Kinetics of ferrite transformation in an Fe-0.28mass%C-3mass%Mo alloy

The morphology and the kinetics of ferrite transformation below the TTT-diagram bay were studied in an Fe-0.28mass%C-3mass%Mo alloy. The three-dimensional reconstruction of the transformed microstructure revealed that the highly irregular morphology of ferrite observed on the polished specimen surface is formed due to extensive impingement and coalescence of thin rod-like crystals. The transformation stasis was observed consistently with previous observations. The fractions of ferrite achieved by fast carbon diffusion-controlled growth (followed by sluggish alloying element-partitioned growth) were calculated under paraequilibrium and local equilibrium. The calculated fractions of ferrite varied with temperature qualitatively in the same manner as those experimentally observed at the stasis. It is likely that above the bay the ferrite growth falls in the low velocity regime of Cahnʹs solute drag theory, whereas below the bay it may fall initially in the high velocity regime, climb up the peak of the drag and enter the stasis. In alloys of lower C and Mo content the growth stasis does not occur presumably because the carbon diffusion-controlled growth can overcome the drag peak.