Incomplete bainite transformation in Fe-Si-C alloys

Bainite isothermal transformation kinetics for Fe-(1.5% and 3%)Si-0.4%C alloys (mass%) was investigated at 400-500 °C and incomplete transformation phenomenon (ICT) of bainite transformation was observed at 450 °C for the 3Si alloy and at 400 °C for the two alloys. Unlike to the ordinary ICT reported in other alloy systems, cementite precipitation with Si partitioning took place from the beginning of ICT. Carbon enrichment in austenite during ICT was measured by three-dimensional atom probe and was found to be higher than T0 or T0′ prediction while significantly deviates from NPLE limits and PE predictions to lower carbon content. Theories for bainite transformation, such as T0 limit, solute drag and WBs limit, were examined based on the experimentally measured carbon content in austenite during ICT. T0′ limit theory is difficult to rationalize the much larger measured carbon content than T0′ prediction. In addition to solute drag effect and spike development in the NPLE mode, solute drag theory should incorporate dissipations caused by other sources in order to account for the estimated 1250-1700 J/mol deviation from PE predictions. In addition, WBs limit theory gives good descriptions on the carbon enrichment in austenite during ICT stage.

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