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Submerged Arc Welding Processes for Steel Pipes

Posted: 02/10/2023 14:59:05 Hits: 7

1. Preparation before welding
Preparation must be made for submerged arc welding before welding the steel pipe, including groove processing of the weldment, surface cleaning of the part to be welded, assembly of the weldment, cleaning of the surface of the welding wire, and drying of the flux.

① Groove processing
The requirements for groove processing shall be carried out according to GB/T 985.2-2008 to ensure that there is no incomplete penetration or slag inclusion at the root of the welding seam, and to reduce the amount of filling metal. Groove processing adopts planers, mechanized or semi-mechanized gas-cutting machines, carbon arc gouging, etc.

② Cleaning of parts to be welded
Weldment cleaning is mainly to remove rust, oil and moisture, and prevent the generation of pores. Generally, it is removed by sandblasting, shot blasting or manual removal, and if necessary, the parts to be welded should be baked with flames. Before welding, rust, scale, and oil on the surface of the groove and within 20mm on both sides of the groove and the parts to be welded should be cleaned.

③ Assembly of weldments
When assembling weldments, it is necessary to ensure that the gap is uniform. The length of the welding seam is generally greater than 30mm, and its quality should be consistent with the requirements for the quality of the main welding seam. Use special tooling and fixtures when necessary.

For the assembly of weldments with straight seams, arc strike plates and lead-out plates should be installed at both ends of the welding seam, and then cut off after welding. The purpose is to obtain a welding seam section with a normal size at the beginning and end of the welding joint. Defects that are prone to arc starting and finishing can be eliminated.

④ Cleaning of welding materials
The welding wire and flux used in submerged arc welding have a great influence on the composition, structure and properties of the welding seam metal. Therefore, the scale, rust and oil stains on the surface of the welding wire must be removed before welding. Pay attention to moisture-proof when storing flux, and dry it according to the specified temperature before use.

2. Process parameters of submerged arc welding
The welding parameters of submerged arc welding mainly include welding current, arc voltage, welding speed, diameters and extension length of welding wires.

① Welding current
Under normal welding conditions, the weld penetration is proportional to the welding current.
With the increase in welding current, the depth of penetration and excess weld metal increased significantly, while the width of the welding seam did not change much. At the same time, the melting amount of the welding wire also increases accordingly, which increases excess weld metal. The width of the welding seam changes little. As the amount of welding increases, excess weld metal increases. As the welding current decreases, the depth of penetration and excess weld metal decreases.

② Arc voltage
With the increase in arc voltage, the welding width increases obviously, while the penetration depth and excess weld metal decrease. However, when the arc voltage is too high, it will not only reduce the penetration depth and cause incomplete penetration, but also lead to poor welding seams, difficulty in slag removal, and even defects such as undercut. Therefore, while increasing the arc voltage, the welding current should also be appropriately increased.

③ Welding speeds
When other welding parameters remain unchanged and the welding speed increases, the welding heat input decreases accordingly, so that the penetration depth of the welding also decreases. If the welding speed is too fast, it will cause defects such as incomplete penetration. To ensure the welding quality, a certain amount of welding heat input must be guaranteed, that is, while increasing the welding speed to increase productivity, the welding current and arc voltage should be increased accordingly.

④ Diameters and extension length of welding wires
When other welding parameters remain unchanged and the diameter of the welding wire increases, the diameter of the arc column increases accordingly, that is, the current density decreases, which will cause the width of the welding seam to increase and the depth of penetration to decrease. On the contrary, the penetration depth increases and the welding seam’s width decreases.

When other welding parameters remain unchanged and the length of the welding wire increases, the resistance also increases; the preheating effect on the protruding part of the welding wire increases, and the melting speed of the welding wire accelerates, resulting in shallow penetration and increased excess weld metal. Therefore, the protruding length of the welding wire must be controlled and should not be too long.

⑤ Inclination angles of welding wires
The inclination direction of welding wires is divided into forward inclination and backward inclination. The direction and size of the inclination angle are different, and the force and heat effect of the arc on the molten pool is also different, thus affecting the shape of the welding seam. When the welding wire is tilted at the back at a certain angle, because the arc points to the welding direction, the weldment in front of the molten pool is preheated; the discharge effect of the arc on the liquid metal in the molten pool is weakened, resulting in a wide welding seam and shallow penetration. On the contrary, the welding seam width is narrow and the penetration depth is deep, but it is easy to cause unfused and undercut at the edge of the welding seam, making the welding seam’s shape worse.

⑥ Others
a. Shapes of grooves
b. Root gaps
c. Thicknesses of weldments and heat dissipation conditions of weldments

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