Microstructure correlation and fatigue crack growth behavior in dissimilar 9Cr/CrMoV welded joint

The fatigue crack growth behavior and the related microstructure of the dissimilar welded joint made from advanced 9Cr and CrMoV steels were systematically investigated in this paper. The dissimilar materials of 9Cr and CrMoV with heavy section were welded by narrow-gap submerged arc welding (NG-SAW) via multi-layer and multi-pass technology. The fatigue crack growth behavior was comparatively studied for different regions including advanced 9Cr and CrMoV base metals (9Cr-BM, CrMoV-BM), heat affected zones of both sides (9Cr-HAZ, CrMoV-HAZ) and weld metal (WM). The results indicated that a lower fatigue crack growth rate (da/dN) as well as a higher fatigue crack growth threshold (Delta Kth) for WM was gained compared to the BMs and HAZs, while a larger da/dN and lower Delta Kth for 9Cr-HAZ were obtained under the same stress ratio. The WM showed a better fatigue crack growth resistance than others, while the 9Cr-HAZ with narrow size became the weaker part of the whole welded joint. Microstructure was revealed that the different microstructure character such as tempered martensite and tempered bainite could account for this diversity in properties. In addition, the observation of fatigue crack growth path displayed that the obstacles of coarse columnar grains and the ductile equiaxed grains in WM caused the higher resistance of fatigue crack growth, while the tempered martensite with and without lath structure led to the higher and lower fatigue crack growth resistance for 9Cr-BM and 9Cr-HAZ, respectively. However, the fatigue crack growth behavior of 9Cr-HAZ would not great influence the whole welded joint for its narrow size. (c) 2015 Elsevier B.V. All rights reserved.