Y-Y Ma et aL/ Engineering Failure Analysis 47(2015)162-177 (b) (d) origin of the crack Fig 8. SEM morphologies of the fracture: (a)total morphology, (b)magnification of the cracks origin. (c)further magnification of the cracks origin and (d) further magnification of the impurity near the cracks origin. tudying a variety of weld imperfections. Kim[ 23 pointed out that the LOPs significantly reduced the fatigue lives of 9 mm thick transverse butt welded specimens without weld reinforcements, showing shorter fatigue life than JSSc-B Sanders and Lawrence [24 studied the effects of lack of penetration(LOP)and lack of fusion( LOF)on the fatigue behavior of butt welds. He concluded that LoP defects can seriously reduce the fatigue life, while inclined loF defects were generally less serious than loP defects Former studies25-27 also proved that LOP had much to do with welding procedure, including welding method, groove, welding speed, welding heat input and so on. In this event, the welding method used here was multi-layer and multi-pas The weld groove is one of the most important facts to affect the quality of welding. Here, the Double-V preparation with proper parameters is recommended, listed in Table 4. As for LOP2 shows in Fig. 7(c), the weld groove is improper because the depth of preparation is too short and the groove angle is quite small according to the standard of IS09692-1: 2003[28]. So, it is difficult for the weld flux to fill the gar ea though other operations are correct. Here, the k preparation with proper parameters is recommended also listed in Table 4.2. Unbalanced microstructure with excessive ferrite As the EBSD analysis result shows, there was a much higher ferrite to austenite ratio in the weld joint, also proved by means of determined metallographically point count method. As known, the toughness of dSS weld joint with higher con- tent of ferrite phase decreases remarkably According former researches [29-31 this phenomenon had much to do with the welding procedure especially the cooling rate or heat input. As Table 1 shows, the content of nitrogen in weld metal is in quirement with the standard, which indicates that the low austenite contend of the weld metal is rather due to the low heat input or high cooling rate employed in the welding than to nitrogen loss. Fig. 12 shows the vertical section of Fe- Cr-Ni ternary diagram based on Chromium-Equivalent presenting the phase transformation of 2205 DSS during the welding procedure Based on the Fig. 12, austenite wi to precipitate at about 1330C, above which all the phas in the weld joint is ferrite. However, during continuous cooling there is not enough time for austenite to precipitate until the temperature has decreased to about 1200C. when a sufficiently amount of nuclei has been formed 32. thus a high coolingstudying a variety of weld imperfections. Kim [23] pointed out that the LOPs significantly reduced the fatigue lives of 9 mm thick transverse butt welded specimens without weld reinforcements, showing shorter fatigue life than JSSC-B. Sanders and Lawrence [24] studied the effects of lack of penetration (LOP) and lack of fusion (LOF) on the fatigue behavior of butt welds. He concluded that LOP defects can seriously reduce the fatigue life, while inclined LOF defects were generally less serious than LOP defects. Former studies [25–27] also proved that LOP had much to do with welding procedure, including welding method, groove, welding speed, welding heat input and so on. In this event, the welding method used here was multi-layer and multi-pass welding and no sufficient heat treatment was conducted after the welding according to the manufacturer. The weld groove is one of the most important facts to affect the quality of welding. Here, the Double-V preparation with proper parameters is recommended, listed in Table 4. As for LOP2 shows in Fig. 7(c), the weld groove is improper because the depth of preparation is too short and the groove angle is quite small according to the standard of ISO9692-1:2003 [28]. So, it is difficult for the weld flux to fill the gap even though other operations are correct. Here, the K preparation with proper parameters is recommended, also listed in Table 4. 4.2. Unbalanced microstructure with excessive ferrite As the EBSD analysis result shows, there was a much higher ferrite to austenite ratio in the weld joint, also proved by means of determined metallographically point count method. As known, the toughness of DSS weld joint with higher con￾tent of ferrite phase decreases remarkably. According former researches [29–31], this phenomenon had much to do with the welding procedure, especially the cooling rate or heat input. As Table 1 shows, the content of nitrogen in weld metal is in requirement with the standard, which indicates that the low austenite contend of the weld metal is rather due to the low heat input or high cooling rate employed in the welding than to nitrogen loss. Fig. 12 shows the vertical section of Fe– Cr–Ni ternary diagram based on Chromium-Equivalent, clearly presenting the phase transformation of 2205 DSS during the welding procedure. Based on the Fig. 12, austenite will start to precipitate at about 1330 C, above which all the phase in the weld joint is ferrite. However, during continuous cooling there is not enough time for austenite to precipitate until the temperature has decreased to about 1200 C, when a sufficiently amount of nuclei has been formed [32], thus a high cooling Fig. 8. SEM morphologies of the fracture: (a) total morphology, (b) magnification of the crack’s origin, (c) further magnification of the crack’s origin and (d) further magnification of the impurity near the crack’s origin. Y.-Y. Ma et al. / Engineering Failure Analysis 47 (2015) 162–177 171