KINETICS OF FERRITE TRANSFORMATION IN AN HSLA STEEL CONTAINING NB

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.