The Relationship of Segregation Structure and Properties in High Speed Steel

Abstract

The relationship between microstructure and properties of high speed steels has been examined, with particular reference to fracture toughness and fatigue crack growth rates. Microstructural variations have been obtained by: (i) variation of heat treatment parameters - austenitising temperatures and tempering conditions; (ii) (ii) inherent microstructural differences resulting from the steels composition and the method of manufacture. The range of microstructures studied in terms of carbide distributions varied from fine homogeneous structures of powder metallurgy steels, to 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 the former 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 under hardening or over tempering increases the fracture toughness value. Fatigue crack growth rates were found to increase rapidly with small changes of the stress intensity ΔK, and were relatively independent of any of the measured carbide parameters.