冷間金型鋼の疲労寿命に及ぼす二次炭化物粒径の影響

抄録

It has been known that mechanical properties of die steels are affected by carbides, especially primary carbides of which shape is coarse and rod-like.It appears that relatively large and spherical secondary carbides which precipitates in austenite, also influence mechanical properties. The effect ofthe particle size (diameter) of secondary carbide on the fatigue life of JIS SKD11 die steels was studied by tension-compression fatigue tests with stress amplitude between 1.1 and 1.8 GPa and frequency of 0.17 Hz. The diameter of secondary carbides was changed by soaking time at 1100°C under the constant width of primary carbide. The fatigue life decreased with an increase in the diameter of secondary carbide. The effect of diameter of secondary carbide on fatigue life depended on stress amplitude in fatigue tests. It became larger in low stress amplitude. This shows that fatigue life is mainly influenced by primary carbide in high stress amplitude and by both primary and secondary carbides in low stress amplitude. The relationship between fatigue life, N, at stress amplitude of 1.1 GPa and the diameter, d/μm, of secondary carbide was shown by the following equation: log N=4.04-0.48d. The fraction of carbides with void, which was observed after fatigue tests, increased in steels with large secondary carbide. Voids were initiated at all of secondary carbides more than 3 μm in diameter and no of carbides less than 1μm. Tensile strength also decreased with an increase in the diameter of secondary carbide. Voids were initiated at interfaces between secondary carbides and matrix in plastic deformation of tensile test. Tensile stress for void initiation was inversely proportional to the square root of the diameter of secondary carbide.