High-throughput investigation of processing–microstructure relation in quenching and partitioning steels via gradient heat treatment and rapid characterization

Abstract

This study presents a new high-throughput method to investigate the relationship between the quenching temperature (QT) and microstructure in quenching and partitioning (Q&P) steels produced by “one-step” Q&P treatment. This approach involves a gradient heat treatment, in which a rod sample is quenched and then a significant temperature gradient is established along the rod for isothermal holding, allowing the exploration of QT from 457 to 280°C in a single heat treatment. Synchrotron X-ray diffraction and high-speed nanoindentation mapping were used to efficiently measure phase fractions and the carbon content in retained austenite (RA) across different QTs. The results show that as QT decreases, a larger fraction of austenite transforms into martensite and bainite during isothermal holding, leading to increased carbon enrichment in the untransformed austenite. The volume fraction of RA initially increases with decreasing QT due to carbon enrichment, then decreases as the untransformed austenite fraction reduces after isothermal holding. The experimental results are compared to the predictions by thermodynamic models, which tend to overestimate the kinetics of phase transformation and carbon partitioning, emphasizing the importance of high-throughput experimental validation.