Investigation of Three-Dimensional Geometry of Butterfly Martensite and Analysis of Relationship between Hardness of Lath/Butterfly Two-Phase Martensite and its Grain Thickness in Medium-Carbon Steel

Medium-carbon steel with approximately 0.6 wt% carbon is widely used for mechanical applications, because the martensite contained in it is harder than that of low-carbon and high-carbon steel. The microstructure of steel martensite varies depending on the carbon content, and the microstructure of medium-carbon steel martensite are a mixture of lath martensite (LM) and butterfly martensite (BM). The geometry of LM grains are thin plates having thicknesses of 0.2–0.5 μm. As for BM, some researches demonstrated that the region of martensite spreads in the depth direction of the observation surface. However, the three-dimensional geometry of BM grains remains unclear. Through cross-sectional observation using focused ion beam (FIB), the present study demonstrates that one BM grain is formed via collision of two plates. Previous research shows that the hardness of LM is inversely correlated with the thickness of the grains. The FIB observation result indicates that geometry of LM and BM grains are both plate-like. In the present study, whether the same relationship is valid for the mixture of LM and BM grains is investigated. The results show that the hardness of medium-carbon steel martensite increases according to the decrease in thickness of constituent grains of the two-phase structure of LM and BM.