Engineering Failure Analysis 47(2015)162-177 Contents lists available at Science Direct VGINEFNING Engineering Failure Analysis ELSEVIER journalhomepagewww.elsevier.com/locate/engfailanal Failure analysis on circulating water pump of duplex stainless steel in 1000 MW ultra-supercritical thermal power unit Yue-Yue Ma, Shi Yan, Zhen-Guo Yang Guo-Shui Qi, Xin-You He b Zhejiang Zhengneng Jiaxing Electric Power Co, LTD, Jiaxing, Zhejiang Province 314201, PR China ARTICLE INFO A BSTRACT With a large number of properties suc mechanical properties and excellent corrosion resistance, 2205 duplex stainle SS)has been extensively used in many eived in revised form 29 September 201 industries for the last decades. However welding procedures will induce embrit Available online 23 October 2014 tlement of the weld joint, seriously decreasing the safety reliability of the weld joint. In this study, lots of unexpected fractures occurred on 2205 DSS which was used as the materia for making circulating water pump(CWP)in an ultra-supercritical thermal power plant of stainless steel (Dss) China for the first time. By means of diverse characterization methods, comprehensive Fracture investigation was carried out on the failed CWP. Analysis results reveal that many lack Circulating water pump penetrations(LOPs)in the weld joint and unbalance ferrite/austenite ratio induced by r welding procedures should be responsible for the fracture of the CWP. And effective countermeasures and suggestions were also proposed. So the analysis results have instructive significance for the fracture prevention of the CwP, even for ensuring safety operation of other equipment under similar seawater environment. e 2014 Elsevier Ltd. All rights reserved. 1 Introduction As one of the largest thermal power plants in the eastern part of China, Jiaxing power plant phase lll has two 1000 MW ltra-supercritical thermal power generating units, which are named by 7# and 8#f, respectively. these two units were put into commercial operation on June 23, 2011 and October 20, 2011, respectively, contributing a lot to the development local economic development. irculating water system is an important facility in a thermal power generating unit, which is used to pump seawater and then to feed seawater into a condenser to cool exhaust steam by heat exchange. Hereby, each of the two units of Jiaxing power plant phase lll was equipped with three CWPs, named 7A, 7B, 7C and 8A, 8B, 8C, respectively, all of which have same structure, designed and manufactured by Hitachi Pump Manufacture(Wuxi)Co., Ltd Fig. 1(a)and Fig. 1(b) show the external appearance and the structure of 8A CWP respectively Since the surrounding of the CWP's shell is natural seawater which usually contains high contents of salts, chloride ions and sediment particles, it requires a high performances such as excellent corrosion resistance and good mechanical proper ties for the material used in the cwps so dss is one of the best choices in this condition. In China. 2205 DSS was the first time used as the material of CWPs in thermal power plants. Corresponding author. Tel: +86 21 65642523: fax: +86 21 65103056. E-mail address: zgyangefudanedu cn(Z -G Yang). http://dxdoiorg/10.1016/jengfailanal2014.09.014 1350-6307/0 2014 Elsevier Ltd. All rights reserved
Failure analysis on circulating water pump of duplex stainless steel in 1000 MW ultra-supercritical thermal power unit Yue-Yue Ma a , Shi Yan a , Zhen-Guo Yang a,⇑ , Guo-Shui Qi b , Xin-You He b aDepartment of Materials Science, Fudan University, Shanghai 200433, PR China b Zhejiang Zhengneng Jiaxing Electric Power Co., LTD, Jiaxing, Zhejiang Province 314201, PR China article info Article history: Received 4 April 2013 Received in revised form 29 September 2014 Accepted 30 September 2014 Available online 23 October 2014 Keywords: Duplex stainless steel (DSS) Fracture Circulating water pump Failure analysis abstract With a large number of properties such as good mechanical properties and excellent corrosion resistance, 2205 duplex stainless steel (DSS) has been extensively used in many industries for the last decades. However, improper welding procedures will induce embrittlement of the weld joint, seriously decreasing the safety reliability of the weld joint. In this study, lots of unexpected fractures occurred on 2205 DSS which was used as the material for making circulating water pump (CWP) in an ultra-supercritical thermal power plant of China for the first time. By means of diverse characterization methods, comprehensive investigation was carried out on the failed CWP. Analysis results reveal that many lack of penetrations (LOPs) in the weld joint and unbalance ferrite/austenite ratio induced by improper welding procedures should be responsible for the fracture of the CWP. And effective countermeasures and suggestions were also proposed. So the analysis results have instructive significance for the fracture prevention of the CWP, even for ensuring safety operation of other equipment under similar seawater environment. 2014 Elsevier Ltd. All rights reserved. 1. Introduction As one of the largest thermal power plants in the eastern part of China, Jiaxing power plant phase III has two 1000 MW ultra-supercritical thermal power generating units, which are named by 7# and 8#, respectively. These two units were put into commercial operation on June 23, 2011 and October 20, 2011, respectively, contributing a lot to the development of local economic development. Circulating water system is an important facility in a thermal power generating unit, which is used to pump seawater and then to feed seawater into a condenser to cool exhaust steam by heat exchange. Hereby, each of the two units of Jiaxing power plant phase III was equipped with three CWPs, named 7A, 7B, 7C and 8A, 8B, 8C, respectively, all of which have same structure, designed and manufactured by Hitachi Pump Manufacture (Wuxi) Co., Ltd. Fig. 1(a) and Fig. 1(b) show the external appearance and the structure of 8A CWP respectively. Since the surrounding of the CWP’s shell is natural seawater which usually contains high contents of salts, chloride ions and sediment particles, it requires a high performances such as excellent corrosion resistance and good mechanical properties for the material used in the CWPs, so DSS is one of the best choices in this condition. In China, 2205 DSS was the first time used as the material of CWPs in thermal power plants. http://dx.doi.org/10.1016/j.engfailanal.2014.09.014 1350-6307/ 2014 Elsevier Ltd. All rights reserved. ⇑ Corresponding author. Tel.: +86 21 65642523; fax: +86 21 65103056. E-mail address: zgyang@fudan.edu.cn (Z.-G. Yang). Engineering Failure Analysis 47 (2015) 162–177 Contents lists available at ScienceDirect Engineering Failure Analysis journal homepage: www.elsevier.com/locate/engfailanal
Y-Y Ma et aL/ Engineering Failure Analysis 47(2015)162-177 163 However, in this event, after only ten months'operation, substantially less than the design lifetime of 30 years, a number of severe fractures occurred on the CwP of thermal power unit 8, as shown in Fig. 2, causing substantial economic losses as well as potential safety problems. By means of visual inspection, it was easy to find that most of the fractures occurred on the weld joints rather than the base materials, just as Fig. 2(a)-(c)show. Material property, manufacturing technology, equip- nt operation, service environment, routine maintenance or other factors, which were the main causes for inducing these abnormal fractures, were urgently investigated. Consequently, a comprehensive failure analysis including a variety of characterization methods was conducted to identify the root cause based on our previous failure analysis experiences 1-9]. 田 1. Appearance and structure of CWP: (a)appearance of SA CWP and(b)structure of the CWP
However, in this event, after only ten months’ operation, substantially less than the design lifetime of 30 years, a number of severe fractures occurred on the CWP of thermal power unit 8, as shown in Fig. 2, causing substantial economic losses as well as potential safety problems. By means of visual inspection, it was easy to find that most of the fractures occurred on the weld joints rather than the base materials, just as Fig. 2(a)–(c) show. Material property, manufacturing technology, equipment operation, service environment, routine maintenance or other factors, which were the main causes for inducing these abnormal fractures, were urgently investigated. Consequently, a comprehensive failure analysis including a variety of characterization methods was conducted to identify the root cause based on our previous failure analysis experiences [1–9]. Fig. 1. Appearance and structure of CWP: (a) appearance of 8A CWP and (b) structure of the CWP. Y.-Y. Ma et al. / Engineering Failure Analysis 47 (2015) 162–177 163
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 of
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, corrosion 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 numbers 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 cylindrical 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 conducted. 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 backscatter 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 chromatography (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 respectively. 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 structure 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
Y-Y Ma et aL/ Engineering Failure Analysis 47(2015)162-177 (c) (d) Fig. 2. Fractures on the CWP's flange: (a) welding joint 1# (b)welding joint 2#, (c) welding joint 3# and (d) base material
Fig. 2. Fractures on the CWP’s flange: (a) welding joint 1#, (b) welding joint 2#, (c) welding joint 3# and (d) base material. Y.-Y. Ma et al. / Engineering Failure Analysis 47 (2015) 162–177 165
Y-Y Ma et aL/Engineering Failure Analysis 47(2015)162-177 (b) Fig 3. Macroscopic appearance of the failed CWP pipe and samples: (a)appearance of the failed connecting pipe, (b)crack A and (c) crack B Chemical composition of the base material and weld joint of the CwP (wte Element SS of the cylindrical body 8.18 290 1.0-23.0 2.5-35 GBT21833 21.0-23.0 45-6.5 2.5-3.5
Fig. 3. Macroscopic appearance of the failed CWP pipe and samples: (a) appearance of the failed connecting pipe, (b) crack A and (c) crack B. Table 1 Chemical composition of the base material and weld joint of the CWP (wt%). Element C Cr Ni Mo N DSS of the flange 0.015 22.26 5.20 3.22 0.17 DSS of the cylindrical body 0.019 22.52 5.47 3.02 0.17 Weld joint 0.35 22.22 8.18 2.90 0.11 ASTM-A790/A790M-09 <0.03 21.0–23.0 4.5–6.5 2.5–3.5 0.08–0.20 GB/T 21833 <0.03 21.0–23.0 4.5–6.5 2.5–3.5 0.08–0.20 166 Y.-Y. Ma et al. / Engineering Failure Analysis 47 (2015) 162–177