Redistribution of carbon and residual stress in low-temperature gaseous carburized austenitic stainless steel during thermal and mechanical loading

The effect of thermal annealing and mechanical loading on the redistribution of carbon- and residual stress-depth profiles in low-temperature carburized 316 L austenitic stainless steel was investigated. Gaseously carburized 316 L austenitic stainless steel was strained with a custom-built four-point bending device and isothermally annealed at temperatures in the range 300–500 °C for up to 200 h. Experimental results show that the carburized case is stable at 300 °C and that carbides precipitate only at the highest temperature investigated. At temperatures above 300 °C, the carbon atoms in the carburized case continue to diffuse inwardly, causing an increase of the case depth. Consequently, the surface concentration of carbon, the surface hardness and compressive residual stress change with time. Four-point bending during isothermal annealing accelerates the diffusion-controlled redistribution of carbon over the carburized case for both tensile and compressive side of the strained specimen. Since the carbon-induced volume expansion of austenite during carburizing is accommodated elasto-plastically, a substantial reduction of the carbon content in the surface-near region leads to a change from compressive to tensile residual stresses in the surface region.