Influence of carbon concentration and reaction temperature upon bainite morphology in Fe-C-2 Pct Mn alloys

Abstract

Isothermal transformation of austenite to bainite was studied by optical, replication, and thin foil transmission electron microscopy (TEM) in both hypo- and hypereutectoid Fe-C-2 wt pct Mn alloys (and a single 3 pct Mn alloy) containing from 0.1 to 1.37 wt pct C in order to characterize and explain the transitions which occur in bainite morphology as a function of carbon content and reaction temperature. A “morphology map” was constructed showing the temperaturecarbon composition(T-x) regions in which four different bainite morphologies predominate: upper bainite, lower bainite, nodular bainite, and inverse bainite. Calculations of the volume free energy changes associated with the nucleation of ferrite, †G v α , and cementite, †G v c , and the parabolic rate constant for growth of ferrite, αα, and cementite, αc, were performed in order to explain the observed morphological transitions. In hypoeutectoid alloys, where |†G v α | ≫ |ΔG v c | and αα ≫ α c , the Widmanstatten ferrite-dominated morphologies of upper and lower bainite predominate. The upper-to-lower bainite transition appears to be associated with the emergence of edge-to-face sympathetic nucleation at high values of |ΔG v α |. In hypereutectoid alloys, the ratios ΔG v α /ΔG v c and αα/αc are considerably smaller; hence, cementite can compete much more readily with ferrite during both nucleation and growth, resulting in the formation of nodular bainite. With decreasing temperature in the hypereutectoid regime,ΔG v α /ΔG v c , and especially αga/αc, increase, resulting in the replacement of nodular bainite by lower bainite at temperatures below about 250 °C to 275°C.