A Study On The Fracture Toughness Of X80 Pipeline Steel Welded Joint

The pipeline project of natural gas transmission from West to East China is theenergy artery connecting East and West China for pipelining the oil-gas resource tosupport the economic development and social progress continuously. With thedeveloping trend of pipeline transportation with high pressure, large diameters andlarge flow capacity, the anti-fracture ability of pipeline steels has an increasinglysignificant effect on ensuring the operation security of oil-gas pipelines. Because ofwelding thermal cycles, the welded joints of pipeline steel tend to initiate cracks andare the weak links for rupture.In this study, the different positions of the welded joint of the longitudinallysubmerged arc welding pipeline of X80steel at the job site of the second project ofnatural gas transmission from West to East China were studied, by the microstructureobservation, the hardness distribution test, the uniaxial tensile test at roomtemperature, to research their changes of the metallographic structures and themechanical properties by the welding process. Then the three-point bending fracturetoughness tests of the different positions of the X80welded joint were conducted,with SEM fractography, to further study their changes of elastic-plastic fracturetoughness values attributing to their changes of the metallographic structures and themechanical properties. The results show that the welded joint of the X80steel isovermatched. X80pipeline steel is acicular ferrite steel, and owns good fracturetoughness. The weld metal is relatively poor in plasticity and fracture toughness,because of the existence of widmanstatten ferrite. The grains of the granular bainite ofthe heat affected zone have been coarsened by different degrees, and formedlarge-size hard-brittle inclusions of M-A constituents, which has more or lessembrittled welded joint of the X80steel. The fusion zone, which has been hardenedand embrittled seriously, is the local fracture risk zone.At last, by building the3D elastic-plastic finite elements models of thethree-point bending fracture specimens, the constraint effect of the crack-tip field ofthe fracture specimens and the strength and toughness matching of the differentpositions of the welded joint were discussed. It is found that increasing the breadth ofthe three-point bending specimen can enhance the three-axial constraint of the crack tip. Although the welded joint of the X80steel is overmatched, the rupture probablyhappens in the weld metal firstly for the critical fracture load of the three-pointbending specimen of the weld metal is lowest.