Effect of strain aging on mechanical properties and engineering critical assessment of X80 girth weld metal

Abstract

Brittle fracture of girth weld metal in high steel grade oil and gas pipelines is the main failure mode of this kind of facilities, which may be related to the strain aging embrittlement of weld metal. To investigate the influence of strain aging on embrittlement mechanism and safety in service, the mechanical properties and engineering critical assessment of X80 girth weld metal was studied. The results demonstrate that strain aging process leads to the increase in dislocation density and low angle grain boundary. After 3 % and 5 % strain aging process, the yield strength increased from the original 565 ± 9 MPa to 675 ± 6 MPa and 754 ± 4 MPa, respectively, and the crack tip opening displacement decreased from 0.22 ± 0.03 mm to 0.11 ± 0.02 mm and 0.06 ± 0.01 mm, respectively. The deterioration of fracture toughness was attributed to the decrease in plastic deformation ability of weld metal, stress concentration at M-A constituent and Cottrell atmosphere. The change of fracture mechanism caused by strain aging increases the risk of brittle fracture of girth weld, which can be assessed by engineering critical assessment.