S-M Hu et al /Case Studies in Engineering Failure Analysis 3(2015) 52-61 2.2. Material examination of heat-exchange tubes Photoelectric direct reading spectrometer was applied to investigate chemical compositions of tube materials, listed in Table 2. The actual compositions of materials are in correspondence to the specifications of 10 carbon steel in GB 9948-2006 Chinese National Standards [15. inclusions. Therefore, the material could be regarded as qualified to the requirement in genet xialgrar ial direction.As it Fig 4 shows the microstructure of matrix imaged by metallographic microscope(MM)in circumferential direction. As it exhibits, the material has typical feature of low-carbon steel: consisting of ferrite and pearlite equia s with no visible 2.3. Failure analysis of heat-exchange tubes 2.3.1.2 heat-exchange To start with, stereo microscope was used to observe the morphologies of 2" tube. Fig. 5(a) is the appearance of outer wall with obvious defects due to uniform corrosion, including rufous corrosion products, red translucent substances and several irregular shallow holes With respect to the inner wall, there are some different morphologies which imply another story. As Fig 5(b)reveals, distinct failure phenomena referring to visible tiny and deep pit has taken place there, in accordance t typical pitting characteris fterwards, microscopic morphologies were imaged by scanning electron microscope(SEM). In Fig. 6(a), corrosion holes by the cut edge of outer wall appear grey in color surrounded by fish-scale lines, indicating that those holes might act as b Fig. 6. Microscopic morphology of defect zones at 2"tube(a)grey-colored pits by the edge of outer wall; (b) cracking of corrosion layers on the inner wa2.2. Material examination of heat-exchange tubes Photoelectric direct reading spectrometer was applied to investigate chemical compositions of tube materials, listed in Table 2. The actual compositions of materials are in correspondence to the specifications of 10 carbon steel in GB 9948-2006 Chinese National Standards [15]. Fig. 4 shows the microstructure of matrix imaged by metallographic microscope (MM) in circumferential direction. As it exhibits,the material has typical feature of low-carbon steel: consisting of ferrite and pearlite equiaxial grains with no visible inclusions. Therefore, the material could be regarded as qualified to the requirement in general. 2.3. Failure analysis of heat-exchange tubes 2.3.1. 2# heat-exchange tube To start with, stereo microscope was used to observe the morphologies of 2# tube. Fig. 5(a) is the appearance of outer wall with obvious defects due to uniform corrosion, including rufous corrosion products, red translucent substances and several irregular shallow holes. With respect to the inner wall, there are some different morphologies which imply another story. As Fig. 5(b) reveals, distinct failure phenomena referring to visible tiny and deep pit has taken place there, in accordance to typical pitting characteristic. Afterwards, microscopic morphologies were imaged by scanning electron microscope (SEM). In Fig. 6(a), corrosion holes by the cut edge of outer wall appear grey in color surrounded by fish-scale lines, indicating that those holes might act as Fig. 6. Microscopic morphology of defect zones at 2# tube (a) grey-colored pits by the edge of outer wall; (b) cracking of corrosion layers on the inner wall. 56 S.-M. Hu et al. / Case Studies in Engineering Failure Analysis 3 (2015) 52–61