Improving fatigue resistance of ultrafine bainitic steel by exploiting segregation-induced bands

Abstract

An ultrafine bainitic steel was designed with segregation-induced martensitic/austenitic (MA) bands. The steel comprises almost 21 % retained austenite (RA); the rest is mainly bainitic ferrite and a small fraction of martensite. The steel exhibits room temperature (RT) yield strength of ∼914 MPa, ultimate tensile strength of ∼1969 MPa, and total elongation to failure of ∼11.5 %. Low cycle fatigue (LCF) behavior was investigated under a fully reversed strain-controlled waveform at RT, with a loading-axis parallel to the MA bands. LCF stress response reveals initial cyclic hardening followed by softening or saturation, depending on the applied strain amplitude. This is attributed to the interplay of strengthening mechanisms, which include RA transforming to martensite and dislocation multiplication/interactions and the softening effects of dislocation annihilation and crack formation/propagation. Damage studies reveal a mixed fracture mode, displaying brittle striations, secondary cracks, quasi-cleavage facets, and dimples. The secondary cracks mainly appear to originate from the main crack and propagate predominantly along the matrix-bands interphase regions or within MA bands. Additionally, surface-initiated cracks along their propagation path were deflected and branched around the interphase regions. This tortuosity reduces the crack propagation rate. Overall, the significant work hardening capability of the steel, along with its banded microstructure, contributes to its superior fatigue resistance compared to non-banded bainitic steels reported in the literature.