Effects of silicon additions on the mechanical properties and microstructure of high speed steels

The effects of silicon additions up to 3.5 wt% on the mechanical properties and microstructure of high speed steels 6W3Mo2Cr4V, W3Mo2Cr4V and W9Mo3Cr4V have been investigated. In order to understand these effects further, a Fe16Mo0.9C alloy is also used. The results show that silicon additions can increase the temper hardness of steels Fe16Mo0.9C, 6W3Mo2Cr4V and W3Mo2Cr4V, but yield an opposite influence on the temper hardness in W9Mo3Cr4V steel. A critical tempering temperature exists for the bending strength of high speed steels containing silicon. If tempering is carried out at temperatures lower than the critical temperature, the bending strength of the high speed steels can be improved by the addition of silicon, otherwise their bending strength is decreased. Transmission electron microscopy reveals that silicon additions can obviously refine secondary hardening carbides and inhibit the formation of M 3C cementite at peak temperature. However, they are also found to accelerate both the depletion of martensite and the formation of coarse M 6C precipitates during tempering. The mechanism whereby silicon additions affect the secondary hardness of high speed steels is discussed in detail, and the types of high speed steel in which silicon additions can be used are suggested.