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 because of low hardness and poor wear resistance. Conventional thermochemical surface treatments for improving the mechanical strength of steels induce Cr carbide precipitation and thus are detrimental to corrosion resistance. A low-temperature (<470?degrees C) plasma treatment has been developed to overcome this problem and to reduce simultaneously the costs and the time of process. This paper reports 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. Microhardness tests and X-ray diffraction indicated that the best results are 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-mu m-thick hardened layer. The results revealed that this layer is not homogeneous because a 2-mu m-thick overlayer of graphitic nature forms on the surface. 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.