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.

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In this paper, we describe the effects of mechanical loading on bcc-to-bcc phase transformations of an Advanced High Strength Steel during cooling. In-situ synchrotron diffraction was employed to measure time–temperature–load diffraction patterns. Calculations were made of the volume fractions of the phases, the transformation kinetics, and the austenite lattice parameter during cooling and simultaneous loading. In addition, volume fractions and lattice parameters of retained austenite at room temperature under different loading conditions were obtained. The results show that applying a load during cooling of the fcc phase significantly increases the volume fraction of a bcc phase before the start of the martensitic transformation. The kinetics of phase transformations were affected by the applied loads. The volume fraction and lattice parameter of retained austenite at room temperature vary in different samples and the highest retained austenite and the largest lattice parameter were obtained in the sample subjected to the highest load.Correction to article: https://doi.org/10.1007/s11661-017-4453-7@en

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.

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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.

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Carbon redistribution is known to occur during martensite ageing.
The two associated processes most discussed in the literature are spinodal decomposition and carbon segregation to defects. In order to elucidate the topic, the ageing and tempering of two Fe–Ni–C alloys have been characterised by means of atom probe tomography and synchrotron radiation diffraction. Upon ageing at room temperature, carbon redistribution is clearly observed, where the process of carbon segregation to defects appears to be most likely to occur. Nevertheless, the possibility of spinodal decomposition is not entirely discarded, and the current work presents a series of discussion points that challenge our current understanding of the thermodynamic of ferrite in steels.@en

Recent findings have indicated that (i) carbon solubility in ferrite is higher than predicted when in equilibrium with austenite, where a non-cubic structure of ferrite has been proposed, and (ii) local variations in carbon concentration consistently form within the matrix, thought to be the product of spinodal decomposition. These observations give rise to the scientific question of how carbon is distributed within ferrite, which remains under discussion in literature. The current study aims at establishing a thermodynamic explanation for the carbon solubility and partitioning within ferrite. Topics on carbon-competing processes such as spinodal decomposition in the Fe-C system and carbon segregation to defects are revisited.@en