Exploring the Influence Mechanisms of Tempering Temperature and N-alloying on Mechanical Properties of AISI M42 High-Speed Steel

The variations in microstructure and mechanical properties of N-free and high-nitrogen (0.18 wt%) M42 HSS with tempering temperatures are investigated. The microstructure of steels after austenitizing primarily consists of martensite, retained austenite (RA), undissolved M2C, and M6C carbides. With increasing tempering temperature (450-600 degrees C), the RA content and martensite lattice strain of steels gradually decrease. At 540 and 600 degrees C, RA is almost or fully transformed into martensite, and massive nanosized M2C secondary carbides are precipitated. The M2C secondary carbides at 600 degrees C obviously coarse compared to those at 540 degrees C. Further, the addition of N increases RA content and martensite lattice strain. With increasing tempering temperature, the bulk hardness and compressive strength of steels first increase and then decrease, whereas their impact toughness shows an opposite trend. This is ascribed to the combined effect of RA decomposition, precipitation and coarsening of secondary precipitates, and decrease in dislocation density and martensite lattice strain. Compared to N-free steel, 0.18N steel tempered at peak-hardening temperature (540 degrees C) and above has higher bulk hardness and compressive strength because of the increased solid solution, dislocation, and precipitation strengthening, but lower impact toughness owing to higher matrix hardness and martensite lattice strain.