The manual arc down welding is suitable for the diameter not less than Φ159mm × 7mm, and the material is carbon steel and ordinary low alloy steel.
Features: high production efficiency, good welding quality, compared with manual upward welding, the work efficiency can be improved by more than 2 times. For example, cellulose-type downward welding electrodes should be used as electrodes for oil and water pipelines, which are widely used in gas pipeline projects, such as Beijing-Tianjin aviation oil, Hainan long-distance transportation and other projects.
One. Features of manual down welding operation
Summarize in five words: "big, fast, thin, many, stable":
The welding current is large; the welding speed is fast; the welding deposit metal is thin; the welding layers are many; the arc movement should be stable.
For example, when the workpiece of Φ377mm×8mm is welded with a large groove, in addition to the first layer of root welding and the second layer of thermal welding, each layer is filled with row welding and cap welding. The groove pattern is shown in Figure 1.
Second, the welding process
1. Equipment: suitable for DC, silicon rectifier, inverter welding machine (with blowing current).
2. Welding materials: The base material is carbon steel (Q195, Q215, Q235, etc.), ordinary low-alloy steel (×52, ×60, etc.), with pipe diameters of 168.3mm, 377mm, 813mm, and 1016mm, respectively.
Welding materials can choose Austria "Bole" electrode E6010 Φ3.2mm, Φ4.0mm.
3. Preparation before welding: Before welding, remove the moisture, oil, and slag after gas cutting at 15~25mm on the inside and outside of the groove of the pipe to reveal the metallic luster (to avoid defects such as pores during the welding process) .
4. Tools and labor protection: grinding machine, grinding wheel, steel wire wheel, goggles, dust cap, long leather sleeve, hand hammer.
Third, the pair
When the slope angle is required to be 30°±2°, the groove cannot be repaired to the concave or convex surface, which will affect the welding quality.
Base material: 20# specification Φ377mm×8mm as an example.
Fourth, tack welding
The inner wall of the pipe fittings is positioned at 4 places with the gantry plate. After the root welding is completed, the positioning plate can be knocked off. The length of the positioning welding seam is about 15mm. If the inner butt joint is selected, the entire root weld bead must be welded before removing the inner butt joint; approximately equal and evenly distributed. The distribution and installation of the positioning block are shown in Figure 2.
Five, welding process parameters
According to the conditions of the construction site and the results of the process evaluation, the selected welding parameters are shown in the attached table.
Six, welding
1. Root welding
Among the four-layer welds, root welding is the most critical. It is necessary to ensure that the root of the weld is penetrated, and the root must not be melted through. It is the basis for the welding of subsequent layers of welds, so the root welding is the most difficult.
Operation method: start welding at the 12 o'clock position of the welding port (Figure 3), and two welders will weld in two directions, left and right, and the welding rod generally does not swing. When the fast melt hole is too long, the arc can be reversed. From 12:00 to 1 (11) point is flat welding, from 1 (11) to 4 (8) point is vertical welding, from 4 (8) point to 6 point is overhead welding, the inclination of the electrode is as shown in Figure 3 shown. The electrode should be lightly pressed at the root of the groove, and it is not necessary to keep a certain arc length. Lead the angle, then start the arc at 5~10mm above the joint, pull it to the joint, and after the part is melted, the penetration sound is heard and the welding is continued.
During on-site construction, from flat welding to vertical welding to overhead welding arc ending, welders can use hot welding quick joints according to their proficiency in operation to improve efficiency and save time.
2. Heat welding
The purpose is to strengthen the root weld bead, and at the same time add a lot of heat, keep the weld at a high temperature, prevent cracks and fractures, and also remove the residual welding slag at the angle of the weld bead. Usually, the welding time interval between heat welding and root welding should be less than 5 minutes, and the interlayer temperature should be greater than 100 ℃.
Operation method: The welding speed and action should be fast and coherent, and the arc length should be kept between 5~8mm.
3. Filler welding
The purpose is to fill the weld bead, but not to bite the edge of the bevel, to create a good foundation for the cap weld.
Operation method: When filling welding, the electrode can swing slightly left and right. If the bevel angle is small, it can be moved straight down without swinging. When welding to the 5 (7) o'clock position, make a slight circular arc movement. Low, the arc length is preferably 2~3mm, and the filler bead should be welded to 1~1.5mm below the surface of the pipe.
4. Cap welding
The purpose is to fill the weld bead, improve the strength and beautify the weld.
Operation method: During cap welding, move the strip downward and swing left and right, but the swing should not be too large, it is appropriate to press the edge of the groove by 1~1.6mm, and the arc length should be short, generally 2~3mm, and welding to overhead welding At the end of the arc, the arc crater should be filled along the tangential direction of the welding point, and the arc should be evenly elongated to avoid surface pores and local superelevation or undercut.
Seven, common welding defects (downward welding)
1. The welding bead is too narrow and too high - the reason: the angle of the electrode is wrong, the welding current is too small, and the speed is too slow.
2. Surface pores - Reason: The arc speed and swing range are too large, and the electrode is damp.
3. Slag inclusion - reason: the cleaning of the weld bead is not thorough, and the angle between its two sides is mainly because the root cleaning current is too small.
4. Undercut - Reason: The current is too large, the arc operation is not in place, and the downward arc pulling speed is too fast.
5. The root joint is concave and not penetrated - the reason: the root welding blowing current is too large, the spatter is large, and the root melting width is too large. The electrode does not press on the root of the groove and burns normally to form a molten hole.
Eight, concluding remarks
The operation characteristics of electrode arc down welding can be summarized in 5 words, namely "large, fast, thin, many and stable".
For example, when welding Φ813mm×2.7mm steel pipe, in the case of large groove, except for the first layer of root welding and the second layer of heat welding, the other layers are filled and covered by row welding.