Factors determining the torsional fatigue strength in bainitic steels with banded microstructures

This study aimed to identify the microstructural factors governing the torsional fatigue strength of bainitic steels. Torsional fatigue tests were performed on two bainitic steels with banded microstructures comprised of soft and hard layers. The soft layers were coarse-grained with low Vickers hardness (HV), while the hard layers were fine-grained with high HV. Both materials possessed similar average HV values but differing band morphologies: a coarse band (CB) with HV = 329 and a fine band (FB) with HV = 314. Interestingly, the FB exhibited a 30 % higher fatigue strength than the CB. Through microscopic observations and finite element analysis, it was established that different fatigue strengths could be attributed to the particular width and array of the bands. The reticular band array in the FB steel raises crack initiation resistance due to the constraint of cyclic plastic deformation. In addition, the narrower spacing of hard layers can impede crack propagation when the extension mode transitions from shear mode to Mode I. In contrast, the columnar array and wider spacing of the bands in the CB steel are likely to provide weaker resistance to crack initiation and propagation, resulting in an inferior fatigue strength.