Papers by Keyword: High Nitrogen Stainless Steel

Abstract: Microstructure and mechanical properties of high nitrogen steels whose chemical composition were Fe-17Cr-12Mn-3Mo and that was produced by using metal injection molding method and nitrogen absorption methods were examined. A compact which is furnace cooled from 1573 K has a brittle surface layer with high chromium and nitrogen contents but the surface layer disappears when the compact is held at 1473 K. The compact which is furnace cooled at 1473 K is observed precipitates in the grains and the grain boundary, while the compact which is quenched at 1473 K shows homogeneous microstructure. In the heat treatments at 1473 K for 2, 5, and 10 h, the nitrogen content becomes higher with increasing of holding time. In the holding times of 5 and 10 h, the microstructure is austenite. In the tensile tests, tensile strength becomes larger with increasing of nitrogen content. In the specimen which is conducted the heat treatment at 1473 K for 10 h, tensile strength is about 1,000 MPa and elongation is 80 %, which shows better balance of strength and elongation than SUS304 and SUS316 steels.

Abstract: 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.

Abstract: Friction stir welding (FSW) was applied to a 0.53% nitrogen stainless steel. The nitrogen content change and the microstructural evolution in the weld were investigated. The nitrogen content analysis revealed that the stir zone (SZ) showed roughly the same nitrogen content as the base material (BM). This result suggests that FSW is an effective welding process for keeping up nitrogen content of high-nitrogen steel weld. The microstructural observation showed that the weld had the BM, the SZ, the partially recrystallized zone (PRZ) and the heat affected zone (HAZ). The coarse grain structure of the BM changed to relatively fine microstructure of equiaxially recrystallized austenite grain structure in the SZ during FSW. The PRZ contained both the fine and coarse grain structures. The detailed TEM observations showed that the particles with about 11m in size were present in the advancing side of the SZ both along grain boundaries and grain interiors. The Cr nitride less than 100 nm was formed in the HAZ along grain boundaries, which indicates that a slight sensitization occurred in the HAZ during FSW.