Oblique 45-Degree Welding: More Demanding Than Full Position

Abstract

According to the national quality inspection special welder test rules and the requirements of the international welder oblique 45 degrees, the fixed welding position of the tube plate corner joint test piece for the welder test has changed from 45 °upward to 45 °downward.

The difficulty has increased, and the welding process has changed accordingly.

As tube sheet welding technology is widely used in the welding of boilers and pressure vessels, this article introduces in detail the welding process, parameters and welding methods of the new 45 ° tube sheet fixed welding.

Oblique 45-Degree Welding: More Demanding Than Full Position 1

1. Preparation before welding

1.1 Test piece

The seamless steel pipe adopts Q345B steel with wall thickness of 5mm and outer diameter of 60mm, and its size is δ5mm ×φ60mm×100mm.

The orifice plate shall be made of 12mm thick Q345B steel plate, with the size of δ12mm × 150mm × 150mm.

The center of the orifice plate shall be drilled through a hole of φ67mm according to the inner diameter of the pipe, with the groove face angle of 35 ° and the blunt edge of 1.5 ± 0.5mm.

1.2 Selection and drying of welding materials

Alkaline electrode shall be selected, and it shall be dried for 1-2h at 350-400 ℃ and kept for 1-2h before welding.

1.3 Cleaning before welding

The cleaning method of the cleaning area before welding is not limited, but the oil, rust and other dirt within 20~30mm of the pipe end and 20~30mm of the orifice groove and both sides of the groove must be cleaned up until the metal luster is exposed.

1.4 Assembly and tack welding

The electrode used for tack welding of specimen assembly shall be the same as that used for formal welding.

The tack weld shall be fixed at one point and eleven points using the left and right two point fixing method. The length of tack weld at each point shall be about 10mm.

The inner wall of the tube and the plate hole of the test piece after assembly and tack welding shall be concentric and free of misalignment.

The tack weld for the assembly of the test piece is generally in the form of a formal weld.

The back is required to be formed without defects.

Before welding, grind the joint with a straight grinder.

2. Key welding points

2.1 Weld bead distribution

Four beads, four layers

2.2 Backing welding

Backing welding shall ensure root penetration to prevent burn through and weld beading.

During welding, positive pole welding is used, which is conducive to controlling the melting hole.

After the arc is ignited, swing the arc up and down for bridging connection.

After the forming of the melting hole, it indicates that the arc ignition work is completed, and the arc breaking and backing layer welding can be carried out.

The arc burns on both sides of the groove at the same time, the blunt edge of the groove melts at the same time, and then the arc is extinguished rapidly.

When the molten pool is about to solidify, the arc is ignited, broken down, and paused at the arc extinguishing point, and repeated repeatedly.

During welding, the arc shall be short, the welding speed shall not be too high, the arc shall stay at the root of the groove slightly, 1/3 of the welding arc shall be kept at the melting hole, 2/3 of the welding arc shall be covered on the molten pool, and the size of the melting hole shall be basically consistent to avoid incomplete fusion, incomplete penetration, high back weld bead or burn through or weld beading at the root of the weld.

During the welding process, the arm and wrist shall be continuously rotated according to the actual position to connect the weld pool with the orifice groove surface and the upper surface of the pipe, and maintain uniform speed movement.

When the welding rod is almost melted, the arc shall be rapidly straightened back to the arc extinction, so that the arc crater is inclined.

If the arc extinction is too fast or the operation is not correct, the cold shrinkage cavity is easy to occur.

Joint: The best and ideal method for joint is to use the hot joint method.

If the first welding rod has just been welded, the welding rod shall be replaced immediately.

When the molten pool is not completely cooled, the arc shall be ignited immediately at 10mm of the arc extinguishing molten pool, and then the arc shall be brought to the arc crater.

The welding shall be continued after the hole is formed again.

After the half circle of weld is completed, the joint shall be polished with a straight grinder before welding.

2.3 Filler welding

The filling welding must ensure the fusion of both sides of the groove.

Before welding the filler layer, clean the slag on the weld bead of the primer layer, and then conduct welding.

When welding the filler layer, short arc welding shall be adopted, which shall be filled in two layers.

Attention shall be paid to the melting condition on both sides to ensure the temperature is balanced, so as to ensure good fusion at the groove of plate and pipe.

The first layer is welded with φ2.5 welding rods, so that the uncleaned slag on both sides of the weld bead can be turned out.

As the new Assessment Rules stipulate that the leg of the tube side in the tube plate corner joint test piece is 0.5T-1T, that is, 2.5-5mm, the weld of the second layer should be 1.5-2mm away from the surface, so as to lay a good foundation for the welding of the covering layer.

2.4 Cover welding

Cover welding must ensure that the weld bead is not undercut and the weld leg is symmetrical.

Before welding, the weld bead of the previous layer shall be cleaned up, and the inclined elliptical electrode shall be used.

During the joint, the arc must be started 10 mm in front of the arc extinguishing bath, and then the arc shall be brought to the arc extinguishing bath.

The arc striking point shall be remelted in the welding project.

When welding the cover weld bead, attention shall be paid to stopping at both sides of the groove to prevent undercut and poor fusion.

The cover arc extinguishing point shall be filled with the weld pool to prevent crater cracks, so as to obtain beautiful weld beads.

3. Conclusion

According to the research and analysis on the welding method and welding process of 45 ° fixed welding of tube sheet corner joint specimen, the technology of single side welding and double side forming is adopted to ensure the size of weld leg and back side forming.

This technology has been comprehensively promoted and applied in welder training and examination.

It not only has fast welding speed and good quality, but also has beautiful weld formation, meeting the requirements of boiler and pressure vessel welding.

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