Solidification Microstructures and Quench/Temper Hardness of Tantalum Added High-Carbon High-Speed Steel Type Cast Alloy

The influence of Ta addition on the solidification microstructure, solute distribution and hardness after quenching and tempering treatments was investigated for a high-carbon high-speed steel type cast 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 to improve the distribution of hard MC carbides in the hypoeutectic range. Electron probe micro-analysis (EPMA) and X-ray diffraction (XRD) identified an oval microstructure as MC carbides containing mainly V and Ta, and a lamellar structure as M2C carbides containing mainly Fe and Mo among the austenite (γ) dendrites. Redistribution of alloying elements during the solidification sequence of primary γ, γ + MC and M2C eutectic structure could be calculated from the Scheil-Gulliver equation and the initial composition. The macro-hardness of the quenched specimens gradually increased with increasing quenching temperature until a maximum was reached. This indicates that macro-hardness of the quenched specimens depends on both the amount and hardness of martensite matrix. All specimens which were tempered at 723–873 K showed secondary hardening. Furthermore, hardening of the specimens was most apparent when specimens containing large amounts of retained γ were tempered at an optimum temperature. For example, the hardness of specimens with added Ta increased to around 900 HV after tempering at 823 K. These results suggest that the macro-hardness of tempered specimens is governed by the maximum amount of carbon in the γ matrix at quenching temperature, the degree of transformation from retained γ to martensite, and the precipitation and distribution of secondary carbides.