| The effects of nonmetallic infusions and microstructures on the sulfide stress-corrosion cracking (SSCC) of welded line-pipe steels have been investigated by using electrical potential method. SSCC tests were conducted in H₂S-saturated aqueous solution containing 5 wt.% NaCl and 0.5 wt.% CH₃ COOH at temperature of 35±2℃ by using smooth U-bend specimens. Sulfide stress-corrosion (SSC) crack initiation and propagation took place in the weld fusion zone (WFZ) of two different kinds of American Petroleum Institute (API) grade X-65 due to the presence of elongated nonmetallic inclusions such as manganese sulfides. However, SSC crack of API grade J-55 initiated and also propagated along the pearlite band nearly perpendicular to the specimen surface of heat affected zone (HAZ) of both upper and lower half regions of the full thickness due to the globular morphology of nonmetallic inclusions such as calcium sulfides in WFZ produced by Ca treatment. And the unstable crack propagation was arrested in the midthickness region due to the orientation change of pearlite band nearly by 90˚, which gives rise to the occurrence of the stripes in the SSCC fracture surface. The SSCC kinetics of line-pipe steel weldments have been divided into the three distinct stages, namely crack incubation, slow crack propagation and unstable crack propagation stages, and were discussed in terms of nonmetallic inclusions and microstructures. It is suggested that the susceptibility to SSCC was rather affected by nonmetallic inclusions than microstructures. |
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