Effect on Quality of LSAW Pipe by Consistent Wire-feeding Operation

Welding wire is the main welding material used in the production of LSAW Pipe. The quality of welding wire plays an important role in the welding quality of welded pipe. Weld metal is formed by base metal, welding wire and flux after melting and solidification; Therefore, the chemical composition of welding wire has a great impact on the strength, toughness, plasticity, chemical composition, hardness, corrosion resistance and so on. The external dimension of welding wire also affects the welding quality. When the diameter of welding wire is too large, it is easy to get stuck in the welding process. When the diameter of the welding wire is small, the contact between the welding wire and the conductive nozzle is not good, the conductivity is poor, the voltage fluctuation is large, and the welding seam is easy to produce slag, edge biting and other defects; When the welding wire has a hard bend or joint, it is easy to get stuck in the welding process, and the arc is broken. When it is serious, the conductive nozzle will be burned, and the weld will have copper cracks.

In automatic welding, wire feeding stability has an important influence on welding process and welding quality. Wire feeding speed must be dynamically balanced with wire melting speed to ensure proper arc length and stability of welding process. To find and eliminate the factors that lead to unstable wire feeding in the welding system, and reduce the welding defects caused by the fluctuation of wire feeding speed in the welding process, is the guarantee of improving the welding quality and production efficiency.

1. Production Example

(1) Case 1. When welding the straight seam submerged arc welded pipe, 350 kg/ coil layer wrapped welding wire was used. There was no problem when the welding wire produced by Factory A was used. When the welding wire produced by Factory B was used, the last two layers frequently got stuck. The investigation found that the width of the welding wire coil produced by factory B is wider than that of factory A, and the welding wire disc has no function of width adjustment, and the welding wire will get stuck in the gap between the movable baffle and the fixed shaft. In order to avoid similar situations, the welding wire disc is improved: the fixed shaft of the welding wire disc is extended by 10mm, and a hole is opened inside the movable baffle which is slightly larger than the diameter of the fixed shaft and 15mm deep. When the width of the welding wire changes, the gap between the movable baffle and the fixed shaft is eliminated. After the improvement of the welding wire plate, when the welding wire produced by different manufacturers is used, the situation similar to the stuck wire no longer occurs.

(2) Case 2. Production in 610mm x 8mm small diameter thin. When welding pipe with straight wall seam, φ3.2mm welding wire is used for pre-welding, the welding current is 850A, and the depth of outer groove is 2.7mm. After pre-welding, the outer groove has been filled, and it cannot be tracked automatically during external welding, and the height of the outer weld is more than 3.5mm. In order to reduce the height of the prewelding weld, φ2.4mm welding wire was used for prewelding, and the current was 650A. During the welding process, the current fluctuated greatly, the weld was high and low, and there were many pores on the surface. It is analyzed that the reason of this situation is that the V-groove of wire feeding wheel is too deep and the wire feeding is not stable. After the wire feeding range of the wire feeding wheel is adjusted from 2.4mm ~5.6mm to 1.6mm ~2.4mm, the welding process is stable and the weld height is uniform.

(3) Case 3. In the production of a batch of φ1016mm x 21mm straight seam submerged arc welding pipe, the slag in the inner weld appears, and the slag is located in the position of the second wire fusion line of the inner weld. According to the statistics of the straight seam submerged arc welded pipes with slag inclusion, most of the straight seam submerged arc welded pipes with slag inclusion are welded by the same internal welding unit. The field investigation shows that sometimes the voltage of the second wire will suddenly increase and return to normal value in the welding process. Check the welding wire, there is no joint or hard bending and other defects causing wire sticking; After checking the wire feeding system, it is found that the pressing wheel of the wire feeding machine does not rotate flexibly, and the slag inclusion disappears after replacing the pressing wheel.

(4) Case 4. Since there are many specifications and a wide range of wall thickness of the straight seam submerged arc welded pipe, the welding parameters vary greatly. In the production, different diameter welding wires should be often selected according to the specifications of the welded pipe to meet the requirements of the welding process. However, it is often the case that the welding voltage fluctuates greatly after the coarse wire is replaced by the fine wire, and the weld edge is more. It is found that the wire feeding tooth marks on the welding wire are shallow and the size of the tooth marks is not uniform. After replacing the wire feeding wheel, the voltage is normal and the biting edge is eliminated.

2. Cause Analysis

Wire feeding stability refers to the performance of keeping the wire feeding speed constant when the motor input power or wire feeding resistance changes. On the one hand, the wire feeding stability is related to the mechanical characteristics of the wire feeding motor and the control accuracy of the drag circuit. On the other hand, it is related to the resistance of the wire feeding process, the wire feeding wheel structure and the wire feeding wheel on the wire driving mode. The welding wire used in straight seam pipe multi-wire submerged arc welding is mostly medium 3.2mm or medium 4.0mm thick welding wire. It adopts steep drop characteristic power supply, matching variable speed wire feeding, and is driven by V-shaped wheel with teeth. The wire feeding speed is taken as the reference signal by the given voltage and regulated by the negative feedback signal of arc voltage. The condition of stable arc combustion is that the melting speed of the wire is equal to the wire feeding speed. The wire feeding system is composed of welding wire disc, guide wire tube, wire feeding machine, wire feeding wheel, pressing wheel, straightening wheel, wire feeding hose, conductive rod, conductive nozzle. Any abnormality of these components will cause the fluctuation of wire feeding speed, resulting in weld defects.

In case 1, the structure of the wire disk is only suitable for a single width layer around the wire. After the wire coil width becomes larger, there is a gap between the movable baffle and the fixed shaft, and the welding wire is easy to produce stuck wire into the gap; When the wire coil width becomes smaller, the wire cannot be clamped. During wire feeding, the wire is easy to jump out of the wire plate baffle, resulting in wire jamming or grounding. After changing the structure of the movable baffle and the fixed shaft, when the width of the wire coil changes within a certain range, it can ensure that the welding wire is clamped and there is no gap between the movable baffle and the fixed shaft, so as to avoid stuck wire.

In Case 2, CO2+Ar gas shielded welding was used for prewelding, flat characteristic power supply was used, and arc length was adjusted by changing welding current. Although in 2.4 ~ 5.6 mm wire feed wheel to send into the 2.4 mm wire, but because the two pieces of wire feed wheel to the formation of V groove deep, when the wire feed wheel, pinch roller wear slightly, wire feeding wheel and the distance between the pinch roller is close to the wire diameter, wire on the contact pressure of small, not enough to overcome the wire resistance, wire feed speed fluctuations lead to welding current is not stable, The weld is not well formed. After the wire feeding range of the wire feeding wheel is reduced, the V-shaped groove formed after the pair of two wire feeding wheels is shallow, and the contact pressure on the wire increases, which is enough to overcome the wire feeding resistance, so the welding current is stable.

In case 3, the wire feeder compression wheel is a bearing roller that regulates the compression force on the wire through a spring. After long-term use of the pressing wheel, the bearing will turn inflexible due to wear, and the wire feeding resistance will increase, the wire feeding speed will decrease, the welding arc will elongate, and the voltage will increase. After REPLACING the new pressing wheel, the bearing rotates flexibly, the wire feeding resistance is small, the wire feeding speed is stable, the welding arc can maintain a certain length, and the voltage is stable.

In Case 4, two wire feeding wheels form a V-shaped groove with teeth, and two rows of tooth marks will form on the welding wire during the wire feeding process. When the depth of the two rows of tooth marks is the same, the length of each tooth mark is the same, and the distance between adjacent tooth marks is the same, the wire feeding speed will be stable. The wire feeding wheel will also have a certain wear in the wire feeding process, the wire feeding wheel will produce a circle of indentation, wire diameter is different, the curvature of the indentation is different, the concave curvature of the thick wire in the wire feeding wheel is small, the concave curvature of the fine wire in the wire feeding wheel is large. Use coarse wire, with wire feeding wheel wear, dent will move toward the bottom of the V groove, wire feeding wheel wear to a certain extent, the dent coarse wire with thin wire to dent overlapping replacing fine wire again, because the wire feed wheel was worn out part of the curvature is small dents, welding wire and the wire feed wheel contact area is smaller, the wire feeding machine is passed on to the wires of wire feed torque smaller, The resistance in the wire feeding system is unchanged, which causes the fluctuation of wire feeding speed, resulting in voltage instability.


3. conclusion

To sum up, the welding wire and wire feeding system have great influence on the welding quality of straight seam submerged arc welded pipe; Therefore, to ensure the welding quality of the straight seam submerged arc welding pipe, the quality of the wire and wire feeding system have the following requirements :

① the surface of the wire can not have oil or rust;

②There can be no hard bending and joint in the welding wire;

 ③ Wire feeding wheel, pressing wheel wear after timely replacement; 

④ Replace the wire feeding wheel when replacing the coarse wire with fine wire; 

⑤Choose a wire feeding wheel that is compatible with the diameter of the welding wire to avoid the lower limit of the wire feeding range.

When welding straight seam submerged arc welded pipe, there is a certain resistance in the wire feeding system. Only when the wire feeding driving force is greater than the resistance, can the continuous and stable wire feeding be ensured. There are two conditions to ensure the driving force of wire feeding: one is the pressure wheel to wire enough pressure, the other is the wire feeding wheel and wire have enough contact area. Wire feeding resistance is related to the wire and wire feeding system, the wire itself exists in the hard bend, joint and wire feeding system in the wire disk, wire feeding wheel, pressure wheel, wire feeding tube and other parts of wear, will lead to increase in wire feeding resistance.it is an effective measure to improve the welding quality and production efficiency of straight seam submerged arc welded pipe to find and eliminate the factors leading to the increase of wire feeding resistance, replace the parts with serious wear in wire feeding system, and ensure the stability of wire feeding speed.

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