Study on estimation of fatigue behavior in (α+γ) dual-phase stainless steel

In this study, we apply repeated stress to (alpha+gamma) dual-phase stainless steel (UNS S31260) and examine changes in surface structure and residual stress by X-ray measurements of each phase with respect to the number of cycles. The following results are obtained. (1) At high stress levels (sigma(a)=375, 400MPa), many alpha'-phases are induced on the surface of a specimen when the number of cycles is increased, and surface cracks are observed immediately before rupture. (2) Residual stress in the alpha-phase does not depend on stress level, however, it converges to a constant value with increasing number of cycles. Residual stress in the gamma-phase converges to a constant value at low stress levels (sigma(a)=325, 350MPa), but shows instability at high stress levels, and strain-induced transformation is considered to occur. (3) When repeated stress at high stress levels accompanying rupture is applied, the volume fraction of the gamma-phase by X-ray analysis tends to decrease substantially with increasing number of cycles. We consider it possible to estimate the fatigue life of the steel from the changes in the volume fraction of the gamma-phase.