164 Y-Y Ma et aL/Engineering Failure Analysis 47(2015)162-177 The mechanism of these fractures on the dss used in CWP was carefully discussed. Finally, effective countermeasures and suggestions were proposed as well Actually, many researchers have focused on the properties of DSS, such as its fatigue behavior, welding property, corro- sion resistance [10-17. but such an engineering practical study of mechanical degradation on DSs applied in CWP of 1000 MW ultra-supercritical thermal power unit has been rarely reported. What's more, the phenomenon that large num- bers of fractures occurred on the flanges of the Cwp is even less reported. Therefore, the analyses and results given in this study have not only important engineering values in failure prevention of the CWPs used under seawater environment, but also practical significance in ensuring safety operation of other equipment under similar condition. 2. Experimental The 8A CWP is located in the Number 3 CWP house of Jiaxing power plant phase Ill, with a vertical structure and th ngth of the underground part is 17.1 m As shown in Fig. 1(b), 8A CWP is mainly composed with two parts, i.e. the pump shell that weighs 42 tons and the shaft that weights 26 tons. The pump shell mainly consists of an inlet bellmouth, a bell pipe four connecting pipes and a bent outlet from bottom to top. Each connecting pipe is constituted of two round flanges and a cylindrical body by means of welding. Hereby the two flanges are located on both ends of the connecting pipe, and each flange is made up with four same flange arcs with a thickness of 46 mm by means of welding technology The cylin- drical body was manufactured by a process of rolling and welding, with an outer diameter of 2200 mm and a thickness of 14 In this event, more than twenty severe cracks were discovered on the surface of 8A CWP. Among the damaged pipes, the everest one is the second connecting pipe counted from bottom to top, whose macroscopic appearance is showed in Fig. 3(a). Two target pairs of cracking samples analyzed in this study were both from the flange of this damaged pipe. One pairs crack occurred on the weld joint of the flange, noted by cracking a and the other occurred on base material of the flange, noted by cracking B. The location of the two samples and the appearances are displayed in Fig 3(a-c) 2. 2. Characterization methods In order to figure out the failure causes and mechanisms, a variety of characterization methods were successively con- ducted Oxygen nitrogen hydrogen(ONH)analyzer, carbon sulfur analyzer (CSA), and inductively coupled plasma atomic emission spectroscopy(ICP-AES) were used to inspect their chemical compositions. Optical microscopy(OM) was utilized to observe their metallographic structures and the austenite/ferrite ratio of the butt weld was obtained by electron back ter diffraction(EBSD)and dyeing calculation method under metalloscope, respectively. The impact toughness of the d used in the CWP was also measured by Charpy impact test and the constituents of the seawater were detected by ion chro- tography(IC)and ICP-AES. Meanwhile, besides further observation of the macroscopic morphologies of the ruptures on the two samples, three-dimensional stereomicroscopy(3D-SM) scanning electron microscopy (SEM)and energy dispersive spectrometry(eds) were adopted to analyze their microscopic morphologies along with micro-area compositio 3. Results and discussion 3.1. Matrix materials 3.1.1. Chemical compositions The chemical compositions of the cylindrical body, the flange and the weld joint of the CWP are listed in Table 1 respec- tively. It can be concluded that the materials used in the cylindrical body and the flange are the same, both of which are in accordance with the requirements of the UNS31803 grade dss [ 18(equals to the 2205 DSS in GB/T 21833-2008 19). Flux cored duplex stainless steel welding wire and gas shielded welding were used according to the manufactory. However, seen in the third row of Table 1, the carbon content at the weld is much higher than that at the cylindrical body and the flange. It meant that the quality of the weld joint was unqualified and it would induce the embrittlement of the weld joint. 3. 1.2. Metallographic structure the materials used in making the cylindrical body and flange are the same kind of DSS, which consists of ferrite and austenite distributing very evenly. The ferrite acts as the matrix, whose color is grey, while the austenite in white color distributes in the ferrite matrix. The grain of the two phases is quite clearly, so is the boundary. Fig 4(c) presents the metallograph ture of the weld joint, which is also consisted of ferrite and austenite but quite different with those of the cylindri and flange. It is obviously that the amount of ferrite is much more than that of the austenite with a dendritic grain shape. By means of EBSD, the ratio of the two phases in the microscopic field can be calculated. Just as the Fig. 5 shows, the amount ofThe mechanism of these fractures on the DSS used in CWP was carefully discussed. Finally, effective countermeasures and suggestions were proposed as well. Actually, many researchers have focused on the properties of DSS, such as its fatigue behavior, welding property, corro￾sion resistance [10–17], but such an engineering practical study of mechanical degradation on DSS applied in CWP of 1000 MW ultra-supercritical thermal power unit has been rarely reported. What’s more, the phenomenon that large num￾bers of fractures occurred on the flanges of the CWP is even less reported. Therefore, the analyses and results given in this study have not only important engineering values in failure prevention of the CWPs used under seawater environment, but also practical significance in ensuring safety operation of other equipment under similar condition. 2. Experimental 2.1. Visual observation The 8A CWP is located in the Number 3 CWP house of Jiaxing power plant phase III, with a vertical structure and the length of the underground part is 17.1 m. As shown in Fig. 1(b), 8A CWP is mainly composed with two parts, i.e. the pump shell that weighs 42 tons and the shaft that weights 26 tons. The pump shell mainly consists of an inlet bellmouth, a bell pipe, four connecting pipes and a bent outlet from bottom to top. Each connecting pipe is constituted of two round flanges and a cylindrical body by means of welding. Hereby, the two flanges are located on both ends of the connecting pipe, and each flange is made up with four same flange arcs with a thickness of 46 mm by means of welding technology. The cylin￾drical body was manufactured by a process of rolling and welding, with an outer diameter of 2200 mm and a thickness of 14 mm. In this event, more than twenty severe cracks were discovered on the surface of 8A CWP. Among the damaged pipes, the severest one is the second connecting pipe counted from bottom to top, whose macroscopic appearance is showed in Fig. 3(a). Two target pairs of cracking samples analyzed in this study were both from the flange of this damaged pipe. One pair’s crack occurred on the weld joint of the flange, noted by cracking A and the other occurred on base material of the flange, noted by cracking B. The location of the two samples and the appearances are displayed in Fig. 3(a)–(c) 2.2. Characterization methods In order to figure out the failure causes and mechanisms, a variety of characterization methods were successively con￾ducted. Oxygen nitrogen hydrogen (ONH) analyzer, carbon sulfur analyzer (CSA), and inductively coupled plasma atomic emission spectroscopy (ICP-AES) were used to inspect their chemical compositions. Optical microscopy (OM) was utilized to observe their metallographic structures and the austenite/ferrite ratio of the butt weld was obtained by electron backscat￾ter diffraction (EBSD) and dyeing calculation method under metalloscope, respectively. The impact toughness of the DSS used in the CWP was also measured by Charpy impact test. And the constituents of the seawater were detected by ion chro￾matography (IC) and ICP-AES. Meanwhile, besides further observation of the macroscopic morphologies of the ruptures on the two samples, three-dimensional stereomicroscopy (3D-SM), scanning electron microscopy (SEM) and energy dispersive spectrometry (EDS) were adopted to analyze their microscopic morphologies along with micro-area compositions. 3. Results and discussion 3.1. Matrix materials 3.1.1. Chemical compositions The chemical compositions of the cylindrical body, the flange and the weld joint of the CWP are listed in Table 1 respec￾tively. It can be concluded that the materials used in the cylindrical body and the flange are the same, both of which are in accordance with the requirements of the UNS31803 grade DSS [18] (equals to the 2205 DSS in GB/T 21833-2008 [19]). Flux cored duplex stainless steel welding wire and gas shielded welding were used according to the manufactory. However, as seen in the third row of Table 1, the carbon content at the weld is much higher than that at the cylindrical body and the flange. It meant that the quality of the weld joint was unqualified and it would induce the embrittlement of the weld joint. 3.1.2. Metallographic structure The metallographic structures of the matrix are displayed in Fig. 4. Fig. 4(a) and (b) present the metallographic structures of the material used in the flange and cylindrical body, both of which show a typical 2205 DSS structure. It is obviously that the materials used in making the cylindrical body and flange are the same kind of DSS, which consists of ferrite and austenite, distributing very evenly. The ferrite acts as the matrix, whose color is grey, while the austenite in white color distributes in the ferrite matrix. The grain of the two phases is quite clearly, so is the boundary. Fig. 4(c) presents the metallographic struc￾ture of the weld joint, which is also consisted of ferrite and austenite, but quite different with those of the cylindrical body and flange. It is obviously that the amount of ferrite is much more than that of the austenite with a dendritic grain shape. By means of EBSD, the ratio of the two phases in the microscopic field can be calculated. Just as the Fig. 5 shows, the amount of 164 Y.-Y. Ma et al. / Engineering Failure Analysis 47 (2015) 162–177