Structure and properties of low temperature plasma carburized austenitic stainless steels

Austenitic stainless steels cannot be conventionally surface treated at temperatures close to 550 °C due to intense precipitation of nitrides or carbides. Plasma carburizing allows introducing carbon in the steel at temperatures below 500 °C without carbide precipitation. Plasma carburizing of AISI 316L was carried out at 480 °C and 400 °C, during 20 h, using CH4 as carbon carrier gas. The results show that carbon expanded austenite (γC), 20 µm in depth, was formed on the surface after the 480 °C treatment. Carbon expanded austenite (γC), 8 µm in depth, was formed on the surface after the 400 °C treatment. DRX results showed that the austenitic FCC lattice parameter increases from 0.358 nm to 0.363 nm for the 400 °C treatment and to 0.369 nm for the 480 °C treatment, giving an estimation of circa 10 at.% carbon content for the latter. Lattice distortion, resulting from the expansion and the associated compressive residual stresses increases the surface hardness to 1040 HV0.025. Micro-scale tensile tests were conducted on specimens prepared with the conditions selected above, which has indicated that the damage imposed to the expanded austenite layer was more easily related to each separated grain than to the overall macro-scale stresses imposed by the tensile test.