Temperature dependent partitioning mechanisms and its associated microstructural evolution in a CMnSiAl quenching and partitioning (Q&P) steel

The effect of temperature (350 ◦C < M < 450 ◦C) on the partitioning mechanisms and the final microstructureevolution in a CMnSiAl quenching and partitioning (Q&P) steel was investigated. The microstructure of boththe Q&P specimens, comprised of distorted BCC or pseudo tetragonal martensite structure with two differentcharacteristics namely (i) tempered or carbon depleted martensite that formed during initial quenching (M< 240 ◦C < M ) and partitioning step and (ii) carbon enriched fresh martensite that formed after partitioningstep and final quenching (RT) together with blocky and inter-lath films of retained austenite. In addition,packets of M/A constituents were observed in Q&P-350-1min specimen and some traces of carbide and platemartensite were observed in Q&P-450-1min specimen. The increase in partitioning temperature led to nearly2% increase in the amount of retained austenite (both blocky and inter-lath) with increased carbon content of0.27 wt.%. Along with carbon partitioning, slight interface mobility/isothermal martensite formation was alsoobserved in the case of specimen partitioned at 350 ◦C, whereas tempering effect was predominantly seen inthe case of specimen partitioned at 450 ◦C. Irrespective of the partitioning temperature, the amount of carbonrequired to stabilize the retained austenite at RT was found to be about 1.15 wt.% and was confirmed throughAPT analysis