High temperature tribological behaviour and microstructural modifications of the low temperature carburised AISI 316L austenitic stainless steel

The influence of low-temperature carburizing (LTC) on the room- and high-temperature dry sliding tribological behaviour 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 behaviour 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 hours. 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.