Effect of Solution Nitriding on Microstructure and Hardness in 12%Cr Martensitic Stainless Steels

Synopsis : Solution nitriding was applied to conventional 12%Cr stainless steels such as JIS SUS4 10L, SUS410, SUS420J1 and SUS420J2, and then the microstructure and hardness were examined for the as-quenched specimens as well as tempered ones. Nitrogen concentration was increased to around 0.28 mass% under the solution nitriding condition of 1473K-0.1MPa N-2 in all of the 12%Cr steels regardless of the carbon content. Solid solution strengthening by nitrogen was relatively small compared with that by carbon in as-quenched martensite. However, solution-nitrided steels exhibited a remarkably large secondary hardening during tempering, which hardly appears in conventional martensitic stainless steels without nitrogen. TEM observation and X-ray analysis for extracted residue indicated that the secondary hardening by nitrogen was caused by precipitation of fine CrN nitride.