Lath And Butterfly Composite Martensite Microstructure Of A Medium-Carbon Steel And Its Quantitative Evaluation

Low-alloy medium carbon steel martensite that is obtained by quenching from austenite is composed of lath and butterfly types of martensite. Lath martensite is the major microstructure in most martensite microstructures of low-alloy medium carbon steels. However, the fraction of butterfly martensite is not negligible. Furthermore, butterfly martensite may affect the evolution of martensite microstructures during quenching, because some of the butterfly martensite in medium carbon steels seems to be formed before the lath martensite. Here we quantitatively evaluated the composite martensite microstructure of a medium carbon steel, and briefly discussed the mechanism of the composite microstructure formation. Sizes of martensite blocks were measured via an automated procedure based on orientation maps obtained by electron backscatter diffraction. The block thickness and area of the butterfly martensite were larger than those of the lath martensite. The coarse-grained appearance of the butterfly martensite was originated from relatively large single variant blocks. This result is also supported by transmission electron microscopy observations. A simple habit plane trace analysis shows that the block shape of the butterfly martensite was distorted by austenite grain boundary. In addition, the butterfly block shape suggests that the growth of the butterfly martensite was earlier than that of the lath martensite.