Gradient Elastic–Plastic Properties of Expanded Austenite Layer in 316L Stainless Steel

The gradient mechanical properties, variation of stress with strain and surface cracking behavior of expanded austenite developed on 316L austenitic stainless steel were investigated by nanoindentation tests, X-ray residual stress analysis and scanning electron microscope observation in four-point bending tests. The results show that the plastic properties of the carburizing layer including true initial yield strengths and strain hardening exponents increase significantly from substrate to surface, while the true elastic modulus just improves slightly. Due to the onset of plastic flow, the residual stresses are almost equivalent to the true initial yield strengths from surface to the depth of ~ 10 μm. The results of four-point bending tests show that surface stress increases linearly with the increase in strain until the strain reaches ~ 1.0%, after that the plastic yield happens. The expanded austenite surface layer is brittle, and the cracks will be created at the strain of ~ 1.4%. The cracking stress is about ~ 2.4 GPa.