The influence of the austenite grain size on the microstructural development during quenching and partitioning processing of a low-carbon steel

Highlights: • Under fixed Q&P conditions, the PAGS influences the phase fractions and retained austenite stability of the final microstructure • Small-grained microstructures promote a more efficient carbon partitioning process than coarser microstructures • For the investigated partitioning conditions and PAGSs, there is a linear dependency between the primary and fresh martensite fractions • The PAGS renders an optimum fraction of primary martensite that minimises fresh martensite and stabilizes sufficient austenite -- Abstract: The influence of the prior austenite grain size (PAGS), varying between 6 and 185 μm, on the microstructural development of a low carbon steel during quenching and partitioning (Q&P) processing is investigated. The effect on the size and morphological aspects of the microconstituents is discussed based on the kinetics of carbon redistribution between martensite and austenite upon partitioning conditions of 400 °C and 50 s. Under fixed quenching and partitioning conditions, decreasing the PAGS leads to a more efficient carbon partitioning process through the smaller and more homogeneously distributed phases developed during the first quench. In contrast, the microstructural heterogeneity obtained with larger PAGSs makes it more difficult to control the degree of carbon enrichment in austenite during partitioning and thus the austenite stability. Additionally, large volumes of fresh martensite are more likely to form in the interior of large-scale austenite grains due to the incomplete carbon homogenisation process. To consider the PAGS in the design of Q&P microstructures the selection of an optimum fraction of primary martensite is proposed, which ensures the minimisation of fresh martensite in the final microstructure and the sufficient stabilisation of the austenite phase. This new methodology facilitates the applicability of the Q&P process providing a controlled and reproducible development of optimised Q&P microstructures.