Influence of the Post-Weld Heat Treatment on the Low-Temperature Toughness of ERW API X70 Line Pipe

• Author / Creator

  Anderson, Neil E.

• Increasingly stricter performance requirements of pipelines are being driven by regulatory bodies, the energy industry, and public interest in protecting public safety, health, and the environment. A critical mechanical property influencing performance is toughness, and particular interest is focused on toughness performance at low temperatures (down to -45 ◦C). Toughness is typically measured by impact testing at the temperature of interest, with industry typically using the Charpy V-Notch impact test.

  A study was conducted to identify the metallurgical factors influencing low-temperature toughness performance of grade API X70 pipe longitudinal weld seams manufactured by
  electric resistance welding. The electric resistance welding bondline is known to have different post-welding heat treatments and there is a specific interest in -45 ◦C performance. The investigation was undertaken using advanced characterization techniques, including
  electron backscatter diffraction and transmission scanning electron microscopy, and found that low-temperature toughness was correlated with post-weld heat treatments.

  Post-welding heat treatment was observed to produce a fine-grained, allotriomorphic ferrite at the bondline. It was found that certain post-welding heat treatments produced predominantly coarse bainitic structures in the weld seam near the bondline, while others produced fine allotriomorphic ferrite homogeneous with the bondline. Martensite-austenite
  (M-A) constituents were also observed. Low-temperature toughness performance was observed to be higher in specimens which exhibited a homogeneous weld
  seam composed of fine-grained, equiaxed, allotriomorphic ferrite, with predominantly high angle grain boundaries (greater than 10◦), minimized M-A size and fraction, and
  lower material strain. No crystallographic texture was observed in any of the specimens. Additionally, low toughness performance was observed in specimens with a soft-zone in
  the weld seam within 100-150 μm of the bondline. This soft-zone is suspected to bedue to the development of a dual-phase microstructure in the weld seam adjacent to the bondline.

  A design-of-experiment study was conducted to gain insight into the microstructural development during post-weld heat treatment, specifically investigating the effect of varying peak temperatures on low-temperature toughness. A Gleeble thermomechanical simulator was used to create Charpy V-Notch specimens and conduct dilatometry investigations. A correlation was found between NbC0.87 precipitate solubility and phase transformation behaviour. These observations agreed with published literature and thermodynamic
  calculations regarding the role of MX type precipitates as austenite grain refiners, soluble Nb in austenite retarding the diffusional austenite-to-ferrite transformations, and soluble C in austenite promoting displacive austenite-to-ferrite transformations. Post-weld heat treatments which exceeded the critical temperature for complete
  precipitate solubility were found to produce a weld seam with a predominantly coarse-grained, bainitic microstructure adjacent to the bondline. This was inhomogeneous
  with the fine-grained, equiaxed, allotriomorphic bondline, and had less desirable low-temperature toughness performance. Post-weld heat treatments below the critical temperature for complete precipitate solubility were found to result in a homogeneous, fine-grained, equiaxed allotriomorphic ferrite weld seam and improved low-temperature toughness performance.

  Inferring from this work, a hypothesis is proposed regarding the role of MX type precipitate formers on the microstructural development, and suggestions are given for the further work required to prove this hypothesis.

• Subjects / Keywords

  • X70
  • toughness
  • thermomechanical
  • microalloyed steel
  • EBSD
  • electron backscattered diffraction
  • Gleeble
  • pipe
  • characerization
  • ERW
  • electric resistance welding
  • microstructure
  • phase transformations

• Graduation date

  Fall 2018

• Type of Item

  Thesis

• Degree

  Master of Science

• DOI

  https://doi.org/10.7939/R3DJ58Z4S

• License

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