Manual downward welding welding process and operation points

Manual arc downward welding is suitable for diameter not less than φ159mm ×7mm. The material is carbon steel and ordinary low alloy steel.
Features: HIGH production efficiency, good welding quality, compared with manual upward welding can improve the efficiency of more than 2 times. Such as: oil and water pipeline welding electrode should be selected cellulose type downward welding electrode, gas pipeline engineering is widely used, such as Beijing-Tianjin jet fuel, Hainan long transport project.


Manual down welding operation features
To summarize in five words: "big, fast, thin, multi-stable":​
High welding current; ​fast welding speed; ​welding cladding metal thin; ​many welding layers; ​arc movement to be steady.
For example: φ377mm ×8mm workpiece welding groove is large, in addition to the first layer of root welding and two layers of hot welding, its in each layer for row welding filling and hat welding. The groove pattern is shown in Figure 1.


Metal Welding Service process

1, equipment: suitable for DC, silicon rectifier, inverter welding machine (with blowing current).

2. Welding material: Base material is carbon steel (Q195, Q215, Q235, etc.), ordinary low-alloy steel (×52, ×60, etc.), pipe diameter is 168.3mm, 377mm, 813mm, 1016mm, respectively.
Welding material can choose Austria "Bole" electrode E6010 φ3.2mm, φ4.0mm.
3. Preparation before welding: before welding, the water and oil at 15~25mm on both sides of the pipe groove, and the slag after gas cutting are completely removed to expose the metallic luster (so as not to produce pores and other defects in the welding process).
4, tools and labor protection: grinder, grinding wheel, steel wire wheel, goggles, dust cap, long leather sleeve, hammer.


Group of
The slope Angle is required to be 30°±2° when grinding, the groove can not be trimmed out of the concave or convex surface, which will affect the quality of welding.
Base material: 20# specification φ377mm ×8mm for example.


Tack welding
The inner wall of the pipe fitting is positioned at 4 places with the dragon door plate. After the root welding is completed, the positioning plate can be struck off. The length of the positioning weld is about 15mm. If the inner mouthpiece is selected, the root weld pass must be completely welded before the inner mouthpiece is removed; ​if THE EXTERNAL MOUTHPIECE IS SELECTED, ONLY 40% OF THE ROOT WELD PASSES SHOULD BE WELDED BEFORE THE EXTERNAL MOUTHPIECE IS REMOVED, AND THE LENGTH OF EACH SECTION SHOULD BE APPROXIMATELY EQUAL AND EVENLY DISTRIBUTED. The distribution and installation of positioning blocks are shown in Figure 2.


Welding process parameters
According to the situation of the construction site and the process evaluation results, the welding parameters selected are shown in the attached table.


1, root welding
The root welding is the most critical among the four layers of welds. It is necessary to ensure that the root of the welds is melted through, and the root shall not be melted through. It is the basis of the welding of each layer of welds, so the root welding is the most difficult.
Operation method: Start welding at 12 o 'clock at the welding spot (Figure 3), and weld downward from two welders to the left and right directions. Generally, the electrode does not swing. When the gap is too large, or the downward pulling too fast welding hole is too long, the arc can be reversed. Point 12 to 1 (11) is flat welding, point 1 (11) to 4 (8) is vertical welding, and point 4 (8) to 6 is invert welding. The dip angle of the electrode is shown in Fig. 3. Electrode to gently press the root of the groove, and don't have to keep certain arc length and distance, it is best to make the arc inside the groove shape, dragging down electrode, the electrode application maintenance of grinding wheel to grind out a v-shaped guide joint Angle, and then 5 ~ 10 mm above the joint arc, pull to the junction, after it's melting, heard through welding to go ahead.
In the field construction, from flat welding to vertical welding and then to the arc welding, 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 pass, add a lot of heat at the same time, keep the weld at a high temperature, prevent cracks and fractures, and remove the residual welding slag of the weld pass Angle. Usually, the welding time interval between hot welding and root welding should be less than 5 minutes, and the interlayer temperature should be more than 100℃.
Operation method: When welding, do the back and forth transport strip, the welding speed and action should be fast and coherent, the arc length should be kept between 5~8mm.

3. Filling welding
The goal is to fill the pass without biting the edge of the bevel, giving a good base for the cap weld.
Operation method: when filling welding, the electrode can slightly swing around, if the groove angle is small, can be straight down the strip, do not swing, welding to 5 (7) o 'clock position when the slight circle arc movement, arc to low, arc length of 2~3mm is appropriate, filling welding path should be welded below the pipe surface 1~1.5mm.
4, cap welding
Objective To fill weld path, improve strength and beautify weld.
Operation method: when welding the hat, run down and swing from side to side, but the swing should not be too large, it is appropriate to press the edge of the groove 1~1.6mm, the arc length should be short, generally 2~3mm, welding to the arc of the inverted welding position, to fill the arc pit along the tangent direction of the welding spot, evenly elongate the arc, avoid surface pores and local high or edge bite.


Common Welding defects (downward welding)
1, the welding pass is too narrow and too high - cause: the electrode angle is wrong, the welding current is too small, the speed is too slow.
2, surface porosity - cause: arc speed and swing range is too large, damp electrode.
3, slag - cause: cleaning welding pass is not complete, from the Angle on both sides, mainly root cleaning hot welding current is too small.
4, bite edge - cause: current is too large, arc is not in place, downward arc speed is too fast.
5, root joint concave, not through welding - cause: root welding aid blowing current is too large, splash, and root melting width is too large. The electrode is not pressed in the groove root and burns normally to form a melting hole.


The operating characteristics of electrode arc downward welding are summarized in 5 words, that is, "large, fast, thin, more stable".
For example, when welding φ813mm × 2.7mm steel pipe, in the case of large groove, in addition to the first layer of root welding and the second layer of hot welding, the rest of the layers are filled and covered by row welding.

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