Effect of nitrogen on the microstructure and hardness of high-carbon high-speed steel type alloy

Influence of nitrogen addition as one of the alloying element on the formation of solidification structure and the improvement of hardness after thermal treatments was investigated for high-speed steel type alloys (Fe-1.7∼2.3%C-5%Cr-5%Mo-5%V). Nitrogen content was successively varied from 48 ppm to 1542 ppm by the mixing of Cr₂N into the molten alloy. Analysis of diffraction pattern reveals the formation of M₂CN carbonitride at N-containing specimen, whereas the eutectic MC carbides containing mainly V solidifies at N-free specimen. The macrohardness of the quenched specimens gradually increases with increasing quenching temperature. Nitrogen addition helps to improve the hardness as carbon does. Macrohardness of the quenched specimen depends on both the amount and hardness of martensite matrix. The specimen, which contains high volume fraction of retained austenite, shows the superior secondary hardening after the optimized twice tempering. Furthermore, the hardening of specimens is enhanced by nitrogen addition. The precipitation of nano-size carbonitride is observed around primary micro-size carbonitride, which indicated that M₂CN carbonitride diffuses nitrogen, causes carbonitride precipitation, and finally develops macrohardness at N-containing specimen.