An investigation was carried out to study the thickening kinetics of f
errite allotriomorph in a high-strength low-alloy (HSLA) steel contain
ing Nb. Using optical microscopy to measure growth distance, it has be
en shown that the movement of the alpha/gamma interface follows a para
bolic relationship with the reaction time. By modelling the steel as a
ternary Fe-C-Mn alloy, the growth rate constants, alpha(e) and alpha(
para). were calculated assuming full equilibrium and paraequilibrium m
odes of transformation, respectively. The parabolic growth rate consta
nt lies within an order of magnitude of the values for alpha(para) up
to about 1000 K with no partitioning of Mn. An abrupt decrease in the
rate of growth of ferrite has been observed at higher temperatures. Th
e parabolic growth rate constant at the highest temperature (1062 K) i
s about an order of magnitude higher than the value of alpha(e) indica
ting that equilibrium partitioning of Mn has occurred. Thus a change f
rom no partitioning at lower temperatures to partial or full partition
ing of Mn at higher temperatures describes the kinetics of ferrite tra
nsformation in this steel.