Functional properties of expanded austenite generated on AISI 316L by plasma nitrocarburizing using different active screen materials

This study investigates the functional properties of the expanded austenite layers generated on AISI 316L austenitic stainless steel resulting from active screen plasma nitrocarburizing using different active screen materials, i.e. steel or solid carbon. Treatments were conducted at 460 degrees C for 5 h in a nitrogen-hydrogen feed gas, whereas for the treatments using a steel active screen, methane was added as a carbon precursor. Additionally, the bias plasma conditions applied at the samples were varied between 0 kW and 1.25 kW. Samples were characterized by complementary microstructural and compositional investigations, surface roughness and hardness measurements, pin-on-disk tribological tests as well as potentiodynamic polarization tests in H2SO4 and NaCl electrolytes. The functional properties of the case are discussed based on the contents of nitrogen and carbon in the expanded austenite and their effective diffusion depths. The results show that the usage of a carbon screen generally produces surfaces with uniform layer thickness, high hardness, improved wear resistance and a delayed tendency to pitting corrosion independent of the bias condition applied to the samples. When applying both screen materials at non-biased condition, the general corrosion resistance is slightly reduced under the conditions used, however, the layers generated using the carbon screen have a wear rate that is 3 times lower. It can be concluded that the carbon screen represents a robust treatment variant for austenitic stainless steels to produce sufficiently thick and wear-resistant surface layers in a short treatment duration, which still have the potential to maintain the corrosion resistance in different environments.