Microstructure and mechanical behaviour of nitrogen alloyed martensitic stainless steel

Abstract

Solid state gaseous nitrogenising was used to produce a wider range of nitrogen alloyed materials in a 10%Cr martensitic alloy. It was possible to achieve maximum nitrogen levels of 0·45 wt-%. A high solution treatment temperature was required to obtain a single phase martensitic structure. The effect of nitrogen content and heat treatment on microstructure in a variety of conditions was studied using transmission electron microscopy and X-ray techniques. CrN, VN, and Cr2N phases occurred during tempering ofmartensitic alloys in the range 400–750°C. High strength material is produced by introducing nitrogen; strength and hardness increased linearly with increasing nitrogen content and superior properties were observed on tempering. The presence of precipitation caused pronounced secondary hardening, and the increase and subsequent sharp decrease in strength with increasing temperature were assumed to be the effect of coherent precipitation and loss of coherency, respectively. The nitrogen alloyed martensitic structure is finer and more uniform than carbon alloyed material, giving a very fine precipitate distribution. Further increasing the nitrogen content of these martensitic alloys will be limited by the effects of brittleness and retained austenite.MST/2071