The Incomplete transformation phenomenon in Fe-C-Mo alloys

Abstract

The overall kinetics of isothermal transformation of austenite to bainite were studied with quantitative metallography and transmission electron microscopy (TEM) in a series of highpurity Fe-C-Mo alloys containing 0.06 to 0.27 wt pct C and 0.23 to 4.28 wt pct Mo at reaction temperatures mainly below that of the bay in the time-temperature-transformation (TTT) curve for initiation of transformation. Ferrite growth kinetics were also determined at temperatures slightly below that of the bay. The incomplete character of the bainite transformation was found to depend upon both the C and Mo concentrations; below threshold combinations of these elements, the incomplete reaction is absent. In cases in which the bainite reaction was incomplete, transformation of austenite resumed and went to completion following the initiation of Mo2C precipitation at α: γ boundaries. The time of transformation stasis increased with the proportions of C and Mo in the alloy. Pearlite was not observed anywhere in the time-temperature-composition (TTC) region investigated. Ferrite growth kinetics at temperatures near the bay do not exhibit simple time laws; this behavior is attributed to a solute drag-like effect (SDLE). Extensive sympathetic nucleation of ferrite is observed at temperatures below that of the bay. The temperature and composition dependence of the incomplete reaction can be explained by a combination of the SDLE and the variation in the sympathetic nucleation rate of ferrite with temperature and the amount of transformation.