DEUTSCHE NORM March Heat treatment of ferrous materials Part 5: Surface hardening

Transcription

1 ICS DEUTSCHE NORM March 2000 Heat treatment of ferrous materials Part 5: Surface hardening Wärmebehandlung von Eisenwerkstoffen Verfahren der Wärmebehandlung Teil 5: Randschichthärten In keeping with current practice in standards published by the International Organization for Standardization (ISO), a comma has been used throughout as the decimal marker. Contents { Foreword Scope Normative references Concepts Principle of method Identification of heat treatment condition Procedure Pretreatment and preparation Austenitizing Quenching Subzero treatment Tempering Secondary treatment Heat treatment media Cooling and quenching media Subzero treatment media Effects of surface hardening Effects on case structure Effects on hardness and effective case depth Effects on shape and dimensions Defects in heat treated products Designing for heat treatment Straightening Testing surface hardened products Foreword This standard has been prepared by Technical Committee Wärmebehandlungstechnik of the Normenausschuss Werkstofftechnologie (Materials Technology Standards Committee). 1 Scope This standard describes the surface hardening of products made of rolled steel, cast iron, or steel powder compacts. Page Continued on pages 2 to 13. Translation by DIN-Sprachendienst. In case of doubt, the German-language original should be consulted as the authoritative text. No part of this translation may be reproduced without the prior permission of DIN Deutsches Institut für Normung e. V., Berlin. Beuth Verlag GmbH, Berlin, Germany, has the exclusive right of sale for German Standards (DIN-Normen). Ref. No. English price group 11 Sales No

2 Page 2 2 Normative references This standard incorporates, by dated or undated reference, provisions from other publications. These normative references are cited at the appropriate places in the text, and the titles of the publications are listed below. For dated references, subsequent amendments to or revisions of any of these publications apply to this standard only when incorporated in it by amendment or revision. For undated references, the latest edition of the publication referred to applies. DIN 6773 Heat treatment of ferrous materials Heat treated parts, representation and indications on drawings *) DIN Heat treatment of ferrous materials Symbols for heat treatment processes DIN Heat treatment of ferrous products Hardening and tempering DIN Heat treatment of ferrous materials Heat treatment methods Hardening and tempering of tools DIN Heat treatment of ferrous metals Forms Orders for heat treatment (WBA) DIN Rockwell hardness testing of metallic materials Modified Rockwell scales Bm and Fm (for thin sheet steel) DIN Determination of the effective case depth of heat treated parts after surface hardening DIN Determination of depth of decarburization of steel DIN Metallographic examination Determination of the ferritic or austenitic grain size of steel and ferrous materials DIN EN Non-destructive testing Penetrant testing Part 1: General principles DIN EN Vocabulary of heat treatment terms for ferrous products DIN EN Safety of machinery Laser processing machines Safety requirements (ISO : 1996, modified) DIN EN ISO Metallic materials Brinell hardness test Part 1: Test method DIN EN ISO Metallic materials Vickers hardness test Part 1: Test method (ISO : 1997) DIN EN ISO Metallic materials Rockwell hardness test (scales A, B, C, D, E, F, G, H, K, N, T) Part 1: Test method 3 Concepts For the purposes of this standard, the heat treatment concepts defined in DIN EN shall apply. 4 Principle of method The surface layer of a ferrous product is austenitized and then cooled at a suitable rate. Martensite is thus formed, increasing the hardness of the surface layer and enhancing strength and wear resistance. The area to be hardened is heated to a temperature above Ac 3 or Ac m by means of either flame, induction, laser beam or electron beam hardening. For each material, the density of the heat flow rate of the heat source and the treatment time produce a specific thermal cycle during which the surface layer is austenitized to a certain depth at a high heating rate followed by a short soaking time as compared to other heat treatment methods. Because of the transformation behaviour of steel, higher heating rates require higher heating temperatures to obtain a sufficiently austenitic condition. The relationship between the heating rate and temperature can be derived from a time-temperature-transformation (TTT) diagram for continuous heating. Hardening actually occurs during the subsequent quenching of the product. Large areas can be hardened either by means of a suitable energy transfer or by moving the product itself. Between the hardened case and the non-hardened core lies a transition zone of several millimetres within which the depth of hardness gradually diminishes. The depth of this zone is influenced by the heating and quenching conditions. In many cases, surface hardening is followed by tempering. 5 Identification of heat treatment condition The heat treatment condition shall be indicated on drawings as specified in DIN Instructions for performing surface hardening shall be formulated using either the WBA form specified in DIN or in a heat treatment plan (WBP). Symbols used to designate the heat treatment method shall be as specified in DIN *) Currently at draft stage.

3 6 Procedure 6.1 Pretreatment and preparation Page 3 Products shall be pretreated and prepared to ensure a material condition suitable for surface hardening, particularly in terms of microstructure and residual stresses, and to obtain the required core strength in the final condition. Prior to laser hardening, it may be necessary to clean the product surface and pretreat it to improve absorption. Prior to electron beam hardening, the surface shall be cleaned and, if necessary, demagnetized Pretreatment Stress relieving If residual stresses (e.g. due to cutting processes) might cause distortion of the product during treatment, it is recommended that stress relieving be carried out. Any resulting distortion can then be corrected by subsequent machining, although there shall be an allowance great enough to eliminate any unwanted changes to the surface layer (e.g. decarburization). The stress relief temperature shall be close to, but shall not exceed, the transformation temperature Ac 1 of the material being treated. In the case of quenched and tempered products, this temperature shall be lower than the tempering temperature in order to maintain strength, and soaking for more than thirty minutes during the heating phase will not be necessary. Heating and cooling shall be carried out slowly to prevent new residual stresses from building up. Cold-worked products should not be stress relieved, but rather normalized, if there is a risk that recrystallization would result in grain coarsening Normalizing Residual stresses in untreated products may also be relieved by normalizing, which at the same time alters the microstructure, thus preventing grain coarsening in critical areas. Normalizing parameters (normalizing temperature and duration, cooling) shall be taken from the steel manufacturer s specifications or other documents Quenching and tempering It may be necessary to subject the product to quenching and tempering prior to treatment to obtain the desired strength and a homogenous material condition. See DIN and DIN regarding the procedure. To ensure that any changes to the surface layer (e.g. decarburization or oxidation) which occur during quenching and tempering do not adversely affect subsequent treatment, the product surface should be machined before further treatment Oxidizing Prior to laser hardening, it may be necessary to oxidize the surface to promote the absorption of the laser beam by the material. Normally, this is done by annealing the material in water vapour at a temperature between 450 C and 550 C Preparation Machining or cutting residues (e.g. oxide layers, residues of cooling lubricants, cleaning agents or preservatives) can impede the surface hardening process, as can chips, burrs, rust, scale and nonferrous metals. The evaporation of residues during electron beam hardening can adversely affect the vacuum, while during laser hardening such residues can affect the transfer of energy to the surface layer. It is therefore necessary to carefully treat and thoroughly clean the products prior to hardening, depending on the degree of surface impurities and the required quality. The surface can be cleaned by washing, deburring, blasting or pickling Washing Normally, products are washed in hot water with suitable cleaning agents. To ensure that the surface is fully cleaned, it may be necessary to subject the surface to water-blast cleaning or ultrasound cleaning prior to washing. After washing, the products shall be thoroughly dried Deburring Burrs caused by machining can be removed by blasting, or chemical or thermal deburring. It should be noted that thermal deburring processes oxidize the product s surface, while in chemical processes the material reacts with the electrolyte, so that in both cases treatment with electron beams or lasers can be impeded. When removing adherent chips, the product should be demagnetized Blasting Dry or wet blasting with suitable cleaning agents can be used to remove burrs, scale, rolling, forging or casting skin, colorants or flux residues.

4 Page Pickling Pickling is suitable for removing rust, scale, or rolling, forging or casting skin. Care should be taken to fully remove all pickling residue, since this can begin to rust. Furthermore, too intensive pickling can leave pits in the surface layer Coating Prior to laser hardening, it may be necessary to supply the product with a coating that promotes laser beam absorption (e.g. using graphite powder) Edge protection Prior to flame or induction hardening, it may be necessary to protect edges in the area to be hardened from overheating. This can be done by fitting suitable copper inserts into undercuts, flutes, slots, holes, etc. 6.2 Austenitizing Surface hardening involves a localized heating of a product s surface layer to austenitizing temperature for a certain length of time, with the heating process being performed once or several times, using one of several heat sources. The heating rate is determined by the energy supplied by the heat source and the heating time. The resulting temperature profile for the heated case is a function of the type and density of the flow rate of the heat source, the exposure time and the type of material being treated. The objective is to maintain a uniform temperature distribution within a localized heated area. Care should be taken to ensure that the maximum temperature within the case does not exceed the melting temperatures of the different phases 1 ) in the material. With surface hardening, the austenitizing temperature is reached within a much shorter time than with furnace heating, due to the relatively high density of heat flow rate (cf. table 1). For sufficient austenitizing, it is therefore necessary to heat to temperatures which are 50 C to 100 C higher than furnace temperatures, taking care that the temperatures in the external regions of the product are below the melting temperature of the material, to avoid unwanted fusion. Table 1: Density of heat flow rate of various heat sources Heat source Density of heat flow rate, Effective case depth, in W/cm 2 in mm Laser beam 10 3 to ,01 to 1 Electron beam 10 3 to ,01 to 1 Induction: MF 2 to to 10 HF 4 0,1 to 2 HF-impulse 0,05 to 0,5 Flame 10 3 to ,5 to 10 Plasma beam 10 4 Salt bath (convection) 20 Air/gas (convection) 0,5 The microstructural changes taking place during the heating process are described in a time-temperaturetransformation (TTT) diagram for continuous heating (see figure 1 for an example). 1 ) For example, 950 C for the phosphide eutectic mixture in cast iron.

5 Page 5 Heating rate, in C/s Temperature, in C Time, in s Complete austenitization Partial austenitization Ferrite + austenite + carbide Ferrite + carbide Quenching/tempering parameters: 825 C for 15 min in water, 600 C for 60 min in air Figure 1: Time-temperature-transformation (TTT) diagram for continuous heating of grade 42CrMo4 steel in the quenched and tempered condition Figure 1 shows that as the heating rate increases, austenite formation and carbide dissolution take place at increasingly higher temperatures. The curves in the TTT diagram can be used to approximate the temperature above which a specific microstructure can be obtained at a given heating rate. The formation of austenite and dissolution of carbide are influenced by the type of alloying elements present and their quantities, as well as by the material condition prior to treatment. Although a complete dissolution of carbide is not generally desirable, enough carbide should be dissolved to ensure the carbon content of the austenite is sufficient to achieve the required hardness. In progressive methods the heat source or the product travels, allowing localized austenitizing with a varying microstructure. Patterns of hardened areas can be created by moving the product. A simple example of a spiral pattern is shown in figure 2. If a treated area is exposed a second time to the heat source, tempering occurs in the adjacent areas, making them subject to cracking. See DIN 6773 regarding the designation of such areas. Hardened Not hardened Figure 2: Example of a spiral pattern of hardened areas on a shaft

6 Page Induction hardening Here the heat required for austenitizing is generated by means of induction. Heating is accomplished by placing a product in the magnetic field generated by an alternating current passing through an inductor, usually watercooled. The rapidly alternating magnetic field induces current within the product and the induced currents then generate heat. A conductive heating method can also be used in which the heated part of the product serves as the inductor. The depth of heating produced by induction is inversely proportional to the frequency of the alternating current. Normally, this frequency is constant. The formation of the heated area is determined by the type and form of the inductor and its coupling, and thus by the distance between the inductor and the product surface. The heated case formed does not always absolutely conform to the shape of the product. The depth of heating is normally controlled by the alternating current power input, the inductive coupling, and the density of the electromagnetic field. To this end, a single-turn or multi-turn induction coil, or a magnetic inductor may be used. Normally, the inductor remains still and the product moves, for instance to cover large areas ( progressive method ). A spinning method in which the product is rotated is often used on symmetrical pieces for concentrated heating. After the metal has been austenitized and the alternating current turned off (or the product has been removed from the inductor), the product is quenched in a suitable medium. Where the depth of heating is not very great, the product can be self-quenched by simply allowing the unheated core to draw off heat from the surface layer Flame hardening Flame hardening is a heating method in which the product surface is austenitized by heating with a torch, which is normally moving while the product remains still or is rotated. The depth of heating is determined by shape of the torch, the type of gas used to create the flame, and the flow rate of the gas. The type of torch used determines the size of the heated area. The torch can be moved back and forth across the product to cover a greater area or to obtain a greater case depth by means of thermal conduction. Furthermore, a moving torch can help ensure that the surface temperature remains below melting temperature. Quenching is carried out after flame hardening in much the same manner as after induction hardening Laser hardening With this method, the heat source is a high-power laser beam. The laser is only partially absorbed by a very thin surface layer and the rest is reflected. The extent of absorption depends on the product material, the laser s wavelength, the surface condition (roughness, degree of oxidation, cleanliness, etc.) and the product temperature. Absorption can be increased by adding coatings or using polarized radiation. The area covered by the beam can be influenced by manipulating the optical components or mirrors used to create the laser beam. The depth of heating is determined by the level of thermal conduction. By moving the laser beam and product in relation to each other, the area of treatment can be moved. Measures are to be taken to protect persons and property from direct and reflected radiation, as specified in DIN EN Electron beam hardening With this method, the heat source is an electron beam formed by means of magnetic lenses and directed at the product s surface. Both the electron beam and the product are in a vacuum. With their kinetic energy, the electrons heat the product to a depth of about 10 mm to 50 mm, with the actual depth of heating being determined by the level of thermal conduction. It should be noted that X-rays are emitted, depending on the accelerating voltage, and sputtering occurs at the surface. The electric charge of the product has to be dissipated via the product and its holder. The size of the treated area can be adjusted by changing the shape of the beam, or by guiding it or splitting it. The location of the area can be adjusted by moving the beam or the product. 6.3 Quenching Quenching is performed using a medium that is suitable for the material s hardness, and for the size and shape of the product. For smaller heating depths and where the relevant product dimension is about ten times the effective case depth, quenching may not be necessary because the bulk of the product acts as an adequate heat sink for self-quenching. For regular quenching, the product can either be dipped in the quenching medium, or nozzles can be used to spray the product with the medium. As with heating, quenching produces differences between the core and case temperatures, which in turn creates stresses that can lead to distortion or cracking. It may therefore be necessary to limit the quenching action.

7 Page 7 Figure 3 shows an example of a continuous-cooling-transformation (CCT) diagram, which illustrates the phase transformations taking place in a grade 42CrMo4 steel during quenching at austenitizing temperature. The regions in which microstructural changes occur are shown as curves, whose position and shape are determined by the steel s material composition and the austenitizing conditions. The expected microstructure of the case at ambient temperature and the relevant hardness can be approximated on the basis of the cooling curves in the diagram. In the case of surface hardening, a full transformation to martensite is desirable. This is only possible if the critical cooling rate, v Km, characteristic for each steel can be reached within the austenitized region. If the hardenability of the material is too low, the case is too deep, or the quenching effect is not sufficient, then other constituents (e.g. bainite, pearlite or ferrite) form in addition to martensite. Hypereutectoid steel can also contain undissolved or preeutectoid carbides, as well as retained austenite. The transformation of austenite into martensite begins once the cooling temperature goes below the M s temperature, and is not complete until the M f temperature is reached, which can be below ambient temperature, depending on the composition of the material and the austenitizing conditions. Temperature, in C Austenitzing temperature: 850 C Austenite Martensite Ferrite Bainite Minutes Time, in s Pearlite Pearlite Bainite content (%) Retained austenite content (%) Hours Figure 3: Continuous-cooling-transformation (CCT) diagram for a grade 42CrMo4 steel 6.4 Subzero treatment The amount of retained austenite at ambient temperature can be reduced by subzero treatment. This may be necessary if tempering would lower the hardness value, or there are special requirements regarding the dimensional stability of the product. However, subzero treatment increases the brittleness of the material, thus reducing tensile or fatigue strength. Because the retained austenite stabilizes immediately after quenching, subzero treatment should be carried out directly following the quenching process. Tempering at low temperatures can also lead to the stabilization of retained austenite. See subclause 8.2 for suitable subzero cooling media. Days

8 Page Tempering Tempering involves heating the product and soaking it at tempering temperature, then cooling it to ambient temperature. Either the entire product or the case only is heated. Tempering should immediately follow the hardening process, although the product should be allowed to cool to ambient temperature first. Normally, tempering is carried out between 180 C and 220 C, rarely above these temperatures. If tempering is performed in a furnace, the soaking time should be at least one hour. 7 Secondary treatment Normally no secondary treatment is performed on surface hardened products aside from mechanical or chemical surface treatment. 8 Heat treatment media 8.1 Cooling and quenching media Quenching can be carried out with or without a quenching medium. Quenching using a medium is necessary if self-quenching will not occur at the required critical cooling rate; this is normally the case for flame or induction hardening, while self-quenching is usually sufficient after laser or electron beam processes Liquid media Common liquid quenching media include water with or without additives, and oil. It should be noted that polymer additives lower the cooling rate as compared to water without additives. The temperature of the quenching medium is maintained within a narrow range, with water normally being used at a temperature between 15 C and 40 C and oil normally being used either at ambient temperature or a temperature above 60 C Gaseous media Still or forced air, and nitrogen may be used as gaseous quenching media. The quenching effect is dramatically lower in gaseous media than in liquids, although it can be increased by raising the pressure or the flow rate. 8.2 Subzero treatment media In conventional freezers, the cooled air cools the products to about 60 C. Special equipment can be used to lower the temperature to 140 C. Temperatures below 60 C may be reached by using dry ice, alcohol mixtures or liquefied gases (e.g. liquid nitrogen, which has a temperature of 196 C). 9 Effects of surface hardening 9.1 Effects on case structure Because the product surface is heated to a temperature well above Ac 3 and Ac m in current practice, the formation of austenite is to be expected, as shown in TTT diagrams for various steels. Throughout the heating process, the degree of austenite formation 2 ) decreases with increasing depth; the phases formed are influenced by the heating and quenching conditions, and the product material. Figure 4 shows a schematic representation of the hardness profile and phases in a quenched and tempered and then surface hardened product. The microstructure of hardened or quenched and tempered materials can be divided into several zones. Starting at the core and moving towards the surface these are: a tempering zone, a mixed zone with martensite, bainite, pearlite, ferrite and carbide, and a martensitic zone. Decarburization can occur in the case, depending on the thermal cycle and material. Laser hardening can cause layers to form which promote absorption, leading to carburization. 2 ) The degree of austenite formation is given by the degree of carbide dissolution, the uniformity of the austenite and the austenitic grain size (cf. DIN ).

9 Page 9 Hardness Fully and uniformly austenitized and quenched with V ö V km Primarily martensite Fully austenitized but quenched with V < V km Martensite, bainite, pearlite Hardness profile Partially austenitized Martensite + bainite + pearlite + ferrite Distance from surface Figure 4: Example of hardness profile and microstructure in a quenched and tempered steel (schematic) 9.2 Effects on hardness and effective case depth Original microstructure with tempering at higher temperatures Quenched and tempered original microstructure The hardness of the case formed by surface hardening is a function of the type, amount and distribution of martensite, bainite, pearlite, ferrite and carbide. Figure 5 shows a schematic hardness profile for two steels with different carbon concentrations. Hardness HV Distance from surface, in mm Figure 5: Hardness profiles for surface hardened steels with different carbon concentrations in the non-tempered original condition (schematic)

10 Page Effects on shape and dimensions Localized heating and quenching can cause extensive dimensional changes and can produce residual stresses in different areas of the product due to thermal expansion and phase transformations. Since this can lead to distortion, products should be fixed in place or prestressed. Furthermore, products which had residual stresses before hardening and which were not sufficiently tempered or stress relieved will become distorted during the hardening process. 10 Defects in heat treated products Defects in heat treated products are rarely due to a single cause. In addition to the heat treatment process itself, possible causes include the material and shape of the product, the machining process and service conditions. Table 2 lists some of the most common defects which occur in practice and which can be attributed to the surface hardening process, assuming the products have been delivered in good condition without any defects. Table 2: Defects and their possible causes Defect Cause Heat treatment error Refer to subclause 1 Surface hardness too 1.1 Insufficiently a) Austenitizing temperature 6.2 low transformed pearlite, too low ferrite, or insufficiently b) Austenitizing time too short dissolved carbides 1.2 Insufficient amount of martensite in case due to formation of a) Austenitizing temperature 6.2 bainite, pearlite or too low ferrite b) Austenitizing time too short 6.2 c) Insufficient quenching 6.3, 8.1 (through medium or selfquenching) d) Too much oxidation of 8.1 edges e) Case is decarburized due to retained a) Austenitizing temperature 6.2 austenite too high (overheating) b) Austenitizing time too long 6.2 c) Insufficient, improperly 6.4 timed, or no subzero treatment d) Insufficient, improperly 6.5 timed, or no tempering e) Carburization due to use of 9.1 coatings 1.3 Martensite too soft, a) Tempering temperature too 6.5 possibly localized high b) Tempering time too long 6.5 c) Overlapping treatment of 6.2 already hardened areas 1.4 Too much retained a) No tempering or tempering 6.5 austenite or too few done at wrong time carbides 1 ) dissolved b) Tempering temperature too 6.5 low c) Tempering time too short Surface hardness too Martensite in case too a) No tempering 6.5 great hard b) Tempering temperature too 6.5 low c) Not tempered enough times Effective case depth 3.1 Austenitization does a) Austenitizing temperature 6.2 too small not cover entire area too low b) Austenitizing time too short 6.2 For 1 ), see page 11. (continued)

11 Page 11 Table 2 (concluded) Defect Cause Heat treatment error 3.2 Too little martensite Quenching rate too slow 6.3, Effective case depth Temperature too high when a) Austenitizing temperature 6.2 too great preheating to austenitizing too high temperature b) Austenitizing time too long Too much distortion Thermal and a) Too quickly or unevenly 6.3, 9.3 transformation stresses too heated and austenitized great or unevenly b) Product not properly 9.3 distributed arranged c) Area to be treated not 6.2 suitably designed 6 Cracking Thermal and a) Too quickly or unevenly 6.2 transformation stresses too heated and austenitized great (localized brittleness) b) Too quickly or unevenly 6.3 quenched c) No tempering 6.5 d) Tempering temperature too 6.5 low e) Tempering time too short 6.5 f) Overlapping treatment of 6.2 already hardened areas 7 Distortion of corners Unintended fusion a) Temperature too high 6.2 and edges or b) Treatment time too long 6.2 warping c) Power of heat 6.2 source too high d) Edges overheated 6.1, ) Applies only to steels having undergone secondary hardening. Refer to subclause 11 Designing for heat treatment The product shape and size are major factors influencing the hardness profile and stresses created during the hardening process, as well as the resulting distortion. By selecting a suitable design the likelihood of distortion and the risk of cracking can be minimized, and often the life of the product can be increased. Abrupt changes in cross section can have different effects on the hardness profile, depending on the method used. The following design principles should therefore be taken into account. A suitable mass distribution can be obtained by avoiding designs with abrupt changes in cross section (cf. figure 6). Instead of abrupt changes in cross section, give preference to rounded or bevelled transitions (cf. figure 7). Where the case extends to an edge of the product, include a chamfer (cf. figure 8). Symmetrical designs should be used wherever possible (cf. figure 9). Unsuitable Suitable Figure 6: Examples of suitable and unsuitable mass distribution

12 Page 12 Unsuitable Suitable Figure 7: Examples of suitable and unsuitable changes in cross section Unsuitable Suitable Figure 8: Examples of suitable and unsuitable edge designs Unsuitable Suitable Figure 9: Examples of asymmetrical and symmetrical designs If it is not possible to design the product so that it is suitable for surface hardening, the product should receive its final form after hardening, as shown in figure 10. Detail X Detail X Removed by machining after hardening Figure 10: Giving the product its final form after hardening (example)

13 12 Straightening When straightening hardened products, it should be noted that the case has practically no deformability and can therefore break even when only slightly deformed. For this reason, straightening of such products should be avoided. Slight distortions can be removed by bending the product using a straightening press, machine or bench, or by subjecting it to selective heating. When thermally straightening localized areas, care should be taken that the case is not tempered by the heating, since this will reduce hardness. It should be noted that the residual stresses induced by straightening may create renewed distortion. Straightening should be performed before tempering, since then the risk of cracking, the formation of residual stresses, and of deformation is lower. 13 Testing surface hardened products When the effective case depth is determined, the product is destroyed. If this is not permitted, a reference specimen, preferably of the same material condition, size and shape as the product, should be heat treated along with the products, or an extra number of products are to be treated. Table 3 lists methods for testing the effectiveness of the heat treatment procedure. If a batch of several products has been treated, sampling should be carried out following statistical principles. The product user shall decide whether test results are suitable for determining the performance characteristics of the product. Table 3: Testing surface hardened products Page 13 Property/characteristic tested Test method 1 Hardness As in DIN , DIN EN ISO , DIN EN ISO and DIN EN ISO Effective case depth As in DIN Extent of unintended fusion Visual examination of cleaned products, without any further pretreatment 4 Soft spots a) Hardness testing b) Visual or macroscopic examination of etched (preferably polished 1 )) or blasted surface 5 Cracking a) Visual examination of cleaned products b) Micrographic examination (macro- or microscopic) c) Penetration testing as in DIN EN d) Ultrasound testing e) Eddy current testing f) Magnetic flaw detection 6 Microstructure: 6.1 Form, number and structure of constituents (martensite, bainite, pearlite, ferrite, retained austenite and carbides) 6.2 Grain size and form Micrographic examination As in DIN Decarburization or carburization of surface As in DIN layer 1 ) Polishing shall be carried out so that the soft spots in the surface layer are not removed.