Stress on the trench corrosion of high-frequency straight seam resistance welding pipe

Direct roll-formed steel high-frequency resistance welding technique is a rapid production of pipeline technology, currently in many areas such as: downhole casing, oil and gas gathering pipe has been widely used. However, this pipe weld area than the base metal and the heat affected zone has higher activity, and therefore determines the pipe weld area of life. Corrosion is a trench for pipe joints special crevice corrosion phenomena, often forming a trench in the weld zone. Under natural conditions, the formation of the trench easily inseam corrosive medium in the state of stagnation, leading to reduced oxygen concentration in the joints gradually oxygen concentration cell and outer seam formed to accelerate the corrosion of the metal seam. At the same time the corrosion process seam metal ion remaining, causing the joints to move within Cl-, someone had to observe the dip in neutral sodium chloride solution in the gap, which contains chloride of 3 to 10 times the body of the solution, with the the corrosion of corrosion products accumulate around the weld, such as Fe2 + hydrolysis tends to produce excess H inseam +, and the role of stagnation makes the seam metal in acidic conditions, has been reported its pH value can reach 2 to 3 so keep the seam metal activation, promote corrosion will be. When its anodic corrosion rate of the base metal and weld depend mainly on its electrochemical activity. High-frequency electric resistance welded pipe (erw steel pipe) as a whole without heat treatment, to achieve circumferential weld residual stress around 350MPa, close to the yield strength of the pipe. When the weld processing into working electrode, the residual stress is reduced to 100MPa or less. Circumferential tensile stress can reduce the corrosion potential of welds, accelerated corrosion, corrosion sensitivity coefficient increases trench. And after the heat treatment of welded pipe products, circumferential weld residual tensile stress smaller, with 10 ~ 100MPa, after processing into working electrode not change its circumferential tensile stress. Such conventional measurement methods can not characterize the welded pipe products in the practical application of corrosion resistance, and can not distinguish between real or not heat treatment on the corrosion resistance of welded pipe.

In the high frequency resistance welding process, within the initial 0.1s, temperature was raised to 1600 , the integration process section MnS decomposed into S or converted to FeS, within the next 4 ~ 6s, the temperature dropped to 400 , then water-cooled to room temperature. S and sulfide-rich seam zone is through the rapid heating, rapid cooling to achieve. For high sulfur content (mass fraction> 0.01%) steel, the corrosion of the trench produce MnS inclusions related; loss of low sulfur content steel trench produces solder alloy, residual stress and increasing carbon content related. Steel used in the test are low-sulfur steel. Uneven temperature field of welding process is a root cause of welding stress and deformation. In the small thickness of the weldments, welding residual stress is substantially plane stress, stress in the thickness direction is small. In the free state the welding plate, in the vicinity of the weld and the tensile stress is generally in the stress away from the weld compared. Residual Stress can affect the strength of the weldment, stiffness, stability, accuracy and corrosion resistance. The elimination of residual stress, the general overall heat treatment. Since the annealing temperature is too high will result in material strength and organizational changes or the second phase precipitates, the annealing temperature is too low to remove the effects of residual stress is unlikely, therefore, to the steel to stress annealing 500  1h, after slow cooling, reducing the welding residual stress member, improve the uniformity of the organization and composition.