Effect of Fe content on type and distribution of carbides in medium-entropy high-speed steels

The effect of iron content on the type and distribution of carbides in the vacuum arc melted Fe-x(AlCoCrCuNiV)(88.05-x)Mo5W6-C-0.95 (x = 69, 76, 83 wt%, respectively) medium-entropy high-speed steels (ME-HSSs) was studied. The homogeneous distribution of granular MC carbides (M refers to the carbides forming transition metal elements, such as W, Mo and V), both at the grain boundary and in the martensitic matrix, was obtained in the Fe-76(AlCoCrCuNiV)(12.05)Mo5W6-C0.95ME-HSS, after quenching at 1210 degrees C followed by triple tempering at 530 degrees C. A maximum hardness of about 841 HV0.5 was achieved, even higher than the maximum hardness of the commercially available M2 (W6Mo5Cr4V2) HSS, 826 HV0.5. The medium-entropy effect was shown to be beneficial to transform the solidified primary carbide network into finely dispersed granular MC. However, when the iron content was further reduced to 69 wt%, the enhanced entropy effect promoted the dispersion of carbides but at the same time led to a decrease in the carbide content, so that the maximum hardness (816 HV0.5) was slightly lower than the maximum hardness of M2-HSS. The results provide a novel alloy system together with a simple heat treatment method to obtain hard HSSs, and more importantly to eliminate the primary carbide network which is harmful to the toughness of traditional HSS.