970 Gong, Yang, Yao and Yang Materials and Corrosion 2011. 62. No a) 89 234 b) b) Figure 4. Macroscopic morphologies of defects on failure pipes: (a) three neighboring concaves, and(b) peanut-like concave morphologies of (1)three neighboring concaves, as well as(2 )one peanut-like concave, on two failure pipes, and the diameters of these concaves were all not exceeding 10 mm. Subsequent investigations would be particularly carried out on the pip bearing the special peanut-like concave, whose location was 5.8m away from the floating head, ie, the rightmost part of the heat Figure 5. Metal structures of the failure pipe: (a)100x;(b)500x 2.2 Matrix material examination speaking, the matrix material of the failure pipe could be regarded qualified Chemical compositions of the matrix material of the failure pipe with peanut-like concave are listed in Table 1, which were in 2.3 3-D stereomicroscopy accordance with the requirements of 10 carbon steel specifica- tions in GB/T 699-1999 standard of China[3](corresponding to By using the Hirox KH-7700 3D digital microscope, morphologies ASTM 1010 steel (4). Etched in agent of HNO3 2 mL and ethanol of the peanut-like concave were further investigated. Figure 6a 98 mL, the metallographic structures of the matrix material are presents the total morphology of this concave with dimension of presented in Fig. 5. It is obvious in Fig 5a that the microstructure nearly 3 x 6mm. It can be easily inferred from Fig. 6b, which consisted of ferrites and pearlites, and the average ASTM grain shows the ridge in the middle part of the"peanut, "that this size of the ferrites was about seven. Furthermore, inclusions in special-shape concave may have been resulted from connection of the ferritic grains were fairly uniform, nevertheless the pearlites two neighboring round concaves. Further magnified, more ad already dissolved to some extent and consequently led to detailed morphologies of the left and the right parts of the increase of cementites content, seen in Fig 5b, which may act as "peanut"were, respectively, revealed. As is shown in Fig. 6c, the susceptible initiating sites of corrosion. However totally densely distributed pits with diameters not exceeding 0. 1mm Table 1. Chemical compositions of the failure pipe(wt%) Elemen P S Failure pipe 0.1 0018 007-0.13 0.17-0.37 <0.030 008-0. 0.050 denotes the content is undefined o 2011 WILEY-VCH Verlag Gmbh Co KGaA, Weinheim www.matcorr.commorphologies of (1) three neighboring concaves, as well as (2) one peanut-like concave, on two failure pipes, and the diameters of these concaves were all not exceeding 10 mm. Subsequent investigations would be particularly carried out on the pipe bearing the special peanut-like concave, whose location was 5.8 m away from the floating head, i.e., the rightmost part of the heat exchanger in Fig. 2b. 2.2 Matrix material examination Chemical compositions of the matrix material of the failure pipe with peanut-like concave are listed in Table 1, which were in accordance with the requirements of 10 carbon steel specifica￾tions in GB/T 699-1999 standard of China [3] (corresponding to ASTM 1010 steel [4]). Etched in agent of HNO3 2 mL and ethanol 98 mL, the metallographic structures of the matrix material are presented in Fig. 5. It is obvious in Fig. 5a that the microstructure consisted of ferrites and pearlites, and the average ASTM grain size of the ferrites was about seven. Furthermore, inclusions in the ferritic grains were fairly uniform, nevertheless the pearlites had already dissolved to some extent and consequently led to increase of cementites content, seen in Fig. 5b, which may act as the susceptible initiating sites of corrosion. However totally speaking, the matrix material of the failure pipe could be regarded qualified. 2.3 3-D stereomicroscopy By using the Hirox KH-7700 3D digital microscope, morphologies of the peanut-like concave were further investigated. Figure 6a presents the total morphology of this concave with dimension of nearly 3
6 mm2 . It can be easily inferred from Fig. 6b, which shows the ridge in the middle part of the ‘‘peanut,’’ that this special-shape concave may have been resulted from connection of two neighboring round concaves. Further magnified, more detailed morphologies of the left and the right parts of the ‘‘peanut’’ were, respectively, revealed. As is shown in Fig. 6c, densely distributed pits with diameters not exceeding 0.1 mm 970 Gong, Yang, Yao and Yang Materials and Corrosion 2011, 62, No. 10 Figure 4. Macroscopic morphologies of defects on failure pipes: (a) three neighboring concaves, and (b) peanut-like concave Table 1. Chemical compositions of the failure pipe (wt%) Element C Si Mn P S Cr Ni Cu Failure pipe 0.109 0.258 0.436 0.018 0.005 0.027 0.007 0.016 10 0.07–0.13 0.17–0.37 0.35–0.65 0.030 0.020 0.15 0.30 0.25 ASTM 1010 0.08–0.13 – 0.30–0.60 0.040 0.050 ‘‘–’’ denotes the content is undefined. Figure 5. Metallographic structures of the failure pipe: (a) 100
; (b) 500

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