High-frequency resistance welding technology

The direct roll formed steel high-frequency resistance welding technology is a rapid production pipeline technology, now in many areas such as: downhole casing, oil and gas gathering and transportation pipe has been widely used. However, this pipe weld area than the base metal and heat affected zone has a higher activity, so the weld zone of the pipe determines the service life. Trench etching is a special pipe joints for crevice corrosion phenomena, usually forming a trench in the weld zone. Under natural conditions, the formation of the trench is easy to make the seam corrosion medium is stagnant state, resulting in dissolved oxygen concentration decreased gradually in the seam and the seam formed outside the oxygen concentration cell, accelerated corrosion of the metal seam. Simultaneously etching the metal ions during the remaining seam, the seam causing Cl- moved, it was immersed in a neutral have been observed within the gap sodium chloride solution, which contains chloride as the main solution 3 to 10 times, with the the corrosion progresses, the corrosion products accumulate in the weld around, such as Fe2 + hydrolysis tends to seam generate excess H +, and the stagnation of the role of making seam metal in the acidic conditions has been reported that the pH can reach 2 to 3, so that the seam metal retainer activation, promote corrosion proceed. When its anodic polarization, the base metal and weld depends on the speed of the electrochemical corrosion activity. Frequency ERW pipe (ERW) if the whole tube without heat treatment, the circumferential weld residual stresses reach 350MPa, close to the yield strength of the pipe. When processed into the working electrode after welding, the residual stress is reduced to 100MPa or less. Circumferential weld tensile stress can reduce the corrosion potential, accelerated corrosion, corrosion susceptibility coefficient increases trench. The product of the pipe after the heat treatment, the residual tensile stress of the circumferential weld small as 10 ~ 100MPa, after processing into the working electrode changes its circumferential tensile stress is not. Such a conventional measurement methods can not characterize pipe products in the practical application of corrosion resistance, can not distinguish between real or not heat-treated for corrosion resistance of welded pipes. However, there is no crevice corrosion under tensile stress reported, this work uses a self-designed fast etching method, studied under constant tensile stress corrosion pipe trench.

Welding process is to produce non-uniform temperature field welding stress and deformation of the underlying causes. Weldment small thickness, the welding residual stress is essentially planar stress, stress is small in the thickness direction. In a free state welding plate, the general vicinity of the weld and the tensile stress, compared away from the weld stress. Residual stress can affect the strength of the weldment, stiffness, stability, accuracy, and corrosion resistance. Elimination of residual stress, the general overall heat treatment. As the annealing temperature is too high will result in material strength and organizational changes or second phase precipitates, the annealing temperature is too low for the removal of little residual stress, and therefore to the stress of the steel 500 ℃ annealing 1h, slow cooling, reducing the welding member of residual stress and improve the homogenization of organization and composition. ERW pipe under the influence of residual stresses in welds susceptible to corrosion, and the greater the degree of residual stress corrosion larger, generally without the entire pipe ERW pipe heat treatment of serious corrosion. 500 ℃ to stress annealing 1h, weld and heat affected zone and base metal corrosion rate close to the same , less prone to corrosion trench.