Study On Characterization Method Of Residual Plastic Strain For Base Metal And Girth Weld Of X70 Pipeline

Brittle fracture failure of high-steel grade pipeline girth weld has been a common safety problem in domestic trunk oil and gas pipelines in recent years.So far,the mechanism of brittle fracture has not been fully explored.Recent studies have found that brittle fracture of pipeline girth weld is associated with small plastic strain in their vicinity.However,the plastic strain is difficult to characterize by conventional physicochemical analysis.In order to reveal the root causes of pipe girth weld brittle fracture accidents,this thesis selects X70 pipe base metal and girth weld as research objects,and performs four-point bending test and tensile test loading.The thesis also proposes the characterization method of residual plastic strain,and performs verification.Metallographic and microhardness analyses are performed on the selected X70 pipe base metal,and EBSD analyses are also performed on these samples to screen the relationship between EBSD diffraction band contrast(Band Contrast,BC)and Kernel Average Misorientation(KAM)parameters and strain.The following conclusions are obtained: when the residual plastic strain is 0-10%,there is no change in the morphology of the metallographic organization,and the hardness has an increasing trend with the accumulation of strain but fluctuated greatly;while BC is easily affected by the sample making,electron microscopy parameters and other factors,and the sensitivity to strain is poor;there is a positive correlation between KAM and residual plastic strain,and the response of KAM to deformation is significant.In order to explore the metallographic organization,hardness and KAM distribution of the girth weld,the undeformed girth weld and the four-point bending loaded girth weld are analyzed,and the study shows that: the undeformed welded joint,except for the fine grain zone,there are significant differences in the composition of the metallographic and hardness,the same side of the fine grain zone in the morphology,hardness and KAM distribution is the most stable,and the hardness value of the fine grain zone is the lowest,the most suitable for assessing the overall residual welded joint Plastic strain distribution;four-point bending loading tests under different gradient strains showed that the fine grained zone is fine and uniform in size,and when the residual plastic strain is at a larger plastic strain and ±2% or more,the fine grained zone KAM increases from 0.09° to 0.26°.Applying the results of the ring weld residual plastic strain investigation to the characterization of an accident fracture,the EBSD analysis of the fine grain zone shows that because the weld metal of the accident fracture has a lower hardness value than the fine grained zone,the fine grained zone of the accident fracture is compared with the fine grained zone of spiral and straight welding,and the residual plastic strain in the fine grain zone of the accident fracture is very small and should be below 2% in the preliminary analysis.