Self-organized Formation of Multilayer Structure in a High Nitrogen Stainless Steel during Solution Treatment

Compared with traditional stainless steels, high nitrogen stainless steels (HNSS), have been widely used due to their high strength, toughness along with excellent corrosion resistance and low cost, formed by partial replacement of Ni (austenite-forming element) by N. The evolution of the microstructure of a Cr19Mn19Mo2N0.7 stainless steel is investigated after solution treatment at 1010, 1060, 1200 or 1250°C for 30min. A complex multilayer structure has been found under a negative pressure vacuum. A white ferritic layer at the surface is formed, and a subsurface layer with full austenitic structure and a bulk microstructure comprising of austenite and ferrite are detected. With increasing solution temperature, the surface layer thickness increases. The formation of the multilayer structure is attributed to an outward diffusion, a diffusive retardation and an abnormal accumulation of nitrogen during solution treatment.