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The tensile properties and toughness of microstructures relevant to the HAZ of X80 linepipe steel girth welds

University of British Columbia

In the transportation of oil and gas products from and over remote locations, such as Canada's Arctic environment, pipelines are used. Girth welding to join sections of steel pipelines creates a substantial heat affected zone (HAZ) within the base pipeline steel. While there is significant concern as to the fracture and mechanical properties of the HAZ as whole, detailed knowledge about the mechanical properties of the microstructural constituents is lacking. For this research, measurements of the temperature time profile in the HAZ of single and dual torch welds were made. This was then used to guide heat treatments of X80 steel in a Gleeble simulator to create samples of 8 different bulk microstructures with differing amounts and morphologies of bainite, ferrite and martensite-retained austenite (MA). From the heat treated samples tensile and Kahn tear test specimens were made for testing at ambient, -20⁰C, and -60⁰C. The highest strength microstructure proved to be the finest, lower bainitic microstructure, while the lowest strength microstructure was the coarsest, upper bainitic sample containing a significant amount of MA. This was observed to be true at all testing temperatures. As part of the tensile behaviour investigation, the Bouaziz dislocation based model for work hardening was applied and shown to fit well across all temperatures and conditions. The Kahn tear test, a machine notched, thin-sheet, slow strain rate test, showed all tests failed in a ductile manner. Relative toughness measurement from this test showed that the fine, lower bainitic microstructure was the toughest and the coarse, ferritic microstructure was the least tough. This work presents a novel measurement of dual torch temperature time profiles in a real HAZ, an extensive mechanical testing program of isolated microstructures relevant to the X80 HAZ at potential pipeline operating temperatures, and an applied a robust model to fit the work hardening behaviour for all conditions. This work has the potential for future application in microstructure evolution-property models, and in a combined mechanical model of the different microstructures to further improve understanding of HAZ mechanical responses.

Text

2015-10-23

Attribution-NonCommercial-NoDerivs 2.5 Canada

http://hdl.handle.net/2429/54408

Materials Engineering

University of British Columbia

UBCV

http://creativecommons.org/licenses/by-nc-nd/2.5/ca/