Effect of Inclusions on High Temperature Fatigue Properties of a Cr-Mo-V Steel under Variable Straining

Abstract

The effect of service loading on the fatigue behavior of a steam turbine rotor steel with large inclusions (1Cr-1Mo-1/4V steel with non-metallic inclusions of about 0.5mm in diameter) was investigated at 550°C. The service loading was simulated by a multiple two-step straining (MTSS). Under the MTSS tests, the life of the defect-free materials was reduced remarkably (1/10-1/100) as previously reported by the authors, and the reduction was even larger for a material containing large inclusions. However, the inclusions had little effect in reducing the life as far as a constant strain range was applied. Therefore the effect of inclusions was effectively evaluated through variable strain testing, not by conventional low cycle fatigue tests. An estimation of fatigue life was carried out based on the assumption of linear cumulative fatigue damage law in terms of the measured plastic strain range. This method tended to overestimate the life by a factor of two for the defected material under programed loading, though it effectively predicted the life for the defect-free ones. This was attributed to the crack initiation period reduced by inclusions. In the case of low cycle fatigue tests the crack initiation life was relatively short (-20%) even for the defect-free materials and therefore little effect of inclusions on the total life was noticed. For a conservative designing of a turbine rotor which might contain defects, it would be of significance to apply a fracture mechanics approach on an assumption that the total life is predominated by the crack growth period.