| Oil and gas pipelines are responsible for the important task of transporting fossil energy such as oil and natural gas,and have become the fifth largest transportation industry in China.As the most important work in pipeline construction,welding is related to the construction cost and quality of the whole pipeline project.At present,arc welding is still mainly used in pipeline welding,which can't further increase the welding speed and improve the welding efficiency,research on a new generation of efficient,safe,low-cost welding technology has become an urgent problem in the pipeline construction.Since laser-arc hybrid welding was proposed,it has been widely used in shipbuilding,automobile industry,engineering machinery and railway locomotive,etc.,and has shown great application potential in pipeline field.Because it has the advantages of single laser welding and single arc welding,and makes up for their respective shortcomings.However,due to the high power laser arc composite welding is mainly laser welding,its adaptability and reliability to complex environment still need to be further verified.This paper proposes the use of a low power peak value pulse laser induced and enhanced arc hybrid heat source,its main advantage is that compared with arc welding,it can improve the welding efficiency,welding quality and automation level.Compared with high-power laser-MAG arc welding,it has low cost,good adaptability and is easy to meet the actual welding requirements.Therefore,it is of great theoretical and practical significance to study the low-power laser-MAG hybrid welding technology,which is mainly based on arc and supplemented by laser for the purpose of improving welding efficiency and saving energy.In this paper,the laser-MAG arc hybrid heat source was used to carry out the study of the over-head welding for 5mm E36 high strength steel and the all position welding for 12 mm thick X52 pipeline steel.The comparative test of one-sided welding with back formation in over-head position for 5mm thick E36 high strength steel shows that the heat input of the laser-MAG arc hybrid heat source for the full penetration is only 71% of the single MAG arc source,therefore,the grains of the weld area and the over-heat area in the heat affected zone of the welded joint are finer.Force analysis shows that gravity is an important factor to influence the weld formation in over-head position,because the hybrid heat source can effectively reduce the volume and the gravity of molten pool,in order to inhibit the defects of back concave and the front ridges on the welding joint and increase the parameters intervals.When the laser power is 480 W,the defocus is 0mm,the space between optical and wire is 2mm,and the butt gap is 1mm,the welding speed is between 650mm/min ~850mm/min,and the arc current is between 180A~220A,the well-formed butt joints can be obtained.The laser-MAG arc hybrid all position welding of 12 mm thick X52 pipeline steel adopts three-layer and three-pass segment welding process.Through reasonable control of welding process parameters and wave process parameters,the good forming of the welding seams in each welding area,including root,filling and cover welding seams at all positions of the pipeline is finally realized.Due to the effect of the thermal cycle,from the cover layer of welds to the root layer of welds,the grains in most areas of the weld joint are gradually refined.The over-heated area of the heat-affected area is prone to produce too much widmanstatten structure,which makes the mechanical properties poor.The hardness test of the welded joint shows that,the hardness in the over-heated zone> the hardness in the weld zone> the hardness in the completely recrystallized zone> the hardness in the incomplete recrystallization zone> the hardness in the base material zone.The hardness in the overheated zone and weld zone of the root layer is obviously higher than that in the corresponding area of the cover layer.The mechanical property tests show that,the tensile strength and impact absorption work of the welded joints in different positions are in compliance with the standard,and there were no cracks in the bend specimens of 45° slope welding?... |
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