Research On Failure Mechanism And Rehabilitation Technology Of High Steel Grade Pipeline Girth Weld

With the completion and production of the high grade natural gas pipeline,which is represented by 2^nd,3^rd Shaanxi-Beijing Gas Pipeline Project,&2^nd,3^rd line West-East natural gas transmission project,girth weld failure occurred during its operation has aroused widespread concern.In this paper,the failure mechanism of high grade steel pipe girth welded joints with different materials and different wall thickness is studied from three aspects:failure mode,failure cause and distribution of strength and toughness.At the same time,take the X80pipeline of the 2nd line West-East natural gas transmission project as the research object.The reliability of rehabilitating girth weld defects of high steel grade pipe by type B sleeve is studied from failure control,and the influence rule of the welding operation on the performance of the main pipe is studied also.The results of the study include the following aspects:1.The main failure mode of girth weld of high steel pipe is fracture,the main failure causes are welding defects and additional loads.Girth weld defects are mainly caused by welding defects and geometric defects,most of which are superimposed.The failure of weld incomplete fusion and low impact toughness of weld are the main reasons for the fracture of girth welds.2.Most of the high steel grade pipe girth welds with different material and unequal thickness are strongly matched.The bending load will cause the failure of the welded joint with a low strength and a thin wall.There is a significant change in the impact toughness distribution in the weld center and HAZ.The impact toughness of the weld center is the lowest,and the trend of the weld center to the base metal is increasing.In the HAZ,the cross area between the fusion line and CGHAZ is poor,which is the weak area of the girth weld heat affected zone.The hardness of the weld zone is higher than that of HAZ and base metal.The hardness distribution in the heat affected zone is distinct,and CGHAZ has a higher hardness value,and it gradually decreases to the direction of base metal.3.There is no industry standard for high steel grade pipe rehabilitation in China,and there are some contradictions and differences between the existing enterprise standards on girth weld rehabilitation.The full-scale hydraulic blasting test shows that the type B type sleeve can effectively rehabilitate the X80 pipeline girth weld defects,and the welding thermal influence of B sleeve side weld and fillet weld has a certain degree of influence on the main pipe material properties.As a whole,the influence of the side welds is smaller,the fillet weld is larger,the grain size coarsening leads to the hardness increase of the main pipe under fillet weld,and there is a certain risk of service during the bending test because of the surface cracking occurs,so it should be monitored and used after rehabilitation.