The influence of V and Ta addition on solidification structure and hardness after quenching and tempering treatments was investigated for high speed steel type alloy (Fe- 1.9%C- 0.5%Mn- 4.9%Cr- 5.0%Mo- 5.0～7.2%V- 0.4～1.4%Ta). The compositions of V and Ta were systematically changed for the purpose of distributing hard MC carbides in the hypoeutectic range. EMPA and XRD analysis identified the lamellar structure as M2C carbide containing mainly Fe and Mo, and the oval microstructure as MC carbide containing mainly V and Ta among austenite dendrite. The macrohardness of the quenched specimen gradually increased with solution treatment and increasing quenching temperature, and then decreased later. This indicates that the macrohardness of the quenched specimen depends on both the amount and hardness of the martensite matrix. All specimens which were tempered at 723K to 873K showed secondary hardening. Furthermore, hardening of the specimen was conspicuous when specimens containing large amounts of residual austenite were tempered at the optimum temperature, as seen with the increase in the hardening of Ta-contained specimen to around 900HV at the tempering temperature of 823K. These results suggest that the macrohardness of tempered specimens is governed by the amount of carbon in the austenite at quenching temperature, the degree of transformation from residual austenite to martensite, and the precipitation of secondary carbides.
