OPTIMIZED STEEL SELECTION FOR APPLICATIONS IN PLASTICS PROCESSING

Transcription

1 OPTIMIZED STEEL SELECTION FOR APPLICATIONS IN PLASTICS PROCESSING C. Ernst Edelstahl Witten-Krefeld GmbH Research Tool Steels Oberschlesienstraße 16 D Krefeld W. Pannes Edelstahl Witten-Krefeld GmbH Q uality Department Tool Steels Auestraße 4 D Witten Abstract A survey on the material properties and examples for the application of some precipitation hardening tool steels in plastics processing are presented. The new steel group "Thyroplast PH" offers an alternative to the conventional hardened and tempered plastic mould steels for applications either as high-volume tools with improved mechanical and corrosion properties or as low-volume tools with improved machinability. After solution annealing and aging the steels of this group reach a hardness of up to 42 HRC and show an improved compressive strength. By modifying the chemical composition, especially by lowering the amount of carbon, weldability and electrical discharge machinability are significantly improved. Sulphur additions help to advance machinability. In detail, the new materials show the following specific properties: Thyroplast PH 42 FM combines good machinability and high hardness. It is recommended for building mould frames and backup plates with high demands on strength as well as for hot runner systems. By remelting, Thyroplast PH 42 SUPRA obtains its high degree of purity which grants excellent polishability 321

2 322 6TH INTERNATIONAL TOOLING CONFERENCE and suitability for texturing. Examples for application are plastic injection and compression moulds. The corrosion-resistant sulphurized steel Thyroplast PH X FM with improved machinability is used for the production of mould frames, constructional parts and plastic moulds with low requirements on polishability which have to show resistance to condensation and cooling waters. Thyroplast PH X SUPRA with improved corrosion resistance and toughness is favoured for building plastic mould inserts which are under corrosive attack by the plastics to be processed. Keywords: Plastic mould steel, precipitation hardening, heat treatment, hardness, machinability, toughness, corrosion resistance, weldability, typical application INTRODUCTION Plastics as innovative materials have been conquering more and more fields of application and have successfully substituted many traditional metallic materials. Worldwide in the year 1998 a total amount of more than 150 million tons of plastics was produced, a third of it in Europe Fig. 1. The substitution of traditionally metallic compounds, especially in the vehicle industry, is one of the most important reasons for this rapid growth. As an example, the use of plastics in car building has increased from 7 % to 12 % during the past 25 years [1]. Besides of this application many household articles, parts of electronic devices such as computer houses, furniture and containers for food and beverages are made of plastics. Plastic products are manufactured by mass production using thermoplastic and duroplastic bulk materials which are processed by injection moulding, blow moulding and compression moulding. The intensive increase in production and consumption of plastics has also influenced the tool steel market with its demand for increasing amounts and good availability of tool steels. The described field of application therefore is an important market segment for the producer of these steels. The tool steel suitable for the different application in plastics processing have to show specific properties and need continual research and development. In the production of plastic items via extrusion, injection, blowing, compressing and also via deep drawing, the mould is of main significance for the efficiency of the process as the costs for the mould influence the production costs in a fundamental way. Extremely important for the tool builder and the tool user are an economic production of the tools and a high lifetime of the mould. On selecting the right plastic mould steel grade from the wide variety

3 Optimized Steel Selection for Applications in Plastics Processing 323 Figure 1. Production of plastics worldwide, in Europe and in Germany. of steels available the different requirements on the tools which depend on the type of application have to be known and respected. The most important properties of plastic mould steels are wear resistance, hardness, corrosion resistance, toughness, polishability, texturizing properties, weldability and machinability. It is difficult to find these properties combined in one steel grade as some of them counteract each other. According to their heat treatment the tool steels suitable for plastic moulding can be divided into four main groups which are listed in Table 1. Most plastic mould steels are delivered in the soft annealed condition with a maximum hardness of 250 HB equivalent to 850 N/mm 2. In this condition, the best machinability is guarantied. A large disadvantage of these steels is the additional heat treatment that has to be carried out by the tool builder after the machining process. Today more and more large parts, e.g. components of car bodies, are made of plastics. The use of case-hardening steels for large tools is not recommended as they cannot provide the required properties over the whole cross-section. Here the use of quenched and tempered steels is the solution. They are completely heat treated by the steel producer and reveal a strength of approximately 1000 N/mm 2 in as-delivered condition. The great advantage is that tools made of quenched and tempered steels do not bear the risk of distortion due to heat treatment after tool building but their

4 324 6TH INTERNATIONAL TOOLING CONFERENCE machinability is decreased compared to the steels machined in soft-annealed condition. Table 1. Steels for plastic moulding, an overview Group Quenched and tempered steels Throughhardenable steels Case hardening steels Nitriding steels Precipitation hardening steels Example Mat.-. Delivery condition and delivery hardness Heat treatment & hardness Q + T, HB not necessary Q + T, HB not necessary Q + T, HB not necessary Q + T, HB not necessary Q + T, HB not necessary Q + T, HB not necessary soft annealed, < 230 HB HRC soft annealed, < 250 HB HRC soft annealed, < 260 HB HRC soft annealed, < 220 HB HRC soft annealed, < 230 HB HRC soft annealed, < 265 HB HRC soft annealed, < 210 HB surface 60 HRC soft annealed, < 250 HB surface 60 HRC Q + T, HB surface 65 HRC Q + T, HB surface 68 HRC PH 42 SUPRA sol. annealed + aged, HRC not necessary PH 42 FM sol. annealed + aged, HRC not necessary PH X SUPRA sol. annealed + aged, HRC not necessary PH X FM sol. annealed + aged, HRC not necessary In this connection, the element sulphur has gained large importance in the past. Sulphur is added to low alloyed steels and to corrosion resistant steels to improve machinability as it has a chip breaking effect. If the tool to be build needs a lot of machining, the sulphur alloyed grade (40CrMnMoS8-6) for example is an appropriate steel with improved machinability. Due to the addition of sulphur, inclusions of MnS are formed which on the one hand improve machinability but on the other hand reduce the toughness of the steel. Further in the nineties calcium treated, quenched and tempered plastic mould steels without sulphur addition were developed that showed good machinability and better isotropy of mechanical-technological proper-

5 Optimized Steel Selection for Applications in Plastics Processing 325 ties [2]. The favourable machinability of the calcium treated steels is based on the formation of a layer consisting of calcium compounds on the cutting edge. At high cutting speeds, the calcium compounds become soft and semiliquid so that the cutting edge is protected with a lubricant, wear protecting layer. Nowadays, calcium treatment for machinability improvement of plastic mould steels is state-of-the-art. Constantly increasing demands in particular on the efficiency and durability of the plastic moulds applied have motivated Edelstahl Witten-Krefeld GmbH (EWK) to include mould steels into the production programme which have a higher hardness in the as-delivered condition compared to conventional hardened and tempered steels and which at the same time still show a good machinability Table 1. These materials are of a group of steels with completely different heat treatment compared to the quenched and tempered steels. The EWK short name "Thyroplast PH" characterizes this group of precipitation hardened steels which is divided into corrosion-resistant and non corrosion-resistant steels each with highest degree of purity (condition SUPRA) or additionally improved machinability (condition FM). QUENCHED AND TEMPERED STEELS VERSUS PH STEELS To meet the demands on the plastic mould steels a heat treatment procedure before or after the machining of the tools is necessary. This procedure involves changes in the microstructure to achieve certain properties which are important either for the following manufacturing steps, such as polishing or texturing, or for the application of the tools. In general the type of heat treatment depends on the chemical composition of the steels Fig. 2. Carbon containing, alloyed steels which are mainly used for making plastic moulds, are quenched and tempered (heating up to hardening temperature, holding at this temperature and quenching with sufficient speed to achieve a martensitic transformation, followed by one or several tempering steps). The less commonly known nickel alloyed steels with a low carbon content need a precipitation hardening procedure, consisting of a solution annealing to dissolve precipitated particles and of an aging procedure aiming at a renewed precipitation of metallic or intermetallic particles with smaller size and fine distribution. Besides of other changes in the material properties an

6 326 6TH INTERNATIONAL TOOLING CONFERENCE Figure 2. Classification according to the heat treatment. increase in tensile strength R m and in hardness are the results of this heat treatment. Of the group of quenched and tempered plastic mould steels which do not need an additionally heat treatment by the tool builder especially the low alloyed steels with material (40CrMnMo7), (40CrMnMoS8-6), (54NiCrMoV6) and (40CrMnNiMo8-6-4) have found widespread use. These steels are usually applied in a highly tempered condition with a hardness between 280 and 325 HB. Due to their low contents of alloying additions they show a high thermal conductivity. A further advantage which helps to decrease the total production costs of a plastic mould is the good machinability which is achieved either by a calcium treatment in the steelworks or by additions of sulphur in a range of up to 0,10 %. To obtain a hardness increase by the formation of martensite, these steels are alloyed with carbon. The interstitial carbon is needed for the distortion of the crystal lattice on changing from the austenitic to the martensitic phase. It also forms carbides which are precipitated pre-eutectically and during the tempering process. According to the necessity of repair welding or welding operations resulting from changes in the design of the tool the hardened and tempered tool steels are welded although the weldability is reduced by their carbon content. Increasing contents of carbon are responsible for a decrease in weldability as a distinct rise in hardness in the heat-affected zone due to

7 Optimized Steel Selection for Applications in Plastics Processing 327 martensitic transformation takes place during cooling from welding temperature [3]. In these hardened areas, cracking of the welding seam is likely to happen if no additional heat treatment is carried out after welding. Compared to the described hardened and tempered tool steels, precipitation hardened plastic mould steels which are aged at temperatures between 480 Cand 550 Cshow an increased as-delivered hardness of 38 to 42 HRC Fig. 3 and therefore a better wear resistance [4]. The high compressive strength and the resistance against surface deformations leads to a good stability of the clamping edges of the mould. Despite of their higher as-delivered hardness precipitation hardened steels can still be machined reasonably. Due to their low carbon contents they show improved weldability. In contrast to the hardened and tempered steels the hardness of the unaffected material and the welded area is not very different. The risk of cracking after welding operations therefore is reduced. Figure 3. Comparison of hardness, quenched and tempered steel and PH-steel. NON CORROSION-RESISTANT STEELS OF TYPE THYROPLAST PH 42 The steels of type Thyroplast PH 42 are medium alloyed materials on the basis of 3 % nickel, 1,5 % manganese, 1 % aluminium and 1 % copper. Thyroplast PH 42 FM as an alternative to the well-known steel additionally is alloyed with more sulphur to promote machinability. The steel Thyroplast PH 42 SUPRA which always is delivered in remelted condition, is

8 328 6TH INTERNATIONAL TOOLING CONFERENCE an alternative to the steels and , combining a high as-delivered hardness of 38 to 42 HRC with excellent polishability Table 2. Table 2. Steels of type Thyroplast PH 42 Standard grade Alternative Chemical composition in % C Mn Ni Cu Al S THYROPLAST 2311 PH 42 SUPRA THYROPLAST 2312 PH 42 FM Higher hardness Improved weldability Improved polishability (SUPRA) Improved machinability (FM) To investigate one of the most important properties of a plastic mould steel which is machinability, drilling tests with twist drills made of high speed steel (HS ) without cooling were carried out. The tests were based on Stahl-Eisen-Prüfblatt SEP 1161 ("Tool life test at elevated temperatures"). As a criterion for tool life, the failure of the cutting edges was employed. In Fig. 4 the tool life versus the cutting speed is shown. For both steels without sulphur addition, steel and Thyroplast PH Figure 4. Machinability investigated in drilling test.

9 Optimized Steel Selection for Applications in Plastics Processing SUPRA, a similar tool life was determined although the hardness of Thyroplast PH 42 SUPRA is approximately 10 HRC higher. Drilling of the steels with sulphur addition, steel and Thyroplast PH 42 FM, leads to a longer tool life. Here especially the conduct of Thyroplast PH 42 FM has to be emphasized as it also has a higher hardness compared to the quenched and tempered steel The ideal applications for the sulphur-alloyed steel Thyroplast PH 42 FM are mould frames, base plates and tool components with an increased amount of machining and at the same time high demands on stability as well as on firmness of the edges. An example for such tools are manifold blocks for hotrunner systems in injection moulding Fig. 5. On the other hand, Thyroplast PH 42 SUPRA shows a minimal sulphur content and on principle is supplied in remelted condition which grants an outstanding polishability. This plastic mould steel therefore especially can be applied for small and medium size plastic moulds and mould inserts with high demands on polishability. Tools for the processing of plastics are occasionally repair welded or welding operations are carried out which result from changes in the design of the tool. The weldability of tool steels which are used for the production of plastic moulds likewise is an important property. To investigate the influences of a welding procedure on the hardness and the microstructure, small flat specimens of the steels Thyroplast PH 42 SUPRA and were produced, their condition being precipitation hardened or quenched and tempered, respectively. By melting a seam via the TIG-process with an amperage of 150 A, a welding process without preheating of the specimens was simulated. The hardness in the cross-section of the molten and heataffected zones was determined by Vickers hardness measurements Fig. 6. In steel a new hardened zone can be detected which is caused by the high temperatures and results from the formation of martensite. This zone is followed by a tempered zone with a significantly decreased hardness compared to the unaffected material. To avoid cracking and problems during polishing and structural etching, a heat treatment is necessary by all means to adjust the different hardnesses and microstructures. In contrast to that, the steel Thyroplast PH 42 SUPRA only shows little variation in hardness after welding. In the heat-affected zone, the welding temperatures lead to an "overaging" effect which causes a slight decrease in hardness. Altogether, the differences in hardness between the unaffected material, the heat-affected

10 330 6TH INTERNATIONAL TOOLING CONFERENCE (a) Manifold block (b) Injection mould (c) Blowing mould Figure 5. (d) Injection nozzle Typical applications of steels Thyroplast PH. zone and the molten zone are clearly much lower compared to steel , so that a post-weld treatment is not necessary. A concluding summary of the properties of the precipitation hardenend plastic mould steels Thyroplast PH 42 SUPRA and Thyroplast PH 42 FM compared to the hardened and tempered standard tool steels and is given in Table 3.

11 Optimized Steel Selection for Applications in Plastics Processing 331 Figure 6. Simulated welding test, hardness after welding. Table 3. Comparison of properties hardness wearresistance polishability photoetchability machinability weldability Thyroplast Thyroplast PH 42 Supra Thyroplast O Thyroplast PH 42 FM O CORROSION-RESISTANT STEELS OF TYPE THYROPLAST PH X Two further developments concerning corrosion-resistant plastic mould steels are worth mentioning. As an alternative to the carbon alloyed tool steels (X36CrMo17) and (X33CrS16) which are generally used in their as-delivered condition with a hardness of around 300 HB two precipitation hardened plastic mould steels are now available Table 4. Similar to the non corrosion-resistant steels described above, these steels are a

12 332 6TH INTERNATIONAL TOOLING CONFERENCE remelted steel with increased polishability named Thyroplast PH X SUPRA and a sulphur-alloyed steel with improved machinability, Thyroplast PH X FM. Table 4. Steels of type Thyroplast PH X Standard grade Alternative Chemical composition in % C Mn Ni Cr Cu, Nb S THYROPLAST 2316 PH X SUPRA THYROPLAST 2085 PH X FM Higher hardness Improved corrosion resistance Improved toughness (SUPRA) Improved machinability (FM) Thyroplast PH X SUPRA is a corrosion-resistant tool steel for plastic moulds and mould inserts which have to meet highest demands on corrosion resistance and hardness. Its chemical composition is based on the American steels according to AISI 15-5 PH and AISI 17-4 PH, steels with a carbon content below 0,07 % that originally were used in applications where high strength, toughness and moderate corrosion resistance were required. These steels especially were used for building components for the aerospace, nuclear and naval industry [5, 6]. Accordingly Thyroplast PH X SUPRA offers some advantages compared to standard quenched and tempered steels such as : a higher hardness, an improved polishability, better weldability and corrosion resistance as well as an excellent toughness. Properly heat treated, that means solution annealed at 1020 Cand aged at around 500 C, Thyroplast PH X Supra consists of a lath martensite matrix into which copper- rich epsilon-phase and fine Nb-containing carbides are embedded [7]. The aging results in a marked hardness and strength increase above that observed in the solution annealed condition. Values of 40 HRC equivalent to 1260 N/mm 2 are reached. This strength increase is not accompanied by any significant decrease in the tensile ductility. Moreover, another big advantage of Thyroplast PH X SUPRA compared to the standard tool steel is its improved toughness at higher hardness levels [8]. Typical values that were determined in impact bending tests applying unnotched specimens are distinctly higher than 250 J Fig. 7.

13 Optimized Steel Selection for Applications in Plastics Processing 333 Figure 7. Toughness of steel Thyroplast PH X SUPRA. Especially the corrosion resistance of Thyroplast PH X SUPRA is distinctly improved compared to steel The results from dipping tests in different corrosive agents such as acetic acid, nitric acid, hydrochloric acid and sulphuric acid are included in Fig. 8 which shows the gravimetrically determined mass loss rate of the specimens. In dependence on the acid applied, the mass loss rate of Thyroplast PH X SUPRA is one or more powers of ten lower than the rate of steel Two of the reasons for Thyroplast PH X SUPRA s improved corrosion resistance are its higher chromium plus nickel content and the lack of coarse carbide particles. Also, a passivation effect of copper has been reported in literature for 15-5 and 17-4 PH steels [5], as copper dissolves as Cu + and Cu ++ -ions, forming stable corrosion products like Cu 2 O on the steel surface. Thus the pitting corrosion resistance is improved by inhibiting the anodic reaction. Thyroplast PH X FM is a new sulphur-alloyed steel with resistance against condensation and cooling waters which offers an alternative to the quenched and tempered steel according to mat Typical for Thyroplast PH X FM is its much lower carbon content of 0,04 % and lower chromium content of 11,5 % compared to steel , resulting in a higher chromium/carbon

14 334 6TH INTERNATIONAL TOOLING CONFERENCE Figure 8. Corrosion resistance of Thyroplast PH X. ratio. As can be seen from the results of dipping tests in artifical sea water, this ratio has a positive effect on the corrosion resistance Fig. 9. Also, resistance against humid conditions, as was tested in an atmospheric chamber where dry and humid conditions were alternated with each other for two weeks testing time, was found to be improved. Figure 9. Sample surfaces after test in artificial sea water.

15 Optimized Steel Selection for Applications in Plastics Processing 335 Due to its sulphur content, the steel shows improved machinability. This property was investigated in drilling tests with twist drills made of high speed steel In this test, holes with a depth of 24 mm were drilled (cutting speed 12 m/min, feed 0,12 mm/r, under cooling). The wear at the cutting edges as well as the flank wear were determined using a microscopical measuring system. The steels in quenched and tempered condition (hardness 325 HB) and Thyroplast PH X FM in precipitation hardened condition (hardness 380 HB) were used as base materials for drilling. Figure 10 shows the highest and lowest wear that occurred at the edges and flanks. In spite of its higher hardness, wear at the cutting edges on drilling steel Thyroplast PH X FM is comparable to the wear that appears when steel with lower hardness is drilled. The flank wear is significantly reduced. A similar behaviour was found in milling tests where gravers made of steel were applied as tools. Here the wear of the cutting edges was twice as high for the steel after a cutting length of 2,5 m compared to the steel Thyroplast PH X FM. Due to its properties, Thyroplast PH X FM in particular is suitable for making mould frames for multicavity dies with increased demands on corrosion resistance (marine climate, cooling water, condensation water) and machinability. A comparison of the properties of the new precipitation hardened steels is given in Table 5. Figure 10. Wear of tools in drilling tests.

16 336 6TH INTERNATIONAL TOOLING CONFERENCE Table 5. Comparison of properties hardness wear resistance corrosion resistance polishability machinability weldability Thyroplast O + Thyroplast PH X Supra O + + Thyroplast Thyroplast PH X FM SUMMARY The new steel group "Thyroplast PH" which shows a higher tensile and compressive strength in as-delivered condition offers an alternative to the conventional quenched and tempered plastic mould steels. After solution annealing and aging the four steels from this group reach a hardness of up to 42 HRC, a heat treatment after machining by the tool builder is not necessary. By modifying the chemical composition, especially by lowering the amount of carbon, weldability and electrical discharge machinability were significantly improved. Sulphur additions help to advance machinability. In detail, the new materials show the following specific properties: Thyroplast PH 42 FM combines good machinability and high hardness. It is recommended for building mould frames and backup plates with high demands on strength. By remelting, Thyroplast PH 42 SUPRA obtains its high degree of purity which grants excellent polishability and suitability for texturing. Examples for application are plastic injection and compression moulds. The corrosion-resistant sulphurized steel Thyroplast PH X FM with improved machinability is used for the production of mould frames, constructional parts and plastic moulds with low requirements on polishability which have to show resistance to condensation and cooling waters. Thyroplast PH X SUPRA with improved corrosion resistance and toughness is favoured for the production of plastic mould inserts which are under corrosive attack by the plastics to be processed. REFERENCES [1] R. BAUN, Die Vielfalt der Werkstoffe ein Garant für den Fortschritt im Automobilbau, in Werkstoffe in der Automobilindustrie (1999), p

17 Optimized Steel Selection for Applications in Plastics Processing 337 [2] H.-J. BECKER, E. HABERLING, P. OTTER, Eigenschaften calciumbehandelter Kunststoffformenstähle, Thyssen Edelstahl Techn. Ber. 15 (1989) 2, p [3] K. RASCHE, Schweißen von Kunststoffformenstählen", Schweizer Maschinenmarkt (1981) 45, p [4] C. ERNST, Substitution des Legierungselementes Kupfer in aushärtbaren Werkzeugstählen für die Kunststoffverarbeitung", Dissertation Ruhr-Universität Bochum (2001). [5] U. KAMACHI MUDALLI et al, Localised corrosion behaviour of 17-4 PH stainless steel", Materials Science and Technology (1990) 6, p [6] H. KREBS, "Entwicklungsstand und Tendenzen bei ausscheidungshärtbaren Edelstählen", Maschinenmarkt 103 (1997) 13, p [7] H. J. RACK, D. KALISH, "The strength, fracture toughness, and low cycle fatigue behaviour of 17-4 PH stainless steels", Metallurgical Transactions (1974) 5, p [8] C. ERNST, Neue aushärtbare Werkzeugstähle", Form + Werkzeug 1 (2002), p