Microstructural evolution and stability during strain-controlled fatigue in a multiphase microalloyed steel

Abstract

Multiphase ferrite-bainite-martensite (F-B-M) microstructure processed through rolling and two-step cooling in a V-microalloyed steel exhibited excellent stability under strain-controlled fatigue. The steel exhibited a steady state response and even cyclic hardening up to a total strain amplitude of 0.6% and displayed a cyclic softening response at total strain amplitudes >0.7%. To understand and determine the degradation mechanisms, a detailed transmission electron microscopic investigation was carried out in fatigue tested samples as a function of total strain amplitude. At low strain amplitudes (<0.4%), microstructural constituents such as the bainite/martensite colonies, and the inter-lath films of retained austenite remained stable. The deformation occurred only in the polygonal ferrite and the lamellar retained austenite, which was sheared into fine, smaller strips. Only some boundaries of bainite/martensite colonies were subjected to shearing. At an intermediate strain amplitude (0.6%), all the coarse bainite/martensite colonies exhibited shear deformation with very fine retained austenite segments distributed over large regions. The bainite/martensite lath interiors were filled with dislocation substructures and sheared inter-lath austenite layers. Austenite grain rotation was noticed, and the grain interior exhibited sheared or segmented layers. Selective inter-lath austenite layers transformed into acicular martensite. At higher amplitudes (0.8%), the F-B-M microstructure displayed homogeneous deformation and cyclic softening. Carbides and lath debris were distributed throughout the matrix and consisted of decomposed bainite and martensite. Formation of acicular and twinned martensite was present due to the TRIP effect. The F-B-M microstructure comprising small colonies of bainite/martensite with fine lath morphology, polygonal ferrite (∼20%), retained austenite with lamellar morphology and inter-lath films (∼5%) along with a fine dispersion of carbonitride precipitates in the matrix appears to be highly resistant to fatigue damage. Hot rolling processes aimed at promoting these microstructural features in the steel, which is also accompanied by texture strengthening, seem to be effective against cyclic softening in fatigue.