1983 Volume 32 Issue 355 Pages 350-354
The author previously reported the fatigue crack initiation process in a low carbon steel, and clarified that the fatigue cracks originated in ferrite grains by an intrusion mechanism.
In this paper, the mechanism of fatigue crack initiation in a high strength steel was investigated using a Ni-Cr-Mo steel (SNCM 439) consisted of sorbitic structure, that is, mixed structure of farrite and granular carbide. The observations of fatigue crack initiation process were made on the surface, the longitudinal section and the fracture surface of specimens using optical and scanning electron microscopes. Then, crystallographic investigations were made on the fracture surface by the use of etch pits.
Based on the experimental results, the conclusions are summarized as follow:
(1) The feature of Stage I cracking was found in the fatigue crack initiation for all the specimens observed, and the initiation process could be explained by an intrusion mechanism.
(2) The fatigue crack originated at a part of ferritic matrix in which granular carbides hardly precipitated, and no influence of inclusion was recognized on the initiation process.
(3) Therefore, it was assumed that mechanism of a fatigue crack initiation in high strength steels with ferritic structure was essentially the same as that of a low carbon steel.