Microstructure and bending fatigue behavior of martensite steel with 0.4%C subjected to different heat treatments

Abstract

The purpose of the present work was to analyze the microstructure evolution of martensite steel with 0.4C (in mass%) under different heat treatments which were marked as OQ900 and OQ1000, and its effect on fatigue fracture behavior. A crystal plasticity finite element method (CPFEM) was developed to analyze the role of multi-scale structures of lath martensite on fatigue crack initiation, and the fatigue lives of the materials with varying substructures are quantitatively compared. The numerical simulation was verified by fatigue tests where special attention was paid on the peculiarity of crack initiation exposed to the microstructural effect. The fracture surfaces of the steels subjected to both high-cycle fatigue (HCF) and low-cycle fatigue (LCF) regimes were also analyzed. The results indicated that OQ900 sample hold a smaller size in equivalent prior austenite grain, packet and block length than OQ1000 sample, which finally led to a slightly higher bending strength for OQ900 sample. Both inclusions and slipping fractures were found as the origins of fatigue crack initiation in the martensite steels with 0.4C, and especially the latter played a dominant role in HCF regime. In this regard, the difference in fatigue life tended clear as a clue to the effect of microstructure on fatigue behavior in HCF regime. The block morphology and its orientation affected the strain localization in an obvious way, where lower fracture indicator parameters (FIPs) value and longer crack initiation life were obtained in OQ900 sample. The EBSD analysis of fatigue-tested samples showed that the block boundaries were the most preferred sites for crack initiation, which were characterized as persistent slip bands. The crack propagation path was distributed in a deflected manner owing to the inhibition effect of boundaries. Overall, OQ900 sample presents a higher fatigue resistance than OQ1000 in HCF regime when the initiation and early propagation life of fatigue cracks were considered.