Abstract
In-situ Neutron Diffraction and Small-Angle Neutron Scattering (SANS) are employed for the first time simultaneously in order to reveal the interaction between the austenite to ferrite phase transformation and the precipitation kinetics during isothermal annealing at 650 and at 700 °C in three steels with different vanadium (V) and carbon (C) concentrations. Austenite-to-ferrite phase transformation is observed in all three steels at both temperatures. The phase transformation is completed during a 10 h annealing treatment in all cases. The phase transformation is faster at 650 than at 700 °C for all alloys. Additions of vanadium and carbon to the steel composition cause a retardation of the phase transformation. The effect of each element is explained through its contribution to the Gibbs free energy dissipation. The austenite-to-ferrite phase transformation is found to initiate the vanadium carbide precipitation. Larger and fewer precipitates are detected at 700 than at 650 °C in all three steels, and a larger number density of precipitates is detected in the steel with higher concentrations of vanadium and carbon. After 10 h of annealing, the precipitated phase does not reach the equilibrium fraction as calculated by ThermoCalc. The external magnetic field applied during the experiments, necessary for the SANS measurements, causes a delay in the onset and time evolution of the austenite-to-ferrite phase transformation and consequently on the precipitation kinetics.
Highlights
Micro-alloyed steels containing nanometer-sized precipitates in a ferrite matrix have recently attracted a lot of interest due to their high performance when being used in lightweight automotive parts [1,2,3,4]
Austenite to ferrite phase transformation is observed in all steels during annealing at 650 and 700 °C, and the transformation kinetics depends on the temperature and on the alloy composition
Faster kinetics at 650 than at 700 °C is measured in all alloys due to the larger driving force for phase transformation at lower temperatures
Summary
Micro-alloyed steels containing nanometer-sized precipitates in a ferrite matrix have recently attracted a lot of interest due to their high performance when being used in lightweight automotive parts [1,2,3,4]. Interphase precipitation during the austenite to ferrite phase transformation has been studied by ex-situ SANS at room temperature in previously heat-treated vanadium micro-alloyed steels, while TEM and APT have been used to reveal the precipitate shape, size and chemical composition, supporting the SANS data analysis [10,16,17]. In these ex-situ SANS experiments, the precipitate evolution during the austenite to ferrite phase transformation is measured using a series of samples that have been annealed for different holding times followed by rapid cooling to room temperature. The precipitation kinetics is quantified and the the coupling between phase transformation and precipitation kinetics is explained
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