Microstructural Evolution, Behavior of Precipitates, and Mechanical Properties of Powder Metallurgical High-Speed Steel S390 During Tempering

The evolution of the microstructure and mechanical properties of a powder metallurgical (PM) high-speed steel (HSS) were investigated using a series of tempering times. A near Baker–Nutting (B–N) orientation relationship (OR) between the MC carbide and martensitic matrix was detected, while no OR between the M₆C carbide and matrix was observed. The average grain size of the investigated alloys was approximately 4 to 6 μm, which was slightly larger than the thermodynamic calculated grain size of 2 μm. The results indicated that the Rockwell hardness, compressive strength, and in-situ nano-hardness of martensite decreased as the tempering times increased. After quadruple tempering, the fracture strain, compressive strength, and flexural strength declined. Triple tempering contributed to the superior combination of compressive strength, fracture strain, Rockwell hardness, and flexural strength of 3245 MPa, 30.6 pct, HRC 62.2, and 2094 MPa, respectively. The investigated alloys demonstrated strong susceptibility to cracks. The results of this study could provide useful information for the application of this HSS.