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Metal Casting Technologies : September 2006
TECHNICAL FEATURE 820 and 900oC to equalize temperatures in different sections and to minimize distortion. Before quenching the steel is held for the minimum time at the final temperature of 1020-1050oC to prevent austenite grain growth. Tempered martensite produced from coarse grained austenite tends to have reduced toughness. Hardening can be achieved by air cooling which avoids grain boundary carbide and/or pearlite formation producing a matrix of martensite and some retained austenite. On air cooling some bainite, which lowers toughness, may form in central areas of thicker sections. This can be prevented by oil quenching but the faster the quench rate the greater is the danger of distortion. In some cases interrupted quenching procedures can be used to overcome this problem -- this is often called "Martempering" which is a misleading description since it is not a tempering treatment. The steel is cooled quickly from the austenitising temperature, to avoid any grain boundary or pearlitic carbide formation, down to an intermediate temperature of 500-550oC in a salt bath. Holding at this intermediate temperature for a short period allows some equalization of temperature resulting in less distortion during subsequent cooling to form martensite. The dangers of distortion & cracking during oil and salt bath quenching can be overcome by the use of high pressure gas quenching in purpose built vacuum furnaces. This also avoids the difficult cleaning, environmental and health & safety difficulties associated with the use of hot oils and molten salts. Gas quenching in a vacuum furnace is achieved by high pressure back filling with argon or nitrogen, or less commonly, hydrogen or helium. Cooling rates during such quenching can be readily controlled by changes in gas, gas pressure or gas speed. Changes in cooling rate during quenching and interrupted quenching can also be applied. Tool steel must be tempered immediately after hardening by slow heating to the desired tempering temperature. Because of the secondary hardening effects of Mo and V hot work tool steels can be tempered at relatively high temperatures giving almost complete relief of residual stresses generated during martensite formation on hardening. For H13 tempering is normally in the range from 550-650oC depending on required hardness. A double, and sometimes triple, tempering procedure is used: the first temper converts most of the retained austenite to martensite. In the second treatment this fresh martensite is then tempered at a slightly higher temperature to give the final working hardness of the die. If required for stress relief, a third temper is carried out at a temperature of some 25-50o below the highest temperature used in the previous tempers. The alloy carbides produced during tempering are not only much finer than the M3C carbides formed in quenched & tempered Plain C Steels but they also coarsen at much slower rates hence strengthening from these carbides is retained at higher temperatures. The microstructural features outlined above are represented in simple schematic form in Figure 2. Some damaging microstructural effects that must be avoided by correct processing and heat treatment are indicated. For example the presence of just small amounts of grain boundary carbide can reduce the toughness by up to 50%. The surface properties of the die steel can be improved by suitable surface treatments. Nitriding and Physical Vapour Deposition are the most practiced techniques since they can be performed at temperatures below the normal tempering temperature of 600oC so that the microstructure of the heat treated steel is not changed. ● www.metals.rala.com.au 48 REFERENCES 1. J.T.H. Pearce. "Some Factors Affecting Die-casting Die Life". METAL Casting Technologies (2004) Vol. 50 No.2 June pp. 33-36 2. J.T.H. Pearce. "An Introduction to Microstructure Control by Heat Treatment in Steel Castings". METAL Casting Technologies (2004) Vol. 50 No.2 June pp. 26-32. 3. J.T.H. Pearce. "Some Metallurgical Aspects in the Heat Treatment of Low and Medium Alloy Steels". METAL Casting Technologies (2005) Vol. 51 No.2 June pp. 18-23. 4. J.H.D. Bautista. "Practical Heat Treatment of Steels". METAL Casting Technologies (2006) Vol. 52 No.1 pp. 46-49. 5. N.J. Culp, D.D. Hullman & R.J. Henry. "Tool Materials". Chapter 18 Metals Handbook Desk Edition, (1992) American Society for Metals, Metals Park, Ohio, US 6. ASM Handbook Vol.4 "Heat Treating", (1997) ASM International, Metals Park, US. 7. J.T.H. Pearce. "Inclusions in Castings". METAL Casting Technologies (2004) Vol. 50 No.4 December pp.17-22. Figure 2. Schematic views of some microstructural features of die steel. Primary Carbides Grain boundry carbides Non-martensitic transformation Retained Austenite (a) Normal quenched and multi-tempered structure consisting of fine tempered alloy carbides in a ferrite matrix. The tempered carbides can only be resolved using electron microscopy. An annealed structure also contains alloy carbides in a ferrite matrix, but the carbides are fewer in number and much coarser such that they can be seen by optical microscopy. (b) Some deleterious microstructural effects that would each severely reduce resistance to cracking and wear. * Retained primary carbides due to segregation and inadequate hot working. *Grain boundary carbides and non martensitic transformation products (pearlite/bainite) due to insufficient quench rates on hardening. * Retained austenite resulting from incorrect tempering.