ベイナイト鋼のねじり疲労限度に及ぼす微視組織の影響

抄録

In order to study the determination factors of the torsional fatigue limit of bainitic steels, torsional fatigue tests were carried out, using two materials; CG (HV = 329) and FG (HV = 314) having different microstructure morphologies. Electron backscattered diffraction (EBSD) observation revealed that coarse microstructure and fine microstructure were co-existed in both the materials. The area fraction of fine microstructure in FG was about 15% higher than that of CG. The torsional fatigue limit of FG was about 80 MPa higher than that of CG, even though they had almost the same Vickers hardness. Shear-mode cracks in these steels initiated and propagated transgranularly in coarse grain region. In addition, some cracks stopped propagating at the grain boundaries. The statistics of extremes predicted that CG and FG have almost the same maximum grain size. This result inferred that the size of crack initiation units in these materials was nearly equivalent to each other. Based on the series of experimental results and observations, the effect of microstructure in bainitic steels on the torsional fatigue limit is discussed.