Influence of Gas Composition and Treatment Time on the Surface Properties of AISI 316L Austenitic Stainless Steels During Low-Temperature Plasma Nitrocarburizing Treatment

摘要

The major drive for the application of low-temperature plasma treatment in nitrocarburizing of austenitic stainless steels lies in improved surface hardness without degraded corrosion resistance. The low-temperature plasma nitrocarburizing was performed in a gas mixture of N_2, H_2, and carbon-containing gas such as CH_4 at 450 deg C. The influence of the processing time (5-30 h) and N_2 gas composition (15-35 percent) on the surface properties of the nitrocarburized layer was investigated. The resultant nitrocarburized layer was a dual-layer structure, which was comprised of a N-enriched layer (gamma N) with a high nitrogen content on top of a C-enriched layer (gamma C) with a high carbon content, leading to a significant increase in surface hardness. The surface hardness reached up to about 1050 HV_(0.01), which is about 4 times higher than that of the untreated sample (250 HV_(0.01)). The thickness of the hardened layer increased with increasing treatment time and N_2 gas level in the atmosphere and reached up to about 25 mu m. In addition, the corrosion resistance of the treated samples without containing Cr_2N precipitates was enhanced than that of the untreated samples due to a high concentration of N on the surface. However, longer treatment time (25 percent N_2, 30 h) and higher N_2 gas composition (35 percent N_2, 20 h) resulted in the formation of Cr_2N precipitates in the N-enriched layer, which caused the degradation of corrosion resistance.