Mechanisms of strength–plasticity enhancement and stress-induced phase transition in a medium-carbon low-alloy steel

A medium-carbon low-alloy steel with designed chemical composition was investigated. The steel exhibits an excellent product of strength and elongation value of 31,832 MPa% through quenching and partitioning treatment, with a tensile strength of 1413 MPa and elongation of 22%. X-ray diffraction analysis and transmission electron microscopy characterizations confirm that the retained austenite in the specimens undergoes stress-induced phase transformation to the martensite and hexagonal phases, namely the transformation-induced plasticity (TRIP) effect is triggered. This TRIP effect, triggered by the stress-induced phase transition of retained austenite, is responsible for the excellent mechanical properties obtained in the steel. For further investigating the stress-induced phase transition mechanism, thermodynamic methods are applied. Gibbs free energy of face-centered cubic-Fe, ε-Fe, ω-Fe and body-centered cubic-Fe associated with the stress-induced phase transition was calculated using molecular dynamics simulations, and a calculation method of strain energy in thermodynamic units for the stress-induced martensitic transformation is presented. The final results reveal the process and thermodynamic mechanism of stress-induced martensitic transformation in medium-carbon steels, in which the hexagonal phase can participate in the process as an intermediate product.