Changes in the Microstructure and Hardness after Heat Treatment of a Billet Made of Powder High-Speed Steel R6M5K5 Containing a Diffusion-Alloyed Additive

In this work, a study was made of the change in the microstructure of a hot-pressed billet from a powder mixture of high-speed steel grade R6M5K5 containing a diffusion-alloyed additive based on tungsten, molybdenum, cobalt, and iron after heat treatment processes: quenching in oil at a temperature of 1230°C and double high tempering at 550°C for 60 min. The study was carried out using electron microscopy and energy dispersive X-ray spectroscopy. After quenching, dissolution of small-sized carbides in the matrix is observed, and after twofold tempering, secondary carbides of the M6C and MC types are already precipitated. The second type of carbides is formed from a separate structural component—a phase based on ferrovanadium, whose powder was used as a source of vanadium. Also, cobalt is unevenly distributed in the matrix, which is concentrated in a separate phase based on a diffusion-alloyed additive. These phases hardly dissolve and are quite stable under these conditions of hot pressing and heat treatment. It is shown that the hardness of blanks changes from 63.1 HRA after hot pressing to 67.2 HRA after quenching and up to 66.3 HRA after double tempering.