Effect of the Distribution of Primary Carbide on Fatigue Strength of Cold Work Die Steels

The effect of distribution of primary carbides on fatigue strength has not been fully clarified for cold work die steels. Eight percent Cr type steel and JIS SKD11 have been studied by changing carbides area fraction. We have tested low cycle fatigue properties and discussed with comparing tensile properties and microfractographs. The test results are summarized as follows. 1) Tensile strength of 8Cr type steel and JIS-SKD11 is decreased by increasing carbide area fraction. But σ0.2 and σ0.02 is same in tested steels. independent to carbide area fraction. 2) Fatigue strength is decreased by increasing carbide area fraction and fatigue strength of 8Cr type cold die steel is higher than that of JIS-SKD11. 3) Fatigue crack is initiated from primary carbide in all specimens. 4) Fatigue life is determined by initiation life of fatigue crack in these cold work die steels. 5) Small crack is initiated at primary carbide in early stage of fatigue test and this small crack is connected to neighborhood carbide, finally this crack changes to propagation mode of fatigue phenomena.