From heterogeneous to homogeneous nucleation for precipitation in austenite of microalloyed steels

This paper studies the influence of strain on precipitate nucleation in austenite for three microalloyed steels with different microalloying element (Nb, V) contents. Precipitation start–time–temperature diagrams were determined by means of hot torsion tests and nucleation periods were measured at strains of 0.20 and 0.35. The increase in the dislocation density caused by the strain was calculated for both strains, and the driving forces for precipitation were also calculated. The results show that the influence of the strain on the nucleation time (t0.05) is dependent not only on the strain magnitude but also on the driving force for precipitation. When the driving force is high, or low in absolute terms, the influence of the strain, i.e. the increase in the dislocation density, gives rise to a notable reduction in the t0.05 value due to heterogeneous nucleation on the dislocation nodes. In contrast, when the driving force is low, or high in absolute terms, the influence of the strain on t0.05 decreases considerably and the nucleation of precipitates becomes preponderantly homogeneous. Therefore, the driving force value is responsible for the transition from heterogeneous nucleation to homogenous nucleation.