Abstract
Torsional fatigue strength of induction hardened steels has been affected by fracture origin. The purpose of this study is to clarify a relationship between fracture origin site in torsional fatigue test and hardness distribution of induction hardened steels.
In the test-pieces of a shallow case hardeninng depth, fracture tended to occur on internal origin. When case hardening depth increased and exceeded a certain level, the fracture origin changed from internal to surface. A new indicator, "projected core hardness", defined as a core hardness projected from internal fracture origin site to surface along stress distribution, was proposed. The site of fractue origin was dependent on the ratio of projected core hardness and case hardness. That ratio of 1 corresponded to the critical condition which fracture occurred on internal or surface origin.
In addition, the site of fractue origin was dependent on stress amplitude, too. The fracture tended to occur on surface origin with the test condition of high stress amplitude. This was because the compressive residual stress at surface decreased with increasing stress amplitude. Torsional fatigue mode maps, shown as a function of the ratio of projected core hardness, case hardness and stress amplitude, were newly proposed. These maps enable to predict the fracture origin site in torsional fatigue test.
