Effect of Defects on High-Temperature Mechanical Properties of Welded Water Walls

• Author / Creator

  Dev Choudhury, Suvan

• Water walls play a crucial role in the operation of refineries, industrial plants, and power plants. However, the repeated start-ups and shutdowns of boilers lead to in- phase thermo-mechanical fatigue failure of the water walls with fatigue cracks often initiating from pre-existing weld defects. There is a lack of comprehensive research on the fatigue failure mechanisms specific to water walls. Most existing studies primarily focus on failure modes related to microstructural degradation and oxide formation, not on fatigue crack propagation under harsh loading conditions. By investigating the fatigue failure mechanism, this thesis sheds light on the formation and progression of blow holes, which is a critical failure mode for the membranes of water walls.

  Subjecting defect-containing water walls to hydrostatic preloading at 1.5 times the maximum allowable working pressure (MAWP) results in localized plastic defor- mation near the pre-existing weld defects. This leads to significant strain hardening and strain aging effects, altering the mechanical properties of the welded membrane specimens. To mitigate such changes, it is recommended to maintain the hydrostatic pressure at approximately 110% of the MAWP to minimize the occurrence of significant mechanical property alterations caused by strain hardening and strain aging at stress concentration points within defect-containing water wall membranes.

  An extensive three-dimensional characterization of pre-existing weld defects in welded membrane specimens has been conducted. The defects are analyzed based on their size, shape, and position within the specimens. Parameters such as sphericity, circularity, and aspect ratio are effective in quantifying defect sharpness. Sharper defects significantly reduce fatigue life compared to rounder defects. Stress intensity factors (SIF) are calculated at each defect location, pointing at potential fatigue crack initiation sites. In the absence of defects, the weld fusion zone is found to be the most susceptible location for fatigue crack initiation. This susceptibility is attributed to the coarse dendritic microstructure in the weld fusion zone, which is less ductile than the base material. The coarse microstructure is prone to the formation of micro-cracks, contributing to the initiation of primary fatigue cracks.

  Finite element modeling is conducted to quantify the fatigue damage and crack propagation and to establish crucial parameters describing experimental results. The fatigue damage parameters are extracted from experimental results to simulate the accumulation and progression of damage. These crack propagation parameters measure blowhole formation, tracking crack initiation and propagation. The cracks are found to have originated from fusion defects, causing membrane disconnection which may have caused localized thermal hotspots and membrane rupture, leaving molten slag residues on the airside of water walls.

• Subjects / Keywords

  • Strain Hardening
  • Strain Aging
  • Water Walls
  • Fatigue Failure
  • X-Ray Tomography

• Graduation date

  Fall 2023

• Type of Item

  Thesis

• Degree

  Doctor of Philosophy

• DOI

  https://doi.org/10.7939/r3-e77m-2882

• License

  This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.