CONTINUOUS HEATING TRANSFORMATION OF BAINITE TO AUSTENITE

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.