Revealing the roles of martensitic transformation in affecting the fatigue resistance of austenitic stainless steel with heterogeneous grain size distribution

The present work investigates the roles of martensitic transformation on the small fatigue crack growth behavior and fatigue resistance of a 316 L austenitic stainless steel. Martensitic transformation with varied block size takes place along crack path and at the crack tip. The large martensite blocks are vulnerable to crack propagation as the cracks can easily penetrate the block interior, leaving flat facets on the fracture surface. In contrast, the ultrafine martensite blocks can provide an enduring resistance on fatigue crack extension and deflect the crack propagation by cracking along their irregular and dense block boundaries. Considering the correlation between martensite block size, mechanical stability and prior austenite grain size, the present work suggests that austenite grain refinement is beneficial for improving fatigue properties of steels containing metastable austenite grains.