Advanced bainitic steels containing retained austenite: Microstructural effects across fatigue stages

In this work, fatigue crack initiation and growth studies have been conducted on two austenite-bearing advanced bainitic microstructures. The approach adopted has implied the consideration of the different fatigue stages: nucleation and short-crack growth, and long-crack growth, including near-threshold conditions. Fatigue flat specimens containing on-purpose designed artificial defects, generated by means of focused ion beam, have been used. The results show that, under plane-stress conditions, the microstructure affects fatigue in different ways depending on the ruling stage: On one hand, shear stresses are key in crack nucleation and short-crack growth. Thus, the activation of dislocation slip in coplanar systems of austenite and bainitic ferrite triggers the nucleation of critical cracks. It suggests that a refinement of bainite blocks and packets is beneficial in terms of fatigue strength. The presence or appearance of weak boundaries, such as prior austenite grain boundaries and interphases between retained austenite and mechanically-induced martensite, also favors crack nucleation. On the other hand, the microstructural scale seems to be directly correlated to the plastic zone size from long cracks at the non-propagation/propagation threshold conditions. In that case, coarser microstructural features contribute to increase the threshold value of the stress intensity factor, compensating the associated hardness reduction.