Modification of AISI 304 Stainless Steel Surface by the Low Temperature Complex Salt Bath Nitriding at 430°C

Abstract

Salt bath nitriding of type 304 austenite stainless steels conducted at low temperature, 430°C, using a complex salt bath heat-treatment. Microstructure and the properties of the nitrided surface were systematically evaluated. Experimental results revealed that when 304 stainless steel subjected to complex salt bathing nitrided at such temperature less than 4 hours, the main phase of the nitrided modify layer was the expanded austenite (S phase) generally. When the treatment times prolonged up to 8 hours and more, S phase was formed and subsequently transformed partially into CrN, and then the secondary CrN phase precipitated. All treatments can effectively increase the surface hardness. And the nitrided layer depth thickened intensively with the increasing nitriding time. The growth of the nitrided layers took place mainly by nitrogen diffusion according to the expected parabolic rate law. Low temperature nitriding can improve the corrosion resistance against diluted hydrochloric acid. After nitriding for 16 hours, the sample has the best corrosion resistance than others though there are many CrN transformed in the layer.