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It is a mode of 7th International Conference on Surface Engineering. AISI 321 stainless steel was salt bath nitrided at 430K for 8 hours,then being long-term aging treatment at different temperature in order to explore the service situation of the extended austenite in the nuclear reactor .To study the changes of microstructures and performances of these samples by using X-ray diffraction (XRD), scanning electronmicroscopy (SEM), micro hardness tester, electrochemical workstation, and universal tensile testing machine. Experimental results showed that a nitrided layer was formed on the surface with the thickness ranging from 7.6 to 22.5μm varying with changing aging treatment time and temperature after dealing with salt bath nitriding. The thickness of the nitrided layer increased with increasing aging time and temperature. According to the ficks second law, the growth of the nitride layer is mainly due to the diffusion of nitrogen, as expected. The active energy of nitrogen atoms in AISI 321 stainless steel is 340.2kJ/mol by calculated form Thermodyn mics formula. The surface micro hardness significantly increased to nearly 1430 Hv 0.098, which is about 4 times harder the non-treated material, and then gradually reduced. The corrosion resistance of the 321 stainless steel is increases with proper aging treatment, but when the aging time more than 15.552×106s, the corrosion resistance decrease. The tensile properties of AISI 321 ASS of strength decrease but the plastic increase a little after salt bathing nitriding and aging. Conclusions: After dealing with salt bath nitriding at 430K and aging treatment, some mechanical properties such as the surface microhardness and the plastic improve effectively when the aging treatment time and temperature prolonged to proper parameter. However, the tensile properties of AISI 321 ASS of strength decrease after salt bathing nitriding and aging. The proper aging treatment also can improve the corrosion resistance.