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 (Δ K th ) for WM was gained compared to the BMs and HAZs, while a larger da / dN and lower Δ K th 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.

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

Failure mechanisms and design of dissimilar welds of 9%Cr and CrMoV steels up to very high cycle fatigue regime

The evolution of grain size and structure morphology across the CrMoV welded joint were investigated. The tensile properties of subzones in the weldment were measured by miniature specimens. And the measured results were compared to the results acquired from the standard round bar specimen. It is found that the strength characterized by miniature specimen is lower than that obtained by standard specimen. The deviation of strength is acceptable. However, the measured Young's modulus is not consistent with the elastic modulus of standard specimen, due to the jig deformation and clamped effects of the miniature specimen. Although HAZ is the weakest location due to the deteriorated microstructure and the reduced strength, the failure pattern is different comparing the measured results of the miniature specimen and that of the standard round bar cross-weld specimen. For the miniature specimen, fracture occurred at the HAZ region close to the base metal (BM). However, the fracture of the standard specimen occurred in the BM instead of the HAZ region. The difference is ascribed to the different constraints of specimens with various geometric dimension.

Evaluation of the microstructure related strength of CrMoV weldment by using the in-situ tensile test of miniature specimen

Hydrogen embrittlement (HE) is a critical issue that hinders the reliability of hydrogenation reactors. Hence, it is of great significance to investigate the effect of hydrogen on fracture toughness of 2.25Cr-1Mo-0.25V steel and weld. In this work, the fracture behavior of 2.25Cr-1Mo-0.25V steel and welds was studied by three-point bending tests under hydrogen-free and hydrogen-charged conditions. The immersion charging method was employed to pre-charge hydrogen inside specimen and the fracture toughness of these joints was evaluated quantitatively. The microstructure and grain size of the specimens were observed by scanning electron microscopy (SEM) and by metallurgical microscopy to investigate the HE mechanisms. It was found that fracture toughness for both the base metal (BM) and the weld zone (WZ) significantly decreased under hydrogen-charged conditions due to the coexistence of the hydrogen-enhanced decohesion (HEDE) and hydrogen-enhanced localized plasticity (HELP) mechanisms. Moreover, the formation and growth of primary voids were observed in the BM, leading to a superior fracture toughness. In addition, the BM compared to the WZ shows superior resistance to HE because the finer grain size in the BM leads to a larger grain boundary area, thus distributing more of the diffusive hydrogen trapped in the grain boundary and reducing the hydrogen content.

Experimental Investigation of the Effect of Hydrogen on Fracture Toughness of 2.25Cr-1Mo-0.25V Steel and Welds after Annealing.

Transition and fracture shift behavior in LCF test of dissimilar welded joint at elevated temperature

In this paper, the fracture toughness and the related microstructure characteristics of dissimilarly welded joint manufactured by advanced 9Cr and CrMoV steels were systematically investigated. The dissimilarly welded joint was fabricated by narrow gap submerged arc welding (NG-SAW) applying multi-layer and multi-pass technique. Fracture toughness, as one of the most important property to assess the reliability of welded joint, was studied for different regions including CrMoV base metal (CrMoV-BM), heat affected zone (HAZ) of CrMoV side (CrMoV-HAZ), weld metal (WM), heat affected zone of 9Cr side (9Cr-HAZ) and 9Cr base metal (9Cr-BM). It was found that the fracture toughness of CrMoV-BM, CrMoV-HAZ and WM was better than that of 9Cr-HAZ and 9Cr-BM. In order to illustrate these results, the microstructure of the whole welded joint was observed by optical microscope (OM), scanning electron microscope (SEM) and transmission electron microscope (TEM) detailedly. It was found that the fine high-temperature tempered martensite and bainite in WM, CrMoV-BM and CrMoV-HAZ contribute to the higher fracture toughness, while lower fracture toughness for 9Cr-BM and HAZ was caused by coarse tempered lath-martensite. Furthermore, the fracture morphology showed that ductile fracture occurred in WM and CrMoV side, while brittle fracture appeared in BM and HAZ of 9Cr side.

Correlation of microstructure and fracture toughness of advanced 9Cr/CrMoV dissimilarly welded joint

Modelling the crack propagation behavior in 9Cr/CrMoV welds

The invention discloses a method for measuring the fracture toughness J of each micro area of a heat affected zone of a welded joint, which is applicable to measurement on fracture toughness of various welded joints. According to the method, by forming a V-shaped groove in a coarse grain zone or a fine grain zone in the welded heat affected zone on a three-point bending test sample, the direction of crack development is effectively controlled to ensure the crack development in the coarse grain zone or the fine grain zone in the heat affected zone, and thus the fracture toughness of the different micro areas in the heat affected zone of the welded joint can be measured. The method is simple, is low in cost, ensures real and reliable measurement results, and has great significance for evaluating the mechanical property of each micro area of the welding joint, confirming a weak section of the welded joint and studying the fracture toughness of the heat affected zone of the welded joint.

Qian Guo

Papers

Correlation of microstructure and fracture toughness of advanced 9Cr/CrMoV dissimilarly welded joint

Modelling the crack propagation behavior in 9Cr/CrMoV welds

Method for measuring fracture toughness J of heat affected zone of welded joint