Effect of tensile stress on the formation of S-phase during low-temperature plasma carburizing of 316L foil

Low-temperature plasma carburizing of austenitic stainless steel can produce a carbon-supersaturated austenite layer, the “S-phase”, on the surface, which has high hardness, excellent wear and fatigue properties, and good corrosion resistance. Although the S-phase was discovered some years ago, the basic understanding of S-phase formation remains incomplete. In this paper, the effect of tensile stresses (0–80 MPa) on the formation and stability of S-phase during carburizing of 316L stainless steel foils at 400, 425 and 450 °C for 10 h has been investigated for the first time. The microstructures were characterized by X-ray diffraction, scanning electron microscopy and transmission electron microscopy and the mechanical properties were evaluated by microhardness and tensile tests. The results showed that the in situ applied tensile stress effectively thickened S-phase layers. The calculated activation energy for carbon diffusion in 316L was reduced from 142.76 to 133.91 kJ mol−1 when a tensile stress of 40 MPa was applied. However, chromium carbides were formed in the outmost surface when the tensile stress exceeded 40 MPa. The results are discussed and explained through appropriate thermodynamic calculations.