Roles of N-Alloying and Austenitizing Temperature in Tuning the Hardness and Strengthening-Toughening Behavior of M42 High-Speed Steel

A novel high-nitrogen (N) (0.18 wt pct) M42 high-speed steel (HSS) was fabricated by pressurized metallurgy. The microstructural characteristics and mechanical properties of N-free and high-N M42 HSS austenitized at 1373 K to 1433 K and tempered at 813 K were comparatively studied, and the strengthening and toughening mechanisms were analyzed in detail. The results showed that the precipitates primarily consisted of undissolved M2C + M6C carbides and nano-sized M2C secondary carbides after tempering. High N addition or increasing austenitizing temperature decreased the fraction of undissolved precipitates, increased the content of retained austenite and the dislocation density and nano-hardness of the tempered martensite matrix, promoted the precipitation of M2C secondary carbides, while barely affecting the average martensite block width. The major strengthening mechanisms of steels were dislocation and precipitation strengthening, and the precipitate-shearing strengthening played a decisive role in precipitation strengthening. High N addition or increasing austenitizing temperature enhanced the bulk hardness and compressive strength of M42 HSS by reinforcing some strengthening factors, but decreased the impact toughness due to the decreased fracture ductility of the tempered martensite matrix. The bulk hardness and compressive strength of high-N M42 HSS were at least 1.0 HRC and 145 MPa higher than those of N-free M42 HSS at the same austenitizing temperature. Overall, high-N M42 HSS could meet higher requirements of modern industry to make cutting tools.