Pre-welding Technology of LSAW Steel Pipe

Longitudinal submerged arc welded steel pipe (LSAW steel pipe) is a welded pipe made from a single medium-thick plate, pressed (rolled) into a tube blank in a mold or forming machine, and double-sided submerged arc welding and expansion . Pre-welding is an integral part of the welding process of straight seam submerged arc welded steel pipes. It performs "shallow welding" along the entire length of the forming seam, and is one of the special processes in the production of straight seam submerged arc welded steel pipes.

There was no pre-welding in the early production of longitudinal submerged arc welded steel pipes. It was not until the second generation of UOE welded pipe units that pre-welding machines began to appear, but at this time the pre-welding was intermittent welding with a spacing of about 300mm. In the third generation (1968~1979) of the development of UOE welded pipe units, pre-welding has been greatly valued and developed, and the discontinuous method has been changed into a continuous method. The pre-welding technology at this stage has laid the foundation for modern pre-welding technology.

Modern pre-welding technology uses continuous, high-speed gas shielded welding (MAG) and weld laser tracking. The welding speed can reach 7m/min, and the weld bead is straight and beautiful.

Pre-soldering process

During pre-welding, the steel pipe blanks are firstly joined together, followed by continuous gas shielded welding, and the status of the weld seam and the quality of the weld are monitored and fed back during welding. The specific process is as follows: the imported roller table accepts the broken tube → adjusts the opening position of the tube blank → delivers the tube blank by the conveying device → closes the tube blank → confirms the seam quality → lowers the welding torch and prepares for welding → starts the laser tracker for tracking → turns on the shielding gas and cooling water valve → start welding (the tube blank is fed at the welding speed) → stop the arc at the terminal and stop welding → shut off the protective gas after a delay → the welding torch rises back to its position → the tube blank is passed to the next process. At this point, a pre-soldering cycle is completed.

In the above process, adjusting the opening position of the broken pipe refers to adjusting the position of the opening seam to the required position, usually at 12 o'clock. This work can be carried out through the camera monitoring system in the electronic control system. To confirm the seam quality is to confirm the amount of misalignment of the seam, the gap of the seam, etc. Only after confirmation can the tracking and welding of the seam be carried out. In order to ensure the welding quality, check the special welding torch before starting the welding. Clean up the spatter on the welding torch in time, and spray some anti-splash agent appropriately. The arc starting and extinguishing of pre-welding are generally carried out on the arc starting plate and the arc extinguishing plate. The forming seam within about 80mm of the pipe end is welded by manual gas shielded welding after pre-welding.

Pre-soldering quality

Pre-welding quality includes seam quality and weld quality.

(1) The joint (that is, the forming seam) has no misalignment or the misalignment is less than the specified value. Generally, the amount of misalignment is ≤ 8% of the plate thickness, and the maximum does not exceed 1.5mm.

(2) It is necessary to ensure that the weld has a suitable penetration depth and deposition amount. It is necessary to ensure that there is no cracking and no burn-through after welding, and the height of the weld must be controlled so that the reinforcement of the external weld will not be affected.

(3) The weld bead is continuous and well-formed to ensure the final external welding quality.

(4) There are no defects such as welding deviation, porosity, cracks, slag inclusions, burn-through and back welding bumps in the weld, and the center deviation of the weld is required to be < lmm.

(5) No arc burns, little spatter, and does not affect the pipe end bevel and surface quality.

(6) The weld seam matches the base metal, and the physical and chemical properties of the weld seam metal meet the quality requirements. Welded by manual gas shielded welding.

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