F-f. Chen et aL/ Engineering Failure Analysis 37(2014)42-52 a Fig. 6. Appearance of tubes 052003-17, 18(a) top of the tube (b)bottom of the tube. 2.1.2. Metallographic structure Fig. 3 is the metallographic structure of one in-service titanium tube. It is the typical metallographic structure of pure a-Ti -equiaxed polygonal grains that are evenly distributed with uniform size and clear grain boundary with no visible 2.1.3. Mechanical tests of titanium tubes The outer wall and inner wall of the in-service titanium tubes were exposed to high purity water steam and sea water espectively. Tests should be done to judge whether their mechanical performance had deteriorated after long time service The sample preparation and tensile testing of titanium tubes were done according to the standard of ASME SA370 [9) Fig 4 is the force-displacement curve. Table 2 shows the measured value of mechanical properties of in-service titanium tubes in unit 1 Table 3 further displays the comparison between the measured values and the standard values of AsME SB338 2]and Chinese GB T 3624-95 10 It can be learned from Fig 4 that the force-displacement curve of the titanium tube basically conforms to the tensile behavior of metal. Meanwhile, the comparison of several mechanical property indexes in Table 3 presents that the yielding strength, tensile strength and elongation percentage do not show any obvious deterioration and that all the performances have met the ASME SB338 standard [7] and Chinese GB/T 3624 standard [10. Therefore, the performance of the tube mate- rial is qualified and has not deteriorated Fig 5 is a group of pictures of microscopic and macroscopic fracture morphologies of an in-service titanium tube after tensile test. The fracture takes on a look of indention( Fig. 5(a))and the fracture surface is not smooth with tangles. also duc tile shear zones and dimples( fig. 5())are seen from the microscopic morphology of fracture edge, which is characteristic of ductile fracture. So the sample must have undergone sufficient plastic deformation before fracture At high magnification, there are hardly any inclusions in the dimples, indicating that the microscopic structure of the material is fine( Fig. 5(b)). Therefore, we can safely conclude that the mechanical performance of the in-service tube is good without indications of Till now, we have ruled out the possibility that the failure was caused by material problems.2.1.2. Metallographic structure Fig. 3 is the metallographic structure of one in-service titanium tube. It is the typical metallographic structure of pure a-Ti -equiaxed polygonal grains that are evenly distributed, with uniform size and clear grain boundary with no visible inclusions. 2.1.3. Mechanical tests of titanium tubes The outer wall and inner wall of the in-service titanium tubes were exposed to high purity water steam and sea water respectively. Tests should be done to judge whether their mechanical performance had deteriorated after long time service. The sample preparation and tensile testing of titanium tubes were done according to the standard of ASME SA370 [9]. Fig. 4 is the force–displacement curve. Table 2 shows the measured value of mechanical properties of in-service titanium tubes in unit 1. Table 3 further displays the comparison between the measured values and the standard values of ASME SB338 [2] and Chinese GB/T 3624-95 [10]. It can be learned from Fig. 4 that the force–displacement curve of the titanium tube basically conforms to the tensile behavior of metal. Meanwhile, the comparison of several mechanical property indexes in Table 3 presents that the yielding strength, tensile strength and elongation percentage do not show any obvious deterioration and that all the performances have met the ASME SB338 standard [7] and Chinese GB/T 3624 standard [10]. Therefore, the performance of the tube mate￾rial is qualified and has not deteriorated. Fig. 5 is a group of pictures of microscopic and macroscopic fracture morphologies of an in-service titanium tube after tensile test. The fracture takes on a look of indention (Fig. 5(a)) and the fracture surface is not smooth with tangles. Also duc￾tile shear zones and dimples (Fig. 5(b)) are seen from the microscopic morphology of fracture edge, which is characteristic of ductile fracture. So the sample must have undergone sufficient plastic deformation before fracture. At high magnification, there are hardly any inclusions in the dimples, indicating that the microscopic structure of the material is fine (Fig. 5(b)). Therefore, we can safely conclude that the mechanical performance of the in-service tube is good without indications of deterioration. Till now, we have ruled out the possibility that the failure was caused by material problems. Fig. 6. Appearance of tubes 052003-17, 18 (a) top of the tube (b) bottom of the tube. 46 F.-J. Chen et al. / Engineering Failure Analysis 37 (2014) 42–52