Brittle Structure Of Fusion Zone And Crack Analysis Of Welded Joints Of Bimetal Lined Pipe

The research objects are mechanical bimetal lined pipe and end metallurgical composite pipe.Two butt welding processes are Low-alloy welding consumables filling capping welding process and nickel-based welding consumables filling capping welding process.The research results will provide theoretical basis and technical support for the selection of welding materials and welding process of bimetal lined pipe,and are helpful for the wide use of the dual metal mechanical composite pipe in the high corrosive oil and gas fields.In low-alloy welding consumables filling capping welding process,welding joints fusion zone of mechanical bimetal lined pipe and end metallurgical composite pipe between filling capping surface layer and transition layer appears martensite.In nickel-based welding consumables filling capping welding process,two kinds of pipe welding joint microstructure has not appeared martensite.The reason of appearing brittle tissue between filling capping surface layer and transition layer is large differences of alloy elements of welding material and element diffusion through chemical composition analysis of welding joint.Element distribution difference is big,because Cr and Ni content in transition is more than in filling capping layer.And martensitic component of fusion zone is uneven,prompting large supercooling degree,but carbon content is low,thus microstructure formed martensite.Brittle tissue's influence on the mechanical properties of welded joint depends on brittleness rate.The brittleness rate means the proportion of brittle hard tissue thickness and welded joint thickness.Experiments show that only rigid organizational impact bending test.Through analysis of Relationship between brittleness rate and bending test results,plasticity and toughness of welded joints meet the requirements when brittleness rate is less than equal to 6.0%.Through the simulation result,Von mises stress was tensile stress.With the increase of the number of layers of weld,welding residual stress first increases and then decreases.During the welding process,residual stress at tip point 1 was maximum.Under the two kinds of welding process,residual stress of ?114mm bimetal lined pipe and after filling capping was small,but residual stress of ?355mm/?508mm bimetal lined pipe and after filling capping was big.Nickel-based welding consumables filling capping process reduced equivalent residual stress peak of ?355/?508mm mechanical bimetal lined pipe and peak after filling capping,and helped to reduce welding cracks.End Metallurgy can effectively reduce welding equivalent residual stress at tip point.The reduction of welding equivalent residual stress in peak was more than 100 MPa.This helped to reduce the possibility of welding cracks.The microstructure of crack initiation formed lath martensite and upper bainite.The hardness of this area was very high.Crack initiation was welding residual stress concentration points and welding residual stress value was high.And remelting seal increased residual stress of welded joints.So this area produced crack.Lath bainite and proeutectoid ferrite of area 2 made crack extend.Crack extended in the fusion zone because of martensite.Coarse microstructure of filling capping layer near fusion zone reducing plasticity and many defects,Particularly slag,made crack extend.When the crack encountered less welding defects and finer grain size of ferrite and pearlite,the crack was not extended.