Influence of Nitrogen Introduced by Solution Nitriding on Microstructure and Room-temperature Mechanical Properties of Grade 91 Steel

The effect of nitrogen introduced by solution nitriding on microstructure and mechanical properties of modified 9Cr-1Mo (Gr. 91) steel at room temperature was investigated. The nitrogen concentration at the sample surface was 0.164 wt% and nitrogen diffused at least 5000 μm after solution nitriding heat treatment at 1200ºC for 48 hours. The martensite with a small amount of MX carbonitride with cF8 structure and retained austenite was formed on 100 μm from the sample surface. The Cr2N phase with hP9 structure containing V, Nb and Mo and Cr23C6 phase with cF116 structure precipitated by tempering. Solute nitrogen improved the hardness, yield strength, ultimate tensile strength and uniform elongation. However, the nitride formation did not contribute to the improvement of hardness and decreased yield strength and ultimate tensile strength. It suggests that the contribution of solid solution strengthening by Cr, V, Nb and Mo is larger than that of precipitation strengthening by nitride at room temperature.