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 prope rties and microstructure of high speed steels 6W3Mo2Cr4V, W3Mo2Cr4V an d W9Mo3Cr4V have been investigated. In order to understand these effec ts further, a Fe-16Mo-0.9C alloy is also used. The results show that s ilicon additions can increase the temper hardness of steels Fe-16Mo-0. 9C, 6W3Mo2Cr4V and W3Mo2Cr4V, but yield an opposite influence on the t emper hardness in W9Mo3Cr4V steel. A critical tempering temperature ex ists for the bending strength of high speed steels containing silicon. If tempering is carried out at temperatures lower than the critical t emperature, the bending strength of the high speed steels can be impro ved by the addition of silicon, otherwise their bending strength is de creased. Transmission electron microscopy reveals that silicon additio ns can obviously refine secondary hardening carbides and inhibit the f ormation of M3C cementite at peak temperature. However, they are also found to accelerate both the depletion of martensite and the formation of coarse M6C precipitates during tempering. The mechanism whereby si licon additions affect the secondary hardness of high speed steels is discussed in detail, and the types of high speed steel in which silico n additions can be used are suggested. (C) 1997 Acta Metallurgica Inc.