Thermal fatigue characteristics of die steel for hot working SKD 61 and improved SKD 61 were investigated. The cylindrical test bars with U-notch were directly immersed into Al alloy melt kept at 923 K, and then cooled quickly to water. Theinvestigation led to the following conclusions: 1) The number of heat cracks of the improved SKD 61 was fewer than that of SKD 61. It was especially pronounced in the hardness of HRC 45 or more. 2) The heat crack length of SKD 61 and the improved SKD 61became small as the hardness increased. The minimum value of heat crack length was obtained the specimen with HRC 50. The total length of heat cracks of the improved SKD 61 became about 1/4 that of SKD 61. The maximum length of it became shout 2/5 of SKD61. 3) The heat crack resistance of the improved SKD 61 is superior to the resistance of SKD 61, because the tensile strength of the improved SKD 61 is higher than that of SKD 61 over the whole testing temperature.
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