The Influence of Quench Cooling Rate on Tempered Hardness of a Forging Die Steel

The influence of cooling rate during quenching on tempered hardness and on some other properties, such as softening characteristics at high temperature, shock resistance and heat-checking was investigated and the following results were obtained: (1) Tempered hardness decreases with increase of quench cooling rate. (2) Softening resistance at high temperature and shock resistance at room temperature decrease when bainite is formed during quenching. (3) Influence of quench cooling rate on activation energy (∼90kcal/mol) for coarsening of carbide during tempering and on heat-check resistance is negligible. (4) There is a little influence of quench cooling rate on the amount of retained austenite. Retained austenite is decreased by tempering at 200∼300°C, and it causes secondary hardening by tempering at 300°C. (5) Carbides in tempered microstructure are identified to M3C, M23C6 and VC, and influence of quench cooling rate on carbides is not observed.