Microstructures and properties of fully automatic welded joints of X70 pipeline steel with slope
A fully automatic welding technique was used to weld X70 pipeline steel with a slope of 25°.The microstructures of the welded joint were observed by an optical microscope(OM)and electron backscattered diffraction(EBSD).Mean-while,the mechanical experiments on the welded joint such as tension,bending,hardness,impact and crack tip opening displacement(CTOD)were performed.The results show that the width of the coarse-grained heat-affected zone(CGHAZ)and the welding residual stress of the upper and lower grooves are different.In the upper groove,the width and the residual stress are both bigger.The welding position obviously affects the impact energy at the fusion line of the inner surface.The impact energy at the fusion line of the inner surface in the flat and vertical welding positions is much lower than that in the o-verhead welding position,which is the weak toughness zone of the welded joint.The impact energy of the weld is high but the CTOD value is low.Thus it is recommended to consider both the impact energy and CTOD value when evaluating the tough-ness of the welded joint.The root welding zone is the dangerous area of the welded joint,which is easy to cause the failure of the girth weld.It is recommended to use equal strength or high-strength matching welding materials in the root welding zone.In the welding procedure qualification we should distinguish mountainous areas and plain areas.When performing welding procedure qualification for welded joints under mountainous conditions,the root welding zone must be paid attention to.
large slopeX70 pipeline steelautomatic weldingmicrostructurestoughness
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