Effects of Different Quenching Cooling Rates at Different Temperature Ranges on Impact Toughness of Hot Work Tool Steels

This study used the steel of JIS SKD61 and the improved steel as experimental materials. The quenching temperature (1020℃) and holding time (1hr) were fixed for quenching at different cooling rates. The cooling process was divided into three stage during quenching, which are the high temperature range (1020℃~700℃), moderate temperature range (700℃~500℃) and low temperature range (500℃~300℃). In order to simulate the actual temperature change in the mold center thicker than 300mm in the field quenching operation, the appropriate quenching cooling rate was reasoned from the CCT curve diagram of various steel grades as the initial condition. The quenching cooling rate of only one temperature interval was changed each time to reduce the cooling rate to 2/3 or 1/2 of initial condition. The quenched sample was tempered to the same target hardness HRC47±_1^(0.5) for impact test to compare the impact values. The microstructure and chemical composition of various samples were analyzed by optical microscope and SEM. The effect of reducing the temperature interval cooling rates on the impact toughness was discussed. The findings showed that the impact value is most likely to decrease when various types of steel are quenched in the condition of low temperature range cooling rate scale down, and then in the condition of high temperature range cooling rate scale down. The quenching in the condition of moderate temperature range cooling rate scale down has slight effect on the impact value. In addition, if the raw material structure contains considerable and continuous net carbide, as well as considerable macroscopic nonmetallicinclusions (above 20μm), the impact value of the steel containing these structures decreases markedly after quench tempering under various conditions.