A preliminary study of thermo-mechanical stability of carbon S-Phase formed in austenitic stainless steel

Carbon S-phase was generated in the surface of AISI316 austenitic stainless steel by plasma carburising at 500°C for 10h in a gas mixture of 1.5%CH₄ and 98.5%H₂. The thermo-mechanical stability of the carbon S-phase was studied by stressing the 'dogbone' tensile specimens in the range of 0-200MPa at temperatures ranging from 400 to 500°C for 100-150h. Post-test characterisation was conducted using XRD, SEM, TEM and micro-indentation. The experimental results demonstrate that when tested at a fix temperature the thickness of the carbon S-phase layer increased with the stress applied to the tensile specimens during the thermo-mechanical stability tests. This indicates that tensile stress promotes the diffusion of carbon in the carbon-S-phase. When stressed at 400°C the microstructure of the carbon S-phase was not affected by the stress level; however, when stressed at 450 and 500°C for 100MPa or above, the corrosion resistance of the carbon S-phase slightly deteriorated. The application of a tensile stress during annealing of S-phase layer can retard the deduction of its hardness. This is believed to be related to the early stage precipitation of carbides in the S-phase, which could be facilitated by the applied tensile stress during thermal annealing.