Fatigue Properties and Dislocation Structures of Nitrided SS 400.

Abstract

Fatigue properties of the nitrided SS 400 are investigated on the basis of the observation of dislocation structures, surface topography and fracture surface by means of optical and electron microscopy. Both slip bands and dislocation structures during stress cycling are not formed in the compound layer. The dislocation morphology of the diffusion layer is composed of the untangled and uniformly distributed dislocations and the clusters of the dislocation dipoles at lower stress amplitude, but well defined cell structures at higher stress amplitude. It is also found that the fatigue cracks of the nitrided specimen are initiated without the formation of slip bands and the separation between the compound layer and the diffusion layer occurs during cyclic loading. On the basis of these observations, a mechanism of fatigue fracture for the nitrided SS 400 is proposed.