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.