The role of heat treatment on the cyclic stress-strain response of ultrafine-grained interstitial-free steel

Thermal stability of ultrafine-grained (UFG) interstitial free (IF) steel was investigated in order to establish heat treatments for obtaining recovered and bimodal UFG IF steel microstructures, as part of an effort to further improve the fatigue response of this material. The initial UFG microstructure was obtained using equal channel angular extrusion at room temperature. Microstructural investigations, such as electron backscattering diffraction and transmission electron microscopy, were carried out in order to observe the microstructural evolution due to heat treatment and fatigue. Cyclic tests at room temperature revealed the role of heat treatment and resulting microstructures on the cyclic stress-strain response of the IF steel. We demonstrate that the fatigue performance could be improved significantly with low temperature annealing. The bimodal microstructure, however, is not beneficial to improving fatigue properties of the UFG IF steel. Finally, the stability of the UFG microstructure, and the absence of localized damage during fatigue is associated with the presence of impurities in the IF steel. (C) 2007 Elsevier Ltd. All rights reserved.