Growth and overall transformation kinetics above the bay temperature in Fe-C-Mo alloys

The kinetics and morphology of isothermal transformation in the vicinity of the time-temperature-transformation (TTT) diagram bay have been investigated with optical and transmission electron microscopy (TEM) in 19Fe-C-Mo alloys at three levels of carbon concentration (approximately 0.15, 0.20, and 0.25 wt pct) and at Mocon centrations from 2.3 to 4.3 wt pct, essentially always at temperatures above or at that of the bay, Tb. Quantitative metallography yielded no evidence for incomplete transformation (stasis) in any of these alloys at T > Tb. Measurements of the thickening kinetics of grain boundary ferrite allotriomorphs (invariably containing either in ter phase boundary or fibrous Mo2C) demonstrated four different patterns of behavior. The customary parabolic time law for allotriomorph thickening in Fe-C and in many Fe-C-X systems was obtained only at higher temperatures and in the more dilute Fe-C-Mo alloys studied. With decreasing temperature and increasing solute concentrations, a two-stage and then two successive variants of a three-stage thickening process are found. In the most concentrated alloys and at temperatures nearest the bay, the second stage of the three-stage thickening process corresponds to growth stasis - the cessation of allotriomorph thickening. Sufficient prolongation of growth statis presumably leads to transformation stasis. A number of models for growth of the carbide-containing allotriomorphs were investigated during attempts to explain the kinetics. It is concluded that growth is controlled by carbon diffusion in austenite but with a driving force drastically reduced by a strong solute drag-like effect induced by Mo segregation at disordered-type austenite: ferrite boundaries