Mechanical behavior and strengthening mechanism of a fine-grained medium carbon steel produced via cyclic oil quenching

Medium-carbon steels are widely used in structural applications. However, the strength of such steels can hardly be over 1100 MPa. In this work, a cyclic oil quenching process was applied to a common medium-carbon steel, i.e., #45 steel, to produce fine-grains, which exhibited a superior tensile strength of over 1690 MPa with moderate ductility. The microstructural results revealed that the refined grains and imbedded dislocations play a critical role in the strength improvement. In addition, the material exhibited an increased strength and ductility under dynamic tension due to the strain rate effect, while however the strain hardening capability decreases with strain rates. For the underlying reasons, the widely reported thermal softening effect and deformation-induced martensitic transformation are ruled out here by microstructure observation. This work brings up a new process to produce high performance steels.