Speaker
Description
Retained austenite provides TRIP (Transformation-Induced Plasticity) steels with their exceptional combination of strength and formability. To retain a sufficient volume fraction of austenite, alloying with silicon (Si) or aluminum (Al) is used to suppress carbide formation during the bainite transformation. This suppression allows the austenite to enrich with carbon, stabilizing it at room temperature.
While Si and Al effectively inhibit carbide precipitation for standard transformation durations (e.g., several minutes), prolonged holding at the isothermal bainite transformation temperature eventually triggers carbide formation within the austenitic regions. These precipitates deplete the carbon concentration in the austenite, destabilizing it and promoting martensite formation upon final quenching. Understanding this decomposition process is critical for optimizing the industrial production of TRIP steels, particularly for large-scale coils where slow cooling rates can result in holding times of several hours, ultimately reducing the TRIP effect.
To investigate this behavior, a steel containing 0.4 wt% C, 1.5 wt% Mn, and 1.5 wt% Si was austenitized at 950 °C and quenched at 50 K/s to bainite transformation temperatures ranging from 350 to 450 °C. Samples were held isothermally for up to 8 hours. To track the microstructural evolution in real-time, in-situ X-ray diffraction (XRD) was performed at DESY (Deutsches Elektronen-Synchrotron). The experimental results are discussed and compared with MatCalc thermodynamic simulations to characterize the kinetics of carbide formation.