Fatigue Properties of SUS316LN Stainless Steel Sheet with Heterogeneous Nano-structure Developed by Heavy Cold Rolling

Fatigue behaviors of SUS316LN austenitic stainless steel with heterogeneous nano-structure developed by heavy cold rolling have been investigated in this study. The tensile strength and the elongation to fracture in the heterogeneous nano-structure SUS316LN were 1552 MPa and 10%, respectively. The fatigue strength of the heterogeneous nano-structure SUS316LN, which was defined at 10(7) cycles, reached double of fatigue strength of conventional austenitic stainless steels. The improvement of fatigue strength can be connected with ultimate tensile strength in the heterogeneous nano-structure SUS316LN. Fish-eye fractures, in which crack initiated at Al2O3 inclusions, were clearly observed on the fracture surfaces. The crack propagation rate was measured based on the striation intervals on fracture surface, the analysis of crack propagation rate revealed that the cracks tend to propagate difficult to sheet thickness direction due to lamella structure whose grain boundaries are low misorientation angles. The fatigue lives before and after crack initiation were also estimated by using the number of cycles at fracture and the crack propagation rate. It was found that most of fatigue life was spent before crack initiation. Therefore, fatigue strength would be able to improve by reducing the number and size of inclusion particles.