Effects of Surface Softening on the Mechanical Properties of an AISI 316L Stainless Steel under Cyclic Loading

The effects of surface softening on fatigue behavior of AISI 316L stainless steel were investigated. Using cold-rolling and electromagnetic induction heating treatment, a gradient structure was fabricated on AISI 316L stainless steel within which the grain size decreased exponentially from micrometers to nanometers to mimic the surface softening. Stress-controlled fatigue tests were applied to both the gradient and homogeneous structures. Compared with the homogeneous sample, surface softening had no evident effect on fatigue behavior when the stress amplitude was greater than 400 MPa, but significantly deteriorated the fatigue behavior at stress amplitude & LE;400 MPa. At high-stress amplitude, fatigue behavior is dominated by crack propagation. When the stress amplitude is lowered, strength reduction and stress concentration caused by surface softening accelerate crack initiation and propagation, resulting in an inferior fatigue behavior.