抄録
In order to clarify the giga cycle faigue behavior of hot forging die steel ultrasonic fatigue tests were conducted up to (10)^9 cycles. The frequency was 20kHz and the R value was-1. The hot forging die forging die steel with different hardness was used. At higher axial stress region fatigue life of the specimen with HRc50.8 was longer than that with HRc47.0. The lower the axial stress, the difference of fatigue life decreased. The fatigue strength at (10)^9 cycles was the same for specimens with different hardness. The effect of ion nitride on fatigue strength was not observed for both steels. Fatigue crack initiated from nonmetallic inclusions located at the subsurface for all tested specimens.
