The Hardened and Tempered Microstructure of High-Speed Tool Steel as a Factor in Tool Performance

Abstract

Abstract The hardened and tempered metallurgical characteristics of a large number of tools representing various types of high-speed steels were studied to determine the probable cause for variation in tool life during conduct of continuous roughing-cut performance tests. The studies included the austenitic grain size, hardened and tempered microstructure, the Rockwell hardness of the finished tools, and the Shepherd hardened fracture grain size, as well as comparison of these properties with tool performance. The results of these studies indicated that, for various lots of a type of high-speed tool steel subjected to the same heat-treatment, variations do occur in tool performance, austenitic grain size, hardened and tempered microstructure, Rockwell hardness, and hardened fracture grain size. Of the variables observed, the hardened and tempered microstructure showed direct correlation with variation in tool performance. Within normal limits there appears to be no relationship between the hardened and tempered microstructure and the austenitic grain size, Rockwell hardness, or hardened fracture grain size. For lots of the same type of high-speed tool steel, subjected to the same heat-treatment, the characteristics of the hardened and tempered microstructure are directly associated with the speed of response, i.e., solution of carbide in the material at the austenitizing temperature. Red hardness being a function of the quantity of carbide dissolved during heat-treatment, the characteristics of the hardened and tempered microstructure are associated with red hardness and therefore are of considerable influence in the performance of tools.