An investigation into the effect of stress on the formation and stability of carbon s-phase on austenitic stainless steel

S-phase can be created in austenitic stainless steels by low-temperature thermochemical treatments, which greatly enhanced their hardness, wear resistance and fatigue properties because of the supersaturation by interstitials. One of the technological challenges associated with S-phase surface engineering is that the maximum layer thickness of the S-phase layers is very thin. The thickness of S-phase is restricted by its metastability and precipitation will occur as a result of prolonged treatment. In this project, the effect of in situ tensile stress on the formation of carbon S-phase on 316L austenitic stainless steel was investigated and it was demonstrated that the tensile stress thickened the S-phase layer by promoting the carbon diffusion in austenitic substrate. However, metastable carbides precipitated when applied tensile stress exceeded 40MPa. The thermo-mechanical stability of carbon S-phase was studied by creeping (tensile stress) and HIPping (compressive stress) tests. The results showed that the compressive stress retard the decomposition of S-phase by impeding the carbon diffusion; on the other hand, tensile stress promoted the carbon diffusion. The residual compressive and shear stresses in carbon S-phase was measured be 2.2 GPa and 132 MPa. The wear behaviour of carbon S-phase was studied by dry and oil lubricated reciprocating wear