The microstructural evolution of a quenched medium-C steel during tempering was analyzed by means of Orientation Imaging Microscopy (OIM). The steel was heat treated in order to develop after quenching fully martensitic microstructures with different prior austenite grain sizes. Main results can be summarized as follows:

1. A very poor effect of austenite grain size on packet size was found. 
2. A finer packet was measured at mid-thickness with respect to surface after external and internal water quenching process. This phenomenon was attributed to the effect of the strain path on the phase transformation during quenching. 
3. The through-thickness microstructural gradient remains substantially unchanged after tempering.
4. High-angle boundary grains do not significantly grow after tempering; on the contrary, low-angle grain boundaries (cells) move, fully justifying the hardness evolution with the tempering temperature.