No abstract

Carburized-quenched steel has a hard layer on the surface and a soft layer in the core. Internal fatigue cracks are observed around the boundary between these two layers under cyclic stress. In this research, we investigated the microstructures (carbon content, prior austenite grains and retained austenite) in the carburized-quenched chromium molybdenum steel bar (JIS-SCM415, diameter = 10 mm) failed by rotating bending test (RBT) at nominal stress amplitude of 716 MPa. After the investigations, we obtained three conclusions: the carbon content in the area from the surface to 0.1 mm depth was higher than other area; the prior austenite grain (PAG) sizes at 0.1 mm depth from the surface was almost the same as that of 0.6 mm depth; and the retained austenite which was indicated from the ratio of γ to α in the cross section ranging from the surface to 0.1 mm depth was decreased by rotating bending fatigue.

Induction-heated steel has hard and soft layers. These layers can cause an internal fatigue crack originating from the boundary of these layers when cyclic stress is applied. Repeated heating is known as a method for improving fatigue strength, and it was applied to induction heating method. Repeatedly induction-heated steel had high fatigue strength compared to single quenching. We performed rotating bending fatigue tests of low carbon steel (JIS-S45C) induction-heated three times, and observed the fracture surfaces and the microstructures of internal fatigue cracks. The internal fatigue cracks originated from the area around the boundary between soft and hard layers surrounding crack origin. Some pearlite and ferrite can be seen. There were pearlite and dimples on the soft layer of internal fatigue crack and clear grains on the hard layer of the crack. From chase-up observation, we revealed that internal fatigue crack originated from soft layer.