A Study on Sulfide Stress Cracking Susceptibility of GMA Girth Welds in X80 Grade Pipes
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The present work evaluates the SSC susceptibility of linepipe steel weld metals produced with various microstructures consisting of different ratios of intragranular (acicular) ferrite and grain boundary ferrite. It is shown that weld metal with high fractions of intragranular ferrite and low grain boundary ferrite passed SSC tests even though their hardness exceeded 250 HV, the widely accepted guideline to prevent SSC fracture initiation. Using a novel combination of hydrogen microprinting combined with SEM and TEM electron microscopy analysis techniques, the intragranular ferrite grain boundaries are shown to provide key hydrogen trapping sites which consist of fine grains and finely dispersed nano-scale carbide precipitates. The presence of a high fraction of trapping sites is suggested to account for the good SSC resistance coupled with high hardness, while increased grain boundary ferrite led to rejection during SSC testing due to inferior fracture toughness associated with coarser grains.
The authors are grateful to CanmetMATERIALS, Nippon Steel & Sumikin Engineering Co., Ltd. (Japan), TransCanada Pipelines, and the Natural Sciences and Engineering Research Council (NSERC) of Canada for supporting this research. The EM research described in this paper was performed at the Canadian Centre for Electron Microscopy at McMaster University, which is supported by NSERC and other government agencies.
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