Critical Assessment: Diffusion-controlled Growth of Ferrite Plates in Plain Carbon Steels

H. K. D. H. Bhadeshia

Abstract

The implementation of the theory of diffusion-controlled growth of ferrite plates in plain-carbon steels is critically assessed. It is found that the use of empirically extrapolated diffusion coefficients, phase boundaries, and thermodynamic functions leads to errors in calculations of growth rate. The errors become most important for low transformation temperatures, leading to exaggerated growth rates. Ways of avoiding these difficulties are suggested, and a new analysis of experimental data indicates that the lengthening of Widmanstatten ferrite plates in Fe-C alloys occurs at a rate which is influenced by the diffusion of carbon in the austenite ahead of the interface, assuming that the plates adopt a tip radius consistent with the maximum growth velocity.

However, there is a systematic discrepancy between theory and experiment: plate-growth theory seems to underestimate the lengthening rate by a few microns per second. This may have something to do with the lath shape of Widmanstatten ferrite, but an analysis using needle-growth theory does not resolve the problem for data obtained at low lengthening rates. In general, plate-growth theory gives a better explanation of experimental data. The growth of bainite sheaves occurs at a rate much faster than expected from carbon diffusion-controlled growth. If the maximum-velocity hypothesis is incorrect (as it is for dendritic solidification), the above-mentioned discrepancies would be larger.

This research paper provides a rigorous critical assessment of the mathematical models used to predict how ferrite plates grow in plain-carbon steels. The author identifies significant computational errors caused by the use of imprecise empirical data and outdated thermodynamic assumptions, which often lead to exaggerated growth rates at lower temperatures. By applying refined thermodynamic models and advanced diffusion theories, the study attempts to reconcile the discrepancy between theoretical predictions and experimental observations. The findings reveal that even with improved calculations, actual lengthening rates often exceed theoretical expectations, possibly due to the specific lath-like geometry of the ferrite. Ultimately, the work offers a more accurate framework for predicting the microstructural development of steel during industrial heat treatments and welding.

Materials Science and Technology, Vol. 1, 1985, 497-504.

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