Very high cycle fatigue behaviors of bainite/martensite multiphase steel treated by quenching-partitioning-tempering process

• The non-inclusion induced crack initiation is observed.
• Effect of retained austenite on VHCF property depends on its size and morphology.
• Local plastic deformation is observed nearby fatigue crack initiation site.
• VHCF property of B/M multiphase steel is enhanced by a novel Q-P-T process.

A bainite/martensite (B/M) multiphase steel was treated by a novel bainite-based Q-P-T treatment with various process parameters to obtain multiphase microstructure composed of bainite, martensite and different types of retained austenite (nanometer-sized film-like and micrometer-sized blocky retained austenite). We investigate here the effect of retained austenite on the very high cycle fatigue (VHCF) behavior, especially the fatigue crack initiation in VHCF regime. Results show that the non-inclusion induced crack initiation is the main fatigue failure mechanism in the B/M steel with blocky retained austenite. The blocky retained austenite transforms easily to martensite due to local plastic deformation under cyclic loading, which is prone to induce the formation of micro-cracks. In contrast, the film-like retained austenite is beneficial to the VHCF property of B/M steel due to the dislocation absorption and transformation induced plasticity effects. Therefore, the B/M steel with film-like retained austenite exhibits excellent VHCF property, namely, the fatigue limit in VHCF regime reaches 770 MPa while the tensile strength is 1410 MPa.