Effect of hardness distribution on crack initiation life and fracture life of induction hardened steels for automotive shafts

摘要

The purpose of this study is to clarify an effect of hardness distribution on the torsional fatigue strength of induction hardened steels. The factors of hardness controlling crack initiation life and breakage life were investigated against the each fracture mode of internal and surface fracture origin. Torsional fatigue strength of induction hardened steels was dependent on each of the hardness and depth of the hardened layer and the hardness of core zone on the internal and surface fracture origin, respectively. In addition torsional fatigue strength increased with increasing these factors of hardness distribution. The crack initiation life of the test-pieces fractured with surface origin could be expressed by a new indicator, "equivalent hardness", defined as a mean hardness weighted with the radius squared. The breakage life of the test-pieces fractured with surface origin in low cycles fatigue test could be expressed by the equivalent hardness, too. This was because the crack initiation life was dominant to the breakage life in low cycles fatigue test. The crack initiation life of the test-pieces fractured with internal origin could be expressed by a new indicator, "projected core hardness", defined as a core hardness projected from internal fracture origin site to surface along stress distribution. The breakage life of the test-pieces fractured with internal origin could be expressed roughly by the projected core hardness, too.