The boundary between the HAZ, and the TMAZ (i.e. the point of which plastic deformation owing to the welding process first occurred) was difficult to identify, as no evidence of deformation without recrystalisation was observed. This suggests that the HAZ/TMAZ boundary occurred in the region heated above the AC3 temperature. This is in accordance with the peak temperature reached during friction stir welding of low carbon steel, which is probably around 1000-1100°C. A typical microstructure from the centre of the weld (in the TMAZ) is shown in Figure 6a, while Figure 6b shows the HAZ in the region of the AC3 isotherm, where areas of partial and complete transformation to austenite during the welding cycle are apparent.