Weldability, Residual Stress And Corrosion Performance Of Pipeline Steel With Dissimilar Welding Wire

The reliability of welded joints of oil and gas pipeline is an important problem that cannot be ignored in petroleum engineering. High strength, large diameter pipeline steel can increase the conveying pressure, improve the transmission efficiency and reduce the transmission pipeline wall thickness, thus, the corresponding welding material consumption are reduced, which decreased the cost of oil and gas transmission pipeline. With the increasing of pipeline steel strength level, the tendency of welding defect and welding residual stress, stress concentration are greater, which resulting in the decrement of fatigue strength as well as the increment of hydrogen induced cracks, brittle fracture, stress corrosion crack failure tendency. The traditional welding residual stress elimination method in some large complicated structure welding process is difficult to put into effect and inevitably increase the cost. So the control of the residual stress distribution and magnitudes in the welded joint of pipeline steel is an important method to ensure the safety of welded structure. Special welding filler material method by using the transformation expansion occurred during a series of organizational change in welding process to compensate the residual tensile stress introduced by thermal shrinkage. This method can reduce or even eliminate residual stress without preheating or post-weld heating treatment directly during the welding process, in addition, this method can not only lower the cost, but also suitable in many large and complex structure, which has a broad application prospects.Theoretical and experimental methods are utilized in this paper to study the effect of alloy elements to the steel properties and martensitic transformation temperature. Combining with the schaeffler diagram, three kinds of welding wire with different martensitic transformation temperature were developed(LTT), namely LTT1,LTT2,LTT3. X65 pipeline steel was used as base material, X65 and three low transformation temperature welding wires are used as filler materials. The welding method was argon tungsten-arc welding(TIG). After welding, optical microscope(OM),Scanning electron microscope(SEM),X-ray diffraction(XRD) were used to analyze the microstructure of welded joint and the diffusion of the alloy element. Results show that the microstructure in welding joint filled will low transformation temperature welding wires are mainly composed by martensite and a small amount of residual austenite(10%~15%). The connection of base metal and weld metal are good, and main alloy elements diffusion was exist from the weld to base metal. Low temperature impact toughness, microhardness, tensile strength test of the welding joint are measured. Results show that, the microhardness and tensile strength of the welding joint filling with low transformation temperature welding wires were higher than the homogeneous filler wire, the impact toughness of three kinds of low LTT welding wires are good, but the impact toughness of LTT3 and LTT1 welded joint is obviously decreased with the decrease of temperature, and the impact toughness of LTT2 welded joint varies little with temperature.Blind hole method was used to analyze the welding residual stress state, results showed the residual stress state in homogeneous welding joints are welding residual tensile stress, the longitudinal peak stress value can reach 450 Mpa, the horizontal peak value can reach 450 MPa, the stress state in welding joint filled with low transformation temperature welding wires are compressive residual stress and the longitudinal peak values are between-300~200MPa. The transverse peak value are between-200~200MPa. In addition, after welding, deformation directions of welded joint filed by low transformation temperature welding wires and homogeneous welding wires are contrary, the distortion radian are LTT1 0.056 rad, 0.083 rad, 0.033 rad, 0.14 rad, the main reason for this result is the residual stress state change.Electrochemical method was used to analyze the corrosion behavior of different welding joint filling with three kinds of low transformation temperature welding wires in saturated CO2 solution containing 0.2mol/L H2 S with different NaCl concentration and pH value. Results show that with the increment of Cl– concentration, the corrosion resistance of the welding joint decreased, with the increment of pH value, the corrosion resistance of welding joint increased, when the solution changed from acidity to medium and alkalescent, the corrosion behaviors are hardly influenced by the pH value.