Die Steels - Buderus Edelstahl Gmbh

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The material concept The right material for all application profiles Optimal use of a die steel is closely linked to understanding its stress profile (Figure 1). The right die steel for forging-die and press-die units and secondary tools is selected bearing in mind economic and qualitative aspects. One major factor is the die’s service life or estimated tool life quantity (see Tables 1 and on page 8). The tool life quantity is generally limited by wear, or by cracking in the case of complicated impressions (see Table 1 on page 8+9). Die wear is significantly reduced as the alloy content of carbide formers such as vanadium, molybdenum, tungsten and chromium is increased, as shown in Figure . One qualitative measure of this is the alloy coefficient (AC) used to rate steel. Higher-grade steels produce favourable results when an optimum relationship between tool life quantity and tool costs is achieved. The tool life quantity can be further optimized by increasing hardness, balanced with the necessary toughness characteristics. Key here is uniform quenching and tempering of the dies throughout their cross-section. Comparing the hardness distribution of the standard die steel 2714 ISO-B to the recently developed new die steel 2714ISO-Bmod. confirms this modified composition improves hardening and tempering characteristics (see Figure and Table on page 10). Figure 1 Die tool stress profile Mechanical stress Thermal stress Mechanically induced state of tension Mechanical die load Relative movement die/forging Deformation • Cracking • Fracture Wear • Deformation • Temperature fatigue cracking Thermally induced state of tension Thermal die load Loss of cohesion Matched material characteristics ensure high tool life quantities This special steel is effective for forging dies with high volumes, regardless of the die size and impression depth. Heat resistance and hardness retention are key material indicators for die steels. Loss of hardness and even transformation of the microstructure at the impression surface is the most frequent cause of wear, deformation and cracking. Figure 2 Effect of alloy content on die wear Die wear (µm) µm 350 300 250 200 150 100 50 2713 2714 2744 2714mod. 2766 2343 2606 2365 2344 2367 Temperature 250°C Alloy coefficient 2Cr + 5W + 10Mo + 40V 0 0 10 20 30 40 50 60 70 80 90 100 110 120 Alloy coefficient
Die steels can be divided into two groups according to their tempering behaviour as illustrated in Figure : Group 1 The low hardness retention of these nickel alloy steels makes them unsuitable for applications involving high temperature load (> 500 °C) with relatively long or frequent contact phases, as for example in presses. Group 2 These steels are much more resistant to thermal load than those in Group 1, and also more wear resistant, in line with the higher carbide content in the matrix. In addition to hardness retention, the level of the Ac1 conversion temperature is significant for the temperature capacity of a die steel (cf. Table on page 10). Steels with lower Ac1 temperatures can be more easily converted and hardened after extended press contact phases. The consequences are premature local wear and cracking in the impression. Measures to improve wear resistance however generally result in higher susceptibility to cracking of the forging die. The tougher nickel alloy steels in group 1 (see table ) are less susceptible to stress cracking than the higher alloyed steels, containing special carbides, described in Group . A low level of thermal conductivity (Table ) can be taken as an indicator of increasing danger of cracking when exposed to critical temperature fluctuations. Figure 3 Comparative hardness profile curve Hardness (HB) HB 400 375 350 325 2714 ISO-B 2714 ISO-B mod. 300 0 100 200 300 400 500 600 700 800 900 1000 Rod diameter (mm) mm Pre-heating the die to 280 °C The best known measures for reducing susceptibility to cracking are increasing material toughness by means of moderate strength when assembled, and sufficiently high pre-heating temperature (see Figure 5). Figure 5 Effect of pre-heating temperature on notch impact energy Notch impact energy ISO-V (J) Figure 4 Hardness retention of the Group 1 and Group 2 die steels Hardness (HRc) J 100 80 60 40 20 HRc HB 60 55 50 45 40 Group 2 die steels, hardness 415 HB 0 200 400 Test temperature (°C ) 600 Alloy groups 1 2 NiCrMoV die steels CrMoV hot working steels Pre-heating temperature for dies ~565 ~485 ~425 ~375 35 ~330 0 200 400 600 800 Tempering temperature (°C ) °C °C Hardness (HB) 5
Page 1 and 2: Die Steels I Production Technology
Page 3: • Rough machining • Full servic
Page 7 and 8: Extreme dimensions for extreme dema
Page 9 and 10: Cracking low high 2714 ISO-Bmod. 23
Page 11 and 12: Material characteristic Important m

Die Steels - Buderus Edelstahl Gmbh

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