The influence of 5% cobalt addition on structure and working properties of the 9-2-2-5, 11-2-2-5 and 11-0-2-5 high-speed steels

The paper presents the results of research on the possibility of application of cobalt addition in concentration of up to 5% for the W-Mo-V and W-V high-speed steels of the 9-2-2, 11-2-2 and 11-0-2 types with economically designed chemical composition and the concentration of Si increased to about 1% of W reduced to 9–11% and concentration of Mo limited to about 2%. It was found out that cobalt in the concentration limited to about 5% results in increasing the secondary hardness effect by 1.6–1.9 HRC, depending on a grade of steel. The reason for the secondary hardness effect is the precipitation of dispersive carbides of the M4C3 type in the matrix of the tempered martensite and martensite transformation of the retained austenite, which occurs during cooling from tempering temperature. The maximum secondary hardness effect about 66.3–67.6 HRC, depending on a grade of steel, increases the hardness by about 2–4 HRC, compared with the hardness in the as-quenched state, and occurs in each steel after tempering at 540°C and austenitizing at 1240°C during 100 s. The higher hardness, which is the characteristic of cobalt steel, is also caused by strengthening the martensitic matrix by cobalt, tungsten and molybdenum in these steels, in which the mentioned elements are present. It was also proved that 5% cobalt addition decreases the primary austenitic grain size and reduces the volume fraction of the retained austenite after quenching and tempering. The 5% cobalt addition results in enhancing the working properties of tools, longer tool life and makes it possible to increase cutting speed by about 50%, compared with the steels of identical concentration of other alloying elements but without cobalt addition. In the investigated steels, however, cobalt does not have any significant influence on erosion resistance, which is independent of the steel hardness after quenching and tempering.