Effect of High Nitrogen Addition on Microstructure and Mechanical Properties of As-cast M42 High Speed Steel

This study systematically investigated the influence of high nitrogen (N) addition (0.205 wt.%) on microstructure and mechanical properties of as-cast M42 high speed steel. The results demonstrate that the conventional and high-nitrogen M42 cast ingots are mainly composed of martensite, retained austenite and various precipitates (M2C, M6C as well as MC in M42 cast ingot or M(C, N) in M42N cast ingot). The addition of N could increase the retained austenite content, trigger the transformation of MC to M(C, N), favor the formation of M2C at the expense of M6C, and improve the distribution uniformity of M6C at the macroscopic scale. Moreover, the addition of N could lead to the reduction of the secondary dendrite arm spacing as well as the decrease of the thickness and area fraction of eutectic carbides, and improve the distribution uniformity of eutectic carbides at the microscopic scale. The M(C, N) particles form directly from the liquid phase prior to the formation of primary austenite, which could act as the heterogeneous nuclei of primary austenite and thus promote the refinement of the as-cast microstructure. The addition of N slightly decreases the macro-hardness and ultimate compression strength of the cast ingot but increases its ductility, which could be ascribed to the increase of retained austenite content and the reduction in the amount of eutectic carbides. Therefore, high N addition can significantly improve the as-cast microstructure of M42 high speed steel, which is promising for the further enhancement of the mechanical property and service life of the final product.