Grain boundary engineering of high-nitrogen austenitic stainless steel

Large amount of nitrogen addition into an austenitic stainless steel can improve the mechanical properties and corrosion resistance remarkably as far as the nitrogen is in solid solution. However, once the nitrogen precipitates as nitride, it results in deteriorations in the properties of the high nitrogen austenitic stain steel. During welding, a high nitrogen austenitic stainless steel is ready to precipitate rapidly immense amounts of chromium nitride in the heat affected zone (HAZ), as intergranular or cellular morphologies at or from grain boundaries into grain interiors. The nitride precipitation reduces seriously the local mechanical properties and corrosion resistance. The present authors have demonstrated that a thermomechanical-processing as grain boundary engineering (GBE) inhibited intergranular chromium carbide precipitation in the HAZ of a type 304 austenitic stainless steel during welding and improved the intergranular corrosion resistance drastically. In the present study, the thermomechanical-processing was applied to a high nitrogen austenitic stainless steel containing 1 mass% nitrogen to suppress the nitride precipitation at or from grain boundaries in the HAZ during welding by GBE. GBE increases the frequency of coincidence site lattice (CSL) boundaries in the material so as to improve the intergranular properties, because of strong resistance of CSL boundaries to intergranular deteriorations. The optimum parameters in the thermomechanical-processing brought a very high frequency of CSL boundaries in the high nitrogen austenitic stainless steel. The GBE suppressed the intergranular and cellular nitride precipitation in the HAZ of the high nitrogen austenitic stainless steel during welding.