Influence of Transformation Temperature on the High-Cycle Fatigue Performance of Carbide-Bearing and Carbide-Free Bainite

This study investigates the high-cycle-fatigue (HCF) behavior of carbide-bearing bainite (CBB) and carbide-free bainite (CFB) fabricated at different transformation temperatures. The fatigue limit of each material is determined via staircase method using a 1 kHz resonant testing machine. A new load increase test is proposed as an efficient alternative to estimate the fatigue limit in HCF regimes. The assessment of the fatigue behavior is accompanied by data-driven microstructural analyses via state-of-the-art computer vision tools. The analyses reveal that the finer carbide distribution, which is obtained at lower transformation temperature, enhances the overall performance of CBB. Electron backscatter diffraction (EBSD) measurements of CFB before and after tensile testing evidence the transformation of retained austenite (RA) to martensite during deformation. The finer film-like and stable RAs, which are promoted via reduction in transformation temperature, enhance the HCF properties by absorbing the energy required for fatigue crack propagation through improved transformation-induced plasticity. However, blocky unstable RA and/or martensite-austenite (MA) islands at prior austenite grain boundaries deteriorate the HCF properties of high-temperature CFB. Furthermore, unindexed regions in EBSD maps are effectively used to differentiate the MA islands of CFB, as validated by scanning electron microscopy (SEM) images and deep learning-based MA island segmentation.