从热力学、动力学角度对Fe-1.5(3.0)%Si-0.4%C (质量分数)合金在400~500 ℃等温贝氏体相变过程中的贝氏体不完全转变阶段可以出现渗碳体析出的现象进行了深入分析。结果表明,贝氏体不完全转变阶段渗碳体的析出动力学非常缓慢。热力学分析表明,缓慢的渗碳体析出动力学与其很小的化学驱动力和/或渗碳体析出需要Si原子配分有关。证实了贝氏体不完全转变现象可以伴随有碳化物析出,但碳化物析出动力学在贝氏体不完全转变阶段需要非常缓慢。

Steels containing bainite microstructure are widely applied in various industrial areas. Incomplete bainite transformation is frequently used to control volume fraction of retained austenite as well as its stability and is also closely related to bainite growth mechanism. It is generally accepted that incomplete bainite transformation could occur when carbide precipitation is absent. On the other hand, some new studies revealed that carbide with very fine size were observed in “carbide-free” bainite in Si added steels. Our previous study on bainite isothermal transformation kinetics together with its microstructural evolution with Fe-1.5(3.0)%Si-0.4%C alloys (mass fraction) at 400~500 ℃ found incomplete bainite transformation phenomenon for the 3.0Si alloy at 450 ℃ and for two alloys at 400 ℃. In contrast with the generally accepted view, cementite precipitation with Si depletion was observed at the beginning of incomplete transformation stage. Further analysis on three dimension atom probe results revealed that the carbide volume fraction as well as amount of C atoms in carbide hardly changes during incomplete transformation stage. Thermodynamic analysis revealed that small chemical driving force for cementite precipitation and/or the necessity of Si partition are two factors accounting for the extremely slow cementite precipitation kinetics. It is thus proposed that incomplete bainite transformation and carbide precipitation could co-exist. Conditions for incomplete bainite transformation are modified as follows. Firstly, bainitic ferrite growth is stopped before reaching equilibrium fraction. In addition, carbide precipitation should be absent or its kinetics should be slow enough.