NiCrMoV鋳鋼におけるき裂の発生・伝ぱ寿命のばらつきに及ぼす材料微視組織の影響

抄録

In order to clarify the cause for statistical scatter of the surface fatigue crack initiation lives and the growth lives in NiCrMoV cast steel, which is used as a large-size turbine rotor shaft, the initial crack growth behavior and the crack initiation sites have been investigated in detail. The rotating bending fatigue tests were carried out for four kinds of stress amplitude including the one just above the fatigue limit in room temperature and in air. The fatigue process was divided to several ranges by its crack length. By analyzing the distribution of the number of cycles required for cracks to propagate in each range, the corresponding crack length range which would strongly affect the scatter of the fatigue lives was examined. From the SEM observation of crack initiation sites, most of the cracks were found to initiate from inclusions, and their size, shape and depth were measured. The relationship between these values and the initiation lives were examined. Moreover, the crack length range which is sensitive to the microstructure was investigated by considering the correlation between the crack propagation rate and the stress intensity factor on the log-log relation.