Cracking in Hot-dip Galvanized Welded Joints of Steel Platform Structures

• Author / Creator

  Di Giovanni, Christopher T

• In a recent construction project, a platform structure made up of 350W and 300W steel underwent a double dip galvanizing process. Prior to the galvanizing process, the platform was fabricated to completion, which included numerous welds throughout the design. After galvanizing, cracks were found originating in the welds and propagating into the base material. To the steel designer, this posed a new and curious problem. Previously, similar structures had undergone the same processing with no cracking. This research project began by investigating the metallurgy and microstructure of the base material, the welds, and of the crack sites. Two key features that were noted were the thicker and inclusion-rich grain boundaries in the base material, and the cracking appeared with little or no deformation of the grains. In addition, the fracture surfaces were studied at high magnification levels and showed features of brittle fracture, which is uncharacteristic of the 350W and 300W steels used. Studies were carried out to asses the material's susceptibility to both hydrogen embrittlement and temper embrittlement, both of which are known to potentially occur during galvanizing. Tension test samples were sectioned from the base material and were charged with hydrogen. During the tension testing, the samples charged with hydrogen did not show any different behavior from the regular samples. All the fracture surfaces were ductile and did not match the surfaces from the galvanizing cracks. Furthermore, hardness levels were too low to be deemed susceptible to hydrogen cracking according to the values seen in literature. Similarly, samples were sectioned from the base material and notched for temper embrittlement testing. The samples were heated to the galvanizing temperature and then fractured. The samples broke suddenly and showed little signs of ductile fracture and, more significantly, had a fracture surface matching the original cracks. To support the notion of temper embrittlement, material chemistry testing found high levels of phosphorus. Phosphorus is a key culprit in temper embrittlement, and given the elevated temperature and stress of the double dip process, it would have been able to diffuse to the grain boundaries causing brittle grain boundary separation. Finally, to quantify the thermal stresses induced from galvanizing, a three dimensional finite element analysis model was created to simulate the double dip process. The model found relatively high stresses but not enough to reach the yield point without an embrittlement factor present.

• Subjects / Keywords

  • Cracking
  • Temper Embrittlement
  • Galvanizing

• Graduation date

  Fall 2017

• Type of Item

  Thesis

• Degree

  Master of Science

• DOI

  https://doi.org/10.7939/R33R0Q725

• 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.