Modelling of High-Speed Steels Fracture Toughness

Abstract

The microstructure of AISI M2 high-speed steel can be substantially modified by a vacuum heat treatment, with the aim to improve the ratio of the hardness to the fracture toughness. This ratio is significantly affected by the volume fractions of undissolved eutectic carbides and retained austenite, as well as the mean distance between these carbides. In this investigation the fracture toughness was determined using specimens with a nonstandard shape, i.e., circumferentially notched and precracked round-bar tensile specimens (K Ic-test specimen). The advantage of using these specimens over standard compact-tension (CT) or single-edge notched-bend (SENB) specimens is their radial symmetry, which makes them particularly suitable for studying the influence of the microstructure of metallic materials on their fracture toughness. The radial symmetry of heat transfer allows the formation of a microstructure that is completely uniform on the circumference of the specimens. The calculated fracture toughness values, obtained using a semiempirical equation based on the stress-modified critical strain criterion and the quantified microstructural parameters, offer the opportunity to better understand the influence of the microstructure of the investigated AISI M2 high-speed steel on its fracture toughness and make it possible to choose the optimum composition and processing conditions for high-speed steel as well as the appropriate heat-treatment process to obtain an optimum combination of the basic characteristics for a given tool application.