High cycle fatigue performance, crack growth and failure mechanisms of an ultrafine-grained Nb plus Ti stabilized, low-C microalloyed steel processed by multiphase controlled rolling and forging

An effort has been made to examine the high cycle fatigue (HCF) properties including crack propagation characteristics and related fracture mechanisms of submicron-grained (SG) Nb + Ti stabilized low C steel processed through advanced multiphase-controlled rolling (MCR) and multiaxial forging (MAF). The HCF and other mechanical properties have been correlated with microstructural features characterized by light optical (LOM), transmission electron (TEM) and scanning electron microscopy (SEM), aided with electron backscatter diffraction (EBSD). TEM analysis near the fracture zones of the fatigue tested samples and corresponding fractographic analysis corroborated well in explaining the improved fatigue life of the SG steel. The fatigue strength was found to have a linear relationship with the tensile strength in both types of processed samples. The fatigue strength of the forged specimens was estimated to be nearly twice than that of the untreated annealed steel, demonstrating significantly different fracture characteristics. Intergranular fracture is found to be dominant in the rolled/forged specimens, in comparison to the transgranular fracture observed in the as-received steel. Such variances in fatigue strength and fracture characteristics have been endorsed to their microstructural constituents. Superior combinations of yield strength (YS), tensile strength (UTS), elongation (% El.) and high cycle fatigue strength (sigma(f)) (YS = 1027 MPa, %El. = 8.3%, sigma(f) = 355 MPa) were obtained in multiphase-controlled 15-cycle multiaxially forged (MAFed) specimens (processed in intercritical alpha+gamma phase regime). An enhancement of the fatigue strength can be ascribed to the formation of evenly dispersed nano-sized fragmented cementite (Fe3C) particles (similar to 35 nm size) present in the SG ferritic matrix (average similar to 280 nm size). The fine dislocation substructures/cells together with the nano-sized Fe3C particles could efficiently block the initiation and propagation of cracks thereby enhancing the fatigue endurance limit of the steel. Superior mechanical properties together with high fatigue resistance in the SG material render the present steels highly beneficial for high-strength structural applications.

Automated summary

This study investigated the high cycle fatigue properties and microstructural characteristics of submicron-grained Nb + Ti stabilized low C steel processed through advanced multiphase-controlled rolling and multiaxial forging. The research found that the forged specimens exhibited significantly improved fatigue strength compared to untreated annealed steel, attributed to the formation of nano-sized fragmented cementite particles in the ferritic matrix. Superior mechanical properties and high fatigue resistance make the SG material ideal for high-strength structural applications.