Microstructure–property relationship in a novel high-boron powder metallurgical tool steel

High-alloy cold work tool steels rely on large additions of refractory elements to achieve high hardness and tempering resistance, resulting in elevated cost and resource consumption. In this study, a novel powder metallurgical high-boron tool steel is developed as an alternative that exploits boron-based hard phases (HP) in contrast to conventional carbide-based tool steels. Based on prior studies, the alloy designated SHB-1 (Fe-0.4C-1B-3.5Cr-1Mo-0.2V-1Si-0.8Mn, in wt.%), was derived empirically and produced by gas atomization followed by hot isostatic pressing and hot swaging. A microstructural analysis of the alloy SHB-1 by SEM, EBSD, EDS, WDS, and XRD reveals a homogeneous dispersion of approximately 15 vol.% HP consisting primarily of tetragonal M 2 B borides and M 23 (C,B) 6 carboborides embedded in a martensitic matrix. Carbon contents were systematically varied by diffusion alloying of the powder with graphite to assess their influence on the HP stability and tempering behavior. Increasing carbon prompted a gradual shift from boride- to carboboride-dominated HP populations while simultaneously increasing the carbon content dissolved in the matrix. Hardness measurements demonstrated high as-quenched hardness values of up to 775 HV10 and pronounced tempering resistance, with peak hardness values exceeding 640 HV10 after tempering at 550 °C. Fracture toughness testing of the master alloy yielded values of 25.5 M P a m after low-temperature tempering and 22.6 M P a m after high-temperature tempering, comparable to conventional high-alloy tool steels. The results demonstrate that high-boron contents combined with powder metallurgy enables a favorable balance of hardness, tempering resistance, and toughness with significantly reduced amounts of critical alloying elements.