Tailoring Primary Carbide Architecture of High-speed Steels by Borderline Heat-treatments

High-speed steels exhibit a good combination of high strength with reasonable toughness when compared to hardmetals. These mechanical properties enable the excellent performance of high-speed steels as tool materials in metalworking applications. The composite microstructure consisting of primary and secondary hardening carbides embedded in a martensitic matrix leads to this mechanical key feature. To increase efficiency parameters, such as tool life, high-speed steels are subject to continuous microstructural development by heat treatments. For tool steels, knowledge of the effect of primary carbide coarsening heat treatments of high-speed steel on primary carbide spacing is still incomplete. In this contribution, specimens made from commercially available high-speed steel were subjected to distinct heat treatments, such as long-term and high-temperature annealing. Specimens quenched and tempered to industrial standards were used for reference. The long-term austenitization resulted in a carbide coarsening to more than twice the carbide size than in the reference state with no associated change in primary carbide volume fraction. The high-temperature austenitization in contrast led to limited carbide coarsening, but a carbide content reduction of roughly 4 vol.%. Both heat treatments led to similar primary carbide spacings. The matrix hardness was the same for all considered microstructure variations, despite significant changes in microstructural features and phase composition.