Formation and microstructural characterisation of S-phase layers in Ni-free austenitic stainless steels by low-temperature plasma surface alloying

The feasibility of generating S-phase surface layers in nickel-free austenitic stainless steels by plasma surface alloying with nitrogen (at 430 °C), carbon (at 430 °C and 500 °C) and both carbon and nitrogen (at 430 °C) has been investigated. The structure, microstructure and composition of the plasma-alloyed surfaces were characterised by X-ray Diffraction (XRD), microscopy, Glow Discharge Optical Emission Spectroscopy (GDOES) and Transmission Electron Microscopy (TEM). The experimental results have demonstrated for the first time that the S-phase can be produced in the surface of nickel-free austenitic stainless steel by low-temperature plasma surface alloying. TEM analysis has revealed that when alloyed with carbon no precipitates can be found within the carbon-rich S-phase layer; however, when alloyed with nitrogen or both carbon and nitrogen some nitride precipitates (Mn3N2 and Cr2N) were found within the nitrogen-rich S-phase layer. Based on experimental results, the response of Ni-free austenitic stainless steel to plasma surface alloying has been compared to the Ni-containing counterpart, and the role of nickel in the formation of S-phase in austenitic stainless steels has been discussed.