Spray Forming법으로 제조된 고속도강의 탄화물 조직과 기계적 성질에 미치는 carbon의 영향

Abstract

Effect of carbon on the carbide microstructure of spray-formed high speed steels, Fe-xC-4.3Cr-6.5Mo-5.5W-5V-8.5Co, was investigated by calculating the phase equilibria via Thermo-Calc program and by analyzing their carbide micronstructure via transmission electron microscopy in particular. The calculation showed that the volume fraction of fine secondary carbides increases with carbon concentration and that fine secondary MC carbides become more prominent over M_6C carbides at larger carbon concentration. The result further showed that the driving force for the secondary MC carbides is distinctively larger than that for the secondary M_6C carbides. These results were in good agreement with the microstructural analysis using particularly TEM showing that the fine secondary carbides are mostly the MC carbides and that their volume fraction increases with carbon concentration. Tempering treatment results in a large increase of the hardness and bend strength of these steels. This is primarily because of the precipitation of fine secondary MC carbides during tempering treatment. This tempering effect was more pronounced at larger carbon content because of the larger amount of precipitation of fine secondary MC carbides. The bend deflection in as-rolled condition decreased with increasing carbon content. This appeared to be closely related to the variation of the amount of coarse primary carbides with carbon content.