The thicker compound layer formed by different NH3-N2 mixtures for plasma nitriding AISI 5140 steel

Abstract

The plasma nitriding of AISI 5140 steel was carried out assisted with hollow cathode discharge at 540 °C for 4 h under different NH3-N2 mixtures. In different NH3-N2 mixtures, the thickness of compound layer is different, and the phase composition, corrosion resistance and microhardness is also different. As well as, the effect of different NH3-N2 mixtures on the microstructure of nitrided layer, phase composition, compound layer thickness and microhardness was characterized carefully. The results show that the thickness of compound layer, microhardness, hardening depth and corrosion resistance are related to NH3-N2 mixtures. At 1:4 ratio, the thickest compound layer obtained is about 12 μm and thicker than that of the conventional compound layer (5–9 μm). At the same time, the depth of surface hardening layer is about 360 μm. The hardness value of nitrided surface is about 975 HV0.1, which is about 3.4 times than that (295 HV0.1) of the substrate. The thickest compound layer has the best corrosion resistance characteristics. Moreover, there is no defect in the compound layer for nitrided samples. Duo to the explosion and high cost of H2, the use of NH3-N2 mixtures greatly improve safely and reduce costs.