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 SKD6 hot work tool steel were investigated. The fatigue life in vacuum was twice as long as that in air. It became clear from the resu lts obtained from the experiment conducted in vacuum that slip bands o n the specimen surface were formed during fatigue and grew up with inc reasing number of cycles. The initial fatigue cracks were generated at the intersections of slip bands and were vertical to loading directio n. The fatigue cracks mainly initiated at internal inclusions when tes ted in vacuum, whereas superficial cracks were observed on the same fr acture surface when applied higher strain amplitude. In contrast, nucl eation sites of fatigue crack when tested in air were specimen surface irrespective of the magnitude of strain amplitude, The improvement of low cycle fatigue properties in vacuum was mainly due to the decreasi ng of growth rate of superficial and internal cracks to the critical c rack length.