Surface modification of austenitic steels by low temperature carburization

Austenitic stainless steels are characterized by good corrosion resistance in different environments but their use is limited due to low hardness and poor wear resistance. Conventional thermo-chemical surface treatments for improving the mechanical strength of steels induce Cr carbide precipitation thus are detrimental to corrosion resistance. A low temperature (< 470 °C) plasma treatment has been developed to overcome the problem with reduced costs and time of process. This paper shows the results of a microstructural characterization performed on a series of AISI 316L steel samples treated by plasma-assisted low temperature carburization in different conditions. Micro-hardness tests and X-ray diffraction (XRD) indicated that the best operative condition is achieved by employing a gas mixture with 2% of CH4 in H2. XPS and AES were used to examine the chemical composition of the 20 m-thick hardened layer. The results revealed that such layer is not homogeneous because a 2 m-thick overlayer forms on the surface exhibiting a typical graphitic nature. Furthermore, only the plasma treatment with 2% of CH4 guarantees that the whole carbon remains in solid solution whereas for higher CH4 amounts in the gas mixture carbide precipitation takes place.