Interaction of martensite and bainite transformations and its dependence on quenching temperature in intercritical quenching and partitioning steels

Highlights: • All of the constituent phase fractions showed multiple-stage variations by controlling quenching temperature. • The attributions of martensite and bainite to austenite retention were significantly related to quenching temperature. • An identical partitioning end between T₀ line and paraequilibrium condition was achieved in all heat treatments. • The carbon gradient in parent austenite influenced subsequent martensite transformation and corresponding structure. -- Abstract: A series of quenching temperatures (QT) were applied to investigate the interaction of bainite and martensite transformations in intercritical quenching and partitioning steels with multiple-stage martensite transformation. It reveals that the fractions of initial martensite, bainite, whole retained austenite and austenite respectively retained by bainite or martensite showed obvious three-stage variations, coincided with first stage, stagnant stage and second stage of martensite transformation. In the case of high QT that near Ms., the retained austenite was mainly ascribed to bainite carbon enrichment. As quenched to relatively low QT in stage-1 region or lower, the austenite retained by martensite carbon partitioning occupied the majority and the decrease of RA was mainly ascribed to inter-lath austenite and packet boundary austenite. In addition, an identical partitioning end was achieved with mean austenite carbon contents between T₀ line and paraequilibrium condition, irrespective of initial martensite and bainite fractions. The heterogeneous carbon distribution in parent austenite deriving from ferrite formation significantly influenced the subsequent martensite transformation and corresponding martensite structure. The martensite block in high carbon region possessed much smaller size, while the corresponding packet size was still larger. Moreover, the significant martensite variant absence and variant combination were also observed.