What are the high requirements for large diameter butt welding elbows during welding?

Large-diameter butt-weld elbows will generally deform after welding. Assuming that the amount of deformation exceeds the allowable value, it will affect the application. The primary reason for this is that the weldment is partially heated and cooled unevenly. Because when welding, the weldment is heated to high temperature only in some areas, the closer to the weld, the higher the temperature, the greater the expansion.

However, the metal in the heating area cannot be expanded freely due to the lower temperature of the surrounding metal; while cooling, it cannot be relaxed due to the control of the surrounding metal. As a result, this part of the heated metal has tensile stress, while other parts of the metal have compressive stress in balance with it.

When these stresses exceed the metal's yield limit, welding deformation will occur; when the metal's strength limit is exceeded, cracks will appear. Welding seam enhancement is too high: this phenomenon occurs when the viewpoint of the welding groove is opened too small or the welding current is too small. Because the stress meets and breaks easily, the fatigue life of the forward pressure vessel requires the welded seam to be shoveled high.

Welding seam is too concave: the strength of the joint is reduced due to the reduction of the welding seam cross section. Weld seam undercut: The depression formed on the weld elbow along the edge of the weld seam is called undercut, which not only reduces the working section of the joint, but also constitutes a serious stress convergence at the undercut. Welding nugget: The molten metal flows onto the unmelted welding elbow at the edge of the molten pool, and the accumulation forms a welding nugget, which is not fused with the welding elbow. The welding nugget has no effect on the static load strength, but will cause the stress to converge and reduce the dynamic load strength. Burn-through: Burn-through means that part of the molten metal leaks out of the weld, or even burns into a hole, which reduces the strength of the joint.

The above five shortcomings exist in the appearance of the weld, which can be found by naked eyes and can be repaired in time. Assuming proficient operation, it is generally preventable. 1. Divided by viewpoint: the most commonly used are 45 °, 90 ° and 180 °, and other non-normal viewpoint large-diameter butt welding elbows such as 60 ° are also included according to engineering needs. 2. According to the production process, it can be divided into: welding large-diameter butt-welding elbow, stamping large-diameter butt-welding elbow, casting large-diameter butt-welding elbow. 3. According to the radius of curvature: it can be divided into long radius and large diameter butt welding elbow and short radius and large diameter butt welding elbow.