Effect of stress ratio on fatigue strength of micro pulse induction hardened steel with very thin case-hardened depth

Abstract

A new super rapid heating induction hardening technique has superior properties, such as very short heat treatment time, accurate case-hardened depth control and less deformation of hardened parts. Therefore, these properties have been useful for the surface improvement of machine parts. In this paper, in order to investigate the influence of stress ratio on the fatigue strength of the induction hardened JIS S45C steel with very thin effective case-hardened depth (ECD) of 0.5mm, cyclic uniaxial loading fatigue tests have been conducted at four different stress ratios (R = -1.0-0.1) by using the notched round-bar specimens. Furthermore, the fatigue strength was discussed by fracture mechanics. The results obtained are summarized as follows: (1) As stress ratio increases from R = -1.0 to 0, the stress range of the "fish-eye" type failure decreased. And, no "fish-eye" appeared under R = 0.1. (2) The center of the "fish-eye" was located in accordance with the all case-hardened depth. Either non-metallic inclusion or facet exists at the center of "fish-eye". (3) In the case of the fatigue crack initiation and propagation from the interior, the fatigue strength was predicted by using the threshold stress intensity factor ΔKth. (4) In the case of the fatigue crack from notched root surface, the attenuation of residual stress under cyclic stress was considered experimentally. Consequently, the fatigue strength was predicted by the same method.