High temperature tribological behavior and microstructural modifications of the low-temperature carburized AISI 316L austenitic stainless steel

The influence of low-temperature carburizing (LTC) on the room- and high-temperature dry sliding tribological behavior of AISI 316L was investigated and discussed along with microstructural modifications of the carburized layer (expanded austenite or S-Phase). The wear tests were carried out by a ball-on-disk testing device at temperatures up to 600 °C, by using alumina balls as the counterface material. The significant hardness increase induced by the LTC treatment, due to S-phase formation, significantly enhanced the tribological behavior of the AISI 316L at room temperature. However, the wear resistance of the LTC-treated steel strongly decreased with increasing temperature, becoming comparable to that of the non-treated AISI 316L already at 150 °C. Such tribological properties can be explained by considering the presence of a continuous and protective oxide layer in the non-treated AISI 316L steel, whose formation is prevented in the LTC one by a thin amorphous carbon layer covering the S-phase. Thermal stability of the S-phase was studied by static annealing in the temperature range 150–600 °C for 2 h. Temperature increase led to carbon diffusion from the S phase to the matrix, but a remarkable effect with consequent weakening of the surface hardened layer occurs only above the treatment temperature. The high temperature involved in the sliding tests and further frictional heating induced by the tribological contact enhanced the tendency towards structural modifications in the carburized layer.