Superior improvement of tensile properties in a low alloy medium silicon DIN 1.5025 steel under single step quenching and partitioning heat treatment compared to fully martensitic conditions

Advanced High Strength Steels (AHSS) are the result of a great effort by researchers to design and develop high strength steels with good formability, which can be replaced by the conventional quenched and tempered steels. In fact, this group of steels is hardened by controlling the phase transformation of austenite to different phases, which is quite different from conventional quenching and tempering steels with fully martensitic microstructure. In the present study, the tensile behavior of low alloy medium silicon DIN 1.5025 steel has been investigated under one step quenching and partitioning heat treatment (Q&P) at quenching temperature 250oC (below the martensitic start temperature: 275oC) for different partitioning times ranged from 5s to 1h compared to direct quenching condition. Microstructural studies, as well as changes in mechanical properties, were carried out by using laser and scanning electron microscopes, along with X-ray diffraction pattern and tensile and hardness tests, respectively. Microstructural studies indicate the formation of microcomposite microstructures containing carbide free bainite-martensite-retained austenite during Q&P heat treatment compared to fully martensitic microstructures. The results indicated that the retained austenite volume fraction increasing up to a maximum value of 17.8% after partitioning for 600s resulted in a superior improvement in the tensile behavior of the microcompositic samples. Superior improvement of mechanical behavior with respect to the partitioning time of microcompositic specimens indicates the beneficial effect of retained austenite phase on tensile properties according to the transformation induced plasticity effect (TRIP) mediated by the transformation of retained austenite to martensite during the tensile test.