Swee Road No , SAN Centre SG-Singapore Telephone: Jurong ASSAB Steels Singapore (Pte) Ltd 18, Penjuru Close SG Singapore Telephone: Sri Lanka GERMANIA COLOMBO PRIVATE Ltd. 451/A Kandy Road LK-Kelaniya Telephone: Syria WARDE STEEL & METALS SARL MET Charles Helou Av, Warde Bldg P.O. Box LB-Beirut Telephone: lebanon@assab.com Taiwan ASSAB Steels (Taiwan) Co Ltd No. 112 Wu Kung 1st Rd. Wu Ku Industry Zone TW-Taipei , Taiwan (R.O.C.) Telephone: Branch offices NANTOU BRANCH No. 10, Industry South 5th Road Nan Kang Industry Zone Nantou , Taiwan (R.O.C.) Telephone: TAINAN BRANCH No. 180, Yen He Street, Yong Kang City Tainan , Taiwan (R.O.C.) Telephone: Thailand ASSAB Steels (Thailand) Ltd 9/8 Soi Theedinthai, Taeparak Road, Bangplee, Samutprakarn 10540, Thailand Telephone: , United Arab Emirates ASSAB INTERNATIONAL AB P.O. Box Jebel Ali AE-Dubai Telephone: Vietnam CAM Trading Steel Co Ltd 90/8 Block 5, Tan Thoi Nhat Ward District 12, Ho Chi Minh City Vietnam Telephone: Other Asia ASSAB INTERNATIONAL AB Box 42 E Solna, Sweden Telephone: Africa Egypt MISR SWEDEN FOR ENGINEERING IND. Montaser Project No 20 Flat No 14 Al Ahram Street-El Tabia EG-Giza Cairo Telephone: Kenya SANDVIK Kenya Ltd P.O. Box Post code KE-Nairobi Telephone: info@sandvik.co.ke Morocco MCM Distribution 4 Bis, Rue Z.I Charguia 1 TN-Tunis Telephone: South Africa Africa (Pty.) Ltd. P.O. Box 539 ZA-1600 Isando/Johannesburg Telephone: Tunisia MCM Distribution 4 Bis, Rue Z.I Charguia 1 TN-Tunis Telephone: Zimbabwe Representative office: Africa (Pty.) Ltd. P.O. Box 539 ZA-1600 Isando/Johannesburg Telephone: Other African Countries ASSAB INTERNATIONAL AB Box 42 SE Solna, Sweden Telephone:

33 Network of excellence Uddeholm is present on every continent. This ensures you high-quality Swedish tool steel and local support wherever you are. Assab is our wholly-owned subsidiary and exclusive sales channel, representing Uddeholm in various parts of the world. Together we secure our position as the world s leading supplier of tooling materials.

34 HAGFORS KLARTEXT U0712XX Uddeholm is the world s leading supplier of tooling materials. This is a position we have reached by improving our customers everyday business. Long tradition combined with research and product development equips Uddeholm to solve any tooling problem that may arise. It is a challenging process, but the goal is clear to be your number one partner and tool steel provider. Our presence on every continent guarantees you the same high quality wherever you are. Assab is our wholly-owned subsidiary and exclusive sales channel, representing Uddeholm in various parts of the world. Together we secure our position as the world s leading supplier of tooling materials. We act worldwide, so there is always an Uddeholm or Assab representative close at hand to give local advice and support. For us it is all a matter of trust in long-term partnerships as well as in developing new products. Trust is something you earn, every day. For more information, please visit or

35 PHOTO-ETCHING OF TOOL STEEL

36 Contents The photo-etching process... 3 Advantages of textured surfaces... 3 Test programme... 4 Summary... 7 This information is based on our present state of knowledge and is intended to provide general notes on our products and their uses. It should not therefore be construed as a warranty of specific properties of the products described or a warranty for fitness for a particular purpose. Classified according to EU Directive 1999/45/EC For further information see our Material Safety Data Sheets. Edition 4, The latest revised edition of this brochure is the English version, which is always published on our web site SS-EN ISO 9001 SS-EN ISO 14001

37 PHOTO-ETCHING OF TOOL STEEL Introduction A wide variety of moulded parts are produced with a patterned or textured surface. Normally, the pattern is reproduced on the moulding surfaces of the tool by the photo-etching process. The photo-etching process Published information about the techniques employed by the specialist photo-etching companies is very limited. Essentially, however, the required pattern is transferred to the moulding surface by a photographic process. The pattern is then etched to the required depth by the application of an appropriate acid, under closely controlled condition. Photo-etching can be performed both on complete tools or on specified areas of the tool only. Photo-etching enables a wide variety of different patterns to be produced in virtually any tool. The patterns may resemble leather or wood graining, for example, or be a straight forward line pattern with varying directions and depths. Typical applications are for car interior fittings, etc., and plastic casings for different kinds of machines and instruments. In recent years, photoetching has become an increasingly popular and practical method for imparting attractive and appealing surfaces to different products. Advantages of textured surfaces A textured surface hides minor surface flaws which may occur in manufacture or during further treatment and fitting. Because of this, the rejection rate for the finished products is lower. Moreover, photo-etching replaces the lengthy and expensive finish polishing process. The product is given an aesthetically attractive surface finish. The surface is easier to grip than a bright surface, which facilitates holding and handling. Irritating reflections are largely avoided. A further advantage is that finger prints and similar marks do not show up as much as on a bright surface. This brochure deals with photoetching as a finishing process, with the possibilities offered by it and with the factors which must be taken into account to ensure a satisfactory result. These factors were determined by a test programme carried out with the cooperation of a leading photo-etching company. Textured mould and moulded part for automobile steering wheel. 3

38 PHOTO-ETCHING OF TOOL STEEL Test programme To ensure that the toolmaker and the tool-user gets the optimum results from Uddeholm tool steels that are photoetched, Uddeholm Tooling has carried out a series of tests. The test programme examined a number of influencing factors, including: Photo-etching of different tool steel grades, annealed and hardened Flame-hardening, welding and EDM Grain flow direction of the tool steel Variations in steel analysis and cleanliness Material size Several different tool steels were studied, by etching plates measuring 50 x 60 mm (2" x 2 1/4"). All surfaces were ground with a 280-grain grinding wheel. In one set of tests, all the specimens were etched under identical conditions in order to grade the etchability in terms of the amount of stock removed from the different material. After this, etching conditions were varied with the aim of producing optimum etching results. The steels listed below have been examined in the first instance as longitudinal specimens (in the rolling direction of the material) in the softannealed state and also according to the parameters shown in the chart. PHOTO-ETCHING OF DIFFERENT STEEL GRADES Results The etching results were assessed taking into account the etching depth, pattern similarity, side-etching effect and surface appearance. The surfaces have not only been visually appraised but also examined at a high magnification in order to detect and study any microscopic differences. Photograph of a patterned surface. Annealed material Depending on the type of etching method used, a special etching media may be needed when etching steels with good corrosion resistance. This is valid for Uddeholm Stavax ESR, Uddeholm Mirrax ESR, Uddeholm Corrax and Uddeholm Elmax. However, owing to its alloy content also Uddeholm Orvar Supreme and Uddeholm Calmax gives weaker etching than other grades when the standard media is used and in view of this the special media is recommended. Uddeholm steel grade AISI Other parameters studied RIGOR A2 Hardness: 60 HRC High retained austenite content. CALMAX Hardness: 57 HRC ORVAR SUPREME H13 Hardness: 52 HRC Rough- and fine-spark-machined. IMPAX SUPREME P20 Analysis variation. Flame-hardened to 54 HRC. Surface and centre of large dimension. Welded with IMPAX electrode. STAVAX ESR 420 Hardness: 300 HB 55 HRC Welded with STAVAX electrode. ELMAX Hardness: 58 HRC Soft-annealed Hardened to 55 HRC Uddeholm Stavax ESR textured with special media. 4

39 PHOTO-ETCHING OF TOOL STEEL The other steels examined show good results upon visual examination after having been etched by the standard process. When the surfaces are examined under the microscope (9 x magnification), some minor differences can be observed. The observed differences normally have no practical significance. They nevertheless show that if a tool with inserts which are to be etched with the same pattern is being made it is advisable for material from the same bar or block to be used in all parts in order to get a pattern of identical and uniform appearance on the moulding. (See Grain flow direction of the tool steel, page 6.) Hardened material All grades were examined in the fully hardened condition. Here, too, the four grades Uddeholm Orvar Supreme, Uddeholm Calmax, Uddeholm Stavax ESR and Uddeholm Elmax differ from the others in respect of etchability. When the surfaces are studied under the microscope, some tendency to streakiness is discernible in some of the hardened specimens. The streaks are parallel to the direction of rolling, and the phenomenon is an expression of the normal rolling direction which appears in alloyed tool steels. The streakiness, however, is of such modest proportion that it lacks significance when using tool steels with normal degrees of segregation, but at the same time it demonstrates the importance of selecting a steel that is as homogeneous and uniformly worked as possible. The presence of a high content of retained austenite in a hardened tool is normally a disadvantage. Etchability, however, is not affected even by a relatively high content of retained austenite according to a test performed on Uddeholm Rigor. Flame-hardened material The influence of flame-hardening on the etching of Uddeholm Impax Supreme was also studied and here there is a decided difference between the locally hardened zone and the hardened and tempered basic material. In the flame-hardened zone, a faint streakiness similar to that in hardened specimens is discernible. In addition, there is a difference in etching depth between flame-hardened and hardened and tempered material. Basic material. Flame-hardened zone. Flame-hardening, therefore, should be carried out after photo-etching, wherever possible. Welding In certain circumstances it may be necessary to weld a tool, for instance for repair purposes. Welding always severely affects the uniform structure of the parent material. The weld metal and the base steel must be similar in composition if a welded surface of a plastic mould is to be textured via photo-etching. If not, the response to etching will vary between the weld and the base metal and this will result in a witness mark on the plastic component. Welds in Uddeholm Impax Supreme, Uddeholm Stavax ESR, Uddeholm Mirrax ESR and Uddeholm Calmax with Impax Weld, Stavax Weld or Calmax Weld (or TIG-Weld) or Corrax TIG-Weld will normally not be discernible after photo-etching. More information on welding is given in the brochure Welding of Tool Steel. Areas which have been welded should always be clearly indicated to the photoetching company. Electrical discharge machining (EDM) If EDM is not carried out in the right way, some defects may remain in the surface of the material. The influence of spark-erosion on photo-etchability has therefore been studied. Specimens with both a rough-sparked and a fine-sparked surface were tempered at 250 C (480 F). Photo-etching on a rough-sparked surface gives a very poor result. Even after a careful fine-sparking operation, it may be difficult to get an acceptable result. Nitrided material When a tool or insert is to be nitrided, this must be done after photo-etching. Photo-etching on a rough-sparked surface. 5

40 PHOTO-ETCHING OF TOOL STEEL Tempering does not give an appreciable improvement. If doubts are entertained as to how the spark-machining has been carried out the material should always be ground or polished to remove any residual traces of the sparking. Special test kits are available for checking removal of residual effects after spark-erosion. Areas which have been spark-eroded should be clearly indicated to the photo-etching company. a very low sulphur content (max. 0,010%). There are, however, similar types of steels with far higher sulphur contents (0,08%), which can give rise to streakiness in photo-etching, as evident from he following photograph. Grain flow direction of the tool steel Calmax has been examined on both the lengthwise and crosswise direction in the soft-annealed state. No appreciable difference between the specimens was observed. For fine patterns, however, experience shows that some difference can occur. Where it is important that photo-etched patterns on different mould parts match exactly, e.g. when using inserts, the following procedure is strongly recommended: 1. Make all parts to be textured from the same bar or block of steel 2. Make sure that all surfaces to be textured have the longitudinal grain flow in the same direction MATERIAL SIZE When manufacturing materials in heavy sections differences in the microstructure of the material can be observed between the surface and the centre. In order to study the influence of these differences on the photo-etchability of Impax Supreme in the size 500 mm (20") dia., specimens from the surface and centre were photo-etched. The photograph shows streakiness in photo-etching of a pre-hardened mould steel with high sulphur content. VARIATIONS IN STEEL ANALYSIS AND CLEANLINESS There are always minor differences in the analysis of every steel to occur from one heat to another. In this context, two extremes in the analysis of Uddeholm Impax Supreme were examined, but no differences in the results of the etching were observable. Normal variations in analysis of Uddeholm Tooling tool steels thus have no influence on photo-etchability. The cleanliness of the steel, and especially its sulphur content, can affect the appearance of photo-etched patterns. Uddeholm Impax Supreme prehardened mould steel is particularly suitable for photo-etching for two reasons: it has a very clean microstructure, being subjected to a vacuum degassing process during manufacture; it also has Surface. Centre. Uddeholm Impax Supreme Ø 500 mm (20"). No difference between the two specimens was observable. A wood-grain texture on a moulded handle for a saucepan. 6

41 PHOTO-ETCHING OF TOOL STEEL Summary Several different grades of Uddeholm Tooling tool steels have been tested for photo-etchability. The results of the etching tests and other experience gained can be summarized as follows: All of the grades examined can be photo-etched with satisfactory results. There are certain microscopic differences, but these normally have no practical significance whatsoever. Uddeholm Orvar Supreme, Uddeholm Calmax, Uddeholm Stavax ESR, Uddeholm Mirrax ESR and Uddeholm Elmax should be etched by a special process. If nitriding is to be carried out it must be done after photo-etching. Flame-hardening prior to photoetching should be avoided, since the pattern will be etched differently in the flame-hardened zone and in hardened and tempered base material. A welded tool can in certain circumstances be photo-etched, but this is conditional upon using the same material in the weld as in the parent material. Spark-machined surfaces should be ground or polished in order to be on the safe side. A poor etching result will be obtained on surfaces marred by residual traces of spark-machining. Areas of tools which have been flame-hardened, welded or sparkeroded should always be clearly indicated to the photo-etching company. If several parts are included in a tool and are to be photo-etched with exactly the same pattern, the same grade of material and the same grain flow direction should be chosen for all the parts. Normal variations in analysis for the same grade of steel have no adverse influence. Steels with a clean microstructure and low sulphur content give the most accurate and consistent pattern reproduction. Different sizes of starting material of one and the same grade do not usually show any differences. Initial machining operations should be followed by stress-relieving prior to finish-machining. Coarser abrasives than 220 grain must not be used on surfaces which are to be photo-etched. Photo-textured body for Polaroid instant camera. Mould material: Uddeholm Stavax ESR. Part of an automobile steering wheel produced from a photo-etched Uddeholm Impax Supreme mould. 7

42 HEAT TREATMENT Europe Austria Representative office Albstraße 10 DE Neuhausen Telephone: Belgium Europark Oost 7 B-9100 Sint-Niklaas Telephone: Croatia BÖHLER Zagreb d.o.o za trgovinu Zitnjak b.b Zagreb Telephone: Telefax: Czech Republic BÖHLER CZ s.r.o. Division Uddeholm U Silnice Praha 6, Ruzyne Telephone: ,8 Denmark A/S Kokmose 8, Bramdrupdam DK-6000 Kolding Telephone: Estonia TOOLING AB Silikatsiidi 7 EE Tallinn Telephone: Finland OY AB Ritakuja 1, PL 57 FI VANTAA Telephone: France Z.I. de Mitry-Compans, 12 rue Mercier, FR Mitry Mory Cedex Telephone: +33 (0) Branch offices S.A. 77bis, rue de Vesoul La Nef aux Métiers FR Besançon Telephone: +33 (0) LE POINT ACIERS - Aciers à outils Z.I. du Recou, Avenue de Champlevert FR GRIGNY Telephone: +33 (0) LE POINT ACIERS - Aciers à outils Z.I. Nord 27, rue François Rochaix FR OYONNAX Telephone: +33 (0) Germany Hansaallee 321 DE Düsseldorf Telephone: Branch offices Falkenstraße 21 DE Bad Soden/TS Telephone: Albstraße 10 DE Neuhausen Telephone: Friederikenstraße 14b DE Harzgerode Telephone: Great Britain DIVISION BOHLER- (UK) LIMITED European Business Park Taylors Lane, Oldbury GB-West Midlands B69 2BN Telephone: Telefax: Greece STASSINOPOULOS- STEEL TRADING S.A. 20, Athinon Street GR-Piraeus Telephone: SKLERO S.A. Heat Treatment and Trading of Steel Uddeholm Tool Steels Industrial Area of Thessaloniki P.O. Box 1123 GR Sindos, Thessaloniki Telephone: Hungary TOOLING/BOK Dunaharaszti, Jedlik Ányos út 25 HU-2331 Dunaharaszti 1. Pf. 110 Telephone/fax: Ireland : DIVISION BOHLER- (UK) LIMITED European Business Park Taylors Lane, Oldbury UK-West Midlands B69 2BN Telephone: Telefax: Dublin: Telephone: Italy Divisione della Bohler Uddeholm Italia S.p.A. Via Palizzi, 90 IT Milano Telephone: Latvia TOOLING LATVIA SIA Piedrujas Street 7 LV-1035 Riga Telephone: latvia@assab.com Lithuania TOOLING AB BE PLIENAS IR METALAI T. Masiulio 18B LT Kaunas Telephone: , The Netherlands Isolatorweg 30 NL-1014 AS Amsterdam Telephone: Norway A/S Jernkroken 18 Postboks 85, Kalbakken NO-0902 Oslo Telephone: Poland BOHLER POLSKA Sp. z.o.o./co. Ltd. ul. Kolejowa 291, Dziekanów Polski, PL Lomianki Telephone: , -203, Portugal F RAMADA Aços e Industrias S.A. P.O. Box 10 PT-3881 Ovar Codex Telephone: Romania BÖHLER- Romania SRL Atomistilor Str. No com. Magurele, Jud. Ilfov. Telephone: Telefax: Russia TOOLING CIS 9A, Lipovaya Alleya, Office 509 RU Saint Petersburg Telephone: Slovakia Bohler-Uddeholm Slovakia s.r.o. divizia Csl.Armády ˇ 5622/5 SK Martin Telephone: +421 (0) Slovenia Representative office Divisione della Bohler Uddeholm Italia S.p.A. Via Palizzi, 90 IT Milano Telephone: Spain Guifré ES Badalona, Barcelona Telephone: Branch office Barrio San Martín de Arteaga,132 Pol.Ind. Torrelarragoiti ES Zamudio (Bizkaia) Telephone: Sweden TOOLING SVENSKA AB Aminogatan 25 SE Mölndal Telephone: Branch offices TOOLING SVENSKA AB Box 45 SE Anderstorp Telephone: TOOLING SVENSKA AB Box 148 SE Eskilstuna Telephone: TOOLING SVENSKA AB Aminogatan 25 SE Mölndal Telephone: TOOLING SVENSKA AB Nya Tanneforsvägen 96 SE Linköping Telephone: TOOLING SVENSKA AB Derbyvägen 22 SE Malmö Telephone: TOOLING SVENSKA AB Honnörsgatan 24 SE Växjö Telephone: Switzerland HERTSCH & CIE AG General Wille Strasse 19 CH-8027 Zürich Telephone: Turkey ASSAB Korkmaz Celik A.S. Organize Sanayi Bölgesi 2. Cadde No: 26 Y. Dudullu Umraniye TR-Istanbul Telephone:

43 HEAT TREATMENT America Argentina ACEROS BOEHLER S.A Mozart Centro Industrial Garin Garin-Prov. AR-Buenos Aires Telephone: Brazil AÇOS BOHLER- DO BRASIL LTDA DIV. Estrada Yae Massumoto, 353 CEP BR-Sao Bernardo do Campo - SP Brazil Telephone: , Canada Head Office & Warehouse BOHLER- LIMITED 2595 Meadowvale Blvd. Mississauga, ON L5N 7Y3 Telephone: Branch Warehouses BOHLER- LIMITED 3521 Rue Ashby St. Laurent, QC H4R 2K3 Telephone: BOHLER- LIMITED 730 Eaton Way - Unit #10 New Westminister, BC V3M 6J9 Telephone: Heat Treating BOHLER- THERMO-TECH 2645 Meadowvale Blvd. Mississauga, ON L5N 7Y4 Telephone: Colombia AXXECOL S.A. Carrera 35 No Apartado Aereo CO-Bogota 6 Telephone: ASTECO S.A. Carrera 54 No Apartado Aereo 663 CO-Medellin Telephone: Dominican Republic RAMCA, C. POR A. P-2289 P.O. Box Miami, Fl Telephone: domrep@assab.com Ecuador IVAN BOHMAN C.A. Apartado 1317 Km 6 1/2 Via a Daule Guayaquil Telephone: IVAN BOHMAN C.A. Casilla Postal Quito Telephone: El Salvador ACAVISA DE C.V. 25 Ave. Sur, no 763 Zona 1 SV-San Salvador Telephone: Guatemala IMPORTADORA ESCANDINAVA Apartado postal 11C GT-Guatemala City Telephone: guatemala@assab.com Honduras ACAVISA DE C.V. 25 Ave. Sur, no 763 Zona 1 SV-San Salvador Telephone: Mexico ACEROS BOHLER S.A. de C.V. Calle Ocho No 2, Letra C Fraccionamiento Industrial Alce Blanco C.P Naucalpan de Juarez MX-Estado de Mexico Telephone: Branch office BOHLER- MONTERREY, NUEVO LEON Lerdo de Tejada No.542 Colonia Las Villas MX San Nicolas de Los Garza, N.L. Telephone: Peru C.I.P.E.S.A Av. Oscar R. Benavides (ante Colonial) No PE-Lima 1 Telephone: peru@assab.com U.S.A. and Warehouse BOHLER- CORPORATION 2505 Millennium Drive Elgin IL Telephone: or Sales phone: Region East Warehouse BOHLER- CORPORATION 220 Cherry Street Shrewsbury MA Region Central Warehouse BOHLER- CORPORATION 548 Clayton Ct. Wood Dale IL Region West Warehouse BOHLER- CORPORATION 9331 Santa Fe Springs Road Santa Fe Springs, CA Venezuela PRODUCTOS HUMAR C.A. Av. Bolivar, Zona Industrial La Trinidad Edificio. Distribuidora Agrofor, C.A. Piso 3, VE-Caracas 1080 Telephone: or humar@assab.com Other Countries in America ASSAB INTERNATIONAL AB Box 42 SE Solna, Sweden Telephone: Asia & Pacific Australia BOHLER Australia McCredie Road Guildford NSW 2161 Private Bag 14 AU-Sydney Telephone: Bangladesh ASSAB INTERNATIONAL AB P.O. Box Jebel Ali AE-Dubai Telephone: North China ASSAB Tooling (Beijing) Co Ltd No.10A Rong Jing Dong Jie Beijing Economic Development Area Beijing , China Telephone: Branch offices ASSAB Tooling (Beijing) Ltd Dalian Branch 8 Huanghai Street, Haerbin Road Economic & Technical Develop. District Dalian , China Telephone: ASSAB Qingdao Office Room 2521, Kexin Mansion No. 228 Liaoning Road, Shibei District Qingdao , China Telephone: ASSAB Tianjin Office No.12 Puwangli Wanda Xincheng Xinyibai Road, Beichen District Tianjin , China Telephone: Central China ASSAB Tooling Technology (Shanghai) Co Ltd No Humin Road Xinzhuang Industrial Zone Shanghai , China Telephone: Branch offices ASSAB Tooling Technology (Ningbo) Co Ltd No. 218 Longjiaoshan Road Vehicle Part Industrial Park Ningbo Economic & Technical Dev. Zone Ningbo , China Telephone: ASSAB Tooling Technology (Chongqing) Co Ltd Plant C, Automotive Industrial lpark Chongqing Economic & Technological Development Zone Chongqing , China Telephone: South China ASSAB Steels (HK) Ltd Room Tower 2 Grand Central Plaza 138 Shatin Rural Committee Road Shatin NT - Hong Kong Telephone: Branch offices ASSAB Tooling (Dongguan) Co Ltd Northern District Song Shan Lake Science & Technology Industrial Park Dongguan , China Telephone: ASSAB Tooling (Xiamen) Co Ltd First Floor Universal Workshop No. 30 Huli Zone Xiamen , China Telephone: Hong Kong ASSAB Steels (HK) Ltd Room Grand Central Plaza, Tower Shatin Rural Committee Road Shatin NT, Hong Kong Telephone: India ASSAB Sripad Steels LTD T 303 D.A.V. Complex Mayur Vihar Ph I Extension IN-Delhi Telephone: ASSAB Sripad Steels LTD 709, Swastik Chambers Sion-Trombay Road Chembur IN-Mumbai Telephone: , ASSAB Sripad Steels LTD Padmalaya Towers Janaki Avenue M.R.C. Nagar IN-Chennai Telephone: ASSAB Sripad Steels LTD 19X, D. P. P. Road Naktola Post Office IN-Kolkata Telephone: +91 ( ASSAB Sripad Steels LTD Ground floor, Plot No Opp IDPL Factory Out Gate Balanagar IN-Hyderabad Telephone: +91 (40) Indonesia PT ASSAB Steels Indonesia Jl. Rawagelam III No. 5 Kawasan Industri Pulogadung Jakarta 13930, Indonesia Telephone:

44 HEAT TREATMENT Branch offices SURABAYA BRANCH Jl. Berbek Industri 1/23 Surabaya Industrial Estate, Rungkut Surabaya 60293, East Java, Indonesia Telephone: MEDAN BRANCH Komplek Griya Riatur Indah Blok A No.138 Jl. T. Amir Hamzah Halvetia Timur, Medan Telephone: /6 BANDUNG BRANCH Komp. Ruko Bumi Kencana Jl. Titian Kencana Blok E No.5 Bandung Telephone: TANGERANG BRANCH Pusat Niaga Cibodas Blok C No. 7 Tangerang Telephone: , SEMARANG BRANCH Jl. Imam Bonjol No.155 R.208 Semarang Telephone: Iran ASSAB INTERNATIONAL AB P.O. Box IR-1517 TEHRAN Telephone: Israel PACKER YADPAZ QUALITY STEELS Ltd P.O. Box 686 Ha-Yarkon St. 7, Industrial Zone IL YAVNE Telephone: Japan KK Atago East Building Nishi Shinbashi Minato-ku, Tokyo , Japan Telephone: Jordan ENGINEERING WAY Est. P.O. Box 874 Abu Alanda JO-AMMAN Telephone: engineeringway@assab.com Malaysia ASSAB Steels (Malaysia) Sdn Bhd Lot 19, Jalan Perusahaan 2 Batu Caves Industrial Estate Batu Caves Selangor Malaysia Telephone: Branch offices BUTTERWORTH BRANCH Plot 146a Jalan Perindustrial Bukit Minyak 7 Kawasan Perindustrial Bukit Minyak Bukit Mertajam, SPT Penang Telephone: JOHOR BRANCH No. 8, Jalan Persiaran Teknologi Taman Teknologi Senai Johor DT, Malaysia Telephone: New Zealand VIKING STEELS 25 Beach Road, Otahuhu P.O. Box , Onehunga NZ-Auckland Telephone: Pakistan ASSAB International AB P.O. Box Jebel Ali AE-Dubai Telephone: Philippines ASSOCIATED SWEDISH STEELS PHILS Inc. No. 3 E. Rodriguez Jr., Avenue Bagong Ilog, Pasig City Philippines Telephone: /2048 Republic of Korea ASSAB Steels (Korea) Co Ltd 116B-8L, 687-8, Kojan-dong Namdong-ku Incheon , Korea Telephone: Branch offices BUSAN BRANCH 14B-5L, , Songjeong-dong Kangseo-ku, Busan , Korea Telephone: DAEGU BRANCH Room 27, 7-Dong2 F Industry Materials Bldg.1629 Sangyeog-Dong, Buk-Ku Korea-Daegu Telephone: Lebanon WARDE STEEL & METALS SARL MET Charles Helou Av, Warde Bldg P.O. Box LB-Beirut Telephone: lebanon@assab.com Saudi Arabia ASSAB INTERNATIONAL AB P.O. Box SA-Riyadh Telephone: assab@emirates.net.ae Singapore Pacific ASSAB Pacific Pte Ltd 171, Chin Swee Road No , SAN Centre SG-Singapore Telephone: Jurong ASSAB Steels Singapore (Pte) Ltd 18, Penjuru Close SG Singapore Telephone: Sri Lanka GERMANIA COLOMBO PRIVATE Ltd. 451/A Kandy Road LK-Kelaniya Telephone: Syria WARDE STEEL & METALS SARL MET Charles Helou Av, Warde Bldg P.O. Box LB-Beirut Telephone: lebanon@assab.com Taiwan ASSAB Steels (Taiwan) Co Ltd No. 112 Wu Kung 1st Rd. Wu Ku Industry Zone TW-Taipei , Taiwan (R.O.C.) Telephone: Branch offices NANTOU BRANCH No. 10, Industry South 5th Road Nan Kang Industry Zone Nantou , Taiwan (R.O.C.) Telephone: TAINAN BRANCH No. 180, Yen He Street, Yong Kang City Tainan , Taiwan (R.O.C.) Telephone: Thailand ASSAB Steels (Thailand) Ltd 9/8 Soi Theedinthai, Taeparak Road, Bangplee, Samutprakarn 10540, Thailand Telephone: , United Arab Emirates ASSAB INTERNATIONAL AB P.O. Box Jebel Ali AE-Dubai Telephone: Vietnam CAM Trading Steel Co Ltd 90/8 Block 5, Tan Thoi Nhat Ward District 12, Ho Chi Minh City Vietnam Telephone: Other Asia ASSAB INTERNATIONAL AB Box 42 E Solna, Sweden Telephone: Africa Egypt MISR SWEDEN FOR ENGINEERING IND. Montaser Project No 20 Flat No 14 Al Ahram Street-El Tabia EG-Giza Cairo Telephone: Kenya SANDVIK Kenya Ltd P.O. Box Post code KE-Nairobi Telephone: info@sandvik.co.ke Morocco MCM Distribution 4 Bis, Rue Z.I Charguia 1 TN-Tunis Telephone: South Africa Africa (Pty.) Ltd. P.O. Box 539 ZA-1600 Isando/Johannesburg Telephone: Tunisia MCM Distribution 4 Bis, Rue Z.I Charguia 1 TN-Tunis Telephone: Zimbabwe Representative office: Africa (Pty.) Ltd. P.O. Box 539 ZA-1600 Isando/Johannesburg Telephone: Other African Countries ASSAB INTERNATIONAL AB Box 42 SE Solna, Sweden Telephone:

45 Network of excellence Uddeholm is present on every continent. This ensures you high-quality Swedish tool steel and local support wherever you are. Assab is our wholly-owned subsidiary and exclusive sales channel, representing Uddeholm in various parts of the world. Together we secure our position as the world s leading supplier of tooling materials.

46 HAGFORS KLARTEXT U0712XX Uddeholm is the world s leading supplier of tooling materials. This is a position we have reached by improving our customers everyday business. Long tradition combined with research and product development equips Uddeholm to solve any tooling problem that may arise. It is a challenging process, but the goal is clear to be your number one partner and tool steel provider. Our presence on every continent guarantees you the same high quality wherever you are. Assab is our wholly-owned subsidiary and exclusive sales channel, representing Uddeholm in various parts of the world. Together we secure our position as the world s leading supplier of tooling materials. We act worldwide, so there is always an Uddeholm or Assab representative close at hand to give local advice and support. For us it is all a matter of trust in long-term partnerships as well as in developing new products. Trust is something you earn, every day. For more information, please visit or

47 HEAT TREATMENT OF TOOL STEEL

48 Contents What is tool steel?... 3 Hardening and tempering... 3 Dimensional and shape stability... 7 Surface treatment... 8 Testing of mechanical properties Some words of advice to tool designers This information is based on our present state of knowledge and is intended to provide general notes on our products and their uses. It should not therefore be construed as a warranty of specific properties of the products described or a warranty for fitness for a particular purpose. Classified according to EU Directive 1999/45/EC For further information see our Material Safety Data Sheets. Edition 6, The latest revised edition of this brochure is the English version, which is always published on our web site SS-EN ISO 9001 SS-EN ISO 14001

49 HEAT TREATMENT The purpose of this brochure is to provide some idea of how tool steel is heat treated and how it behaves. Special attention is paid to hardness, toughness and dimensional stability. What is tool steel? Uddeholm has concentrated its tool steel range on high alloyed types of steel, intended primarily for purposes such as plastics moulding, blanking and forming, die casting, extrusion, forging and wood-working. Conventional high speed steels and powder metallurgy (PM) steels are also included in the range. Tool steel is normally delivered in the soft annealed condition. This is to make the material easy to machine with cutting tools and to give it a microstructure suitable for hardening. The microstructure consists of a soft matrix in which carbides are embedded. In carbon steel, these carbides consist of iron carbide, while in the alloyed steel they are chromium (Cr), tungsten (W), molybdenum (Mo) or vanadium (V) carbides, depending on the composition of the steel. Carbides are compounds of carbon and these alloying elements and are characterized by very high hardness. A higher carbide content means higher resistance to wear. In alloy steels, it is important that the carbides are evenly distributed. Other alloying elements are also used in tool steel, such as cobalt (Co) and nickel (Ni), but these do not form carbides. Cobalt is normally used to improve red hardness in high speed steels, nickel to improve through-hardening properties. Hardening and tempering When a tool is hardened, many factors influence the result. SOME THEORETICAL ASPECTS In soft annealed tool steel, most of the alloying elements are bound up with carbon in carbides. In addition to these there are the alloying elements cobalt and nickel, which do not form carbides but are instead dissolved in the matrix. When the steel is heated for hardening, the basic idea is to dissolve the carbides to such a degree that the matrix acquires an alloying content that gives the hardening effect without becoming coarse grained and brittle. = Iron atoms = Possible positions for carbon atoms 2,86 A Unit cell in a ferrite crystal Body centred cubic (BCC) 3,57 A Unit cell in an austenite crystal Face centred cubic (FCC) 2,85 A Unit cell in a martensite crystal 2.98 A Note that the carbides are partially dissolved. This means that the matrix becomes alloyed with carbon and carbide-forming elements. When the steel is heated to the hardening temperature (austenitizing temperature), the carbides are partially dissolved, and the matrix is also altered. It is transformed from ferrite to austenite. This means that the iron atoms change their position in the atomic lattice and make room for atoms of carbon and alloying elements. The carbon and alloying elements from the carbides are dissolved in the matrix. If the steel is quenched sufficiently rapid in the hardening process, the carbon atoms do not have time to reposition themselves to allow the reforming of ferrite from austenite, i.e. as in annealing. Instead, they are fixed in positions where they really do not have enough room, and the result is high microstresses that can be defined as increased hardness. This hard structure is called martensite. Thus, martensite can be seen as a forced solution of carbon in ferrite. When a steel is hardened, the matrix is not completely converted into martensite. Some austenite is always left and is called retained austenite. The amount increases with increasing alloying content, higher hardening temperature and longer soaking times. After quenching, the steel has a microstructure consisting of martensite, retained austenite and carbides. This structure contains inherent stresses that can easily cause cracking. But this can be prevented by reheating the steel to a certain temperature, reducing the stresses and transforming the retained austenite to an extent that depends upon the reheating temperature. This reheating after hardening is called tempering. Hardening of a tool steel should always be followed immediately by tempering. It should be noted that tempering at low temperatures only affects the martensite, while tempering at high temperature also affects the retained austenite. After one tempering at high temperature, the microstructure consists of tempered martensite, newlyformed martensite, some retained austenite and carbides. 3

50 HEAT TREATMENT Precipitated secondary (newly formed) carbides and newly formed martensite can increase hardness during hightemperature tempering. Typical of this is the so called secondary hardening of e.g. high speed steel and high alloyed tool steels. Hardness C B Tempering temperature A = martensite tempering B = carbide precipitation C = transformation of retained austenite to martensite D = tempering diagram for high speed steel and high alloy tool steel A+B+C = D The diagram shows the influence of different parameters on the secondary hardening. Tool steel should always be doubletempered. The second tempering takes care of the newly formed martensite formed after the first tempering. Three tempers are recommended for high speed steel with a high carbon content. D A HOW HARDENING AND TEMPERING IS DONE IN PRACTICE Distortion due to hardening must be taken into consideration when a tool is rough-machined. Rough machining causes local heating and mechanical working of the steel, which gives rise to inherent stresses. This is not serious on a symmetrical part of simple design, but can be significant in asymmetrical machining, for example of one half of a die casting die. Here, stress relieving is always recommended. Stress relieving This treatment is done after rough machining and entails heating to C ( F). The material should be heated until it has achieved uniform temperature all the way through and then cooled slowly, for example in a furnace. The idea behind stress relieving is that the yield strength of the material at the elevated temperature is so low that the material cannot resist the inherent stresses. The yield strength is exceeded and these stresses are released, resulting in a greater or lesser degree of distortion. The correct work sequence is: rough machining, stress relieving and semifinish machining. The excuse that stress relieving takes too much time is hardly valid. Rectifying a part during semifinish machining of an annealed material is with few exceptions cheaper than making dimensional adjustments during finish machining of a hardened tool. combined. Heating and cooling rates can be compared with salt bath. The Al-oxides and gas used as protective atmosphere are less detrimental to the environment than salt bath. It is important that the tools are protected against oxidation and decarburization. The best protection is provided by a vacuum furnace, where the surface of the steel remains unaffected. Furnaces with a controlled protective gas atmosphere or salt baths also provide good protection. If an electric muffle furnace is used, the tool can be protected by packing it in spent charcoal or cast iron chips. It should be observed that these packing materials can have a carburizing effect if the steels have a low carbon content, such as conventional hot work steels. Vacuum furnace 4 Tempered once. Tempered twice. 1000x Uddeholm Rigor, hardened and tempered. Heating to hardening temperature The fundamental rule for heating to hardening temperature is that it should take place slowly. This minimizes distortion. In vacuum furnaces and furnaces with controlled protective gas atmosphere, the heat is increased gradually. When molten salt baths are used, preheating is employed, whereas heating is automatically slow in a muffle furnace when steel is packed in castiron chips. In a fluidized bed the advantages of salt bath and protective atmosphere are Salt bath furnace Batch type furnace with a controlled atmosphere

51 HEAT TREATMENT Wrapping in stainless steel foil also provides good protection when heating in a muffle furnace. Decarburization results in low surface hardness and a risk of cracking. Carburization results in a harder surface layer, which can have negative effects. Holding time at hardening temperature It is not possible to state exact recommendations briefly to cover all heating situations. Factors such as furnace type, furnace rating, temperature level, the weight of the charge in relation to the size of the furnace etc., must be taken into consideration in each case. We can, however, give one recommendation that is valid in virtually all situations: when the steel has reached hardening temperature through its entire thickness, hold at this temperature for 30 minutes. An exception to this rule is for thin parts heated in salt baths at high temperature, or high speed steel. Here the entire period of immersion is often only a few minutes. Quenching The choice between a fast and slow quenching rate is usually a compromise; to get the best microstructure and tool performance, the quenching rate should be rapid; to minimize distortion, a slow quenching rate is recommended. Slow quenching results in less temperature difference between the surface and core of a part, and sections of different thickness will have a more uniform cooling rate. This is of great importance when quenching through the martensite range, below the M s temperature. Martensite formation leads to an increase in volume and stresses in the material. This is also the reason why quenching should be interrupted before room temperature has been reached, normally at C ( F). However, if the quenching rate is too slow, especially with heavier crosssections, undersirable transformations in the microstructure can take place, risking a poor tool performance. Water is used as a quenching medium for unalloyed steels. 8 10% sodium chloride (salt) or soda should be added to the water in order to achieve optimum cooling efficiency. Water hardening can often cause problems in the form of distortion and quench cracks. Oil hardening is safer, but hardening in air or martempering is best of all. Oil should be used for low alloyed steels. The oil should be of good quality, and preferably of the rapid quenching type. It should be kept clean and must be changed after a certain period of use. Hardening oils should have a temperature of C ( F) to give the best cooling efficiency. Lower temperatures mean higher viscosity, i.e. the oil is thicker. Hardening in oil is not the safest way to quench steel, in view of the risks of distortion and hardening cracks. These risks can be reduced by means of martempering. In this process, the material is quenched in two steps. First it is cooled from hardening temperature in a salt bath whose temperature is just above the M S temperature. It is kept there until the temperature has equalized between the surface and the core, after which the tool can be allowed to cool freely in air down through the martensite transformation range. When martempering oil hardening steels, it should also be kept in mind that the material transforms relatively rapid and should not be kept too long at the martempering bath temperature. This can lead to excessive bainite transformation and the risk of low hardness. High alloy steels can be hardened in oil, a martempering bath or air. The advantages and disadvantages of the different methods can be discussed. Oil gives a good finish and high hardness, but it also maximizes the risk of excessive distortion or cracking. In the case of thick parts, quenching in oil is often the only way to achieve maximum hardness. Martempering in salt bath produces a good finish, high hardness and less risk of excessive distortion or cracking. For certain types of steel, the tempera- Temperature Surface M S Martensite A C3 A C1 Core The quenching process as expressed in a TTT graph ture of the salt bath is normally kept at about 500 C (930 F). This temperature ensures a relatively mild thermal shock, but a sufficient cooling rate to avoid phase transformations. Full martensite transformation has, in many cases, time to occur when the steel is cooled in air from the martempering bath temperature. However, if the dimensions are big, it is often necessary to use a forced quenching rate depending of the hardenability of the steel. Temperature Surface M S Martempering A C3 A C1 Core Martensite Time 5

52 HEAT TREATMENT Air quenching entails the least risk of excessive distortion. A tendency towards lower hardness is noticeable at greater thicknesses. One disadvantage is a poorer finish. Some oxidation takes place when the material comes into contact with air and cools slowly from the high hardening temperatures. The choice of quenching medium must be made from job to job, but a general recommendation could perhaps be made as follows: Temperature Hardening temperature Cooling rates for various media 6 M S Oil Surface Room temperature Air Core Salt bath Time A martempering bath is the safest in most cases. Air is used when dimensional stability is crucial. Oil should be avoided and used only when it is necessary to achieve satisfactory hardness in heavy sections. Three well known quenching methods have been mentioned here. Some new concepts have been introduced with modern types of furnaces, and the technique of quenching at a controlled rate in a protective gas atmosphere or in a vacuum furnace with gas is becoming increasingly widespread. The cooling rate is roughly the same as in air for protective gas atmosphere, but the problem of oxidized surfaces is eliminated. Modern vacuum furnaces have the possibility to use overpressure during quenching which increases the quenching speed. The surfaces are completely clean after a vacuum hardening, With these techniques, as with quenching in air, the risks of excessively slow cooling must be borne in mind, even for vacuum furnaces if no overpressure is used. The effect is that surface hardness is normally lower than expected. Hardness in the centre of heavy sections is even lower. This effect can be critical with high speed steel and hot work steel, where a centre section can be cooled so slowly that carbide precipitation takes place on the way down. Here, the matrix becomes depleted of carbon and carbideforming alloying elements. The result is reduced hardness and strength of the core. Tempering The material should be tempered immediately after quenching. Quenching should be stopped at a temperature of C ( F) and tempering should be done at once. If this is not possible, the material must be kept warm, e.g. in a special hot cabinet, awaiting tempering. The choice of tempering temperature is often determined by experience. However, certain guidelines can be drawn and the following factors can be taken into consideration: hardness toughness dimension change. If maximum hardness is desired, temper at about 200 C (390 F), but never lower than 180 C (360 F). High speed steel is normally tempered at about 20 C (36 F) above the peak of the secondary hardening temperature. If a lower hardness is desired, this means a higher tempering temperature. Reduced hardness does not always mean increased toughness, as is evident from the toughness values in our product brochures. Avoid tempering within temperature ranges that reduce toughness. If dimensional stability is also an Convection type tempering furnace important consideration, the choice of tempering temperature must often be a compromise. If possible, however, priority should be given to toughness. How many tempers are required? Two tempers are recommended for tool steel and three are considered necessary for high speed steel with a high carbon content, e.g. over 1%. Two tempers are always recommended. If the basic rule in quenching is followed to interrupt at C ( F) then a certain amount of austenite remains untransformed when the material is to be tempered. When the material cools after tempering, most of the austenite is transformed to martensite. It is untempered. A second tempering gives the material optimum toughness at the hardness in question. The same line of reasoning can be applied with regard to retained austenite in high speed steel. In this case, however, the retained austenite is highly alloyed and slow transforming. During tempering, some diffusion takes place in the austenite, secondary carbides are precipitated, the austenite becomes lower alloyed and is more easily transformed to martensite when it cools after tempering. Here, several temperings can be beneficial in driving the transformation of the retained austenite further to martensite. Holding times in connection with tempering Here also, one should avoid all complicated formulae and rules of thumb, and adopt the following recommendation: After the tool is heated through, hold the material for at least 2 hours at full temperature each time.

53 HEAT TREATMENT Dimensional and shape stability DISTORTION DURING THE HARDENING AND TEMPERING OF TOOL STEEL When a piece of tool steel is hardened and tempered, some warpage or distortion normally occurs. This distortion is usually greater at high temperature. This is well known, and it is normal practice to leave some machining allowance on the tool prior to hardening. This makes it possible to adjust the tool to the correct dimensions after hardening and tempering by grinding, for example. How does distortion take place? The cause is stresses in the material. These stresses can be divided into: machining stresses thermal stresses transformation stresses. Machining stresses This type of stress is generated during machining operations such as turning, milling and grinding. (For example, such stresses are formed to a greater extent during cold forming operations such as blanking, bending and drawing.) If stresses have built up in a part, they will be released during heating. Heating reduces strength, releasing stresses through local distortion. This can lead to overall distortion. In order to reduce this distortion while heating during the hardening process, a stress relieving operation can be carried out prior to the hardening operation. It is recommended that the material be stress relieved after rough machining. Any distorsion can then be adjusted during semifinish machining prior to the hardening operation. Thermal stresses These stresses are created when a piece is heated. They increase if heating takes place rapidly or unevenly. The volume of the steel is increased by heating. Uneven heating can result in local variations in volume growth, leading to stresses and distortion. As an alternative with large or complex parts, heating can be done in preheating stages in order to equalize the temperature in the component. Linear expansion mm/100 mm 0,8 0,6 0,4 0, C Temperature Effect of temperature on the linear expansion of Uddeholm ORVAR 2 Microdized, soft annealed An attempt should always be made to heat slowly enough so that the temperature remains virtually equal throughout the piece. What has been said regarding heating also applies to quenching. Very powerful stresses arise during quenching. As a general rule, the slower that quenching can be done, the less distortion will occur due to thermal stresses. It is important that the quenching medium is applied as uniformly as possible. This is especially valid when forced air or protective gas atmosphere (as in vacuum furnaces) is used. Otherwise temperature differences in the tool can lead to significant distortion. Transformation stresses This type of stress arises when the microstructure of the steel is transformed. This is because the three microstructures in question ferrite, austenite and martensite have different densities, i.e. volumes. The greatest effect is caused by transformation from austenite to martensite. This causes a volume increase. Excessively rapid and uneven quenching can also cause local martensite formation and thereby volume increases locally in a piece and give rise to stresses in this section. These stresses can lead to distortion and, in some cases, quenching cracks. Volume Transformation to martensite Transformation to austenite M s A C1 A C3 Temperature Volume changes due to structural transformation Yield strength Rp0,2 MPa C Temperature Effect of temperature on the yield strength of Uddeholm Orvar 2 Microdized, soft annealed 7

54 HEAT TREATMENT HOW CAN DISTORTION BE REDUCED? Distortion can be minimized by: keeping the design simple and symmetrical eliminating machining stresses by stress relieving after rough machining heating slowly during hardening using a suitable grade of steel quenching the piece as slowly as possible, but quick enough to obtain a correct microstructure in the steel tempering at a suitable temperature. The following values for machining allowances can be used as guidelines. Machining allowance Grade of steel on length and diameter as % of dimension ARNE 0,25 % RIGOR 0,20 % SVERKER 21 0,20 % SVERKER 3 0,20 % CARMO 0,20 % SLEIPNER 0,25 % CALDIE 0,25 % VANADIS 4 Extra 0,15 % VANADIS 6 0,15 % VANADIS 10 0,15 % VANADIS 23 0,15 % VANCRON 40 0,20 % CALMAX 0,20 % GRANE 0,15 % STAVAX ESR 0,15 % MIRRAX ESR 0,20 % ELMAX 0,15 % CORRAX 0,05 0,15 % ORVAR 2 Microdized 0,20 % ORVAR SUPREME 0,20 % VIDAR SUPERIOR 0,20 % QRO 90 SUPREME 0,30 % HOTVAR 0,40 % DIEVAR 0,30 % ROLTEC SF 0,15 % TOUGHTEC SF 0,15 % WEARTEC SF 0,15 % Note: Uddeholm Corrax is a precipitation hardening steel. Machining allowance is needed to compensate for shrinkage during ageing. The shrinkage depends on ageing temperature (see product information brochure). No distortion occurs. SUB-ZERO TREATMENT Tools requiring maximum dimensional stability in service can be sub-zero treated as follows: Immediately after quenching, the tool should be sub-zero treated to 70 8 to 80 C ( 95 to 110 F), soaking time 1 3 hours, followed by tempering. The sub-zero treatment leads to a reduction of retained austenite content. This, in turn, will result in a hardness increase of 1 2 HRC in comparison to not sub-zero treated tools if low temperature tempering is used. For high temperature tempered tools there will be little or no hardness increase and when referencing the normal tempering curves, a 25 to 50 C (45 to 90 F) lower tempering temperature should be chosen to achieve the required hardness. Tools that are high temperature tempered, even without a sub-zero treatment, will normally have a low retained austenite content and in most cases, a sufficient dimensional stability. However, for high demands on dimensional stability in service it is also recommended to use a sub-zero treatment in combination with high temperature tempering. For the highest requirements on dimensional stability, sub-zero treatment in liquid nitrogen is recommended after quenching and after each tempering. Surface treatment NITRIDING The purpose of nitriding is to increase the surface hardness of the steel and improve its wear properties. This treatment takes place in a medium (gas or salt) which gives off nitrogen. During nitriding, nitrogen diffuses into the steel and forms hard, wear resistant nitrides. This results in an intermetallic surface layer with good wearing and frictional properties. Nitrided case shown at a magnification of 100X Uddeholm Orvar 2 Microdized Nitriding is done in gas at about 510 C (950 F) and in salt or gas at about 570 C (1060 F) or as ion nitriding, normally at around 500 C (930 F). The process therefore requires steels that are resistant to tempering in order not to reduce the core strength. Examples of applications Nitriding is used in some cases on prehardened plastic moulds in order to prevent indentation and defects on the parting faces. It should be noted, however, that a nitrided surface cannot be machined with cutting tools and can only be ground with difficulty. A nitrided surface will cause problems in weld repairing as well. Nitriding can also have a stress relieving effect. Heavily machined parts may, therefore, undergo some distortion during nitriding due to the release of residual stresses from machining and in such a case, a stress relieving between rough and finish machining is recommended. The life of forging dies can be increased by nitriding. It must be noted, though, that the treatment can give rise to higher susceptibility to cracking in sharp corners. Furthermore, the edge of the flash land must be given a rounded profile. Extrusion dies of Uddeholm Orvar 2 Microdized can be nitrided to advantage especially in the case of aluminium alloys. Exceptions can be profiles with sharp corners and thin sections of the dies. NITROCARBURIZING A widely known method is nitriding in a salt bath. The temperature is normally 570 C (1060 F). Due to aeration the cyanate content of the bath can be better controlled and the nitriding effect is very good. A nitrocarburizing effect can also be achieved in gas atmosphere at 570 C (1060 F). The results after these methods are comparable. The total nitriding time must be varied for different tool types and sizes. In the case of large sizes, the heating time

55 HEAT TREATMENT to the specified nitriding temperature can be considerably longer than in the case of small tools. ION NITRIDING This is a new nitriding technology. The method can be summarized as follows: The part to be nitrided is placed in a process chamber filled with gas, mainly nitrogen. The part forms the cathode and the shell of the chamber the anode in an electric circuit. When the circuit is closed, the gas is ionized and the part is subjected to ion bombardment. The gas serves both as heating and nitriding medium. The advantages of ion nitriding include a low process temperature and a hard, tough surface layer. The depth of diffusion is of the same order as with gas nitriding. Ion nitriding plant CASE HARDENING In this method, the steel is heated in a medium that gives off carbon (gas, salt or dry carburizing compound). The carbon diffuses into the surface of the material and after hardening this gives a surface layer with enhanced hardness and wear resistance. This method is used for structural steel, but is not generally recommended for alloy tool steels. HARD CHROMIUM PLATING Hard chromium plating can improve the wear resistance and corrosion resistance of a tool. Hard chromium plating is done electrolytically. The thickness of the plating is normally between 0,001 and 0,1 mm (0, ,004 inch). It can be difficult to obtain a uniform surface layer, especially on complex tools, since projecting corners and edges may re- ceive a thicker deposit than large flat surfaces or the holes. If the chromium layer is damaged, the exposed steel may corrode rapidly. Another advantage of the chromium layer is that it greatly reduces the coefficient of friction on the surface. During the chromium plating process, hydrogen absorption can cause a brittle surface layer. This nuisance can be eliminated by tempering immediately after plating at 180 C (360 F) for 4 hours. SURFACE COATING Surface coating of tool steel is becoming more common. Not only for cold work applications, but also for plastic moulds and hot work dies. The hard coating normally consists of titanium nitride and/or titanium carbide. The very high hardness and low friction gives a very wear resistant surface, minimizing the risk for adhesion and sticking. To be able to use these properties in an optimal way one has to choose a tool steel of high quality or a powder metallurgy manufactured steel as substrate. The two most common coating methods are: PVD coating: performed at C ( F) (PVD = Physical Vapour Deposition). CVD coating: performed at about 1000 C (1830 F) (CVD = Chemical Vapour Deposition). Coated tools Certain demands are put on the tool steel depending on: coating method, the design of the tool and the tolerances needed. PVD coating is used for the highest demands on tolerances. When using this method a tool steel with high tempering resistance must be used and the surface coating has to be performed as the last operation, after the heat treatment. At CVD coating, hardening and tempering are done after the coating. When using the CVD method there is a risk for dimensional changes. The method is therefore not recommended for tools with requirements of very narrow tolerances. The most suitable steels for the mentioned methods are Uddeholm Vanadis 4 Extra, Uddeholm Vanadis 6, Uddeholm Vanadis 10, Uddeholm Vanadis 23 and Uddeholm Caldie. Surface coating of tools and moulds should be discussed from case to case considering the application, coating method and tolerance requirements. 9

56 HEAT TREATMENT Testing of mechanical properties When the steel is hardened and tempered, its strength is affected, so let us take a closer look at how these properties are measured. HARDNESS TESTING Hardness testing is the most popular way to check the results of hardening. Hardness is usually the property that is specified when a tool is hardened. It is easy to test hardness. The material is not destroyed and the apparatus is relatively inexpensive. The most common methods are Rockwell C (HRC), Vickers (HV) and Brinell (HBW). We shouldn t entirely forget the old expression file-hard. In order to check whether hardness is satisfactory, for example above 60 HRC, a file of good quality can provide a good indication. Rockwell (HRC) In Rockwell hardness testing, a conical diamond is first pressed with a force F 0, and then with a force F 0 +F 1 against a specimen of the material whose hardness is to be determined. After unloading to F 0, the increase (e) of the depth of the impression caused by F 1 is determined. The depth of penetration (e) is converted into a hardness number Vickers (HV) In Vickers hardness testing a pyramidshaped diamond with a square base and a peak angle of 136 is pressed under a load F against the material whose hardness is to be determined. After unloading, the diagonals d 1 and d 2 of the impression are measured and the hardness number (HV) is read off a table. When the test results are reported, F Vickers hardness is indicated with the letters HV and a suffix indicating the mass that exerted the load and (when required) the loading period, as illustrated by the following example: HV 30/20 = Vickers hardness determined with a load of 30 kgf exerted for 20 seconds. 136 d 1 Brinell (HBW) In Brinell hardness testing, a Tungsten (W) ball is pressed against the material whose hardness is to be determined. After unloading, two measurements of the diameter of the impression are taken at 90 to each other (d 1 and d 2 ) and the HBW value is read off a table, from the average of d 1 and d 2. When the test results are reported, Brinell hardness is indicated with the letters HBW and a suffix indicating ball diameter, the mass with which the load d 2 Principle of Vickers hardness testing F 0 F 0 +F 1 =F F 0 D h 0 h e HRC F Surface of specimen 100 h 0 d h 0,2 mm 0 Hardness scale h e HRC Principle of Brinell hardness testing Principle of Rockwell hardness testing (HRC) which is read directly from a scale on the tester dial or read-out. was exerted and (when required) the loading period, as illustrated by the following example: HBW 5/750/15 = Brinell hardness determined with 5 mm Tungsten (W) ball and under load of 750 kgf exerted for 15 seconds. 10

57 HEAT TREATMENT TENSILE STRENGTH Tensile strength is determined on a test piece which is gripped in a tensile testing machine and subjected to a successively increasing tensile load until fracture occurs. The properties that are normally recorded are yield strength R p0,2 and ultimate tensile strength R m, while elongation A 5 and reduction of area Z are measured on the test piece. In general, it can be said that hardness is dependent upon yield strength and ultimate tensile strength, while elongation and reduction of area are an indication of toughness. High values for yield and ultimate tensile strength generally mean low values for elongation and reduction of area. Tensile tests are used mostly on structural steels, seldom on tool steels. It is difficult to perform tensile tests at hardnesses above 55 HRC. Tensile tests may be of interest for tougher types of tool steel, especially when they are used as high strength structural materials. These include e.g. Impax Supreme and Orvar 2 Microdized. IMPACT TESTING A certain quantity of energy is required to produce a fracture in a material. This quantity of energy can be used as a measure of the toughness of the material, a higher absorption of energy indicating better toughness. The most common and simplest method of determining toughness is impact testing. A rigid pendulum is allowed to fall from a known height and to strike a test specimen at the lowest point of its swing. The angle through which the pendulum travels after breaking the specimen is measured, and the amount of energy that was absorbed in breaking the specimen can be calculated. Several variants of impact testing are in use. The various methods differ in the shape of the specimens. These are usually provided with a V- or U-shaped notch, the test methods being then known as Charpy V and Charpy U respectively. For the most part, tool steel has a rather low toughness by reason of its high strength. Materials of low toughness are notch sensitive, for which reason smooth, un-notched specimens are often used in the impact testing of tool steels. The results of the tests are commonly stated in joules, or alternatively in kgm (strictly speaking kgfm), although J/cm 2 or kgm/cm 2 is sometimes used instead, specially in Charpy U testing. There are several other variants of impact testing which are used outside Sweden, e.g. DVM, Mesanger and especially in English speaking countries Izod. Some words of advice to tool designers CHOICE OF STEEL Choose air-hardening steels for complex tools. DESIGN Avoid: sharp corners notch effects large differences in section thicknesses. These are often causes of quench cracks, especially if the material is cooled down too far or allowed to stand untempered. Unsuitable design Preferred alternative Fillet HEAT TREATMENT Choose suitable hardnesses for the application concerned. Be particularly careful to avoid temperature ranges that can reduce toughness after tempering. Keep the risk of distortion in mind and follow recommendations concerning machining allowances. It is a good idea to specify stress relieving on the drawings. 11

58 HEAT TREATMENT Europe Austria Representative office Albstraße 10 DE Neuhausen Telephone: Belgium Europark Oost 7 B-9100 Sint-Niklaas Telephone: Croatia BÖHLER Zagreb d.o.o za trgovinu Zitnjak b.b Zagreb Telephone: Telefax: Czech Republic BÖHLER CZ s.r.o. Division Uddeholm U Silnice Praha 6, Ruzyne Telephone: ,8 Denmark A/S Kokmose 8, Bramdrupdam DK-6000 Kolding Telephone: Estonia TOOLING AB Silikatsiidi 7 EE Tallinn Telephone: Finland OY AB Ritakuja 1, PL 57 FI VANTAA Telephone: France Z.I. de Mitry-Compans, 12 rue Mercier, FR Mitry Mory Cedex Telephone: +33 (0) Branch offices S.A. 77bis, rue de Vesoul La Nef aux Métiers FR Besançon Telephone: +33 (0) LE POINT ACIERS - Aciers à outils Z.I. du Recou, Avenue de Champlevert FR GRIGNY Telephone: +33 (0) LE POINT ACIERS - Aciers à outils Z.I. Nord 27, rue François Rochaix FR OYONNAX Telephone: +33 (0) Germany Hansaallee 321 DE Düsseldorf Telephone: Branch offices Falkenstraße 21 DE Bad Soden/TS Telephone: Albstraße 10 DE Neuhausen Telephone: Friederikenstraße 14b DE Harzgerode Telephone: Great Britain DIVISION BOHLER- (UK) LIMITED European Business Park Taylors Lane, Oldbury GB-West Midlands B69 2BN Telephone: Telefax: Greece STASSINOPOULOS- STEEL TRADING S.A. 20, Athinon Street GR-Piraeus Telephone: SKLERO S.A. Heat Treatment and Trading of Steel Uddeholm Tool Steels Industrial Area of Thessaloniki P.O. Box 1123 GR Sindos, Thessaloniki Telephone: Hungary TOOLING/BOK Dunaharaszti, Jedlik Ányos út 25 HU-2331 Dunaharaszti 1. Pf. 110 Telephone/fax: Ireland : DIVISION BOHLER- (UK) LIMITED European Business Park Taylors Lane, Oldbury UK-West Midlands B69 2BN Telephone: Telefax: Dublin: Telephone: Italy Divisione della Bohler Uddeholm Italia S.p.A. Via Palizzi, 90 IT Milano Telephone: Latvia TOOLING LATVIA SIA Piedrujas Street 7 LV-1035 Riga Telephone: latvia@assab.com Lithuania TOOLING AB BE PLIENAS IR METALAI T. Masiulio 18B LT Kaunas Telephone: , The Netherlands Isolatorweg 30 NL-1014 AS Amsterdam Telephone: Norway A/S Jernkroken 18 Postboks 85, Kalbakken NO-0902 Oslo Telephone: Poland BOHLER POLSKA Sp. z.o.o./co. Ltd. ul. Kolejowa 291, Dziekanów Polski, PL Lomianki Telephone: , -203, Portugal F RAMADA Aços e Industrias S.A. P.O. Box 10 PT-3881 Ovar Codex Telephone: Romania BÖHLER- Romania SRL Atomistilor Str. No com. Magurele, Jud. Ilfov. Telephone: Telefax: Russia TOOLING CIS 9A, Lipovaya Alleya, Office 509 RU Saint Petersburg Telephone: Slovakia Bohler-Uddeholm Slovakia s.r.o. divizia Csl.Armády ˇ 5622/5 SK Martin Telephone: +421 (0) Slovenia Representative office Divisione della Bohler Uddeholm Italia S.p.A. Via Palizzi, 90 IT Milano Telephone: Spain Guifré ES Badalona, Barcelona Telephone: Branch office Barrio San Martín de Arteaga,132 Pol.Ind. Torrelarragoiti ES Zamudio (Bizkaia) Telephone: Sweden TOOLING SVENSKA AB Aminogatan 25 SE Mölndal Telephone: Branch offices TOOLING SVENSKA AB Box 45 SE Anderstorp Telephone: TOOLING SVENSKA AB Box 148 SE Eskilstuna Telephone: TOOLING SVENSKA AB Aminogatan 25 SE Mölndal Telephone: TOOLING SVENSKA AB Nya Tanneforsvägen 96 SE Linköping Telephone: TOOLING SVENSKA AB Derbyvägen 22 SE Malmö Telephone: TOOLING SVENSKA AB Honnörsgatan 24 SE Växjö Telephone: Switzerland HERTSCH & CIE AG General Wille Strasse 19 CH-8027 Zürich Telephone: Turkey ASSAB Korkmaz Celik A.S. Organize Sanayi Bölgesi 2. Cadde No: 26 Y. Dudullu Umraniye TR-Istanbul Telephone:

59 HEAT TREATMENT America Argentina ACEROS BOEHLER S.A Mozart Centro Industrial Garin Garin-Prov. AR-Buenos Aires Telephone: Brazil AÇOS BOHLER- DO BRASIL LTDA DIV. Estrada Yae Massumoto, 353 CEP BR-Sao Bernardo do Campo - SP Brazil Telephone: , Canada Head Office & Warehouse BOHLER- LIMITED 2595 Meadowvale Blvd. Mississauga, ON L5N 7Y3 Telephone: Branch Warehouses BOHLER- LIMITED 3521 Rue Ashby St. Laurent, QC H4R 2K3 Telephone: BOHLER- LIMITED 730 Eaton Way - Unit #10 New Westminister, BC V3M 6J9 Telephone: Heat Treating BOHLER- THERMO-TECH 2645 Meadowvale Blvd. Mississauga, ON L5N 7Y4 Telephone: Colombia AXXECOL S.A. Carrera 35 No Apartado Aereo CO-Bogota 6 Telephone: ASTECO S.A. Carrera 54 No Apartado Aereo 663 CO-Medellin Telephone: Dominican Republic RAMCA, C. POR A. P-2289 P.O. Box Miami, Fl Telephone: domrep@assab.com Ecuador IVAN BOHMAN C.A. Apartado 1317 Km 6 1/2 Via a Daule Guayaquil Telephone: IVAN BOHMAN C.A. Casilla Postal Quito Telephone: El Salvador ACAVISA DE C.V. 25 Ave. Sur, no 763 Zona 1 SV-San Salvador Telephone: Guatemala IMPORTADORA ESCANDINAVA Apartado postal 11C GT-Guatemala City Telephone: guatemala@assab.com Honduras ACAVISA DE C.V. 25 Ave. Sur, no 763 Zona 1 SV-San Salvador Telephone: Mexico ACEROS BOHLER S.A. de C.V. Calle Ocho No 2, Letra C Fraccionamiento Industrial Alce Blanco C.P Naucalpan de Juarez MX-Estado de Mexico Telephone: Branch office BOHLER- MONTERREY, NUEVO LEON Lerdo de Tejada No.542 Colonia Las Villas MX San Nicolas de Los Garza, N.L. Telephone: Peru C.I.P.E.S.A Av. Oscar R. Benavides (ante Colonial) No PE-Lima 1 Telephone: peru@assab.com U.S.A. and Warehouse BOHLER- CORPORATION 2505 Millennium Drive Elgin IL Telephone: or Sales phone: Region East Warehouse BOHLER- CORPORATION 220 Cherry Street Shrewsbury MA Region Central Warehouse BOHLER- CORPORATION 548 Clayton Ct. Wood Dale IL Region West Warehouse BOHLER- CORPORATION 9331 Santa Fe Springs Road Santa Fe Springs, CA Venezuela PRODUCTOS HUMAR C.A. Av. Bolivar, Zona Industrial La Trinidad Edificio. Distribuidora Agrofor, C.A. Piso 3, VE-Caracas 1080 Telephone: or humar@assab.com Other Countries in America ASSAB INTERNATIONAL AB Box 42 SE Solna, Sweden Telephone: Asia & Pacific Australia BOHLER Australia McCredie Road Guildford NSW 2161 Private Bag 14 AU-Sydney Telephone: Bangladesh ASSAB INTERNATIONAL AB P.O. Box Jebel Ali AE-Dubai Telephone: North China ASSAB Tooling (Beijing) Co Ltd No.10A Rong Jing Dong Jie Beijing Economic Development Area Beijing , China Telephone: Branch offices ASSAB Tooling (Beijing) Ltd Dalian Branch 8 Huanghai Street, Haerbin Road Economic & Technical Develop. District Dalian , China Telephone: ASSAB Qingdao Office Room 2521, Kexin Mansion No. 228 Liaoning Road, Shibei District Qingdao , China Telephone: ASSAB Tianjin Office No.12 Puwangli Wanda Xincheng Xinyibai Road, Beichen District Tianjin , China Telephone: Central China ASSAB Tooling Technology (Shanghai) Co Ltd No Humin Road Xinzhuang Industrial Zone Shanghai , China Telephone: Branch offices ASSAB Tooling Technology (Ningbo) Co Ltd No. 218 Longjiaoshan Road Vehicle Part Industrial Park Ningbo Economic & Technical Dev. Zone Ningbo , China Telephone: ASSAB Tooling Technology (Chongqing) Co Ltd Plant C, Automotive Industrial lpark Chongqing Economic & Technological Development Zone Chongqing , China Telephone: South China ASSAB Steels (HK) Ltd Room Tower 2 Grand Central Plaza 138 Shatin Rural Committee Road Shatin NT - Hong Kong Telephone: Branch offices ASSAB Tooling (Dongguan) Co Ltd Northern District Song Shan Lake Science & Technology Industrial Park Dongguan , China Telephone: ASSAB Tooling (Xiamen) Co Ltd First Floor Universal Workshop No. 30 Huli Zone Xiamen , China Telephone: Hong Kong ASSAB Steels (HK) Ltd Room Grand Central Plaza, Tower Shatin Rural Committee Road Shatin NT, Hong Kong Telephone: India ASSAB Sripad Steels LTD T 303 D.A.V. Complex Mayur Vihar Ph I Extension IN-Delhi Telephone: ASSAB Sripad Steels LTD 709, Swastik Chambers Sion-Trombay Road Chembur IN-Mumbai Telephone: , ASSAB Sripad Steels LTD Padmalaya Towers Janaki Avenue M.R.C. Nagar IN-Chennai Telephone: ASSAB Sripad Steels LTD 19X, D. P. P. Road Naktola Post Office IN-Kolkata Telephone: +91 ( ASSAB Sripad Steels LTD Ground floor, Plot No Opp IDPL Factory Out Gate Balanagar IN-Hyderabad Telephone: +91 (40) Indonesia PT ASSAB Steels Indonesia Jl. Rawagelam III No. 5 Kawasan Industri Pulogadung Jakarta 13930, Indonesia Telephone:

60 HEAT TREATMENT Branch offices SURABAYA BRANCH Jl. Berbek Industri 1/23 Surabaya Industrial Estate, Rungkut Surabaya 60293, East Java, Indonesia Telephone: MEDAN BRANCH Komplek Griya Riatur Indah Blok A No.138 Jl. T. Amir Hamzah Halvetia Timur, Medan Telephone: /6 BANDUNG BRANCH Komp. Ruko Bumi Kencana Jl. Titian Kencana Blok E No.5 Bandung Telephone: TANGERANG BRANCH Pusat Niaga Cibodas Blok C No. 7 Tangerang Telephone: , SEMARANG BRANCH Jl. Imam Bonjol No.155 R.208 Semarang Telephone: Iran ASSAB INTERNATIONAL AB P.O. Box IR-1517 TEHRAN Telephone: Israel PACKER YADPAZ QUALITY STEELS Ltd P.O. Box 686 Ha-Yarkon St. 7, Industrial Zone IL YAVNE Telephone: Japan KK Atago East Building Nishi Shinbashi Minato-ku, Tokyo , Japan Telephone: Jordan ENGINEERING WAY Est. P.O. Box 874 Abu Alanda JO-AMMAN Telephone: engineeringway@assab.com Malaysia ASSAB Steels (Malaysia) Sdn Bhd Lot 19, Jalan Perusahaan 2 Batu Caves Industrial Estate Batu Caves Selangor Malaysia Telephone: Branch offices BUTTERWORTH BRANCH Plot 146a Jalan Perindustrial Bukit Minyak 7 Kawasan Perindustrial Bukit Minyak Bukit Mertajam, SPT Penang Telephone: JOHOR BRANCH No. 8, Jalan Persiaran Teknologi Taman Teknologi Senai Johor DT, Malaysia Telephone: New Zealand VIKING STEELS 25 Beach Road, Otahuhu P.O. Box , Onehunga NZ-Auckland Telephone: Pakistan ASSAB International AB P.O. Box Jebel Ali AE-Dubai Telephone: Philippines ASSOCIATED SWEDISH STEELS PHILS Inc. No. 3 E. Rodriguez Jr., Avenue Bagong Ilog, Pasig City Philippines Telephone: /2048 Republic of Korea ASSAB Steels (Korea) Co Ltd 116B-8L, 687-8, Kojan-dong Namdong-ku Incheon , Korea Telephone: Branch offices BUSAN BRANCH 14B-5L, , Songjeong-dong Kangseo-ku, Busan , Korea Telephone: DAEGU BRANCH Room 27, 7-Dong2 F Industry Materials Bldg.1629 Sangyeog-Dong, Buk-Ku Korea-Daegu Telephone: Lebanon WARDE STEEL & METALS SARL MET Charles Helou Av, Warde Bldg P.O. Box LB-Beirut Telephone: lebanon@assab.com Saudi Arabia ASSAB INTERNATIONAL AB P.O. Box SA-Riyadh Telephone: assab@emirates.net.ae Singapore Pacific ASSAB Pacific Pte Ltd 171, Chin Swee Road No , SAN Centre SG-Singapore Telephone: Jurong ASSAB Steels Singapore (Pte) Ltd 18, Penjuru Close SG Singapore Telephone: Sri Lanka GERMANIA COLOMBO PRIVATE Ltd. 451/A Kandy Road LK-Kelaniya Telephone: Syria WARDE STEEL & METALS SARL MET Charles Helou Av, Warde Bldg P.O. Box LB-Beirut Telephone: lebanon@assab.com Taiwan ASSAB Steels (Taiwan) Co Ltd No. 112 Wu Kung 1st Rd. Wu Ku Industry Zone TW-Taipei , Taiwan (R.O.C.) Telephone: Branch offices NANTOU BRANCH No. 10, Industry South 5th Road Nan Kang Industry Zone Nantou , Taiwan (R.O.C.) Telephone: TAINAN BRANCH No. 180, Yen He Street, Yong Kang City Tainan , Taiwan (R.O.C.) Telephone: Thailand ASSAB Steels (Thailand) Ltd 9/8 Soi Theedinthai, Taeparak Road, Bangplee, Samutprakarn 10540, Thailand Telephone: , United Arab Emirates ASSAB INTERNATIONAL AB P.O. Box Jebel Ali AE-Dubai Telephone: Vietnam CAM Trading Steel Co Ltd 90/8 Block 5, Tan Thoi Nhat Ward District 12, Ho Chi Minh City Vietnam Telephone: Other Asia ASSAB INTERNATIONAL AB Box 42 E Solna, Sweden Telephone: Africa Egypt MISR SWEDEN FOR ENGINEERING IND. Montaser Project No 20 Flat No 14 Al Ahram Street-El Tabia EG-Giza Cairo Telephone: Kenya SANDVIK Kenya Ltd P.O. Box Post code KE-Nairobi Telephone: info@sandvik.co.ke Morocco MCM Distribution 4 Bis, Rue Z.I Charguia 1 TN-Tunis Telephone: South Africa Africa (Pty.) Ltd. P.O. Box 539 ZA-1600 Isando/Johannesburg Telephone: Tunisia MCM Distribution 4 Bis, Rue Z.I Charguia 1 TN-Tunis Telephone: Zimbabwe Representative office: Africa (Pty.) Ltd. P.O. Box 539 ZA-1600 Isando/Johannesburg Telephone: Other African Countries ASSAB INTERNATIONAL AB Box 42 SE Solna, Sweden Telephone:

61 Network of excellence Uddeholm is present on every continent. This ensures you high-quality Swedish tool steel and local support wherever you are. Assab is our wholly-owned subsidiary and exclusive sales channel, representing Uddeholm in various parts of the world. Together we secure our position as the world s leading supplier of tooling materials.

62 HAGFORS KLARTEXT U0712XX Uddeholm is the world s leading supplier of tooling materials. This is a position we have reached by improving our customers everyday business. Long tradition combined with research and product development equips Uddeholm to solve any tooling problem that may arise. It is a challenging process, but the goal is clear to be your number one partner and tool steel provider. Our presence on every continent guarantees you the same high quality wherever you are. Assab is our wholly-owned subsidiary and exclusive sales channel, representing Uddeholm in various parts of the world. Together we secure our position as the world s leading supplier of tooling materials. We act worldwide, so there is always an Uddeholm or Assab representative close at hand to give local advice and support. For us it is all a matter of trust in long-term partnerships as well as in developing new products. Trust is something you earn, every day. For more information, please visit or

63 GRINDING OF TOOL STEEL

64 This information is based on our present state of knowledge and is intended to provide general notes on our products and their uses. It should not therefore be construed as a warranty of specific properties of the products described or a warranty for fitness for a particular purpose. Classified according to EU Directive 1999/45/EC For further information see our Material Safety Data Sheets. Edition 7, The latest revised edition of this brochure is the English version, which is always published on our web site SS-EN ISO 9001 SS-EN ISO 14001

65 GRINDING OF TOOL STEEL CONTENTS Introduction 4 Grinding wheel design 4 How the grinding wheel works 6 The grinding machine 9 Grinding fluid 9 The tool steel 10 Recommendations for grinding of Uddeholm tool steel 13 Cutting speed and feed 14 Grinding wheel dressing 15 Examples of suitable grinding wheels

66 GRINDING OF TOOL STEEL Introduction The high alloy content of tool steel means that such steel are often more difficult to grind than conventional structural steel. In order to achieve successful results when grinding tool steel, it is necessary to choose the grinding wheel with care. In turn, choosing the right grinding wheel and grinding data requires an understanding of how a grinding wheel works. This brochure provides a quite detailed description of the make-up of the wheel, of how it works when grinding and of the parameters that determine the final result. It also includes recommendations for grinding wheels for use with Uddeholm tool steel. Grinding wheel design In principle, a grinding wheel consists of the following components: Abrasive Binder Air pores Abrasive Air pores Binder Figure 1. The arrangement and proportions of abrasives grains, air pores and bond bridges (made up of binder) determine grinding wheel characteristics. Certain special grinding wheels, such as metallically bonded diamond wheels, contain no air pores. It is the composition and variation of the above components that determines the characteristic of a grinding wheel. An identification system, which has now been ratified as an interna- tional standard by ISO, indicates the composition of grinding wheels. The identification consists of numerals and letters in a particular sequence, defining the abrasive, grain size, grade and binder. Example: A 46 H V Abrasive Grain size Grade Binder ABRASIVE It is important that the abrasive fulfils requirements in respect of: hardness sharpness thermal resistance chemical stability Today, the following four main groups of abrasives (all synthetic) are used, fulfilling the above requirements to greater or lesser extents. 1. Aluminium oxide designation A (SG) 2. Silicon carbide designation C 3. Cubic boron nitride designation B 4. Diamond designation SD Abrasives have different application areas, depending on their particular characteristics, as shown partially in the table below. THERMAL DURABILITY HARDNESS IN AIR ABRASIVE KNOOP C Aluminium oxide Silicon carbide CBN Diamond Aluminium oxide, is the abrasive most commonly used for grinding steel, and is available in several variants. It can be alloyed with other oxides, of which the most common is titanium oxide. The table below shows how the characteristics of aluminium oxide abrasive can be varied by alloying it. ABRASIVE COLOUR PROPERTIES Normal corundum Brown, grey Mixed corundum Yellowbrown Red alumina Red White alumina White Harder Tougher Unfortunately, the colour of a grinding wheel does not always necessarily indicate the type of abrasive used in it, due to the fact that some grinding wheel manufacturers colour their abra-sives and binders. There is also another type of aluminium oxide named ceramic or sintered aluminium oxide. This abrasive has a fine crystalline structure, which means that the grains retain their sharpness better. However, its use requires higher grinding pressure. A typical application for it is grinding tool steel in rigid grinding machines. Examples of this type of abrasive are SG (Seeded Gel) from Norton and Cubitron from 3M. 2. Silicon carbide is an abrasive that is used primarily for grinding cast iron and austenitic stainless steel, although it can also be used for hardened tool steel. It occurs in two main variants: the black silicon carbide and a somewhat harder green variant, which is more brittle than the black material. 3. Cubic Boron Nitride (CBN) is produced in approximately the same way as synthetic diamond, and is an abrasive that is used primarily for grinding hardened high-carbide tool steel and high-speed steel. A drawback of CBN is its high price almost twice that of synthetic diamond. 4. Diamond is seldom used, despite its high hardness, for grinding tool steel as a result of its low thermal resistance. Diamond is used primarily for grinding cemented carbide and ceramic materials. 4

67 GRINDING OF TOOL STEEL ABRASIVE GRAIN SIZE The grain size of the abrasive is an important factor in selecting the correct grinding wheel. Grain sizes are classified in accordance with an international mesh size in mesh/inch, ranging from 8 (coarse) to 1200 (superfine). Grain sizes for grinding tool steel are generally in the range mesh. Coarse grain sizes are used for rapid rate of removal, when grinding large workpieces, grinding softer materials or when the contact surface of the grinding wheel is large. Fine grain sizes are used to produce high surface finish, when grinding hard materials or when the contact surface of the grinding wheel is small. The surface smoothness of the ground part depends not only on the grain size of the grinding wheel. The sharpness of the wheel, the bonding material used and the hardness of the wheel also play a considerable part in determining the surface finish produced. In the case of diamond and CBN grinding wheels, European grinding wheel manufacturers indicate grain size by the diameter of the abrasive grains in microns, while American and Japanese manufacturers indicate it in mesh size. GRINDING WHEEL GRADE The grade of a grinding wheel refers to its hardness, i.e. how securely the abrasive grains are held by the binder. It does not, therefore, depend on the hardness of the abrasive used in the wheel. The grade of a grinding wheel is determined primarily by the quantity of binder used in the wheel. A higher proportion of binder reduces the amount of air pores and produces a harder wheel. The grade of a wheel is indicated by a letter, indicating the hardness in alphabetical order: E = very soft composition Z = very hard composition. For tool steel, the most commonly encountered compositions are within the hardness range G K. Indication of the grade is sometimes followed by a numeral, which indicates the spread of the abrasive particles in the wheel. GRINDING WHEEL BINDERS The following binders are used to bind the grains in a grinding wheel: Vitrified designation: V Resinoid,, B Rubber,, R Metal,, M Vitrified grinding wheels are those most commonly used for grinding tool steel. Resinoid is used as a binder in grinding wheels intended for high peripheral speeds, such as certain CBN wheels. Rubber-bonded wheels are used for high specific grinding pressures, such as for control wheels in centreless grinding. Metallic binders are used for diamond and certain CBN wheels. Such wheels can withstand very high peripheral speeds. The photo shows the difference between a CBN wheel and a conventional grinding wheel. As a result of the high price of CBN, wheels made from it consist of a thin layer of abrasive applied to a central hub, usually of aluminium. 5

68 GRINDING OF TOOL STEEL How the grinding wheel works Grinding is a cutting process in which the cutting edges are formed by the grains of abrasive. The same principles apply for grinding as for other chip-cutting methods, although various factors mean that it is necessary to consider the theory of grinding somewhat differently. Conditions that are special for grinding. The cutting tool has an irregular cutting geometry and the abrasive grains are irregularly placed, which means that cutting, ploughing and sliding will occur, see figure 2. The cutting geometry can change. The method of working of an abrasive tool includes a certain degree of self-sharpening, which means that grains of abrasive break or are replaced as they wear. Negative cutting angles. The irregular blunt shapes of the grains mean that the rake angles are often negative. Cutting Chip Workpiece Abrasive grain Grinding direction A very large number of cutting edges. Very high cutting speed. The most common cutting speed for precision grinding, 35 m/s = 2100 m/min., is far above what is normal for other cutting processes. Very small chips, i.e. very small cutting depth for each cutting edge. GRINDING FORCES The grinding forces that act on each individual grain of abrasive are referred to as specific forces. A mean value of the specific forces can be obtained by dividing the total force by the number of cutting edges, which depends on the size of the contact area and the number of cutting edges in the grinding path. The specific forces determine various effects, including the degree of selfsharpening of the grinding wheel, i.e. its working hardness. The total force is the force arising between the grinding wheel and the workpiece. GRINDING WHEEL WEAR The grains of abrasive are initially sharp, but in the same way as with all other cutting edges they wear down in use and become blunt. Finally, the grains will have become so blunt that they have difficulty in penetrating into the material of the workpiece. They cease to remove material and generate only heat. The grinding wheel is then said to be burning the material, which can cause cracks in it. For a grinding wheel to work correctly, the stresses in the binder and the strength of the binder must be so balanced that, as the grains become as blunt as can be accepted, they are pulled out of the binder and are replaced by new, sharp grains. The grinding wheel, in other words, sharpens itself. Self-sharpening also occurs through grain breakage, which creates new cutting edges. The degree of self-sharpening, i.e. whether the grinding wheel is hard or soft, is affected by the composition of the wheel (its design hardness) and by the conditions under which it is working. AVERAGE CHIP THICKNESS Although the chips removed by grinding are small and irregular, the mean value of their thickness at any time is relatively constant. This value varies, depending on the type of grinding operation and in response to the changes in grinding data. If a grinding wheel is cutting larger chips, this means two things: 1. Higher loading on each cutting edge, i.e. higher specific forces. This increases the self-sharpening characteristic of the wheel and Ploughing Abrasive grain Grinding direction Small chip Large chip Workpiece Sliding Abrasive grain Grinding direction Low forces on the abrasive grain High forces on the abrasive grain 6 Friction heat Workpiece Figure 2. Different conditions during grinding (highly schematic). Cutting angles are generally negative. Fine surface Rough surface Figure 3. A large chip size results in a rougher surface finish on the workpiece.

69 GRINDING OF TOOL STEEL gives it the characteristics of a softer wheel. 2. The surface of the part being ground is coarser, see Figure 3. A reduction in the average chip thickness represents the opposite. It is therefore important to understand how changes in grinding data and other conditions affect the average chip thickness. STOCK REMOVAL RATE When grinding, the amount of chips removed per unit of time can most easily be expressed as mm 3 /s. This is often referred to as the stock removal rate, and depends on the machine feed, the composition of the grinding wheel, its cutting speed (peripheral speed) and (in certain cases) on the dimensions of the workpiece. It is often more meaningful to talk about stock removal rate rather than about table feed speed, feed depth etc., and it is also quite easy to calculate. Cost considerations often dictate that the stock removal rate should be as high as possible. If the stock removal rate is increased without increasing the number of grains of abrasive performing the work, e.g. by greater infeed depth, the chip size will also naturally be increased. CUTTING SPEED The peripheral speed of a grinding wheel has a direct effect on the number of cutting edges that actually perform the machining work. If, for example, the cutting speed is doubled, twice as many grains of abrasive will pass the workpiece per unit of time. If the workpiece speed is not increased, the mean chip thickness will decrease, thus also reducing the cutting forces on each grain. Selfsharpening will be less effective, i.e. the grinding wheel will be effectively harder, producing a finer surface finish, but with greater risk of burning the surface. Conversely, reducing the speed of the wheel will increase the chip thickness, with the result that the grinding wheel behaves as a softer wheel. Generally, both peripheral velocity and workpiece speed are increased in order to increase the total rate of removal. THE G-RATIO OF A GRINDING WHEEL The G-ratio of a grinding wheel refers to the relationship between the amount of material removed and the amount of grinding wheel consumed. The G-ratio is a measure of how effectively a grinding wheel works with the particular workpiece material. GRINDING WHEEL CONTACT SURFACE It is at the contact surface between the grinding wheel and the workpiece that the actual cutting operation occurs. A large contact surface means that a greater number of cutting edges participate in the process, thus reducing the chip size and specific forces. Similarly, a reduced contact surface area results in greater chip size and higher specific forces. In principle, the contact surface is in the shape of a rectangle. Its extent in the cutting direction is referred to as the contact length or contact arc, while its extent perpendicular to the cutting direction is referred to as the contact width. The contact length depends primarily on the type of grinding operation. In addition, it depends on the diameter of the grinding wheel, the cutting depth and in all cases except for surface grinding the dimensions of the workpiece. Differences in the contact length are the main reason for having to select different grinding wheel compositions for different grinding operations. If, when performing internal grinding, a grinding wheel is used that has a diameter only a little less than that of the ground hole, the contact length will be very large, resulting in low cutting force per grain. If the wheel is to sharpen itself properly, it must be of a softer composition than one intended for external cylindrical grinding of a similar part. In this latter case, the contact length is shorter, which means that there are higher cutting forces on each grain. The contact width may be equal to the width of the grinding wheel as, for example, in plunge grinding. However in operations such as surface grinding with a moving table, only part of the Cylindrical grinding Surface grinding Internal grinding Segmental surface grinding Figure 4. Differences in contact length for different grinding operations. 7

70 GRINDING OF TOOL STEEL grinding wheel is actually cutting and this part changes as the wheel wears down. It is sometimes possible to reduce the contact width, if this is required, by truing of the grinding wheel. This reduces contact surface area, resulting (as already described) in a greater chip thickness, higher loading on the abrasive grains and an effectively softer grinding wheel. THE NUMBER OF CUTTING EDGES IN THE CONTACT AREA The number of cutting edges in the contact area is a factor that has a considerable effect on the chip thickness and thus on the grinding process. A large number of cutting edges per unit area mean that the work of removing material is spread over a larger number of grains, reducing the chip thickness and the specific forces. The grain size of the abrasive also affects the number of cutting edges, which is the reason for the common observation that fine-grained cutting wheels seem to be harder. Dressing is a conditioning of the wheel surface to give the desired cutting action. Dressing the wheel exposes sharp cutting edges. One and the same grinding wheel can be given completely different grinding characteristics through application of different dressing tools or different dressing methods. Dressing is therefore a particularly important parameter in achieving good grinding performance. Dressing resulting in a smooth surface on the wheel results in the cutting edges of the grains of abrasive being close together, while dressing resulting in a rough surface of the wheel gives the wheel a more open structure. Dressing provides a means of making the same grinding wheel give completely different grinding results. The degree of self-sharpening affects the structure of the grinding wheel surface, i.e. the number of cutting edges per unit of area. A grinding wheel that has a high selfsharpening performance has a different, more open structure than one having poorer self-sharpening performance. creates space for chip formation. In practice this can be done by pushing a wet aluminium oxide stone into the wheel for a few seconds. DRESSING AND TRUING GRINDING WHEELS Dressing and truing of a grinding wheel are often considered to be the same thing because they are often performed as one operation. Truing is made to produce any profile which may be required on the face of the wheel and to ensure concentricity. There are many different tools available for dressing and truing grinding wheels, e.g. crushing rolls and diamond tools. CBN wheels are best dressed using a diamond coated roller. Certain types of grinding wheels, e.g. resinoid bonded CBN wheels, need to be opened after dressing. This reveals the abrasive particles and 8

71 GRINDING OF TOOL STEEL The grinding machine The type of grinding operation and the machine available has a considerable effect on the choice of appropriate grinding wheel composition. A grinding machine should be as rigid as possible, in order to allow it to work at high grinding pressures. This is because it is the rigidity of the grinder and the method of clamping the workpiece that determine the permissible grinding pressure and therefore restrict the choice of wheels. If the machine is not sufficiently rigid, a softer grinding wheel composition or a smaller contact area between the grinding wheel and the workpiece should be chosen, in order to achieve the required degree of self-sharpening performance. The speed of the grinder also affects the choice of grinding wheel. CBN wheels often require peripheral speeds of 45 m/s in order to provide good cutting performance. Emulsions. These consist of water with an ad-mixture of 2 5% of oil in an extremely finely distributed form. Sulphur or chlorine additives may also be used as EP additives. Cutting oils. These are composed of a mineral oil base with EP-type additives. Cutting oils provide effective lubrication but poorer cooling. Water solutions are most suitable when grinding with diamond wheels. Emulsions are used nowadays for the majority of grinding operations because they are ecologically beneficial and perform adequately. Cutting oils give the best results for profile and plunge grinding with fine grained wheels, e.g. when grinding threads. Cutting oil also provides the longest life for resinoid bonded CBN wheels, although high-oil emulsions are often chosen in the interests of pollution reduction. Grinding fluid When grinding, as with all other cutting operations, a cutting fluid is used primarily to: cool the workpiece act as a lubricant and reduce friction between the chips, workpiece and grinding wheel remove chips from the contact area There are three main types of cutting fluids that can be used when grinding. Water solutions. These are liquids that consist of water with synthetic additives in order to increase its wetting performance and prevent corrosion. Such fluids contain no oil and provide good cooling performance but poorer lubrication performance. Fine gridning of details in hardened Udddeholm Mirrax ESR 9

72 GRINDING OF TOOL STEEL The tool steel The alloying constituents of a tool steel have a considerable effect on its ease of grinding. The Uddeholm range of tool steel extends from low-alloy steel, such as Uddeholm UHB 11, to high-alloy steel, such as Uddeholm Vanadis 10. There is seldom any problem in grinding low-alloy tool steel. At the other end of the scale, however, the high-alloy carbide-rich steel can cause problems when being ground, and require a careful choice of grinding wheel and operating parameters. The higher the wear resistance of a steel, the more difficult it is to grind. The wear resistance of a steel, and thus also its ease of grinding, are determined by its basic hardness and by the size, hardness and quantity of the carbides in it. In order to enhance the wear resistance of a tool steel, the steel is alloyed with carbide-forming alloying elements, of which the most important are chromium and vanadium. The steel must also have a high carbon content if carbides are to be formed. The diagram, Figure 5, shows the hardness of the basic phases found in a tool steel, the hardness of the most common carbides found in tool steel and the hardness of commonly used grinding abrasives. As can be seen in the figure, it is only diamond and CBN that are harder than all the carbides that are found in a tool steel. However, as mentioned earlier, diamond is unsuitable for grinding steel. The quantity and the size of carbides in a steel has a very considerable effect on the ease of grinding of the material. The greater the number of, and the larger the carbides, the more difficult the material is to grind. This is the reason why tool steel produced by powder metallurgy processes, having smaller carbides, is easier to grind than a conventionally produced steel having a similar composition. In practice, powder metallurgy is employed to increase the quantity of carbide in a tool steel, i.e. such steel are more highly alloyed than conventional steel, which generally means that they are more difficult to grind. The effect of hardness on ease of grinding is also dependent on the quantity of carbide-forming alloying elements in the steel. Hardness kp/mm Ferrite Austenite Martensite Cementite Molybdenum carbide Chromium carbide Niobium carbide Tungsten carbide Vanadium carbide Titanium carbide Aluminium oxide Silicon carbide Cubic boron nitride Diamond As can be seen in Figure 6, hardness has a greater effect on grindability for high-carbide steel. Grindability index 100 A 10 C 1 0,1 B Figure 5. The hardness of grinding abrasives, basic phases found in a tool steel and carbides found in tool steel. Figure 6. The effect of hardness on grindability for: A a low-alloy tool steel of Arne type B a material of Sverker type C material of Vanadis 10 type. 10

73 GRINDING OF TOOL STEEL In order to obtain good grinding performance with high-alloy carbiderich tool steel, it is important to select the correct grinding wheel. Materials in the Uddeholm Vanadis range, for example, contain a large quantity of vanadium carbides. To cut through a vanadium carbide requires an abrasive that is harder than aluminium oxide or silicon carbide. CBN wheels are therefore recommended as first choice for grinding this material. The fact that, despite this, material can be removed from Uddeholm Vanadis steel by grinding with aluminium oxide or silicon carbide is due to the fact that it is the material enclosing the carbides that is ground away, so that the carbides are torn out of the basic material of the steel. However, this occurs at the price of high wear of the grinding wheel and a risk of poor grinding performance. The formation of grinding cracks, which tend to occur perpendicular to the direction of grinding, usually means the tool has to be scrapped. Hardened steel are more sensitive to grinding cracks than non-hardened steel. A material that has been only hardened, and not tempered, must never be ground: hardened materials should always be tempered before grinding. Formation of grinding cracks can be explained as follows: Almost all the energy used in grinding is converted into heat, partly through pure friction and partly as a result of deformation of the material. If the correct grinding wheel has been chosen, most of the heat will be removed in the chips, with only a smaller part heating up the workpiece. The diagram below shows the hardness profile through the surface of a tool steel, incorrectly ground in such a way as to produce re-hardening. Hardness, HRC ,10 0,20 0,30 0,40 0,50 Depth below ground surface, mm Figure 7. Hardness profile through the surface layer of an incorrectly ground tool. GRINDING CRACKS AND GRINDING STRESSES The wrong choice of grinding wheels and grinding parameters results in a considerable risk of causing cracks in the workpiece. Generally, grinding cracks are not as easy to see as in Photo 2. It is usually necessary to examine the part under a microscope, or with magnetic powder inspection, in order to see the cracks. Re-hardened layer in an incorrectly ground tool. Grinding cracks. Incorrect grinding of a hardened tool steel can result in such a high temperature at the ground surface that the tempering temperature of the material is exceeded. This results in a reduction in the hardness of the surface. If the temperature is allowed to rise further, the hardening temperature of the material can be reached, resulting in rehardening. This produces a mixture of non-tempered and tempered martensite in the surface layer, together with retained austenite, as shown in Photo 3. Very high stresses arise in the material, often resulting in the formation of cracks. 11

74 GRINDING OF TOOL STEEL The surface exhibits a high hardness due to the untempered martensite. An overtempered zone occurs just below the surface, where the hardness is lower than the basic hardness of the workpiece. Incorrect grinding, resulting in a modified surface layer, often reveals itself through burn marks discoloration of the ground surface. In order to avoid burning and grinding cracks, it is necessary to keep down the temperature of the ground part, e.g. by means of good cooling, and to employ properly dressed grinding wheels that cut the material with sharp cutting edges instead of simply generating heat through friction. A simple example of how incorrect grinding can cause cracks is shown in Figure 8. A hardened punch with a head is to be cylindrical-ground, with the head (b) being ground flat in the same operation. Alternative A shows the use of a grinding wheel trued with a 90 edge. The grinding wheel, which is suitable for cylindrical grinding of the surface (a), produces a good result on surface (a). Here the contact surface is small so the self sharpening performance is good. The head, on the other hand, which is to be ground flat, presents a larger contact surface to the grinding wheel. The specific forces on the abrasive grains are low so that the wheel does not selfsharpen. Instead, surface (b) is subjected mainly to rubbing and the heat generated can cause grinding cracks. Alternative B shows a better way to grind the punch. In this case, the side of the grinding wheel has been trued as shown so that the contact surface at (b) is smaller. This results in improved self-sharpening and cooler grinding. Case C shows the preferred way to grind this part. The grinding wheel is set at an angle, so that the two contact surfaces are of approximately the same size. The retained austenite content of a hardened material can also affect the grinding result. High retained austenite levels increase the risk of crack formation when grinding. The majority of grinding operations leave residual stresses in the ground surface. These stresses are usually at a maximum close to the surface, and can cause permanent deformation of the ground part when grinding thin materials. Of the three examples shown in Figure 9, Example 1 is most at risk in respect of crack formation. It exhibits tensile stresses in the surface which can, if they exceed the material s ultimate tensile strength, result in the material cracking. Examples 2 and 3 are not as dangerous the surface stresses are compressive stresses, which result in improved fatigue strength of the ground parts. A B C b b b a a a Better Best Figure 8. Incorrect grinding can often result in grinding cracks. It is, unfortunately, very difficult to produce a simple check to determine the stress pattern set up in the ground part unless the stresses are so high that grinding cracks are visible. Grinding stresses can be reduced by stress-relief tempering after grinding. The tempering temperature should be about 25 C below the previous tempering temperature in order to avoid any risk of reducing the hardness of the workpiece. Another way of reducing grinding stresses is to tumble or blast the ground parts. + + Tension Compression + Example 1 Depth below the surface Example 2 Depth below the surface Example 3 Depth below the surface Figure 9. Three typical examples of stress distribution in a ground surface. 12

75 GRINDING OF TOOL STEEL Recommendations for grinding of Uddeholm tool steel GRINDING OF HIGH-CARBIDE TOOL STEEL The high carbide content of highcarbide tool steel gives them excellent wear resistance, and require special recommendations in respect of grinding operations and selection of grinding wheels. For the majority of grinding operations, CBN wheels are the best choice for such steel. There are two different types of carbide rich tool steel, conventionally made steel and powder steel. The main differences that affect the grinding properties are the hardness, size and distribution of carbides, see Figure 10 below. Powder steel, such as Uddeholm Elmax, Uddeholm Vanadis and Uddeholm Vancron, have in spite of the high alloying level relatively good grinding properties due to the small carbide/nitro carbide size. The small carbides will give the grinding wheel good self-sharpening properties. Conventionally made steel, such as Uddeholm Rigor, Uddeholm Sleipner and Uddeholm Sverker, Conventionally made high-carbide steel have not so good self-sharpening properties as powder steel due to the bigger carbide size. However, the lower carbide hardness and carbide content will compensate for the grinding properties. Figure 11 shows the results of surface grinding trials on Uddeholm Vanadis 10 with aluminium oxide, fine crystalline aluminium oxide and CBN grinding wheels. As can be seen in Figure 11, material is removed more quickly, and the G-ratio is higher, using CBN wheels. These wheels have a colder cut, with less risk of burning the surface. If the material is to be profileground, bear in mind that a considerable quantity of heat will be generated. Experiments have shown that vitrified CBN wheels are preferable for this application. These wheels also Stock removal rate mm 3 /s , ,3 Al 2 O 3 Al 2 O 3 -SG CBN operate well for other grinding operations, provided that a high peripheral speed can be maintained. Where boron nitride wheels cannot be used, the type of grinding wheel must be chosen with care. White aluminium oxide or green silicon carbide wheels are recommended. Fine-crystalline aluminium oxide wheels, such as the Norton SG, give good results if the grinding set-up is rigid. When grinding high-carbide steel, the grinding wheel should always be somewhat softer in order to ensure good self-sharpening performance. In addition, the following points must be borne in mind: the grinder must be vibration-free, rigid and in good condition the workpiece must be securely clamped. Use a steady rest when grinding long, thin workpieces use sharp conical diamonds when dressing Al 2 O 3 and SiC wheels. The dressed finish must be rough maintain a high peripheral speed of grinding wheels ensure an adequate supply of coolant to the grinding zone if grinding is carried out without a coolant, select a grinding wheel that is one grade softer than would have been used if grinding was performed with coolant never grind a hardened workpiece before it has been tempered Carbides G-ratio Work piece Powder steel 150 Carbides Work piece Figure 10. Carbide size and distribution in high-carbide tool steel (highly schematic). 30 0,68 2,4 Al 2 O 3 Al 2 O 3 -SG CBN Figure 11. Surface grinding of Uddeholm Vanadis 10 with various grinding wheels. (Grinding wheel width: Al 2 O 3 40 mm, CBN 20 mm.) 13

76 GRINDING OF TOOL STEEL GRINDING OF CONVENTIONAL TOOL STEEL This group covers all the other conventionally produced tool steel. Providing that common grinding recommendations are followed, problems are seldom encountered when grinding these tool steel. For these steel, ordinary aluminium oxide grinding wheels are perfectly suitable. CBN wheels can also be used if the steel are to be ground in the hardened and tempered condition. GRINDING OF PRECIPITATION HARDENING STEEL Precipitation hardening steel, such as Uddeholm Corrax, behaves in a little different way than other tool steel when grinding. It tends to clog the grinding wheel, especially if the grinding wheel is hard and has a close structure. The clogging can cause problems like low material removal rate and rough surface finish. To prevent the clogging, observe following recommendations: the wheel should have an open and porous structure use a softer wheel grade (hardness) than for other types of tool steel the wheel dressing should be done frequent and rough the coolant concentration should be high (>5%) for efficient lubrication Conventional Al 2 O 3 wheels are recommended, but SiC wheels can be a better choice for high surface finish when a small amount of material is to be ground. No particular difference in grindability between solution treated and aged condition. In the table with recommended grinding wheels, page 16 17, suitable standard type of grinding wheels are recommended. However, if a lot of grinding is to be done in this type of steel, it is recommended to select a wheel with a more open structure than a standard wheel type. Cutting speed and feed GRINDING WHEEL SPEED (CUTTING SPEED) When using small grinding machines, the spindle speed often restricts choice of cutting speed. A common safety limit for vitrified grinding wheels is 35 m/s. However, some grinding wheels are approved for peripheral speeds of 125 m/s. A common cutting speed for surface and cylindrical grinding is m/s. Varying the peripheral speed of the wheel makes it possible to modify its grinding performance. Increasing the peripheral speed of the wheel while retaining the same workpiece speed means that the wheel behaves as if it was harder. Reducing the peripheral speed makes the wheel seem softer. A suitable peripheral speed for resinoid CBN wheels is m/s. For vitrified CBN wheels, a cutting speed 45 m/s is often necessary. When grinding high-carbide tool steel, the peripheral speed of the grinding wheel should be high. Tests on cylindrical grinding of Uddeholm Elmax have shown that the G-ratio of the grinding wheel dropped from 127 to 28 when the peripheral speed was dropped from 60 m/s to 30 m/s. Cutting speed, in other words, has a considerable effect on the economics of grinding. WORKPIECE SPEED For surface grinding, the speed of the workpiece should be m/min. For conventional cylindrical grinding, this speed should be m/min. This speed should be reduced for smaller diameter workpieces, for which 5 10 m/min is suitable. Varying the workpiece speed also provides a means of modifying the grinding performance of the wheel. Increasing the speed of the workpiece makes the wheel seem softer, while reducing its speed produces a harder wheel. CROSS-FEED The cross-feed speed of a grinding wheel, i.e. its sideways motion, is higher for rough grinding than for fine grinding. In the case of cylindrical grinding, the cross-feed should be about 1/3 1/2 of the width of the wheel for each revolution of the workpiece. For fine surface finish, this ratio should be reduced to 1/6 1/3 of the width of the grinding wheel per revolution of the workpiece. If a very high standard of surface finish is required, cross feed can be further reduced to 1/8 1/10 of the grinding wheel width. When surface grinding with a straight wheel, choose a transverse feed of 1/6 1/3 of the width of the grinding wheel for each stroke. Again, reduce this feed for high surface finish requirements. Note that when the cross-feed is increased, the active contact surface area between the grinding wheel and the workpiece becomes larger, resulting in an apparent increase in hardness of the grinding wheel. 14

77 GRINDING OF TOOL STEEL INFEED The infeed of the grinding wheel depends on the type of wheel and the rigidity of the grinder and/or workpiece clamping. Guide values for cylindrical grinding using conventional grinding wheels are: Rough finish ~0.05 mm/pass. Fine finish ~ mm/pass. The above feeds should be halved for cylindrical grinding using CBN wheels. For surface grinding using a straight grinding wheel, the feed depths for conventional wheels are: Rough finish ~ mm/pass. Fine finish ~ mm/pass. The feed depths when using CBN wheels are: Rough finish ~0,010 0,040 mm/pass. Fine finish ~0,005 0,010 mm/pass. When using grinding wheels having fine-crystalline aluminium oxide abrasive, such as the Norton SG type, feed depth should be increased somewhat over the above values in order to achieve higher grinding pressure and hence good selfsharpening performance. Grinding wheel dressing During dressing a helix along the wheel periphery is made. The lead of helix which the dressing tool is being fed affects the structure of the grinding wheel. The lead of helix depends both of the r.p.m. of the grinding wheel and the speed of the dressing tool. The following are rules of thumbs for grinding wheel dressing with single point diamonds and similar tools. Rough dressing Fine dressing Diamond infeed (mm) 0,02 0,04 0,01 0,02 Diamond transverse rate (mm/wheel rev.) 0,15 0,30 0,05 0,10 Diamond is sensitive for high temperatures. Therefore, dressing with diamonds should always be carried out with plenty of coolant. The coolant should always be turned on before the diamond touches the wheel. Single point diamond dressing tool should be systematically rotated to maintain the sharpness. Suitable grinding wheels The examples of grinding wheels in the tables, page 16 17, have been made in consultation with grinding wheel manufacturers, and are based on our own and others experience. However, it must be emphasised that the choice of grinding wheel is strongly dependent on the type of grinding machine, rigidity of clamping and the size of the workpiece, which means that the recommendations should be seen as starting points, from which each particular process should be optimized. GRINDING PROBLEMS REMEDIES The table shows the most important actions to solve different grinding problems. SYMPTOM Chatter marks Finish too coarse Burning, grinding cracks Short wheel life Flecking on surface finish REMEDY Check the wheel balance. Ensure that the diamond is sharp. Ensure that the diamond is fixed. Use fine, slow traverse dress. Decrease work speed. Use finer grit wheel. Use harder grade wheel. Ensure that the diamond is sharp. Use coarse dress. Ensure that the coolant reaches the contact point. Use softer grade wheel. Ensure that the cutting speed is sufficient. Reduce depth of cut and feed. Use harder grade wheel. Check coolant filtration. Flush wheel guard. 15

78 GRINDING OF TOOL STEEL 16 Example of suitable grinding wheels The grinding wheels are of SlipNaxos,Tyrolit 2), Norton and Unicorn type. The designations, however, essentially comply with international standards. SURFACE GRINDING SURFACE GRINDING STEEL GRADE CONDITION CENTERLESS STRAIGHT WHEEL SEGMENT Conventional steel: ALVAR Soft 33A 60 LVM 43A 46 HVZ 43A 24 FVZ ALVAR 14 annealed 2) 89A 60 2 K5A V217 2) 91A 46 I8A V217 2) 88A 36 H8A V2 ARNE SGB 60 MVX 3SG 46 G10 VXPM 86A 30 G12 VXPM CALDIE 51A 601 L5V MRAA WA 46 HV WA 24 GV CALMAX DIEVAR FORMAX HOTVAR Hardened 62A 60 LVZ 48A 46 HVZ 48A 46 FVZP MIRRAX ESR 2) 89A 60 2 K5A V217 2) 97A 46 2 H8A V217 2) 97A 46 1 H10A V2 ORVAR SUPREME SGB 60 MVX SGB 46 G10 VXPM 86A 36 F12 VXPC ORVAR 2 MICRODIZED 48A 601 L8V LNAA WA 46 GV WA 36 GV POLMAX QRO 90 SUPREME REGIN 3 STAVAX ESR THG 2000 UHB 11 UNIMAX ORVAR SUPERIOR VIDAR SUPERIOR VIDAR 1 VIDAR 1 ESR HOLDAX Pre-hardened 33A 60 LVM 43A 46 HVZ 43A 24 FVZ IMPAX HI HARD 2) 97A 60 1 K5A V217 2) 89A46 2 I7A V217 2) 88A 36 H8A V2 IMPAX SUPREME SGB 60 MVX SGB 46 G10 VXPM 86A 36 F12 VXPC NIMAX 51A 601 L5V MRAA WA 46 HV WA 24 GV RAMAX HH RAMAX LH Precipitation hardening steel: Solution 33A 60 KVM 43A 46 GVZ 43A 36 FVZ treated or 2) 97A 60 2 K5A V227 15C 46 HVD 15C 36 GVD CORRAX aged SGB 60 KVX 2) 89A 46 1 H8A V217 2) 89A 362 I 10A V237 P20 SPH 50 48A 601 J8V LNAA 3SG 46 G10 VXPM 1TGP 36 F12 VXPC WA 46 GV WA 24 GV High carbide steel: ELMAX Soft annealed 33A 60 LVM 43A 46 HVZ 43A 36 FVZ RIGOR 2) 97A 60 2 J5A V227 2) 455A 36 2 K15 V3 P22 2) 454A 46 K13 V3 SLEIPNER SGB 60 LVX 3SG 46 G10 VXPM 53A 30 F12 VBEP SVERKER 3 51A 601 L5V MRAA WA 46 HV WA 24 GV SVERKER 21 VANADIS 4 EXTRA VANADIS 6 VANADIS 10 VANADIS 23 VANADIS 30 VANADIS 60 VANCRON 40 RIGOR Hardened 48A 60 LVZ B151 R50 B3 420A 46 FVQP SLEIPNER 820A 60 LVQ 420A 46 G12VQP 2) 89A 362 I8A V2 SVERKER 21 2) 97A 60 1 K5A V227 2) 51B126 C50B Vib-Star 3SG 36 HVX VANADIS 23 SGB 60 LVX 2) 455A 36 2 K15 V3 P22 WA 36 HV VANADIS 30 48A 601 L8V LNAA SGB 46 HVX VANCRON 40 43A 601 L8V LNAA 3SG 46G10 VXPM B126 V18 KR237 27A 46 HV ELMAX Hardened 48A 60 LVZ B151 R50 B3 420A 46 FVQP SVERKER 3 820A 60 LVQ 420A 46 G12VQP 2) 454A 46 K13 V3 VANADIS 4 EXTRA 2) 97A 60 K5A V217 2) 51B126 C50B Vib-Star 3SG 46 FVSPF VANADIS 6 SGB 60 LVX 2) 455A 36 2 K15 V3 P22 WA 46 FV VANADIS 10 48A 601 L8V LNAA C150 QBA VANADIS 60 43A 601 L8V LNAA SGB 46 HVX B126 V18 KR237 27A 46 HV

79 GRINDING OF TOOL STEEL CYLINDRICAL STEEL GRADE CONDITION GRINDING INTERNAL GRINDING PROFILE GRINDING Conventional steel: ALVAR Soft 33A 46 KVM 77A 60 K9VZ 42A 100 IVZ ALVAR 14 annealed 2) 89A 60 2 K 5A V217 2) 89A 60 2 K6 V112 2) 89A 801 G11A V237 P25 ARNE 19A 60 KVS 32A 46 L5 VBE 32A 100 KVS CALDIE 48A 46 LV WA 46 JV WA 100 LV CALMAX DIEVAR FORMAX HOTVAR Hardened 48A 60 KVZ 77A 80 K9VZ 42A 1003 HVZ MIRRAX ESR 2) 92A 60 2 I6 V111 2) AH 120 K6 VCOL 2) 89A H11A V2 ORVAR SUPREME SGB 60 KVX 32A 60K5 VBE 32A 100 KVS ORVAR 2 MICRODIZED WA 60 JV WA 60 IV WA 120 JV POLMAX QRO 90 SUPREME REGIN 3 STAVAX ESR THG 2000 UHB 11 UNIMAX ORVAR SUPERIOR VIDAR SUPERIOR VIDAR 1 VIDAR 1 ESR HOLDAX Pre-hardened 33A 46 KVM 77A 60 K9VZ 42A 100 IVZ IMPAX HI HARD 2) 89A 60 2 K 5A V217 2) 97A 60 2 K6 V112 2) 89A 80 1G11A V237 P25 IMPAX SUPREME 19A 60 KVS 32A 46 L5 VBE 32A 100 KVS NIMAX 48A 46 LV WA 46 JV WA 100 LV RAMAX HH RAMAX LH Precipitation hardening steel: Solution 42A 60 JVZ 42A 60 J9 VZ 42A 100 HVZ treated or 15C 60 IVD 15C 60 IVD 2) 89A 80 1G11A V237 P25 CORRAX aged 2) 89A 60 2 J5A V217 2) 64B91 K11 V333 VV 32A 100 JVS SPH 50 SGB 60 JVX 32A 46 K5 VBE 77A 100 J8V LNAA 77A 461 K7V LNAA 25A 601 J85VP MCNN High carbide steel: ELMAX Soft annealed 62A 60 KVZ 77A 60 K9 VZ 42A 100 IVZ RIGOR 2) 454A 80 J11 V3 2) AH 120 K6 VCOL 2) F13A 54 FF22V Strato SLEIPNER SGB 60 KVX 32A 46 L5 VBE 32A 100 KVS SVERKER 3 48A 46 LV WA 46 JV WA 100 LV SVERKER 21 VANADIS 4 EXTRA VANADIS 6 VANADIS 10 VANADIS 23 VANADIS 30 VANADIS 60 VANCRON 40 RIGOR Hardened B151 R50 B3 B151 R75 B3 B126 R100 B6 SLEIPNER 48A 60 KVZ 430A 80 J VQA 820A 1003 GVQ SVERKER 21 2) 51B126 C50B Vib-Star 2) 51B126 C100 B54 2) B126 C75 B53 VANADIS 23 2) 454A 80 J11 V3 2) C202 H5A V18 2) 89A 80 1 G11A V237 P25 VANADIS 30 SGB 60 KVX CB150 TBA CB150 TBE VANCRON 40 3SGP 70 JVX 3SG 60 JVX 5SG 80 KVX B126 V18 KR191 B126 V24 KR237 B126K V24 KR237 27A 60 JV 27A 60 HV 27A 100 JV ELMAX Hardened B151 R50 B3 B151 R75 B3 B126 R100 B6 SVERKER 3 420A 54 JVQ 430A 80 J VQA 820A 1003 GVQ VANADIS 4 Extra 2) 51B126 C50B Vib-Star 2) 51B126 C100 B54 2) B126 C75 B53 VANADIS 6 2) 454A 80 J11 V3 2) C202 H54 V18 2) F13A 54 FF22V Strato VANADIS 10 CB150 QBA CB150 TBA CB150 TBE VANADIS 60 SGB 60 KVX 3SG 60 JVX 5SG 80 JVX 3SGP 70 JVX B126 V24 KR237 B126K V24 KR237 B126 V18 KR191 27A 60 HV 27A 100 IV 27A 60 IV 17

80 GRINDING OF TOOL STEEL 18

81 Network of excellence is present on every continent. This ensures you high-quality Swedish tool steel and local support wherever you are. ASSAB is our wholly-owned subsidiary and exclusive sales channel, representing Uddeholm in various parts of the world. Together we secure our position as the world s leading supplier of tooling materials.

82 / TRYCKERI KNAPPEN, KARLSTAD is the world s leading supplier of tooling materials. This is a position we have reached by improving our customers everyday business. Long tradition combined with research and product development equips Uddeholm to solve any tooling problem that may arise. It is a challenging process, but the goal is clear to be your number one partner and tool steel provider. Our presence on every continent guarantees you the same high quality wherever you are. ASSAB is our wholly-owned subsidiary and exclusive sales channel, representing Uddeholm in various parts of the world. Together we secure our position as the world s leading supplier of tooling materials. We act worldwide, so there is always an Uddeholm or ASSAB representative close at hand to give local advice and support. For us it is all a matter of trust in long-term partnerships as well as in developing new products. Trust is something you earn, every day. For more information, please visit or your local website.

83 EDM OF TOOL STEEL

84 This information is based on our present state of knowledge and is intended to provide general notes on our products and their uses. It should not therefore be construed as a warranty of specific properties of the products described or a warranty for fitness for a particular purpose. Classified according to EU Directive 1999/45/EC For further information see our Material Safety Data Sheets. Edition 3, The latest revised edition of this brochure is the English version, which is always published on our web site SS-EN ISO 9001 SS-EN ISO 14001

85 EDM OF TOOL STEEL Contents Introduction... 3 The basic principles of EDM... 4 The effects of the EDM process on tool steels... 4 Measuring the effects... 6 Achieving best tool performance... 9 Polishing by EDM Summary

86 EDM OF TOOL STEEL Introduction The use of Electrical Discharge Machining (EDM) in the production of forming tools to produce plastics mouldings, die castings, forging dies etc., has been firmly established in recent years. Development of the process has produced significant refinements in operating technique, productivity and accuracy, while widening the versatility of the process. Wire EDM has emerged as an efficient and economic alternative to conventional machining of apertures in many types of tooling, e.g. blanking dies, extrusion dies and for cutting external shapes, such as punches. Special forms of EDM can now be used to polish tool cavities, produce undercuts and make conical holes using cylindrical electrodes. EDM continues to grow, therefore, as a major production tool in most tool making companies, machining with equal ease hardened or annealed steel. Uddeholm Tooling supplies a full range of tool steels noted for consistency in structure. This factor, coupled with very low sulphur levels ensures consistent EDM performance. This brochure gives information on: The basic principles of EDM The effects of the EDM process on tool steels Achieving best tool perform ance 4 The basic principles of EDM Electrical discharge machining (spark erosion) is a method involving electrical discharges between an anode (graphite or copper) and a cathode (tool steel or other tooling material) in a dielectric medium. The discharges are controlled in such a way that erosion of the tool or work piece takes place. During the operation, the anode (electrode) works itself down into the workpiece, which thus acquires the same contours as the former. The dielectric, or flushing liquid as it is also called, is ionized during the course of the discharges. The positively charged ions strike the cathode, whereupon the temperature in the outermost layer of the steel rises so high (10 50,000 C/ 18 90,000 F) as to cause the steel there to melt or vaporize, forming tiny drops of molten metal which are flushed out as chippings into the dielectric. The craters (and occasionally also chips which have not separated completely) are easily recognized in a cross section of a machined surface. See figure 1. Four main factors need to be taken into account when considering the operating parameters during an EDM operation on tool steel: the stock-removal rate the resultant surface finish electrode wear the effects on the tool steel. The influence of the EDM operation on the surface properties of the machined material, can in unfavour- able circumstances jeopardize the working performance of the tool. In such cases it may be necessary to subordinate the first three factors, when choosing machining parameters, in order to optimize the fourth. The effects of the EDM process on tool steels The influence of spark erosion on the machined material is completely different to that of conventional machining methods. As noted, the surface of the steel is subjected to very high temperatures, causing the steel to melt or vaporize. The effect upon the steel surface has been studied by Uddeholm Tooling to ensure that the tool maker may enjoy the many benefits of the EDM process, while producing a tool that will have a satisfactory production life. In the majority of cases, it has been impossible to trace any influence at all on the working function of the spark-eroded tool. However, it has been observed that a trimming tool, for example, has become more wear resistant, while in some cases tool failure has occurred prematurely on changing from conventional machining to EDM. In other cases, phenomena have occurred during the actual electrical discharge machining that have caused unexpected defects on the surface of the tool. This due to the fact that the machining has been carried out in an unsuitable manner. Fig.1. A rough-machined EDM surface with a cross section through chips and craters. Material: Uddeholm Orvar 2 Microdized.

87 EDM OF TOOL STEEL Surface strength an important factor All the changes that can be observed are due to the enormous temperature rise which occurs in the surface layer. In the surface layer, it has been observed that the four (main) factors associated with the all-important surface strength of the steel are affected by this temperature increase: the microstructure the hardness the stress condition carbon content. Figure 2 shows a section from a normal rough-spark-machined surface with the typical, different structural changes. invariably follows the direction of the crystals. In normal rough machining, this layer has a thickness of about µm. The carbon content in the surface layer can also be affected, for instance, by carburization from the flushing liquid or from the electrode, but decarburization can also occur. Rehardened layer In the rehardened layer, the temperature has risen above the austenitizing (hardening) temperature and martensite has been formed. This martensite is hard and brittle. Tempered layer In the tempered layer, the steel has not been heated up so much as to reach hardening temperature and the only thing that has occurred is tempering-back. The effect naturally decreases towards the core of the material see the hardness curve in figure 2. In order to study the structural changes incurred with different machining variables, different tool steels see table 1 were roughmachined and fine-machined with graphite electrodes. Melted and resolidified layer The melted and resolidified layer produced during the EDM process is also referred to as the white zone, since generally no etching takes place in these areas during metallographic preparation. Figure 3, nevertheless, shows clearly that it is a rapidly solidified layer, where long pillar crystals have grown straight out from the surface of the metal during solidification. A fracture occurring in this layer Fig. 3. Pillar crystals formed during solidification x Melted and resolidified layer Rehardened layer Tempered layer H v Unaffected matrix 200 X Typical hardness distribution in the surface layer Fig. 2. Section from a spark-machined surface showing changes in structure. Material: Uddeholm Rigor, hardened to 57 HRC. 5

88 EDM OF TOOL STEEL Austenitizing, time 20 min Tempering, time 2 x 30 min Hardness Uddeholm Temperature Temperature Hardened Annealed steel grade AISI C F C F HRC HB ARNE O CALMAX RIGOR A SVERKER 21 D IMPAX SUPREME P ORVAR SUPREME H Table 1. The tool steels were tested in the hardened and tempered condition, and some of them also in the annealed condition. Note: As Uddeholm Corrax is a precipitation hardening steel the EDM surface has different characteristics. The white layer consists of melted and resolidified material with a hardness of approx. 34 HRC. There will be no other heat affected zone of importance. Measuring the effects The thicknesses of the heataffected zones have been measured. The hardnesses in these zones have also been measured, as have crack frequencies and crack depths. Strength values have been obtained through bending tests. The layer thicknesses appear to be largely independent of both steel grade and electrode material. On the other hand, there is a definite difference between the specimens which have been hardened and those which were in the softannealed condition. Figure 4 shows, in the form of graphs, the layer thicknesses and fissure frequency with different pulse durations for Uddeholm Orvar Supreme. In the annealed material, the zones are thinner and the fissures fewer. The brittle, hardened zone is scarcely present at all (figure 4b). The layer thicknesses can vary considerably, from 0 µm to maximum values slightly below the R max specified in the machining directions. In the rough-machining stages (t i 100µ sec), the thicknesses of the layers vary far more substantially than in the fine-machining stages. The thickness of both the melted and the hardened zone increases with spark duration, which appears to be the most important single controlling variable. Figure 5 shows Thickness µm Thickness µm Graphite electrode Fig. 4a. Layer thicknesses and Melted zone Hardended zone Matrix fissure frequency in the surface layer in electrical discharge machining of hardened (52 HRC) Uddeholm Orvar Supreme at different pulse durations t i µ sec (A) 3(B) (C) No. of cracks per cm: (A) in melted zone (B) in hardened zone (C) in matrix Graphite electrode Melted zone Hardended zone Matrix (A) 1000 t i µ sec (B) (C) No. of cracks per cm: (A) in melted zone (B) in hardened zone (C) in matrix the beneficial effect of fine-finishing, i.e. to produce a very thin remelted and heat-affected zone. 100 x Fig. 4b. As above, but for electrical discharge machining of Uddeholm Orvar Supreme in the annealed condition. Fig. 5. Fine-machined Uddeholm Rigor, pulse duration 10µ sec. 6

89 EDM OF TOOL STEEL Structures of spark-machined layers With longer pulse duration, the heat is conducted more deeply into the material. Higher current intensity and density (and thus spark energy) do, indeed, give a higher amount of heat in the surface, but the time taken for the heat to diffuse, nevertheless, appears to have the greatest significance. The pictures below show how the surface zones are changed in Uddeholm Sverker 21 (in hardened and tempered condition) with different pulse durations and electrode materials. Figur 6a. Copper electrode t i = 10 µs. Magnification 500 x The cause of arcing Short off-times, or pause times, give more sparks per unit of time and thus more stock removal. During the off-time, the dielectric fluid Figur 6d. Copper electrode t i = 200 µs. Magnification 500 x t i = 500 µs. Magnification 500 x Figur 6e. Graphite electrode must have time to become deionized. Too short an off-time can result in double sparking ignitions which lead to constantly burning arcs between the electrode and the work piece, resulting in serious surface defects. The risk of arcing is increased if flushing conditions for the dielectric fluid are difficult. As a result of arcing, i.e. a condition in which arcs are formed between local parts of the electrode and the workpiece, large craters or burns are formed in the surface. These have frequently been confused with slag inclusions or porosity in the material. Figures 7 and 8 show the surface of a tool with a section through one of the suspected pores. One of the primary causes of this type of defect is inadequate flushing, or machining of narrow slots, etc., resulting in chips and other loose particles forming a bridge between the electrode and the workpiece. The same effect can be obtained with a graphite electrode which bears traces of foreign material. On modern machines featuring socalled adaptive current control, the risk of arcing has been eliminated. t i = 10 µs. Magnification 500 x Figur 6b. Graphite electrode Figur 7. The suspected pores can be seen on the surface of the tool 1:1 Figur 6c. Graphite electrode t i = 100 µs. Magnification 500 x Figur 8. A section through one of the suspected pores 65 x 7

90 EDM OF TOOL STEEL Fissure frequency also increases with pulse duration With times in excess of 100µ sec, all steels reveal several cracks in the melted layer. High-carbon and/ or air-hardening steels show the highest frequency of fissures. The annealed specimens contain no cracks at all in the matrix. The number of cracks which continue down into the hardened zone is roughly 20%, while only a very few cracks penetrate into the matrix. In the matrix, the fissure depth is seldom more than about some tens of a µm. Here too, it applies that cracks in the matrix are mainly encountered in the highly-alloyed cold-working steels. Table 2 shows the occurrence rate of fissures in a number of tested tool steels. The difference in stock-removal rate amounts to a maximum of approx. 15% between the different grades of tool steel with the same machine setting data. The hardnesses in the different layers can also vary considerably, but in principle the same pattern applies to all grades. Figure 9 shows a typical hardness distribution. Table 2. The table shows the occurrence rate of fissures. The difference in hardness and volume between the layers gives rise to stresses which, upon measurement, have been found to have the same depth as the affected surface layers. These stresses can be substantially reduced by extra heattreatment operations. Renewed tempering (235 C/ 455 F 30 min) of the specimen in figure 9 resulted in lowering of the hardness level to the curve drawn with a broken line. If electrical discharge machining is properly performed with a final fine-machined stage, surface defects are largely eliminated. If this is not possible for one reason or another, or if it is necessary for all effects to Melted Hardened zone zone Matrix High-alloy cold-work steel SVERKER type Hot-work steel ORVAR type Cold-work steels RIGOR and ARNE types Plastic-moulding steel IMPAX SUPREME type be eliminated, some different related operations can be used: Stress-relief tempering at a tempering temperature approx. 15 C (30 F) lower than that previously used tempering temperature, lowers the surface hardness without influencing the hardness of the matrix. Grinding or polishing will remove both the surface structure and cracks, depending of course on how deeply it is done (approx µm in fine-machining). Graphite electrode t i = 200 µ sec HV Hardness immediately after EDM Hardness after retempering µm Fig. 9. Typical hardness distribution in hardened Uddeholm Sverker 21 immediately after EDM and then after re-tempering. 8

91 EDM OF TOOL STEEL Bending test To evaluate the likely effect of the remelted layer, surface irregularities and cracks produced in the EDM process on the strength of a tool, a bending test was carried out. Various combinations of EDM surface finish and post treatments, e.g. stress-relieving/polishing, were tested on 5 mm square test pieces of Rigor at 57 HRC. The test pieces were spark-machined on one face to different EDM stages and bent severely, with the EDM surface on the outside of the bend. Figure 10 shows that the sample with a fine-spark machined finish which had been polished afterwards gave the best result. The rough spark-machined sample, without any post treatment, had the lowest bending strength. Bending strength N/mm Background to the bending test results The hard, re-solidified rehardened layers cause, in the first instance, those cracks which are formed upon application of the load and in the second instance those which were already present to act as initiators of failure in the matrix. At 57 HRC, the matrix is not tough enough to stop the cracks from growing and consequently the failure occurs already on the elastic part of the load curve. Normally, there should have been a certain amount of plastic bending of a test bar in this material. Achieving best tool performance EDM using solid electrodes (copper/graphite) As noted, in most cases where the EDM process has been carefully carried out no adverse effect is experienced on tool performance. As a precautionary measure, however, the following steps are recommended: EDM OF HARDENED AND TEMPERED MATERIAL A Conventional machining B Hardening and tempering C Initial EDM, avoiding arcing and excessive stock removal rates. Finish with fine-sparking, i.e. low current, high frequency. D (i) Grind or polish EDM surface or D (ii) Temper the tool at 15 C (30 F) lower than the original tempering temperature. or D (iii) Choose a lower starting hardness of the tool to improve overall toughness EDM OF ANNEALED MATERIAL Rough spark-machined Rough spark-machined Stress-relieved Fine spark-machined Fine spark-machined Stress-relieved Fine spark-machined, Polished A Conventional machining B Initial EDM, as C above. C Grind or polish EDM surface. This reduces the risk of crack formation during heating and quenching. Slow pre-heating, in stages, to the hardening temperature is recommended. Fig. 10. Bending strength at different EDM stages and with different subsequent operation. Material Uddeholm Rigor 57 HRC. The shaded areas show the spread of the results measured. Note: When EDM d in solution annealed condition the toughness of Uddeholm Corrax is not affected. It is recommended that all EDM ing of Uddeholm Corrax is done after aging since an aging after EDM ing will reduce the toughness. It is recommended that the white layer is removed by grinding, stoning or polishing. 9

92 EDM OF TOOL STEEL Wire EDM The observation made about the EDM surface in earlier pages are also mostly applicable to the wire EDM-process. The affected surface layer, however, is relatively thin (<10 µm) and can be compared more to finesparking EDM. Normally there are no observable cracks in the eroded surface after wire erosion. But in certain cases another problem has been experienced. After heat treating a through hardening steel the part contains high stresses (the higher the tempering temperature, the lower the stresses). These stresses take the form of tensile stresses in the surface area and compressive stresses in the centre and are in opposition to each other. During the wire erosion process a greater or lesser amount of steel is removed from the heattreated part. Where a large volume of steel is removed, this can sometimes lead to distortion or even cracking of the part. The reason is that the stress balance in the part is disturbed and tries to reach an equilibrium again. The problem of crack formation is usually only encountered in relatively thick cross section, e.g. over 50 mm (2") thick. With such heavier sections, correct hardening and double tempering is important. In certain cases the risk can be reduced through different precautions. 1: To lower the overall stress level in the part by tempering at a high temperature. This assumes the use of a steel grade with high resistance to tempering. 2: By drilling several holes in the area to be removed and to connect them by saw-cutting, before hardening and tempering. Any stresses released during heat treatment are then taken up in the pre-drilled and sawn areas, reducing or eliminating the risk of distortion or cracking during wire-erosion. Fig. 13 illustrates how such pre-cutting may be done. Fig. 13. Pre-drilled holes connected by a saw-cut, before hardening and tempering, will help to prevent distortion or cracking when wire eroding thick sections. Fig. 12. This block of D2 steel, approx. 50 x 50 x 50 mm (2" x 2" x 2"), cracked during the wire EDM operation. Fig. 11. Wire erosion of a hardened and tempered tool steel blanking die. 10

93 EDM OF TOOL STEEL Wire erosion of cutting punches When producing a cutting punch by wire erosion, it is recommended (as with conventional machining) to cut it with the grain direction of the tool steel stock in the direction of the cutting action. This is not so important when using PM steels due to their non-directional grain structure. Polishing by EDM Today some manufacturers of EDMequipment offer, by a special technique, possibilities to erode very fine and smooth surfaces. It is possible to reach the surface finish of about 0,2 0,3 µm. Such surfaces are sufficient for most applications. The greatest advantages are when complicated cavities are involved. Such cavities are difficult, time consuming and therefore expensive to polish manually, but can be conveniently done by the EDM- machine during a night-shift, for example. Investigations made on our grades Uddeholm Impax Supreme, Uddeholm Orvar Supreme, Uddeholm Stavax ESR and Uddeholm Rigor show that the hard re-melted white layer produced is very thin and equal in the these grades. The thickness is about 2 4 µm. Since there is no sign of any heat-affected layer, the influence of the EDM on mechanical properties is negligible. Summary In summing up it can be said that properly executed electrical discharge machining, using a rough and a fine machining stage in accordance with the manufacturer s instruction, eliminates the surface defects obtained in rough machining. Naturally, certain structural effects will always remain, but in the vast majority of cases these are insignificant, provided that the machining process has otherwise been normal. Structural effects, more-over, need not necessarily be regarded as entirely negative. In certain cases the surface structure, i.e. the rehardened layer, has on account of its high hardness improved the resistance of the tool to abrasive wear. In other cases it has been found that the cratered topography of the surface is better able to retain lubricant than conventional surfaces, resulting in a longer service life. If difficulties in connection with the working performance of spark-machined tools should arise, however, there are some relatively simple extra operations that can be employed, as indicated above. A slightly striped appearance has been re-ported in materials rich in carbides, such as high-carbon coldwork steels and high-speed steels, where there is always a certain amount of carbide segregation or in material with high sulphur content. The difference in bending strength between rough-spark-machined and fine-spark-machined test pieces is largely due to the difference in the distribution of the cracks and to the presence of the in spots distributed white layer on the fine-sparkmachined specimens. The rougher surface finish of the rough-machined specimen has not really been significant. Regardless of circumstances, such surface irregularities are relatively harmless as crack initiators compared with the solidification cracks. During the polishing of the fine-machined test piece which was carried out, the depth of the white and rehardened layer was merely reduced and not completely eliminated. Further polishing would probably result in complete restoration of the bending strength. Highly stressed tools and parts thereof, e.g. very thin sections that are far more liable to bending, can justify an extra finishing operation. The lower the hardness in the matrix, the less sensitive the material will be to adverse effects on the strength as a result of electrical discharge machining. Lowering of the hardness level of the entire tool can, therefore, be another alternative. Fig. 14. This Uddeholm Stavax ESR mould insert was finished by EDM polishing. 11