Carbides in high-speed steels containing silicon
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8/10/04
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Carbides in High-Speed Steels Containing Silicon FUSHENG PAN, MITSUJI HIROHASHI, YUN LU, PEIDAO DING, AITAO TANG, and D.V. EDMONDS The effects of silicon additions up to 3.5 wt pct on the as-cast carbides, as-quenched carbides, and as-tempered carbides of high-speed steels W3Mo2Cr4V, W6Mo5Cr4V2, and W9Mo3Cr4V were investigated. In order to further understand these effects, a Fe-16Mo-0.9C alloy was also studied. The results show that a critical content of silicon exists for the effects of silicon on the types and amount of eutectic carbides in the high-speed steels, which is about 3, 2, and 1 wt pct for W3Mo2Cr4V, W6Mo5Cr4V2, and W9Mo3Cr4V, respectively. When the silicon content exceeds the critical value, the M2C eutectic carbide almost disappears in the tested high-speed steels. Silicon additions were found to raise the precipitate temperature of primary MC carbide in the melt of high-speed steels that contained d-ferrite, and hence increased the size of primary MC carbide. The precipitate temperature of primary MC carbide in the high-speed steels without d-ferrite, however, was almost not affected by the addition of silicon. It is found that silicon additions increase the amount of undissolved M6C carbide very obviously. The higher the tungsten content in the high-speed steels, the more apparent is the effect of silicon additions on the undissolved M6C carbides. The amount of MC and M2C temper precipitates is decreased in the W6Mo5Cr4V and W9Mo3Cr4V steels by the addition of silicon, but in the W3Mo2Cr4V steel, it rises to about 2.3 wt pct.
I. INTRODUCTION
HIGH-SPEED steels are still very important tool materials in spite of a rapid development of contemporary tool materials such as sintered carbides, special ceramic materials, and extremely hard materials, i.e., boron nitride and diamond. This is because their hardness, ductility, and machinability are still satisfactory when they are used for cutting tools as well as for cold forming tools.[1–4] The worldwide crisis of the 1980s in the market of alloying elements sparked numerous investigations of high-speed steels. The objectives of these investigations were to limit the content of expensive or not easily available elements such as W, Mo, V, and Co, and to try possible substitution by cheaper and more easily available ones, such as Si, Al, Ti, and Nb.[1,4–7] Some qualitative work on the effects of silicon, aluminum, niobium, and titanium on the microstructure of high-speed steels has been carried out worldwide in order to decrease the content of expensive or not easily available elements and to improve the properties further, which has led to a better understanding of high-speed steels. On the basis of these investigations, some new types of high-speed steels have been developed. Little work on the quantitative effects of silicon, aluminum, niobium, and titanium on the microstructure of high-speed steels, however, has been made, which is very necessary not only for development of new types of low-cost high-speed steels but also
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