Microscopic and nanoscopic observations of metallurgical structures around inclusions at interior crack initiation site for a bearing steel in very high-cycle fatigue

In some high‐strength steels, a fatigue crack tends to occur at the interior inclusion after a long‐term sequence of the cyclic loadings at low stress levels, although the crack takes place at the surface in the usual life region at high stress levels. Thus, we have the duplex S–N curves consisting of the respective S–N curves for usual life region and very high‐cycle regime. It is well known that a significant fracture surface having the fine granular morphology is formed around the interior inclusion at the crack initiation site. This surface area is sometimes called as ‘fine granular area’. In this work, metallurgical structures around the interior inclusion at the fatigue crack initiation site were carefully observed by combining several special techniques such as focused ion beam technique and high‐resolution scanning electronic microscopes. Based on the current observation results, it was found that the microstructure around the interior inclusion was changed into the penny‐shape fine granular layer from the usual martensitic structure during long‐term cyclic loadings. Then, debondings along with the boundaries of the matrix and the fine granular layer have produced the small cracks inside the metallic material, and these interior cracks caused the final fatigue fracture after definite loading cycles of the crack propagation.