Research On Microstructure And Properties Of X70Steel SAW Welding Joint

Oil and gas pipelines were constructed in bad weather, sparsely populated,geological conditions that are very complicated, not only caused difficulties for theconstruction of long distance pipeline, but also put forward higher requirements onwelding technology. Oil and gas pipelines equipments such as pig launcher&receiver,pressure pipe, tee, lots of those products use X70steel, but because of the materialmanufacturer or different welding parameters (heat input) selection problem, thewelding joint performance can not meet the requirements, therefore it is particularlynecessary to research the organization and properties of X70steel welding joints.In this research, the effect of heat input on X70steel welding joint microstructureand properties is studied. Comprehensively contrasted the welding joint and thewelding joint under different heat treatment states, including the welding jointperformance, microstructure, diffusion hydrogen content and residual stress analysis.It provides the basis for the establishment of welding procedure and heat treatmentprocess for X70steel products.The results show that: the welding heat input increasing, the pro-eutectoid ferritein the weld ferrite grain size increases, the acicular ferrite microstructure in the welddecreased, and resulting in mechanical properties of welding joints decreased.Because of X70steel as TMCP steel, welding heat input increasing, can cause thewelding grain increased significantly in the heat affected zone, so heat input should bestrictly controlled during welding.From the perspective of diffusion hydrogen, heatinput increasing can reduce the content of diffusible hydrogen in weld metal, but notobviously.Methods to reduce the diffusion hydrogen may adopt the preheating, slowcooling after welding, post-weld heat treatment or eliminate hydrogen treatment afterwelding.Welded joints after heat treatment, welding longitudinal residual stress andtransverse residual stress are greatly reduced, the effect of heat treatment to eliminateresidual stress can reach40~60%.After560℃heat treatment, the residual stress inwelding joints have been significantly decreased, and raise the heat treatmenttemperature to reduce the welding residual stress has little effect.