Into the quenching & partitioning of a 0.2C steel: An in-situ synchrotron study

Abstract

Quenching and partitioning (Q&P) is an effective way for retaining austenite at room temperature. The actual mechanisms responsible for austenite stabilization are still under debate, since it is impossible to track carbon diffusion with the conventional metallographic tools. The present work depicts several Q&P heat treatments performed on a 0.2 C commercial grade steel by in-situ High Energy X-Ray Diffraction (HEXRD). More specifically, the effect of three different initial quenching temperatures on the microstructural evolution occurring during Q&P was scrutinized in details.It was shown that about 50% of the initial carbon partitions effectively to austenite. A carbon enrichment up to 0.8 wt% is sufficient to retain austenite at room temperature as grain size refinement contributes to further stabilize austenite. The origin of carbon enrichment in retained austenite depends on the initial quench temperature (QT). For low QT, corresponding to an initial martensite fraction larger than 0.75, austenite carbon enrichment is ensured by carbon partitioning from supersaturated martensite. For higher QT, austenite carbon enrichment results from both carbon partitioning from martensite and carbon rejected during the bainite transformation. The former mechanism proceeds rapidly and already starts during the reheating stage to the partitioning temperature. The latter is slower as the carbon enrichment is coupled to bainite formation.