Effect of Strain Aging on Mechanical Properties of Microalloyed (X70) UOE Steel Pipes

• Author / Creator

  Ma, Jun

• Strain aging of microalloyed steel pipes, manufactured for oil and gas transmission, can occur during the anti-corrosion fusion bonded epoxy coating process (180°C - 250°C) and/or during long term storage or use. The primary mechanism of strain aging involves the segregation of interstitial solute carbon/or nitrogen atoms to mobile dislocations and subsequent locking of these dislocations. The aging process is influenced by many factors including aging temperature and time, steel composition/microstructure (as defined by the C/Nb ratio), position through the pipe wall thickness (i.e., inner diameter, center line or outer diameter) and imposed plastic strain. In this work, a Box-Behnken statistical design is conducted to determine the relationship between both longitudinal and transverse tensile properties (yield strength, ultimate tensile strength and yield to tensile strength ratio) and the aging variables mentioned above. Using the statistical software Minitab Academic (version 17), quadratic equations and response surfaces correlating the significant aging variables with changes in the longitudinal and transverse mechanical properties of the steel pipes are developed. The effect of strain aging on micro-hardness and Charpy impact toughness is also investigated in this study. Additionally, the effect of macro location in the UOE pipe (i.e., circumferential location at 90° or 180° relative to the weld) on the response to strain aging is assessed. It is found that a transition from continuous to discontinuous yielding, featured by the upper yield point followed by Luders strain, occurs for several types of steels due to strain aging. Aging temperature, C/Nb ratio and through thickness position (or imposed pre-strain) are statistically significant in determining most longitudinal and transverse tensile properties. The change in micro-hardness correlates linearly with the change in the yield strength. The effect of strain aging for the test conditions studied on Charpy impact toughness of the steel is relatively small. The changes in the yield strength and yield to tensile ratio are affected by the microstructural features of the steels, where greater changes are observed with the microstructure with smaller grain size and lower volume percentage of pearlite. The effects of through wall thickness position and imposed plastic strain on strain aging are relatively complex because of the generation and interaction between dislocations due to pipe forming and/or artificial plastic deformation. The aging response observed at 90° relative to the weld does not correlate well with the aging response observed at 180° relative to the weld. This suggests that strain history differences incurred during the UOE pipe forming process influences the strain aging response of the pipe.

• Subjects / Keywords

  • Box Behnken Design
  • Dislocation locking
  • Mechanical properties
  • Strain aging

• Graduation date

  Fall 2016

• Type of Item

  Thesis

• Degree

  Master of Science

• DOI

  https://doi.org/10.7939/R39882S6G

• 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.