Fatigue properties of carburized alloy steel in very high cycle regime under torsional loading

•An ultrasonic torsional fatigue testing machine was developed.•A very high cycle fatigue diagram of carburized alloy steel was obtained.•Carburizing is effective for enhancing very high cycle fatigue life.•Crack deflection was assessed based on fracture mechanics.

A very high cycle fatigue diagram and crack propagation behavior of carburized SCM420H are obtained using an ultrasonic torsional fatigue testing machine and are compared with those of non-carburized SCM420H. Fatigue crack initiation and growth are observed in the very high cycle regime for both carburized and non-carburized SCM420H. Results show that carburizing of SCM420H is effective for fatigue life enhancement in the very high cycle regime. Furthermore, crack deflection from mode II to mode I occurs under torsional fatigue. This behavior is discussed based on fracture mechanics.