Co 첨가가 고속도강의 탄화물 미세조직과 기계적 성질에 미치는 영향

Abstract

Effect of Co addition on the carbide microstructure of high speed steels was investigated by calculating the phase equilibria via Thermo-Calc and by analyzing their carbide microstructure via transmission electron microscopy in particular. The calculation showed that the nucleation driving force for MC carbide is much larger than that for M_6C so that MC will be precipitated as secondary carbides during tempering treatment. The result further showed that, with the addition of Co, the stable region of MC carbide in the pseudo-binary phase diagram is enlarged and the volume fraction of fine secondary MC carbides increases. These results were in good agreement with those of the microstructural analysis using particularly TEM showing that the fine secondary carbides are mostly the MC carbides and that their volume fraction increases with the Co addition. Tempering treatment results in a large increase of the hardness and bend strength of the steels. This is primarily because of the precipitation of fine secondary MC carbides during tempering treatment. This tempering effect was more pronounced with the Co addition because of the large amount of precipitation of fine secondary MC carbides.