Carbon partitioning in bainite is a central mechanism controlling this transformation. Depending on whether C atoms form precipitates in the ferrite platelets or remain dissolved in a solid solution, the resulting microstructure is denoted as either lower or upper bainite. In the present analysis, a simulation study is conducted with long-range diffusion of C and simultaneous precipitation of carbides. The computational analysis suggests that carbides always form within the ferritic subunits before carbon redistribution into the surrounding austenite commences, independent of temperature. Since these carbides can rapidly dissolve again at higher bainite transformation temperatures, they are commonly not experimentally observed in upper bainite. At lower temperatures, carbide re-dissolution takes significantly more time, and carbides are, therefore, commonly identified, with the corresponding microstructure denoted as lower-bainite. Since carbides shall always form in the ferrite platelets, a classification of upper and lower bainite based on the existence of carbides should be reconsidered.

中文翻译：

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上贝氏体和下贝氏体的故事：同时发生的碳扩散和析出的计算分析

贝氏体中的碳分配是控制这种转变的中心机制。根据 C 原子是在铁素体片晶中形成沉淀物还是保持溶解在固溶体中，所得的微观结构被表示为下贝氏体或上贝氏体。在本分析中，对C的长程扩散和碳化物的同时析出进行了模拟研究。计算分析表明，在碳开始重新分布到周围奥氏体中之前，碳化物总是在铁素体亚基内形成，与温度无关。由于这些碳化物在较高的贝氏体转变温度下可以快速再次溶解，因此通常不会在上贝氏体中通过实验观察到它们。在较低温度下，碳化物的再溶解需要更长的时间，因此碳化物通常被识别，相应的显微组织表示为下贝氏体。由于碳化物总是在铁素体片中形成，因此应重新考虑根据碳化物的存在对上贝氏体和下贝氏体进行分类。