Fracture Toughness of Aisi M2 High-Speed Steel and Corresponding Matrix Tool Steel.

Abstract

The influence of microstructural variations on the fracture toughness of two tool steels with compositions 6%W-5%Mo-4%Cr-2%V-0.8%C (AISI M2 high-speed steel) and 2%W-2.75%Mo-4.5%Cr-1%V-0.5%C (VASCO-MA) was investigated. Samples of the two steels were heat treated to produce a wide range of microstructure, hardness, and fracture toughness values. In the as-hardened condition, the M2 steel has a higher fracture toughness than the MA steel, although the latter steel is softer. It was observed that crack propagation is always transgranular in the M2 steel, whereas in the MA samples the mode is intergranular, when the hardening temperature is above 1095 C (2000 F). Very thin carbide films along grain boundaries of MA steel promote intergranular crack propagation. In the tempered condition, the MA steel has a higher fracture toughness than M2. The crack path in tempered MA is fully transgranular. The thin film carbides in the grain boundaries of the as-hardened samples become thicker and lose continuity, thus promoting transgranular, rather than grain boundary fracture. When the hardening temperature is below 1095 C (2000 F), tempering of both steels causes embrittlement. Tempering results in a reduction of fracture toughness as well as hardness. The fracture of such embrittled samples contains markedly more cleavage surfaces than found in the as-hardened samples. The fracture toughness of both steels was found to be enhanced by increasing the grain size. The steel samples with intercept grain size of 5 (average grain diameter of 30 microns) or coarser exhibited 2 to 3 MPaSQRT.(m (2 to 3 ksiSQRT.(in.) higher fracture toughness than samples with intercept grain size of 10 (average grain diameter of 15 microns) or finer. Tempering temperature has no effect on the fracture toughness of M2 and MA steels as long as the final tempered hardness of the steels is constant. Retained austenite has no influence on the fracture toughness of as-hardened MA steel, in which grain boundaries were the preferred crack path. For the M2 steel, in which crack propagation is always transgranular, a high content of retained austenite appears to raise the fracture toughness. There is a temperature of austenitization for each tool steel after which the retained austenite content in the as-quenched samples is a maximum. The increase of austenite content up to this temperature appears to be controlled by dissolution of carbide, hence, enrichment of alloying elements in the matrix, whereas increasing grain size seems to be the controlling factor for the decrease in austenite content after austenitizing at temperatures above that corresponding to the maximum retained austenite content. A new analytical procedure for estimating volume fractions of carbides was developed. The relationship between Rockwell-C hardness and Diamond Pyramid Hardness of the tool steels was found to be significantly different from that given in ASTM Specification E140-78, and a new correlation is proposed.