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Metal Casting Technologies : September 2006
www.metals.rala.com.au 46 feedstock is used. Both the mean size and overall number of non-metallic inclusions are significantly reduced. These two techniques involve arc re-melting of a consumable ingot in a vacuum (VAR) or through a special refining slag (ESR) to clean the steel, and hence to provide better service performance. Steels processed in this way are often referred to as Premium Grade. Their production costs are higher so they are more expensive however this extra material cost for the die is likely to be well worth the extra outlay thanks to significant service performance in terms of increased heat checking resistance and longer die life. Use of the Premium Grade can increase die life by up to 2.5 times compared to non-refined ingot material. For both refined and non-refined ingot material segregation of alloying elements occurs during solidification resulting in a non-uniform distribution of C and elements such as Cr, Mo, and V that provide hardenability. This gives rise to primary carbide networks within the ingot microstructure. The steel must therefore be heavily hot worked to break up and dissolve this carbide network and to redistribute C, Cr, Mo and V, etc. uniformly through the structure. Any residual carbides will seriously damage resistance to cracking as well as reducing the alloy content of the austenite matrix leading to inferior heat treatment response and non-uniform hardness in dies. Hence major tool steel producers will invariably explain in their product literature that their steel ingots are given at least a 5 to 1 reduction in area during hot working to ensure homogeneity before forging die blocks. As an alternative production route for small sized material powder metallurgy (PM) can be used to produce die steels with no segregation and with microstructures that are much finer and more uniform than those from the best refined ingot route. PM steels are claimed to give equivalent wear resistance with up to ten time's greater toughness when compared to conventional ingot route steels. PM steels are made by hot isostatic pressing of alloy steel powder that has been produced by gas atomization of liquid steel. During atomization the droplets are only 50-100 microns in size so they solidify extremely quickly with very fine homogeneous microstructures. The powder compact is then conventionally hot worked to produce stock material. PM grade H13 has been successfully used for shot sleeves. Tool steel producers have developed their own alloy specifications. For applications such as prototype or short run dies some grades are resulphurised to contain up to 0.1% S. This allows the steel to be supplied and machined in the hardened condition such that no further heat treatment, other than perhaps a stress relief, is needed before use. In PM steels the addition of Sulphur to improve machinability is very effective since a fine homogeneous distribution of sulphides can be obtained. To improve heat resistance some special H11 and 13 grades contain higher levels of Mo, up to 3% compared to normal maxima of 1.6 & 1.75%. Some grades have %Si contents reduced from 1% down to 0.3% for better cracking resistance. Likewise V levels are reduced to minimize free carbides and improve toughness. Cobalt additions are used in standard H19 grade and to provide a special H10 grade. The aim of all the special recipes is to try to achieve more effective combinations of hardness and toughness together with best resistance to thermal shock and heat checking. The problem is that although improved toughness can be achieved by reducing the heat treated hardness, the lower hardness unfortunately results in reduced thermal fatigue resistance. One particular advantage of ESR and VAR refined grades is that their improved toughness allows higher hardness levels to be used for dies without increasing the chance of cracking. OUTLINE OF HEAT TREATMENT H11 and H13 are normally forged at start temperatures of 1100-1120oC, during forging reheating is used to make sure that the stock temperature does not fall below 950oC. Furnace cooling is used after forging. Finished forgings are then fully annealed at 780-850oC and furnace cooled at a rate slower than 20oC per hour down to 550-600oC, followed by still air cooling. During annealing, and subsequent austenitising for hardening, controlled atmospheres must be used to prevent decarburization and scaling. Traditional practice is to pack the steel in pre-treated cast iron chips or coke but modern practice uses purpose built controlled atmosphere furnaces or preferably vacuum heat treatment furnaces since the latter afford the best temperature and cooling rate control especially for hardening. Decarburization must be avoided since a reduced C level at the surface reduces wear and soldering resistance of the die and can be misleading during hardness checks to monitor the tempering condition. In the fully annealed condition the microstructure consists of fine spheroidal alloy carbides in a ferrite matrix giving an overall hardness of 200HB to facilitate machining. After rough machining stress relieving is carried out at around 650oC depending on the section, followed by cooling in still air. This helps to reduce distortion during hardening. The key variables in hardening are austenitising conditions and temperature prior to quenching, quenching rate and subsequent tempering conditions. For a given grade of steel the temperatures and times recommended by steel suppliers or given in engineering handbooks depend on section size and design of the die or tool, and on the required level of the working hardness/toughness balance. During austenitising Cr and Mo carbides decompose and the atoms of C and alloying elements go into solid solution in the austenite, but Vanadium Carbide remains undissolved. The higher the final austenitising temperature the better is the dissolution of alloy carbides: hence for H13 dies a temperature of 1050oC is said to give better performance than 1020oC if excessive austenite grain growth is avoided. Dies must be carefully heated up to the hardening temperature usually via two or three pre-heating steps at 600,