Stability of Ultra-fine Microstructures during Tempering

The stability of ultra-fine microstructure during tempering at 650 ℃ was investigated on a Nb-containing steel. The steel had undergone 5 passes controlled rolling, then was relaxed (air cooled) to 730 ℃ and cooled in water. The evolution of microstructure was that, in early stage of tempering, no obvious change was detected by means of optical microscopy while dislocation cells were formed inside bainitic laths. With further tempering, bainitic laths started to coalesce in some regions. Finally, polygonal ferrite was formed while hardness decreased dramatically. Some samples taken from the same primary plate were reheated at 930 ℃ for 0.5 h followed by quenching into water before tempering. Despite their lower original hardness, the reheated samples softened Faster during tempering. Ferrite was quickly formed in the reheated samples. These results indicate that the evolution of microstructures towards equilibrium during tempering of the steel is mainly determined by whether dislocations are pinned rather than the dislocation density.