According to the National Quality Inspection Special Welder Test Rules and the requirements of the international standard for welding at a 45-degree angle, the fixed welding position of the tube plate corner joint test piece for the welder test has been changed from a 45-degree upward angle to a 45-degree downward angle.
This change has increased the difficulty level and has resulted in a change in the welding process.
As tube sheet welding technology is widely utilized in the welding of boilers and pressure vessels, this article provides an in-depth explanation of the welding process, parameters, and methods for the new 45-degree tube sheet fixed welding.
1. Preparation before welding
1.1 Test piece
The seamless steel pipe is made of Q345B steel with a wall thickness of 5mm and an outer diameter of 60mm. Its dimensions are 5mm (thickness) x 60mm (diameter) x 100mm (length).
The orifice plate is made of a 12mm thick Q345B steel plate and measures 12mm (thickness) x 150mm (width) x 150mm (length).
The center of the orifice plate has a drilled hole with a diameter of 67mm, which aligns with the inner diameter of the pipe. The angle of the groove face is 35°, and the blunt edge measures 1.5mm ± 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 the specimen assembly must be the same as that used for formal welding.
The tack weld should be fixed at one point and eleven points using the two-point fixing method on the left and right. Each point’s tack weld length should be approximately 10mm.
The inner wall of the tube and the plate hole of the test piece must be concentric and free from misalignment after assembly and tack welding.
The tack weld for the test piece assembly should be in the form of a formal weld.
The surface must be free from defects after forming.
Before welding, the joint should be smoothed with a straight grinder.
2. Key welding points
2.1 Weld bead distribution
Four beads, four layers
2.2 Backing welding
The root penetration during back welding must be ensured to prevent burn-through and bead formation. Positive pole welding is used during the process, which helps control the formation of the melting hole.
To initiate the welding process, ignite the arc and move it up and down for bridging the connection. Once the melting hole has formed, it signifies the completion of the arc ignition work and the arc breaking and backing layer welding can commence.
The arc burns on both sides of the groove simultaneously, causing the blunt edge of the groove to melt, and is rapidly extinguished. During welding, the arc should be kept short, the welding speed should not be too high, and the arc should be positioned slightly at the root of the groove, with 1/3 of the welding arc in the melting hole and 2/3 of the arc covering the molten pool.
The size of the melting hole should be consistent to avoid incomplete fusion, incomplete penetration, high back weld bead, burn-through, or bead formation at the root of the weld.
To maintain uniform movement and connect the weld pool with the orifice groove surface and the upper surface of the pipe, continuously rotate the arm and wrist during the welding process.
When the welding rod is almost melted, rapidly straighten the arc back to the arc extinction point so that the arc crater is inclined. If the arc extinction is too fast or the operation is incorrect, it can result in a cold shrinkage cavity.
The hot joint method is the best and ideal method for jointing. If the first welding rod has just been welded, it should be immediately replaced.
If the molten pool is not completely cooled, immediately ignite the arc 10mm from the arc extinguishing molten pool, then bring the arc to the arc crater. Continue welding after the hole has formed again.
After completing half of the weld circle, polish the joint with a straight grinder before proceeding with the welding.
2.3 Filler welding
The filler welding must ensure a proper fusion of both sides of the groove.
Before welding the filler layer, clean the slag from the weld bead of the primer layer, and then proceed with the welding process.
When welding the filler layer, short arc welding method should be used, and the filler should be applied in two layers.
Pay attention to the melting conditions on both sides to ensure a balanced temperature, which will result in a good fusion at the groove of the plate and pipe.
The first layer should be welded with a 2.5mm welding rod, to turn out any uncleaned slag on both sides of the weld bead.
As per the new Assessment Rules, the leg of the tube side in the tube plate corner joint test piece should be 2.5mm to 5mm (0.5T to 1T), the weld of the second layer should be 1.5mm to 2mm away from the surface, to provide a solid foundation for the covering layer welding.
2.4 Cover welding
To ensure a successful cover weld, it is important to prevent undercutting and achieve symmetrical weld legs. Before beginning, it is necessary to clean the previous layer’s weld bead. An inclined elliptical electrode should be used during the welding process.
When starting the arc, it should be initiated 10mm in front of the arc extinguishing bath and then brought to the bath. The arc striking point should be melted during the welding project.
When welding the cover weld bead, it is important to stop at both sides of the groove to prevent undercutting and poor fusion. The cover arc extinguishing point should be filled with the weld pool to avoid crater cracks, resulting in a visually appealing weld bead.
According to research and analysis of the welding method and process for 45° fixed welding of a tube sheet corner joint specimen, the technology of single-side welding and double-side forming is adopted to guarantee the size of the weld leg and back-side formation.
This technology has been comprehensively implemented in welder training and certification programs.
It boasts not only fast welding speed and high quality, but also an attractive weld formation, meeting the standards for boiler and pressure vessel welding.
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