2002 Volume 66 Issue 7 Pages 745-753
Quantitative microstructure analyses by atomic force microscopy (AFM) were performed for two kinds of JIS-SCM440 steel with the tensile strength of 1580 MPa; the modified-ausformed and tempered (MAQT) martensite and the conventional quench-tempered (CQT) martensite, where the critical diffusible hydrogen content for delayed fracture in the MAQT martensite was 0.53 mass ppm and higher than that of the CQT martensite (0.13 mass ppm). Size distribution of cementite particles and martensite blocks was measured to understand the relationship between microstructure and hydrogen embrittlement resistance. The average cementite size along prior austenite grain boundaries for the MAQT martensite was 0.51 times of that for the CQT martensite. Coarse film-like cementites above 200 nm in length markedly decreased for the MAQT martensite. As the result, occupied fraction of grain boundary cementite particles for the MAQT martensite was decreased to 41% from 51% of the CQT martensite. Inner-prior austenite grain cementite and block were refined for the MAQT martensite. The average cementite size of inner-prior austenite grains for the MAQT martensite was 0.58 times of that for the CQT martensite. The average block width was 0.38 μm for the MAQT martensite, while it was 0.49 μm for the CQT martensite. It is suggested that the refinement of the MAQT martensite structure along and inner-prior austenite grain boundary plays an important role in improvement of hydrogen embrittlement resistance.
