A thermodynamic study of inverse bainitic transformation

The effect of Cr, Mn, and the isothermal holding temperature on inverse bainitic transformation in hypereutectoid steels is investigated. Thermodynamic driving force is calculated for the onset of nucleation of cementite and ferrite from parent austenite, Hultgren extrapolation of A_e3 and A_cm phase boundaries, and the molar Gibbs energy change for austenite to ferrite transformation. For a given carbon concentration above the eutectoid carbon concentration, inverse bainite is favored at a lower Cr and higher Mn concentrations in the steel. With the increase in Cr concentration, the inverse bainitic start temperature has been found to increase. Cr partitioning from parent austenite to form Cr₇C₃ or Cr₂₃C₆ occurs only at prolonged transformation time (10000 s and above), by when the inverse bainitic transformation is complete. Cementite is the favored carbide nucleating from parent austenite during the inverse bainitic transformation. With the increase in Mn concentration, both the inverse bainitic start and finish temperatures have been found to decrease. For a given chemical composition, inverse bainite is generally favored below the pearlitic transformation temperature. Inverse bainitic transformation gets suppressed when the isothermal transformation temperature is lowered, in which case conventional upper/lower bainite is the favored transformation product.