Y-Y Ma et aL/ Engineering Failure Analysis 47(2015)162-177 169 Fig. 6. SEM morphologies of a sample's cross-section after Charpy impact test:(a)total morphology under low magnification and( b)dimples under high Table 2 Element constituents of the seawater(ppm, equals to mg/L). Fe Seawater 565×103 1.22x102 0.14 <0.002 <0.002 0025 33. 2. SEM and EDS In order to find the root cause of the fracture, Section 2 of sample a that was cleaned thoroughly by ultrasonic cleaning was studied further by SEM and EDS. The total morphology of the origin site showed in Fig 8(a) was selected to be observed thoroughly. Fig 8(b) shows the overall morphology of the origin site, where an uneven surface can be observed More def inite cracking origin site can be located in a further magnified scheme in Fig 8(c), besides, some impurities near the cracking rigin site can also be found, displayed by Fig 8(d Additionally, chemical compositions near the cracking origin site(sites 001, 002, 003 in Fig. 9)were detected by EDs. Based on the results in Table 3, very high carbon elements were detected in all the three sites, demonstrating the fracture urface of the weld joint was contaminated by organic substances under the seawater environment. Further experiments were conducted on the fillet weld Fig 10 shows the overall appearance of the fillet weld connecting the flange and the cylindrical body. With a high magnification, multiple microcracks were observed by SEM, which strongly enhanced the conclusion about the embrittlement of the weld joint 3.4. Rupture analysis of sample b Fig 11 shows the appearances of sample B, which does not locate on the weld joint. Fig. 11(a) shows the overall morphol- ogy of this crack on the sealing surface and Fig. 11(b)exhibits the two corresponding cross sections. As Fig. 11(c)and(d3.3.2. SEM and EDS In order to find the root cause of the fracture, Section 2 of sample A that was cleaned thoroughly by ultrasonic cleaning, was studied further by SEM and EDS. The total morphology of the origin site showed in Fig. 8(a) was selected to be observed thoroughly. Fig. 8(b) shows the overall morphology of the origin site, where an uneven surface can be observed. More def￾inite cracking origin site can be located in a further magnified scheme in Fig. 8(c), besides, some impurities near the cracking origin site can also be found, displayed by Fig. 8(d). Additionally, chemical compositions near the cracking origin site (sites 001, 002, 003 in Fig. 9) were detected by EDS. Based on the results in Table 3, very high carbon elements were detected in all the three sites, demonstrating the fracture surface of the weld joint was contaminated by organic substances under the seawater environment. Further experiments were conducted on the fillet weld. Fig. 10 shows the overall appearance of the fillet weld connecting the flange and the cylindrical body. With a high magnification, multiple microcracks were observed by SEM, which strongly enhanced the conclusion about the embrittlement of the weld joint. 3.4. Rupture analysis of sample B Fig. 11 shows the appearances of sample B, which does not locate on the weld joint. Fig. 11(a) shows the overall morphol￾ogy of this crack on the sealing surface and Fig. 11(b) exhibits the two corresponding cross sections. As Fig. 11(c) and (d) Fig. 6. SEM morphologies of a sample’s cross-section after Charpy impact test: (a) total morphology under low magnification and (b) dimples under high magnification. Table 2 Element constituents of the seawater (ppm, equals to mg/L). Element Cl Mg Al Cu Fe Ti Mn Seawater 5.65 103 1.22 102 0.14 <0.002 0.92 <0.002 0.025 Y.-Y. Ma et al. / Engineering Failure Analysis 47 (2015) 162–177 169