Papers by Author: S.Q. Yuan

Abstract: After bainitic transformation, the dislocations formed in deformed austenite remained to be pinned by the precipitates so that thermostability of the bainitic ferrite was improved. Coarsening of the precipitates accompanied by their distribution density change occurred during reheating. After long reheating, further precipitates nucleated in bainite. Dislocations inside laths getting rid of pinning of precipitates and their polygonization play the precursor to the evolution of microstructures, in which lath boundaries disappeared gradually.

Abstract: Cooled in water after isothermal relaxation of deformed austenite for different time, a Nb-bearing microalloyed steel always exhibited synthetic microstructures of bainitic ferrite, granular bainite and acicular ferrite. When these samples were reheated to and held at 650°C or 700 °C, the non-equilibrious microstructures tended to evolve into equilibrious ones, accompanied by obvious change of hardness. The rate of microstructures evolution was closely related to relaxation time of deformed austenite. The sample relaxed for 60s displayed the highest thermal stability, while microstructure evolution was quickest in the sample relaxed for 1000s even though it was softest before reheating. By hardness measurement, it was found that softening was not only process occurring during reheating, in which hardness fluctuated with time. There were two peaks in hardness-time curve of each sample having undergone relaxation, while single peak occurred in the curve of the sample not being relaxed. These results indicate that thermal stability of microstructures is determined by their history of formation.