Microstructure-property Relationship in the Quenching and Partitioning (Q&P) Steel

A low-carbon low-alloy steel was subjected to two-step quenching and partitioning (Q&P) treatments which resulted in multiphase microstructures comprising varying amounts of primary martensite, retained austenite, fresh martensite and bainitic-ferrite. The Q&P microstructures were quantified using an alternative methodology that utilized well-established semi-empirical relations and the experimental amount of retained austenite and its carbon content estimated using X-ray diffraction. The dependence of the tensile properties of Q&P samples on their microstructural features was investigated with particular emphasis on bainitic-ferrite. The best combination of strength and elongation was attained corresponding to the quenching temperature which resulted in the maximum amount of retained austenite possessing higher stability. Attempts were also made to calculate the yield strength of the Q&P steel in terms of its microstructure by considering the contributions from various strengthening mechanisms in order to understand the experimental observations. Though the calculated yield strength of the Q&P samples was slightly overestimated, the overall variation of yield strength with QT was predicted correctly. Primary martensite showed the highest contribution to the YS of Q&P microstructure for the lower QTs. The increase in the YS for the higher QTs was attributed to an increase in the amount of bainite at the expense of primary martensite.