Bending Fatigue Crack Observation in Boronized Steels

Boronizing is one of the chemical methods for case hardening of steels. The hardness of the boronized layer exceeds 1400 Vickers hardness, and it has a high resistance, especially, against wear.In the present study, plane and rotating bending fatigue tests of boronized steels with various thicknesses of boronized layers were conducted. Effects of the thickness of boronized layer on crack initiation and the propagation mechanism were investigated by fatigue crack observation.The results are summarized as follows:(1) Many cracks propagating uprightly to the tensile axis and those propagating parallel to the tensile axis along which a boronized layer was spalled, were observed.(2) Fatigue cracks were observed also in the specimen which was not fractured in cyclic number 107. But all those cracks were arrested either in the boronized layer or a boundary between the boronized layer and the matrix. Fracture was controlled by whether the crack propagated through the boundary or not.(3) Fatigue strength of boronized specimens consisting of Fe2B uni-layer was better than that of boronized specimens consisting of FeB+Fe2B bi-layer. The maximum fatigue strength was obtained, when the Fe2B boronized area was cantrolled so as to be about 1.5% of the total area.