Retzl, P., Kozeschnik, E., & Ott, B. R. (2025). Decomposition of carbide-free bainite during prolonged holding at bainite transformation temperatures. In FEMS EUROMAT 2025 Book of abstracts (pp. 513–513).
TRIP-assisted steels rely on retained austenite in their microstructure, which enables the so called TRIP-
effect. Alloying with sufficient amounts of Si or Al is employed to suppress carbide formation during
the bainite transformation to allow for enrichment of austenite with carbon during bainite formation
and thereby stabilizing it down to room temperature. While Si or Al can suppress carbide formation in
carbide-free bainite for durations typically employed for bainite transformation (e.g., several minutes),
prolonged holding at the transformation temperature eventually leads to carbide formation in the
austenite areas. These precipitates reduce the carbon concentration in austenite, destabilizing it and
promoting martensite formation upon quenching. To understand this decomposition process is crucial
to optimize large-scale production of TRIP steels, where steel often cools down in large coils and
therefore the time to reach room temperature can be up to several hours, leading to a reduced TRIP-
effect.
To investigate this behaviour, samples of a steel containing 0.4 wt% C, 1.5 wt% Mn, and 1.5 wt% Si
were austenitized at 950 °C and then quenched with 50 K/s to bainite transformation temperatures
ranging from 450 to 350 °C. The samples where held at these temperatures for up to 10 hours. The
heat-treatment was done on a Dilatometer Baehr Type 805. A combination of XRD and EBSD
investigations was used to examine the decomposition process.
