| With economic development and social progress,the demand for oil and natural gas continues to increase.Pipeline transportation is the most economical,safe,efficient and energy-saving transportation method for oil and natural gas.According to the "Medium and Long-term Oil and Gas Pipeline Network Plan" issued by the National Energy Administration,by 2025,China’s oil and gas pipeline network will reach 240,000 kilometers.According to the distribution of cities in my country and the characteristics of natural geography,most of the eastern,southern and southwestern regions are dominated by mountainous hills,and the proportion of pipelines laid in the mountainous areas will increase.Fully automatic welding has the advantages of high welding efficiency,easy control of welding process parameters,stable welding process,excellent weld performance,and real-time collection and transmission of welding parameters.Therefore,this article takes X70 steel with a large slope as the research object.When the welding slope is 25°,internal welding machine root welding +external welding machine thermal welding,capping and external welding machine root welding + external welding machine thermal welding and capping are used.Two welding processes are used for welding.When the welding slope is 30°,the two welding processes of the external welding machine automatic downward welding and the external welding machine automatic upward welding are used for welding.The microstructure and fracture toughness of the welded joint are studied.The results are as follows: The formulation and implementation of the automatic welding process for mountain pipelines provides theoretical and guiding significance.Non-destructive testing,microstructure observation,fracture analysis,tensile,bending and other mechanical performance tests of the welded joints of the four welding processes have shown that the results of the non-destructive testing under the four welding processes meet the standard requirements.The structure of the affected zone is dominated by ferrite and bainite.Under the influence of gravity,the heat-affected zone on both sides of the weld is asymmetrical.The width of the heat-affected zone of the lower groove is larger than that of the upper groove,but the coarse-grained zone of the heat-affected zone of the upper groove The width is greater than the down bevel.The tensile strength of the joint under the root welding process of the internal welder reaches more than 645 MPa,and the test results such as bending and impact are all qualified,and the welded joint has good toughness.The tensile strength of the joint under the root welding process of the external welder with a slope of 25°reaches more than 640 MPa.Due to the high hardness and high residual stress of the fusion line at the junction of root welding and heat welding,stress will be generated in the larger residual stress part when subjected to external force Concentration leads to cracks,resulting in sample fracture during the bending test.Due to the coarse grains and M-A islands in the structure of the fusion line of the vertical bevel welding,the fusion zone becomes brittle,resulting in less than the impact energy output at-20℃ 60 J,does not meet the standard requirements,and is inferior to the joint toughness under the root welding process of the internal welder.The tensile strength of the joint under the upward welding process of the external welding machine reached more than 650 MPa,and a sample cracked during the bending test.The length was 2.3 mm,which was less than the standard requirement of 3 mm.In the impact test,the impact energy at the weld and fusion line is less than 60 J,the impact energy is low,and the toughness is insufficient.The tensile strength of the welded joint under the downward welding process of the external welder reaches more than 630 MPa,and the bending and impact test results are qualified.It has good toughness and is superior to the upward welding process.The average CTOD of the joint weld and the heat-affected zone under the root welding process of the internal welder are 0.43 mm and 0.50 mm,respectively,and the average CTOD of the joint weld and the heat-affected zone under the downward welding process are 0.36 mm and 0.69 mm,respectively.The joint has Good fracture toughness.The average CTOD of the joint weld and the heat-affected zone under the up-welding process are 0.16 mm and 0.54 mm,respectively.The average CTOD of the joint weld and the heat-affected zone under the root welding process of the external welder is 0.20 mm and 0.61 mm,respectively.The fracture toughness of the zone is better,and the weld zone is worse.CTOD test results show that the sampling position(flat welding,vertical welding,overhead welding)and sampling method(W=B,W=2B)have little effect on the final CTOD value.The CTOD value is mainly related to the width of the extension zone in the fracture.In the same joint and the same position,the larger the extension zone,the larger the CTOD value and the better the fracture toughness. |
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