SAE5120 is a mobile gear component material in the automobile industry of USA, and its service cycles are beyond 10^9.A piezoelectric gigacycle fatigue machine is employed to do fatigue tests in VHCF regime in the condition of 20 kHz frequency, stress ratio R=0.1, and room temperature. The effects of heat and carburizing and low pressure carburizing treatment to VHCF fatigue strength were investigated. Moreover, the scanning electron microscopy(SEM) observation of fracture surface was analyzed to investigate the mechanism of fracture related to surface treatments, microstructure and inclusion. The long crack propagation behaviour was described by the Paris equation and parameters of the model were evaluated by the crack striation spacing of the high magnification SEM photos.
