First Year Assessment of Pablo García entitle “The fatigue behavior of advanced high-strength steels processed via quenching and partitioning” – In the Seminar Room at 12:00 pm

The increasingly demanding environmental and safety goals imposed to the automotive industry have fostered fierce competition to develop new multi-phase steel types that are stronger, tougher, more formable, and fatigue-resistant. Among the various approaches in study, steels processed via quenching and partitioning (Q&P) are one of the most promising. In particular, the Q&P annealing route succeeds in raising the retained austenite (RA) content of martensitic microstructures. Even so, whereas the behavior of Q&P steels under monotonic loading is well-established, it is yet to be studied for the case of cyclic loading. Fatigue performance is crucial for automotive steels, since the thickness reduction associated to car lightweighting makes components more sensitive to fatigue. In this context, the present project aims to explore the relationships between microstructure and fatigue in Q&P steels, with the purpose of deriving microstructural design guidelines for improved fatigue performance in this material. Special attention will be dedicated to the role of RA under all homogeneous cyclic loading, fatigue crack initiation and propagation. Whereas factors affecting RA stability in multi-phase steels under monotonic loading are well-established, no studies have been conducted for the case of cyclic loading. Preliminary results presented here show that partitioning time decreases the stability of RA in Q&P steels under cyclic load. Further ex-situ fatigue tests are expected to give insight into the relative importance of stability factors, as well as into the unclear effect of RA in retarding fatigue crack initiation and propagation in multi-phase steels. In addition, results here cast doubt on the currently accepted view that the marked anisotropy of martensitic steels lies in MnS stringers, and demonstrate the detrimental effect of corrosion pits generated by component fabrication on shortening the fatigue life of AHSSs. As for the latter, a method has been derived aiming to predict the conditions whereby corrosion pitting may affect fatigue life.