Effect of plasma diffusion process on surface microstructure and magnetic properties of 316L stainless steel
Luo Jiandong, Yang Yingyi, Lin Yuzhou
2021, 46(7):
207-211.
doi:10.13251/j.issn.0254-6051.2021.07.040
Abstract
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Surface hardening of the 316L stainless steel was treated by using the process of plasma nitriding (PN), plasma carburizing(PC) and plasma nitrocarburizing (PNC), respectively. Microstructure, microhardness and magnetic properties of the tested steel were analyzed by means of optical microscope (OM), X-ray diffractometer (XRD), electron probe microscopic analyzer (EPMA), microhardness tester and vibrating sample magnetometer (VSM), respectively. The results show that the expanded austenite phase can be obtained on the surface of 316L stainless steel treated by all the three processes of PC, PNC and PN, and the content of interstitial atoms increases gradually, the lattice expansion increases, which is 1.7%, 4.1% and 5.5%, respectively. The specimens treated by PNC and PN process can obtain thicker infiltrated layer and higher surface hardness, which is about 4.5 times of the substrate. In addition, the specimens treated by PN process show ferromagnetism, while the specimens treated by PC and PNC process show weak magnetism, which are consistent with the prior austenitic stainless steel. Compared with PNC process, the specimens treated by PN process exhibit weaker interaction between Cr and N and slightly larger lattice expansion, but the ferromagnetism are significantly enhanced. The lattice expansion that induces the magnetic phase transition of the 316L stainless steel treated by plasma diffusion process is the primary factor, which makes the expanded austenite transform from paramagnetism to ferromagnetism. The lattice expansion increases the Fe-Fe atomic distance and reduces the overlap of 3d orbitals, which resuits in the increase of magnetic moment of Fe atoms, and the ferromagnetism is significantly enhanced. The critical value of lattice expansion that causes the magnetic phase transition is between 4.1% and 5.5%. The interaction between Cr and N of that is the secondary factor. The optimum non-magnetic strengthening process for 316L stainless steel is PNC process, the process parameters are as follows: temperature of 420 ℃, cathode voltage of 600 V, pressure of 200 Pa, atmosphere ratio of N2:H2:CH4=20:77:3, holding time of 6 h, at which the specimens can obtain a strengthened layer with large thickness, high hardness and weak magnetic properties.
