EFFECT OF TESTING ATMOSPHERE ON LOW-CYCLE FATIGUE OF HOT WORK TOOL STEEL AT ELEVATED-TEMPERATURE

Effects of atmosphere and strain amplitude on low cycle fatigue behavi or of AISI H11 hot work tool steel were investigated. The fatigue life in vacuum was twice as long as that in air. From the result of the fa tigue test in vacuum, it became clear that slip bands on the specimen surface were formed during fatigue and they grew up with increasing nu mber of cycles. The initial fatigue cracks were generated at the inter section of slip bands and were vertical to loading direction. The fati gue cracks mainly initiated at internal inclusions when tested in vacu um, whereas superficial cracks were observed on the same fracture surf ace when applied higher strain amplitude. In contrast, nucleation site s of fatigue cracks when tested in air were specimen surface irrespect ive of the magnitude of strain amplitude. The extension of low cycle f atigue life in vacuum was mainly due to the decreasing of growth rate of superficial and internal cracks to the critical crack length.