Abstract
Fully reversed torsional fatigue tests were conducted to elucidate the behaviour of shear-mode crack initiation and propagation in one martensitic and two bainitic steels. The relationship between the crack initiation site and microstructure was investigated by means of an electron backscatter diffraction (EBSD) technique. From the S-N diagram, two notable results were obtained: (i) the shear-mode crack was initiated on the prior austenitic grain boundary in martensitic steel, while in bainitic steels, the crack was initiated along the {110} plane; one of the slip planes of bcc metals, and (ii) the torsional fatigue limit of lower bainitic steel with finer grains was 60 MPa higher than that of upper bainitic steel with coarser grains even though the hardnesses were nearly equivalent. The mechanism determining the torsional fatigue strength in these steels is discussed from the viewpoint of microstructure morphology.
Highlights
Rolling contact fatigue (RCF) is one of the critical failure modes in the components with Hertzian contact surfaces such as ball bearings, back-up rolls and rail and wheel systems in trains
The improvement of cleanness does not result in the improvement of the strength in case that nonmetallic inclusions no longer affect the crack initiation in RCF process [3, 4]
In the case of 100 μm hole, no crack initiated from the hole, whereas in the case of 200 μm, openingmode cracks propagated from the hole in the directions oblique to the specimen axis, demonstrating that a defect smaller than 100 μm in size does not degrade the fatigue strength of BR
Summary
Rolling contact fatigue (RCF) is one of the critical failure modes in the components with Hertzian contact surfaces such as ball bearings, back-up rolls and rail and wheel systems in trains. The improvement of cleanness does not result in the improvement of the strength in case that nonmetallic inclusions no longer affect the crack initiation in RCF process [3, 4]. To improve the RCF strength effectively, two factors should be considered: the threshold size of detrimental inclusion for each microstructure, and an alternative fracture origin when cleanness is below the threshold. Number of studies on shear-mode (modes II and III) crack is limited, though shear-mode crack growth plays a significant role in the RCF process [5,6,7,8] Such a transition of the crack initiation site occurs in the shear-mode cracking; the phenomenon is still unclear. The first purpose of this study is to elucidate the transition behaviour of the initiation site in the case of shear-mode cracking
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