Carbon redistribution is known to occur during room temperature aging of Fe-C martensite. One of the proposed mechanisms in the literature by which carbon redistributes is spinodal decomposition, a thermodynamically driven reaction in which the alloy undergoes separation into carbon-rich and carbon-poor regions, giving rise to modulations in carbon concentration. Despite the substantial experimental evidence supporting the occurrence of spinodal decomposition in Fe-C, its theoretical formulation requires attention. In the present study, a theoretical framework based on the regular solution model is built for evaluating the thermodynamics of the Fe-C system, with particular emphasis on the interstitial nature of carbon atoms within the ferrite lattice. Assuming a defect-free lattice, the model explains a miscibility gap in the Fe-C system. The limitations of the current model are critically evaluated.

Understanding carbon redistribution in steels is crucial in developing advanced high strength steels. For instance, Quenching & Partitioning (Q&P) processes rely on the partitioning of carbon from martensite into austenite, where at the end of the heat treatment the carbon-enriched austenite shows higher stability at room temperature. Recent literature gives increasing evidence of carbide precipitation occurring during partitioning despite the addition of silicon, conventionally thought to suppress carbide precipitation. The aim of the present study is to gain insight into carbon-competing processes by applying a series of Q&P heat treatments, with particular focus on the partitioning stage, where the role of silicon in the stability of austenite is evaluated. Various characterisation techniques are combined in order to unveil the microstructural changes. While carbide precipitation does appear to occur in the presence of silicon, it is found that silicon plays an active role in the stabilisation of the austenite during the partitioning reaction.

The decrease in dislocation density and hence the high strength loss can be modelled during martensite tempering as a recovery process. In this work, an innovative approach is presented to understand the role of silicon associated with the inhibition of the recovery rate. A phenomenological model is presented, where a combination of cross-slip and solute drag is identified as the main governing mechanism for recovery up to 450 (Formula presented.) C, from where it is postulated that recrystallisation occurs.