Relationship between structure and fracture behaviour in 6W–5Mo–2V type high-speed steel

Abstract

The relationship between the microstructure and properties of high-speed steels has been examined, with particular reference to fracture toughness (as measured by K_IC) and fatigue-crack growth rates. Different microstructures have been obtained by varying the heat-treatment parameters - austenitizing temperatures and tempering conditions; there are also inherent microstructural differences resulting from the steels’ composition and the method of manufacture. The range of microstructures studied varied, in terms of carbide distributions, from the fine homogeneous structures of powder-metallurgy steels to the heavily segregated microstructures typical of large bar sizes of cast and hot-worked high-speed steels. It has been shown that the presence of carbide segregation in the form of bands and ‘hooks’ significantly enhances the steel's fracture toughness, particularly at low hardnesses and regardless of the orientation of carbide ‘banding’ to the crack plane. This effect was found to be a direct result of local variations in the excess volume fraction of carbides and the path taken by the crack at fast fracture. Fracture toughness is shown to be a linear function of hardness for homogeneous and segregated steels, when they are tempered at their secondary-hardening peaks. Variations between the toughness values of powder-metallurgy steels at a given hardness level are related to the fine detail of their carbide distributions and volume fractions. It is also shown that the presence of retained austenite and the reduction of the matrix alloy content, by either underhardening or overtempering, increases the fracture-toughness value. Fatigue-crack growth rates were found to increase rapidly with small changes in ∆K and were relatively independent of any of the measured carbide parameters. The practical relevance of these results is discussed.