Crack Propagation Behavior and Change of Residual Stress on the Process of Bending Fatigue Test of Carburized Steel

The elementary factors which determine the fatigue strength of the carburized steel were studied by investigating the fatigue crack propagation behavior and the change of residual stress during the fatigue test. The fatigue strength for finite life of the carburized steel is determined by the propagation property of the crack within 0.3 mm depth from the surface. The fatigue limit is determined by the crack arrest property of the crack 0.1-0.2 mm depth from the surface. The compressive residual stress increases during fatigue process. The absolute value of maximum compressive residual stress increased during fatigue process is large value such as about 50% of value of stress amplitude and depth from the surface of maximum compressive residual stress is almost same with non-propagation crack length observed of fatigue limit specimen. Therefore, it is considered that compressive residual stress is dominant factor for crack arrest property of the crack 0.1-0.2 mm depth from the surface. The larger the stress amplitude is, the larger the amount of increase of compressive stress during fatigue process is. The reason for increase of compressive stress during fatigue process is that stress/strain induced transformation from retained austenite to martensite causes volume extension.