Bainite Transformation in Heterogeneous Steels
Proceedings of an International Conference: Phase Transformations '87,
Institute of Metals, London, Edited by G. W. Lorimer, 1988, pp. 207-210.
S. A. Khan and H. K. D. H. Bhadeshia
This scientific paper investigates the elemental redistribution that occurs during the formation of bainite in high-silicon steels. Using atom-probe analysis, the researchers examined the "incomplete-reaction phenomenon", where the transformation stops before reaching chemical equilibrium.
The study provides evidence that bainitic ferrite forms with a significant carbon supersaturation, suggesting a growth mechanism that is largely independent of carbon diffusion. While manganese was found to redistribute at the interfaces after long periods of heating, the lack of such movement during the initial transformation refutes theories involving solute-drag effects.
Ultimately, the findings support a displacive mechanism for bainite growth, where the structural change occurs too rapidly for substantial alloying elements to partition between the different phases.
Review Quiz
1. What is the primary focus of the research conducted on 300M steel in this study?
The research investigates the effect of chemical segregation (heterogeneity) in austenite on its transformation to bainite. Specifically, it examines how the maximum volume fraction of bainite formed at various temperatures differs between homogeneous and heterogeneous steel samples.
2. Why is the high silicon content in 300M steel significant for the study of bainite transformation?
The high silicon concentration suppresses the precipitation of cementite during the formation of upper bainite. This suppression ensures that almost all of the carbon remains in the residual austenite, simplifying the theoretical analysis of the transformation results and making the bainite transformation well-separated from other reactions.
3. Define the "incomplete-reaction phenomenon" as described in the source context.
The incomplete-reaction phenomenon occurs when the bainite transformation stops well before the austenite reaches its equilibrium or paraequilibrium carbon concentration. The transformation ceases when the carbon concentration of the residual austenite reaches a specific level (xT0) defined by the T0 curve on the phase diagram.
4. What does the T0 curve represent in the context of phase transformations?
The T0 curve on a temperature versus carbon concentration plot represents the locus of all points where ferrite and austenite of the same composition have equal free energy. This curve serves as the thermodynamic limit for the transformation, as the growth of bainite is expected to stop when the carbon concentration reaches this limit.
5. How does chemical heterogeneity affect the spatial distribution of bainite formation?
In heterogeneous alloys, bainite nucleates preferentially in solute-depleted regions. Solute-rich regions may act as barriers to the expansion of bainite, as they do not transform as easily, potentially trapping excess carbon in the austenite and preventing further transformation.
6. What was the observed difference in the maximum volume fraction of bainite between homogeneous and heterogeneous samples?
Experimental data consistently showed that the maximum volume fraction of bainite obtained at any given transformation temperature (between 320–420°C) is lower in heterogeneous samples compared to homogeneous samples.
7. Describe the "slice model" used to quantitatively model the transformation behaviour.
The model divides the heterogeneous alloy into "slices" of varying compositions based on the as-received steel. A T0 curve is calculated for each slice, and the transformation is allowed to proceed until the carbon concentration in that slice reaches its specific xT0 value.
8. What is the difference between the "permeable" and "impermeable" versions of the slice model?
The impermeable slice model assumes that carbon is not allowed to redistribute between slices of different compositions. The permeable model (or the version where diffusion is permitted) assumes that the activity of carbon is equalised across all slices during each increment of the transformation.
9. How did the experimental martensite start (Ms) temperatures of residual austenite compare to the calculated values?
The measured Ms temperatures for the residual austenite in partially transformed specimens were found to be significantly higher than the thermodynamically calculated Ms temperatures. This discrepancy indicates a non-uniform distribution of carbon within the residual austenite.
10. What conclusion did the researchers reach regarding the distribution of carbon in residual austenite?
The researchers concluded that carbon is distributed non-uniformly throughout the residual austenite. Because some regions of the austenite are carbon-poor, they transform to martensite at higher temperatures than would be expected if the carbon were distributed homogeneously.
