2012 年 2012 巻 57 号 p. 1-8
The purpose of this study is to clarify the effect of increase in the grain boundary strength on the torsional fatigue strength of induction hardened steels. The effect of lowering phosphorus content and addition of boron, molybdenum or silicon , as metallurgical factors of increasing the grain boundary strength, on torsional fatigue strength was investigated. Torsional fatigue fracture life in the test condition of high cycles fatigue is prolonged by lowering phosphorus content, or by adding boron, molybdenum or silicon, respectively. In these fatigue test, the crack propagation life against total fracture life is prolonged particularly. The fraction of intergranular fracture in the propagation region of mode I fracture is decreased by lowering phosphorus content or addition of boron etc. This is caused by increase in the grain boundary strength due to lowering phosphorus content or addition of boron etc. Consequently, improvement of torsional fatigue fracture life by lowering phosphorus content or addition of boron etc. is because that static final fracture in the process of fatigue crack propagation is disturbed due to improvement of fatigue fracture toughness caused by increase in the grain boundary strength.