The application of high heating rates in tempering treatments can provide a valuable tool for refining carbide sizes, mainly those located at high angle grain boundaries. This work analyses the influence of heating rates ranging from 1 to 300 degrees C/s during the tempering treatment of a 0.42%C low alloy steel. The results indicate that when high heating rates are combined with short holding times, predicting hardness will require the inclusion of the heating up and cooling down cycles in addition to the holding time and temperature used in the definition of the conventional Hollomon-Jaffe tempering parameter (TP). The effect of heating rate on carbide size distribution has been quantified, distinguishing between particles located at high (HAB) and low (LAB) misorientation angle boundaries. The former correspond to those carbides nucleated at prior gamma grain, martensite block or packet boundaries whereas the latter refer to those nucleated within martensite laths and at lath boundaries. The refinement obtained has been evaluated from the point of view of hardness behaviour. (C) 2014 Elsevier Ltd. All rights reserved.