Effect of phase transformation sequence on the low cycle fatigue properties of bainitic/martensitic multiphase steels

The three microstructures of bainite-fresh martensite, pre-martensite-bainite and carbide-free bainite were obtained by modulating the phase transformation sequence by the heat treatment process in the medium-carbon multiphase steel. The effects of martensitic and bainitic phase transformation sequences on the low cycle deformation behavior of multiphase steels were investigated. The results showed that the sample of the fresh martensite transformation process (295 degrees C-15 min) exhibited the best fatigue life among all processes under strain control. The samples of the preformed martensite transformation process (250 degrees C-295 degrees C) and the carbidefree bainite transformation process (295 degrees C-120 min) exhibited similar fatigue life. The sample of 295 degrees C-15 min possesses good resistance to plastic destabilization, resulting in higher fatigue life. The results of the EBSD crack propagation path analysis demonstrate that high carbon M/A islands block crack propagation in the sample of 295 degrees C-15 min, and high angular grain boundaries skew the crack propagation path. Fracture scans observed that secondary cracks in the sample of 250 degrees C-295 degrees C extend inward with aggregation of voids, which are more likely to form crack sources.