Y Gong Z-G Yang/ Materials and Design 32(2011)671-681 (a) (b) Fig 8. External appearances of piping in the economizer(a) header and steam piping and(b)accumulation of ash dust. (a (b) Fig. 9. External appearances of piping in the air preheater (a uniform corrosion and (b)accumulation of rust. was more serious? As the steam temperature in the air preheater steel equaling to Din X15CrNiSi2012 in German Standards. The was relatively lower, the liquid phase concentration of the steam existence of Si element in these two metals can facilitate superie within it was thereby even higher. It is a common sense that resistance to oxidation at high temperature. According to the anal- ron-based materials are most apt to rust under wet and oxygen- ysis results, the two matrix metals were both qualified. rich environment. Consequently, serious uniform corrosion tool Etched in agent of CuSO4 4 g, HCl 20 ml and ethanol 20 ml, the place on the piping wall, but it would not affect the normal service metallographic structures of the inlet tube matrix metal are dis of the whole equipment. played in Fig. 10. As is shown in Fig. 10a, this material exhibited a duplex microstructure of austenite and 8 ferrite. However, as is 3. Failure analysis shown in Fig. 10b, corrosion products had penetrated into the material, i.e. the evidence of intergranular corrosion. As a result, Based on the above investigations, conclusion can be put for- the duplex microstructure would be gradually destroyed with the ard that corrosion extent of the whole fw cfb boiler ncrease of the amount of the corrosion products, and finally result serious enough. Among them, attention should be mainly n intergranular fracture on the boundaries between austenites and two components, i.e. the nozzle with its inlet tube and the ferrites under stresses doors of the refeed valve. Thus, following failure analysis will be fo- cused on them two 3. 1. 2. SEM and eDs After cutting and sampling, cross-section of the fractured inlet tube is shown in Fig. 1la, from which a brown rust layer as well as an obvious width gradient can be observed. The two phenomena 3. 1.1. Matrix metals inspection both verified the assumption mentioned above that the fracture Chemical compositions of the matrix metals of the nozzle and may have been caused by the interaction between corrosion and the inlet tube for primary air are listed in Table 1, which are erosive wear. Further magnified under sEm, two different sorts of respectively in accordance with the requirements of zG3Cr25Ni20 layers that respectively represented the matrix metal ( the com- [25 and 1Cr20Ni14Si2 [26 specifications in Chinese National Stan- pacted part in the middle, light grey color)and the corrosion prod dards. ZG3Cr25N120 represents a kind of heat-resistant cast steel, ucts(the pitted part on two sides, deep grey color)can be seen in while 1Cr20Ni14Si2 is a kind of heat-resistant austenitic stainless Fig. 11b, and the widths of the corrosion products layers had Table 1 Chemical compositions of the nozzle and the inlet tube(wt.). 051 0.20-035 18.0-220 ≤050 1Cr20Ni14Si2 0.20 120-150 50-2 ≤1.50 "l denotes the content lower than 0.5 wt%, the same below.was more serious? As the steam temperature in the air preheater was relatively lower, the liquid phase concentration of the steam within it was thereby even higher. It is a common sense that iron-based materials are most apt to rust under wet and oxygen￾rich environment. Consequently, serious uniform corrosion took place on the piping wall, but it would not affect the normal service of the whole equipment. 3. Failure analysis Based on the above investigations, conclusion can be put for￾ward that corrosion extent of the whole FW CFB boiler was not serious enough. Among them, attention should be mainly paid to two components, i.e. the nozzle with its inlet tube and the manhole doors of the refeed valve. Thus, following failure analysis will be fo￾cused on them two. 3.1. Nozzle 3.1.1. Matrix metals inspection Chemical compositions of the matrix metals of the nozzle and the inlet tube for primary air are listed in Table 1, which are respectively in accordance with the requirements of ZG3Cr25Ni20 [25] and 1Cr20Ni14Si2 [26] specifications in Chinese National Stan￾dards. ZG3Cr25Ni20 represents a kind of heat-resistant cast steel, while 1Cr20Ni14Si2 is a kind of heat-resistant austenitic stainless steel equaling to Din X15CrNiSi20.12 in German Standards. The existence of Si element in these two metals can facilitate superior resistance to oxidation at high temperature. According to the anal￾ysis results, the two matrix metals were both qualified. Etched in agent of CuSO4 4 g, HCl 20 ml and ethanol 20 ml, the metallographic structures of the inlet tube matrix metal are dis￾played in Fig. 10. As is shown in Fig. 10a, this material exhibited a duplex microstructure of austenite and d ferrite. However, as is shown in Fig. 10b, corrosion products had penetrated into the material, i.e. the evidence of intergranular corrosion. As a result, the duplex microstructure would be gradually destroyed with the increase of the amount of the corrosion products, and finally result in intergranular fracture on the boundaries between austenites and ferrites under stresses. 3.1.2. SEM and EDS After cutting and sampling, cross-section of the fractured inlet tube is shown in Fig. 11a, from which a brown rust layer as well as an obvious width gradient can be observed. The two phenomena both verified the assumption mentioned above that the fracture may have been caused by the interaction between corrosion and erosive wear. Further magnified under SEM, two different sorts of layers that respectively represented the matrix metal (the com￾pacted part in the middle, light grey color) and the corrosion prod￾ucts (the pitted part on two sides, deep grey color) can be seen in Fig. 11b, and the widths of the corrosion products layers had al￾Fig. 8. External appearances of piping in the economizer (a) header and steam piping and (b) accumulation of ash dust. Fig. 9. External appearances of piping in the air preheater (a) uniform corrosion and (b) accumulation of rust. Table 1 Chemical compositions of the nozzle and the inlet tube (wt.%). Element C Cr Ni Si Mo Mn Fe Nozzle 0.33 23.64 19.38 1.78 0.51 0.55 53.81 ZG3Cr25Ni20 0.20–0.35 24.0–28 18.0–22.0 62.0 60.50 62.0 Rest Tube 0.09 21.05 11.00 1.54 0.22 0.73 65.37 1Cr20Ni14Si2 60.20 19.0–22.0 12.0–15.0 1.50–2.50 /a 61.50 Rest a ‘‘/” denotes the content lower than 0.5 wt.%, the same below. Y. Gong, Z.-G. Yang / Materials and Design 32 (2011) 671–681 675