In the same way as for the weld metal, the small, strongly oriented precipitates formed in the interior of martensite laths of the CGHAZ during cooling (Fig. 2 (d)) would be M3C. A few spherical precipitates –probably of the Nb-rich MX type- are also observed in this region. These coarse MX precipitates would remain undissolved during the complete thermal cycle due to their low solubility product (Tamura et al. (2001)). Fig. 3 shows the hardness profile across the weld metal, reflecting (especially the profile obtained at 16 mm from the bed surface) the microstructural variations occurring during the multipass welding process. Such variations could correspond to the formation of the columnar grained, coarse grained and fine grained recrystallized regions. During welding, transformation to austenite in the FGHAZ is complete, but the peak temperature reaches a value only slightly above Ac3; thus, the carbide particles inherited from the initial state are hardly dissolved and contribute to keep fine the austenite grain size, i.e. 3 to 4 μm (Fig. 2(e)). At the same time, as a large fraction of carbon remains bound to undissolved carbides, autotempering would be severely reduced, and in fact the small, oriented rod-like precipitates are hardly detected (Fig. 2 (f)). This region of the HAZ is the widest one. On the other hand, the ICHAZ is characterized by a peak temperature located between Ac1 and Ac3 and hence by a partial transformation to austenite. This means the coexistence of transformed and non-transformed regions, i.e. small prior austenite grains transformed to fresh martensite and large regions of overtempered, non transformed martensite respectively (Figs. 2 (g) and (h)). In these last regions, microscopic observations suggest that precipitates
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