Structural-phase transformations and changes in the properties of AISI 321 stainless steel induced by liquid carburizing at low temperature

Abstract

Structural and phase transformations occurring due to the supersaturation of austenite with interstitial atoms (carbon and/or nitrogen) are the key priority in the study of AISI 300 series austenitic steels. It is also very important to achieve a greater hardening depth by nitriding or carburizing. For this purpose, a method of salt bath carburizing at low temperature was proposed. The aim of this work is to perform a detailed analysis of structural-phase transformations and their effect on the properties of AISI 321 austenitic steel subjected to liquid carburizing at a temperature of 780°С. Optical and scanning electron microscopy, optical profilometry, X-ray diffraction analysis, energy-dispersive microanalysis, electron backscatter diffraction analysis, instrumented microindentation and microhardness measurement are used as methods of investigation. It has been discovered that, along with carbon-rich (up to 0.46 wt% C) austenite, chromium carbide Cr23C6, cementite Fe3C, ε-martensite, and α-martensite are formed in the surface layer of the carburized AISI 321 steel. Carbides are present both at the grain boundaries and within the austenite grains. Martensite formed in the carburized AISI 321 steel is induced by deformation, and the martensitic transformation path is γ → ε → α′ (two-stage transformation). In its turn, plastic deformation occurs during cooling that follows carburizing, and this is a relaxation mechanism of high thermal stresses. Liquid carburizing of AISI 321 steel also multiplies the microhardness of the steel surface from 200 ± 7 to 890 ± 110 HV0.025, with the total hardening depth being about 500 μm. The hardened layer is gradient and characterized by increased resistance to elastic-plastic deformation, this being important for increasing the contact endurance and wear resistance of the steel. It has also been found that, due to high carbon concentration, the corrosion resistance of the carburized steel does not deteriorate significantly.