Carbide evolution and its effect on the impact toughness of the 9Cr/CrMoV dissimilar welded joint during the aging treatment

Abstract

The microstructure evolution in the weld metal (WM) and its effect on the impact toughness for the 9Cr/CrMoV dissimilar welded joint with various aging time was systematically investigated. With the aging time increased from 0 h to 3,000 h, the specimen aged at 470 ℃ for 1,000 h revealed the lowest impact energy (13.5 J). Moreover, the impact fracture morphology details were obtained by scanning electron microscopy (SEM). The results showed that the quasi-cleavage fracture appeared for the specimens in the original and aged (aged at 470 °C for 3,000 h) states, while the cleavage fracture could be observed for the specimen aged at 470 °C for 1,000 h. Besides, the microstructure observation exhibited that the main microstructure in the WMs for the specimens in the original and aged states were all the tempered martensite. The columnar and equiaxed grain zones formed owing to the multi-layer and multi-pass techniques. Based on the characterization analysis of the precipitates for the aged specimens, the coarsening of the carbides was determined as the crucial factors to decrease the impact energy. On the contrary, with the aging time increased, the newly precipitated fine carbides could produce the positive effects on the impact toughness. In the original state, the impact energy of the specimens was about 29.2 J. With the aging time increased to 1,000 h and 3,000 h, the impact energy became 13.5 J and 21.0 J, respectively.