Effects of Notch, Carbon Content and Boron Addition on Torsional Fatigue Property of Induction Hardened Middle Carbon Steels

The purpose of this study is to clarify effects of notch, carbon content and boron addition on torsional fatigue property of induction hardened middle carbon steels. The torsional fatigue strength of smooth specimen increased with increasing carbon content. On the other hand, there is no effect of carbon content for notched specimen. In the torsional fatigue test of notched specimen of steel with high carbon content, a static final fracture is caused immediately in mode I associated with intergranular fracture after the crack occurs from the notch. This is because fatigue fracture toughness of higher carbon steel is not so high. The torsional fatigue strength of notched specimen of higher carbon steel is increased by the addition of boron. The fraction of intergranular fracture in the crack growth region of mode I fracture is decreased by the addition of boron. This is caused by the increase in grain boundary strength due to the addition of boron. Improvement of torsional fatigue strength of notched specimen of higher carbon steel by the addition of boron is attributed to the delayed static final fracture in the process of fatigue crack growth due to the improvement of fatigue fracture toughness caused by the increase in grain boundary strength.