Isothermal Heat Treatment of AISI 430 Ferritic Stainless Steel after High Temperature Gas Nitriding

Abstract It has been known that the ferritic stainless steel can be changed to martensitic stainless steel when nitrogen is added. However the high hardness of martensitic stainless steel prevents the plastic deformation. In this study, instead of martensite, the surface microstructure was changed into nitrogen pearlite to increase the plastic deformation easily by isothermal heat treatment after high temperature gas nitriding (HTGN) the AISI 430 ferritic stainless steel. The isothermal treatment was carried out at 780oC for 4, 6, and 10 hrs, respectively, after HTGN treatment at 1100oC for 10 hrs. The surface layer of isothermal-treated steel appeared nitrogen pearlite composed with fine chromium nitride and ferrite. Hence, the interior region that was not affected by nitrogen per-meation exhibited ferrite phase. When quenching the isothermal treated steel at 1100oC, martensitic phase formed at the surface layer. The hardness of surface layer of isothermal-treated steel and quenched steel mea-sured the value of 150~240 Hv and 630 Hv, respectively