The influence of 5% cobalt addition on structure and working properties ofthe 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 applicatio n of cobalt addition in concentration of up to 5% for the W-Mo-V and W-V hi gh-speed steels of the 9-2-2, 11-2-2 and 11-0-2 types with economically des igned 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 gra de of steel. The reason for the secondary hardness effect is the precipitat ion of dispersive carbides of the M4C3 type in the matrix of the tempered m artensite and martensite transformation of the retained austenite, which oc curs during cooling from tempering temperature. The maximum secondary hardn ess effect about 66.3-67.6 HRC, depending on a grade of steel, increases th e hardness by about 2-4 HRC, compared with the hardness in the as-quenched state, and occurs in each steel after tempering at 540 degreesC and austeni tizing at 1240 degreesC during 100 s. The higher hardness, which is the cha racteristic of cobalt steel, is also caused by strengthening the martensiti c matrix by cobalt, tungsten and molybdenum in these steels, in which the m entioned elements are present. It was also proved that 5% cobalt addition d ecreases the primary austenitic grain size and reduces the volume fraction of the retained austenite after quenching and tempering. The 5% cobalt addi tion results in enhancing the working properties of tools, longer tool life and makes it possible to increase cutting speed by about 50%, compared wit h the steels of identical concentration of other alloying elements but with out cobalt addition. In the investigated steels, however, cobalt does not h ave any significant influence on erosion resistance, which is independent o f the steel hardness after quenching and tempering. (C) 2001 Elsevier Scien ce B.V. All rights reserved.