Preparation and properties of "expanded"α phase layer on AISI431 stainless steel

AISI431 martensitic stainless steel was treated by low-temperature plasma modification. Microstructure of the layers was characterized by optical metallography and X-ray analysis, the hardness and corrosion resistance of the layer were tested by microhardness test and corrosion polarization curves. The results show that the "expanded"α phase can be formed on the surface of the stainless steel by both nitriding and carburizing, the nitrided layer mainly consists of Fe₄N and "expanded"α phase with nitrogen, carburized layer mainly consists of "expanded"α phase with carbon, and the thickness of nitrided layer is higher than that of the carburized layer. After low-temperature modification, the hardness of the stainless steel increases significantly, and the hardness of the nitrided layer is higher than that of the carburized layer. However, under the present nitriding or carburizing conditions, the corrosion resistance of the stainless steels all decreases slightly, comparing to the nitrided specimen, the corrosion resistance of the caburized specimen is weaker, and the passivation films formed on these two kinds of the layer during the process of corrosion all possess the character of n type of semiconductor.