Effect of tempering upon the tensile properties of a nanostructured bainitic steel

The tensile properties of a nanostructured carbide-free bainitic steel formed at 200-250 degrees C are compared against those after tempering sufficiently to remove the retained austenite. Although significant ductility is observed following tempering, a comparison of tempered and untempered samples shows that it is in fact reduced when a comparison is made at identical strength. The shape of the stress-strain curves shows clear evidence that the capacity for work hardening is reduced with the loss of austenite. The nanostructure of the steel transformed at 250 degrees C is examined by transmission electron microscopy, to compare the as-transformed to the tempered structure. In this case after tempering at 500 degrees C the energy absorbed during the tensile test is lower, due to the lower strength. Reduction of strength is caused by the slight coarsening of the bainite plates, and lower dislocation density after tempering. Considering the formation of carbide particles in high strength steel, impressive ductility is exhibited even in the tempered condition. (C) 2014 Elsevier B.V. All rights reserved.