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

The effect of nitrogen addition on the microstructure formation and hardness during solidification and heat treatment was investigated and the possibility of nitrogen as an alloying element was discussed in terms of alloy chemistry for high-carbon high-speed tool steel type cast alloys. Nitrogen with a concentration from 48 ppm to 1542 ppm was successfully introduced by mixing Cr₂N into a molten alloy. Analysis of the diffraction pattern revealed that the primary V₂CN carbonitride crystallized upon the addition of nitrogen, whereas eutectic carbides mainly formed in N-free specimens. The chemical composition of the carbonitride is also affected by the addition of nitrogen. With increasing quenching temperature, the Vickers hardness gradually increased to a peak and then decreased. Nitrogen addition helps to increase the hardness similarly to carbon. A N-containing specimen also exhibited superior secondary hardening after tempering. It is known that a large amount of residual austenite finally transforms to a hard martensite phase after tempering. According to the results of XRD analysis, nitrogen addition increases the volume fraction of retained austenite in the matrix at a higher holding temperature. Furthermore, the precipitation of nanosize carbonitride was observed around the primary V₂CN carbonitride in addition to the standard precipitation. Therefore, this carbonitride precipitation may induce the superior secondary hardening and ultimately increase the macrohardness of N-containing specimens.