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The transformation of a bainitic Fe-Mn-Si-C alloy into austenite has been studied using dilatometry, transmission electron microscopy and microanalytical techniques. The formation of austenite was investigated using two different starting microstructures, the first consisting of a mixture of bainitic ferrite and residual austenite, and the second of a mixture of tempered bainitic ferrite and carbides. Results from isothermal austenitization experiments confirm earlier work on a different alloy, that because of the incomplete reaction phenomenon associated with bainite growth, there is a large temperature hysteresis before the reverse transformation to austenite becomes possible. Continuous heating experiments revealed an identical austenitization behaviour for both initial microstructures when the heating rate utilized was small. This is because any residual austenite then tends to transform into pearlite or to decompose into ferrite and discrete particles of carbides before the sample reaches a temperature where austenite growth becomes thermodynamically feasible. Consequently, the two initial microstructures become identical by the time T-gamma is reached. At faster heating rates the residual austenite remains stable during heating and then commences to grow as the appropriate elevated temperature is reached. A slightly higher degree of superheating is found to be necessary in the absence of residual austenite in the starting microstructure, since austenite nucleation is then necessary prior to growth. Since the excess superheating is rather small, the results indicate that nucleation does not appear to be a major hurdle to the formation of austenite in the alloy studied.
This research investigates the kinetics of reaustenitisation within steel weld deposits, focusing on how different initial microstructures like bainite and acicular ferrite influence phase changes. The authors developed a thermodynamic model and used dilatometry to track how austenite grows through carbon diffusion across metal interfaces. By analyzing Time-Temperature-Transformation (TTT) curves, the study reveals that higher temperatures and specific structural surface areas significantly accelerate the rate of transformation. Additionally, the text examines continuous heating transformation, noting that chemical segregation in unhomogenised welds causes them to react at lower temperatures than more uniform samples. Ultimately, the work provides a quantitative framework to predict how heating rates and alloy compositions determine the final microscopic properties of welded steel.
Materials Science and Engineering A, Vol. A131, 1991, pp. 99-113.
This study guide examines the kinetics of reaustenitisation in steel weld deposits, specifically focusing on the transformation from microstructures consisting of acicular ferrite plus austenite (αa+γ) and bainite plus austenite (αb+γ).
| Term | Definition |
|---|---|
| Acicular Ferrite (αa) | A needle-like microstructure that serves as a starting point for reaustenitisation studies. |
| Bainite (αb) | A plate-like microstructure with a high γ/α interface density. |
| Parabolic Rate Constant (α1) | A value describing the rate at which the austenite layer thickens over time. |
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School of Engineering and Materials Science, Queen Mary University of London Mile End Road, London E1 4NS, U. K. Materials Science & Metallurgy, University of Cambridge 27 Charles Babbage Road, Cambridge CB3 0FS, U.K. |