Effects on the Long-Term Aging of Low Temperature Salt Bath Nitrided AISI 321 Stainless Steel

Abstract

AISI 321 stainless steel was salt bath nitrided at 430K for 8 hours, then being long-term aging treatment at different temperature. To study the changes of microstructures and performances of these samples by using X-ray diffraction (XRD), scanning electron microscopy (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 22.5 μ m 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 fick’s 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 Thermodynamics formula. The surface micro hardness significantly increased to nearly 1430 Hv0.098, 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.