Investigation of Heat Treatment Influence on Microstructure of High-Speed Steel and its Resulting Mechanical Properties

The influence of heat treatment of high-speed steel AISI M2 on its microstructure is investigated. This steel had the following chemical composition (mass content in %): 0.89% C, 0.20% Si, 0.26% Mn, 0.027% P, 0.001% S, 3.91% Cr, 4.74% Mo, 1.74% V and 6.10% W. By means of heat treatment, the microstructure of high-speed steel can be changed. The aim was to optimize the ratio between hardness and fracture toughness KIc. It has also been experimentally confirmed that different microstructural parameters have a significant effect on the measured fracture toughness KIc, [1-5]. Those steels are very often applied in construction of tools with complex loading conditions, especially for cold working applications. In such working conditions they have to satisfy higher and higher demands regarding their toughness and fracture toughness while, at the same time, maintaining the equal mechanical properties, or increasing them (such as, for example, hardness). A high fracture toughness KIc means that those tools will be more resistant to impact loadings as well as to the propagation of fatigue cracks. If a crack tends to propagate in a material with large undissolved eutectic carbides, separated by large mean distance, the plastic deformation of these ligaments is responsible for the energy dissipation that determines the crack resistance of the material. By applying the different heat treatment procedures on high-speed steels it is possible to obtain a microstructure which has the same mechanical properties, but greater fracture toughness KIc. This is possible to be achieved by optimizing the parameters of heat treatment. During the investigation a research about influence of microstructural parameters on the above mentioned mechanical and fracture properties is conducted in the form of influence of the volume fraction of undissolved eutectic carbides, their mean diameter, the mean distance between the carbides, as well as the volume fraction of retained austenite in the matrix. The calculated fracture toughness values, which were obtained by using a newly developed semi-empirical equation, derived by author [3], agreed well with the experimental results obtained by present experimental investigations. References [1] F. Cajner, V. Leskovsek and D. Pustaic: Key Engineering Materials, Materials Structure & Micromechanics of Fracture VII, edited by Pavel Sandera, Vols. 592-593 (2013), p. 680- 683 [2] V. Leskovsek, B. Ule and B. Liscic: Journal of Materials Processing Technology, Vol. 127 (2002), p. 298- 308 [3] V. Leskovsek: Optimization of vacuum heat treatment of HSS, Ph.D. Thesis, University of Zagreb (1999). [4] V. Leskovsek, B. Ule and A. Rodic: Met. Alloys Technology, Vol. 27 (1-2) (1993), p. 195-204 [5] B. Ule, V. Leskovsek and B. Tuma: Engring. Fracture Mechanics, Vol. 65 (2000), p. 559-572 [6] BS 7448: Part 1, Fracture mechanics toughness tests: BSI (1997). [7] ESIS P2-92: ESIS Procedure for determining the fracture behavior of materials, ESIS, Delft University of Technology (1992).