First, the lack of fusion of steel pipe welding defects
During welding, the molten pool metal is discharged to the tail under the action of the arc force to form a groove. When the arc moves forward, the groove is filled with molten pool metal. If the liquid metal layer at the groove wall has solidified at this time, the heat of the filled molten pool metal cannot melt the metal again, and then a lack of fusion is formed. Lack of fusion often occurs on the side wall of the welding groove to form a side wall lack of fusion. It appears between the layers of multi-layer welding to form an interlayer lack of fusion. It appears at the root of the weld to form a root lack of fusion. This lack of fusion cannot be seen on the surface of the weld and must be detected with the help of ultrasonic or radiographic detection.
In flat welding, lack of fusion often occurs along the groove surface of the parent material or between the layers of multi-layer welding. In horizontal welding, lack of fusion often occurs along the upper and lower groove surfaces of the parent material and between the layers of the weld. It is also easy to produce a lack of fusion at the edge of each layer of the weld along the upper groove surface. Vertical welding is usually CO2 gas electric vertical welding, which is automatic welding. During automatic welding, due to the large thickness of the base material and the welding wire not swinging or not swinging enough, the temperature of some parts along the groove surface far from the welding wire is too low, resulting in unfused. This phenomenon often occurs on both sides of the groove surface of the base material.
The reduction of the bearing area due to unfused is very obvious, and the stress concentration is also relatively serious. Its harm is second only to cracks.
Second, the cause of unfused steel pipe welding defects
The cause of unfused is too low welding heat input, arc blow, improper operation, rust and dirt on the side wall of the groove, and incomplete slag cleaning between weld layers. In addition, if the welding current is too large and the welding speed is too slow, the molten iron after the welding wire is melted will flow to a place far away from the molten pool. The molten iron will contact the surrounding base material and cover the surface of the low-temperature weld, which will also cause confusion. Another situation is that when the groove is wide, the swing amplitude of the welding wire is not large enough, resulting in low temperatures on both sides of the weld. The molten iron after the welding wire is quickly cooled and covered on the groove, causing confusion.
Third, the control method of infusion of steel pipe welding defects
The main measure to prevent confusion is to master the welding operation technology and pay attention to the angle of the wire and the edge residence time so that the edge of the groove is fully melted to ensure fusion.
(1) Use correct welding process parameters: The welding current should be appropriate. If the current is too high, the welding wire will melt too quickly, and the molten iron will flow to the front of the welding wire and cover the surface of the weld. Since the surface temperature of the weld is too low, the molten iron covering it will solidify before it can fuse with the base material, resulting in unfused welds. On the contrary, if the molten pool is too small and the temperature around the molten pool is too low, unfused welds will also occur at the edge of the molten pool. Secondly, control the welding speed. The welding speed should be fast rather than slow. The appropriate welding speed should be determined based on the wire diameter, current size, groove form, and welding position.
(2) Choose the appropriate welding angle: When flat welding, the welding gun should be perpendicular to the weld horizontally and have a forward inclination of about 20° with the longitudinal direction of the weld, that is, the welding direction. If it is manual vertical welding, the welding gun should be perpendicular to the weld horizontally and have an inclination of 0°~10° with the welding direction. During horizontal welding, CO2 welding does not produce slag and has no lifting effect on the molten pool, which makes it easy for the molten iron to flow downward and cause incomplete fusion. Therefore, the welding gun angle should be perpendicular to the welding direction, and the angle with the parent material should not be too small, otherwise, it is easy to cause incomplete fusion on the upper groove surface.
(3) Ensure the swing amplitude of the welding wire: During welding, a certain swing amplitude of the welding wire should be guaranteed according to the thickness of the parent material and the groove form. Especially in flat welding and vertical welding, when the thickness of the parent material is large, the swing amplitude of the welding wire is particularly important.
(4) Determine the number of welding layers according to the thickness of the parent material, and try to weld multiple layers and multiple passes: The thickness of each layer should be strictly controlled. This is also related to the welding speed. The faster the welding speed, the smaller the thickness of the welding layer, which can avoid incomplete fusion; the slower the welding speed, the thicker the welding layer, which is easy to cause incomplete fusion.
(5) Strengthen the cleaning of the groove and interlayer
Fourth, the incomplete penetration of steel pipe welding defects
Incomplete penetration is the phenomenon in which the root of the welded joint is not fully penetrated. Incomplete penetration defects may occur in both single-sided welding and double-sided welding. During the CO2 gas-shielded welding with a short-circuit transition of fine wire, the phenomenon of incomplete penetration is also easy to occurs due to the low heat input of the workpiece.
Fifth, the causes of incomplete penetration of steel pipe welding defects
The main reasons for incomplete penetration are low welding current, too fast welding speed, inappropriate groove size, or the welding wire not aligned with the center of the weld. Specifically, it includes the following aspects:
(1) The welding current is too small, the welding speed is too fast, and the heat input is small, resulting in a decrease in the resistance heat generated, which makes the arc penetration power insufficient and the edge of the weld cannot be fully melted.
(2) The welding arc is too long, and the molten droplets from the electrode metal not only transition to the molten pool but also transition to the unmelted parent metal.
(3) There are oxides, rust, oil, water, and other contaminants on the surface of the weld.
(4) When welding the pipeline, the pipe mouth assembly does not meet the requirements, such as a small pipe mouth assembly gap, small groove angle, too thick or uneven pipe mouth blunt edge, etc.
(5) The weldment dissipates heat too quickly, causing the molten metal to crystallize too quickly, resulting in insufficient fusion with the parent metal.
(6) The electrode coating is eccentric, damp, or affected by the weather.
(7) The operator is not skilled, such as improper electrode angle and rod feeding method, lack of experience in controlling the molten pool, etc.
(8) The joint grinding and assembly do not meet the requirements. Incomplete penetration is also a relatively dangerous defect, and its harm is second only to cracks.
Sixth, the control method of incomplete penetration of steel pipe welding defects
(1) Choose a reasonable groove form: Manual CO2 welding with a large groove and small gap is easier to operate than a small groove and large gap, which is conducive to improving penetration. For butt welds with gaskets, in order to fully penetrate the root, the groove without blunt edges is better than that with blunt edges, and an expansion gap should be left between the gasket and the part. In order to ensure the root of the joint is fully penetrated, the weld structure design should avoid dead corners that the welding wire cannot reach.
(2) Select the correct welding current and welding voltage: When performing CO2 gas-shielded welding of T-shaped joints, since it is difficult to weld in the flat welding position, it can be placed in the horizontal welding position for welding.
(3) Prevention of incomplete penetration of pipeline welds: In the construction of large pipelines, incomplete penetration defects of pipeline welds are not allowed. Once incomplete penetration occurs during inspection, it should be immediately judged as unqualified. The prevention measures for incomplete penetration defects of pipeline welds are as follows:
① Under the premise of meeting the welding process, select the best combination of welding current, pipe mouth assembly gap, blunt edge, and groove angle.
② Clean the oxides, rust, oil stains, and other impurities on the welding surface.
③ At the start of the weld and the joint, you can first use a long arc preheating and then press the arc welding, and the root of the weld should be fully fused.
④ After each arc stop, use an angle grinder to grind the joint. The grinding length is generally 15~20mm, and a smooth transition is formed.
⑤ When performing root welding, the diameter of the molten hole must be strictly controlled. For welds that require single-sided welding and double-sided forming, the operator should always control the diameter of the molten hole to 2.5~3mm and maintain the hook speed to move the wire. Only in this way can the internal weld be formed beautifully and meet the quality requirements.
⑥ When using electrode arc welding to weld the root downward, when the ambient wind speed is greater than 5m/s, windproof measures must be taken to ensure welding quality.
Post time: Mar-13-2025