Y-Y Ma et aL/Engineering Failure Analysis 47(2015)162-177 ■ Ferrite:70%■ Austenite23.0% Fig. 5. EBSD of the butt weld. 3.1.3. Mechanical test In order to identify whether the dSs used in the water circulating pump is qualified, the impact toughness of samples from the flange and the cylindrical body were tested respectively by the method of Charpy impact test. It revealed that the Charpy impact value of the flange and the cylindrical body is both greater than 300 J exhibiting a superior toughness quality. To further investigate into the toughness of the base material, SEM morphology analysis was applied. As seen in Fig 6, a number of dimples were found on the Chary impact fractography under magnification of 500, confirming the excel lent ductility of the dSs used in CWP. Based on the analysis above, it was concluded that the 2205 dSS used in the CwP was fully qualified and the root cause of the failure did not ascribe to the selection of materials. Thus, the scope of the root cause for the failure was narrowed down to the quality of the weld. 3. 2. Environmental media By means of ICP-AES and IC, the main element constituents of the seawater are revealed in Table 2, which conformed to the normal compositions of natural seawater- a high content of chloride ions. Judging from the appearance of the CWP. there is no severe corrosion, thus selecting 2205 DSS as the base material of the CwP is right and essential because of the strict demand for corrosion resistance 3.3.1. Macroscopic morphologies Fig 7 shows the macro morphologies of sample A. Just as Fig. 7(a)shows, besides the butt weld in the flange there is also a weld joint that joins the flange and the cylindrical body, and these two weld joints are converged in a point. It also shows hat the crack propagated along the weld joint of the flange, noted by the arrow in Fig. 7(a). Further studies were conducted on the cross-sections of this fracture. Fig. 7(b) shows sample A's two corresponding cross sections(marked by Sections 1 and 2), on each lie a long and deep ditch respectively. This kind of ditch throughout the weld joint of the flange is a serious defect in welding, which is so called lack of penetration(LOP). Besides the LoP inside the butt joint(marked by LOPI), an obvious crevice, which is also a LOP, can be found in the weld joint connecting the cylindrical body and the flange(marked by LOP2) ust as Fig. 7(b)represents. As known, there is more residual stress in the weld joint, especially for those without a well heat treatment after welding 20. As Fig. 7(b)shows, the two serious defects mentioned above are linked to an area, greatly increasing the stress concentration and residual stress, and the intersection becomes the weakest area of this weld joint. According to the characteristics of the fracture surface, it can be learnt that the crack initiated from the intersection of he two weld joints, and propagated along the weld joint of the flange, as showed in Fig. 7(b). With the help of 3D-SM hologies of the section surface can be observed more clearly. As Fig. 7(c)shows, the welding flux did not fill the gap between the cylindrical body and the flange, leaving a serious defect that the cylindrical body and the flange were jointed only by two small areas. Judging by the morphologies of cross section, crack origin site can be located, as illustrated in Fig. 7(d), which is the weakest area of the weld joint.3.1.3. Mechanical test In order to identify whether the DSS used in the water circulating pump is qualified, the impact toughness of samples from the flange and the cylindrical body were tested respectively by the method of Charpy impact test. It revealed that the Charpy impact value of the flange and the cylindrical body is both greater than 300 J, exhibiting a superior toughness quality. To further investigate into the toughness of the base material, SEM morphology analysis was applied. As seen in Fig. 6, a number of dimples were found on the Chary impact fractography under magnification of 500, confirming the excel￾lent ductility of the DSS used in CWP. Based on the analysis above, it was concluded that the 2205 DSS used in the CWP was fully qualified and the root cause of the failure did not ascribe to the selection of materials. Thus, the scope of the root cause for the failure was narrowed down to the quality of the weld. 3.2. Environmental media By means of ICP-AES and IC, the main element constituents of the seawater are revealed in Table 2, which conformed to the normal compositions of natural seawater – a high content of chloride ions. Judging from the appearance of the CWP, there is no severe corrosion, thus selecting 2205 DSS as the base material of the CWP is right and essential because of the strict demand for corrosion resistance. 3.3. Rupture of the sample A 3.3.1. Macroscopic morphologies Fig. 7 shows the macro morphologies of sample A. Just as Fig. 7(a) shows, besides the butt weld in the flange, there is also a weld joint that joins the flange and the cylindrical body, and these two weld joints are converged in a point. It also shows that the crack propagated along the weld joint of the flange, noted by the arrow in Fig. 7(a). Further studies were conducted on the cross-sections of this fracture. Fig. 7(b) shows sample A’s two corresponding cross sections (marked by Sections 1 and 2), on each lie a long and deep ditch respectively. This kind of ditch throughout the weld joint of the flange is a serious defect in welding, which is so called lack of penetration (LOP). Besides the LOP inside the butt joint (marked by LOP1), an obvious crevice, which is also a LOP, can be found in the weld joint connecting the cylindrical body and the flange (marked by LOP2), just as Fig. 7(b) represents. As known, there is more residual stress in the weld joint, especially for those without a well heat treatment after welding [20]. As Fig. 7(b) shows, the two serious defects mentioned above are linked to an area, greatly increasing the stress concentration and residual stress, and the intersection becomes the weakest area of this weld joint. According to the characteristics of the fracture surface, it can be learnt that the crack initiated from the intersection of the two weld joints, and propagated along the weld joint of the flange, as showed in Fig. 7(b). With the help of 3D-SM, morphologies of the section surface can be observed more clearly. As Fig. 7(c) shows, the welding flux did not fill the gap between the cylindrical body and the flange, leaving a serious defect that the cylindrical body and the flange were jointed only by two small areas. Judging by the morphologies of cross section, crack origin site can be located, as illustrated in Fig. 7(d), which is the weakest area of the weld joint. Fig. 5. EBSD of the butt weld. 168 Y.-Y. Ma et al. / Engineering Failure Analysis 47 (2015) 162–177