The graph shows a continuous cooling test in which the austenitizing temperature was approximately 1040°C (1900°F). It shows the effect of intermediate cooling rates on the microstructure and toughness of H13 tool steel austenitized at 1075°C (1970°F). (a) Continuous cooling transformation (CCT diagram) shows variation in microstructure with varying cooling rates. (b) Variation in toughness as a function of carbides ejected from austenite. Hardness was maintained at a constant value.
Hot-work die steels containing 5% Cr are normally used (H13 is the most common in the United States) because they have:
• A good combination of toughness and strength for safety and service
• Good hardenability in thick sections
• Good hot hardness and strength to resist cyclic stressing at operating temperatures up to 400 °C (750 °F)
• Good resistance to tempering during service that gradually lowers the surface hardness and the fatigue strength
Carbide precipitation, other than that produced during tempering, is known to be deleterious to the fatigue life of H13 tool steel. Fast quenching rates required to suppress intermediate precipitation of carbides (such as proeutectoid grain-boundary carbide, pearlite, or bainite) also produce large thermal gradients within the tool. Fast quenching rates can cause distortion or even cracking. The larger the tool, the greater the risk of cracking.
Volume 4D, Heat Treating of Irons and Steels ? Tool Steels ? Heat Treating of Hot-Work Tool Steels ? Examples of Heat-Treating Procedure for Hot-Work Tools
