The Microstructure of Continuously Cooled Tough Bainitic Steels

The influence of bainite morphology on the impact toughness behaviour of a continuously cooled cementite free low carbon C-Mn-B type of steel has been examined. Different bainite morphologies were obtained by cooling from the austenite regime to ambient temperature with different cooling rates. The resulting microstructures have been quantitatively analysed using light optical microscopy (LOM), scanning electron microscopy (SEM) and electron back scatter diffraction (EBSD) techniques. The relationship between the steel morphology and impact toughness has been discussed. It could be shown that bainitic microstructures formed mainly by lath-like upper bainite, consisting of thin and long parallel ferrite laths, exhibit higher impact toughness values than those with a granular bainite morphology, consisting of an equiaxed ferrite structure and discrete islands of M/A phases. Granular bainite shows evidence of low resistance to crack propagation during cleavage fracture because of its larger crystallographic packet size. Moreover, the stress concentration associated with coarse M/A grains is considered as a possible factor contributing to the premature crack nucleation.