Y-Y Ma et aL/ Engineering Failure Analysis 47(2015)162-177 Fuwa【 Cylindrical body Fig 10. SEM morphologies of the fillet joint: (a)overall morphology (24x)(b)magnification of the fillet joint(300x). (c)further magnification(1000x )and (d )microcracks under higher magnification(3000x). rate would prevent the forming of austenite. According to Adams [33, cooling rate in weld joints could be expressed V=2nKp (1) where Ve-cooling rate (C/s), At12/8=cooling time between 1200C and 800C(s), K- thermal conductivity(W m/oC). p=density (g/cm), C=heat capacity (/kg C), HI= heat input U /mm). f=thickness(mm). To-pre-heating temperature(oC). According to Eqs. (1)and (2). the use of pre-heating can help to an increase of At12/8. which contributes to the forming of austenite in the weld joint Badji [34 pointed out that throughout the weld regions of 2205 DSS, the optimal mechanical properties and an acceptable ferrite/austenite ratio corresponds to annealing at 1050C. Just mentioned above, there was no heat treatment after each welding, thus, an post weld heat treatment(Pwht)at 1050C is recommended to solve this 4.3. Fracture process and crack propagation analysis As mentioned before, the pump shell of the CwP weights 42 tons and locates vertically to the ground. When it was on operation, there was always accompanied with a vibration. So a cycle load was applied on the whole pump including the weld joint during its operation. With so many serious defects like LOPs and unbalanced ferrite/austenite ration discussed above, the weld joint was too weak to withstanding the cycle load, thus a premature fatigue fracture occurred. Due to the fracture of the flange, an unevenly load was distributed on the whole CWP thus an unbalance of the CWP occurred, causing other severe fractures. So the crack occurred on the base material belongs to a secondary fracture. The process of the fractures is illustrated in Fig. 13.rate would prevent the forming of austenite. According to Adams [33], cooling rate in weld joints could be expressed as follows: Vc ¼ 2pKqC f HI 2 ðT ToÞ 3 ð1Þ Z 1200 800 dT Vc ¼ ðHIÞ 2 4pKqCxf 2 1 ð800 ToÞ 2 1 ð1200 ToÞ 2 " # ð2Þ where Vc = cooling rate (C/s), Dt12/8 = cooling time between 1200 C and 800 C (s), K = thermal conductivity (W m/C), q = density (g/cm3 ), C = heat capacity (J/kgC), HI = heat input (J/mm), f = thickness (mm), T0 = pre-heating temperature (C). According to Eqs. (1) and (2), the use of pre-heating can help to an increase of Dt12/8, which contributes to the forming of austenite in the weld joint. Badji [34] pointed out that throughout the weld regions of 2205 DSS, the optimal mechanical properties and an acceptable ferrite/austenite ratio corresponds to annealing at 1050 C. Just mentioned above, there was no heat treatment after each welding, thus, an post weld heat treatment (PWHT) at 1050 C is recommended to solve this problem. 4.3. Fracture process and crack propagation analysis As mentioned before, the pump shell of the CWP weights 42 tons and locates vertically to the ground. When it was on operation, there was always accompanied with a vibration. So a cycle load was applied on the whole pump including the weld joint during its operation. With so many serious defects like LOPs and unbalanced ferrite/austenite ration discussed above, the weld joint was too weak to withstanding the cycle load, thus a premature fatigue fracture occurred. Due to the fracture of the flange, an unevenly load was distributed on the whole CWP thus an unbalance of the CWP occurred, causing other severe fractures. So the crack occurred on the base material belongs to a secondary fracture. The process of the fractures is illustrated in Fig. 13. Fig. 10. SEM morphologies of the fillet joint: (a) overall morphology (24), (b) magnification of the fillet joint (300), (c) further magnification (1000) and (d) microcracks under higher magnification (3000). Y.-Y. Ma et al. / Engineering Failure Analysis 47 (2015) 162–177 173