Formation of Lamellar Carbides in Alloy 617-HAZ and Their Role in the Impact Toughness of Alloy 617/9%Cr Dissimilar Welded Joint

The relationship between the microstructure and impact toughness, especially the formation of the lamellar carbides and its effect on the impact toughness of the heat-affected zone of Alloy 617 (Alloy 617-HAZ), was systematically investigated. Impact test was performed on Alloy 617/9%Cr dissimilar welded joints aged for different time at 620 °C. The results showed that the characteristic zones of the whole welded joint revealed decreased impact toughness with increasing the aging time. The impact energies in weld metal showed high stability despite the aging process, while the Alloy 617-HAZ for all testing specimens presented the lowest impact toughness with various aging time. Meanwhile, the mixed grains were found in the base metal (BM) and heat-affected zone of Alloy 617, together with the twins with different widths located in the interior of the large grains. The columnar and equiaxed grain zones appear alternately due to the multilayer and multi-pass welding technique. In particular, compared with Alloy 617-BM, coarsened and even lamellar carbides were observed in Alloy 617-HAZ. These special carbides can separate the grains and therefore result in the formation of intergranular fracture morphology in Alloy 617-HAZ. Besides, the microhardness distribution was tested with a proper load, and the results exhibited that the weld possessed the highest hardness for all specimens with various aging time. The higher microhardness in Alloy 617-HAZ compared with that in Alloy 617-BM was attributed to the high density of the dislocations, which could be detected by transmission electron microscope. As a result, the coarsened carbides aggregated in the grain boundaries to form the lamellar layer are deemed as the main factor to reduce the impact toughness in Alloy 617-HAZ.