| Pipeline steel has been widely applied in the oil and gas transportation,because of good mechanical properties.But they are more and more used in the situation of high corrosion medium,so are required to have good strength and corrosion resistance.Bimetal lined pipes have emerged by the consideration of situation and economy.Because of different properties and clearance between the base material and the cladding material,the welding process in the bimetal lined pipe causes some key problems,such as melting loss of alloy elements,carbon atoms migration and complicated stress state.Sealing process or surfacing process are used before the combination of end for reducing difficulties of welding.The welded joints by two different welding technology of bimetal lined pipe of L415/316 L are comprehensively researched about microstructure and mechanical property,the values of residual strength about bimetal lined pipes that have different diameters,and fracture toughness in-40℃.Microstructure,strength,bending property and impact property of the welded joints were investigated by using optical microscope,impact testing machine and universal testing machine.The results are as follows,microstructure about different layers of the welded joints with tearing phenomenon after tensile test of L415/316 L bimetal lined pipe that have the eminent qualities are good by using alloy welding materials,but there exist some martensite on filling layer and shell cover by using alloy and carbon steel welding materials,thus deteriorating the properties,emerging crack on bending and bad impact toughness.Because of metallurgical bonding,there is not tearing phenomenon on the welded joints after tensile test.The residual stress distribution of the outside surface about the welded joints of L415/316 L bimetal lined pipe is investigated by using strain gage and Indentation Method.The results are as follows,the residual stresses of the outside surface of L415/316 L bimetal lined pipes are changed into tensile stresses in weld zone,near weld zone and base metal zone.The same diameter as the bimetal lined pipes,the welding residual stresses of weld zone and near weld zone on outside surface after using alloy and carbon steel welding materials are higher than the welding residual stresses of weld zone and near weld zone on outside surface after using alloy welding materials.And the higher welding residual stress of weld zone and near weld zone on outside surface increases along with the increase of the diameter of L415/316 L bimetal lined pipes that have the same welding technology on the pipe end.Comprehensively considering the pipe shape and the actual direction of weld,two kinds of three-point bend specimens are designed.High-frequency fatigue testing machine as well as self-own force and displacement,extensometer calibration instrument and crack tip opening displacement testing technology are applied to test fracture toughness of base metal,welding,and heat affected zone of welded joints at-40℃.The results are as follows,the base metal and the welding have evidently stable crack extension region,and good toughness with an average value of 1.0281 mm on the base metal,0.5769 mm on the welding joint of using alloy welding materials,as well as 0.7027 mm on the welding joint of using alloy and carbon steel welding materials.The fracture toughness on fine-grained is good,while bad fracture toughness with an average value of 0.2394 mm on the welded joints of alloy welding materials as well as 0.3036 mm on the welding joint of using alloy and carbon steel welding materials on coarse-grained region due to difference of microstructure.The analysis of metallographic,morphologies of fracture specimen by crack tip as well as morphology and ingredients of inclusions in the fracture zones show that form of microstructure,grain size,the way of crack propagation and the number of inclusions have affection on the fracture toughness. |
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