Discussion on welding defects of thin-walled straight seam submerged arc welded pipes

1. Welding defects of straight seam submerged arc welded pipes

1.1 Burn-through

Burn-through is a hole formed by the liquid molten pool metal flowing out from the back of the weld during welding. Burn-through often occurs during the welding of thin-walled welded pipes. Burn-through in the production process of welded steel pipes can be eliminated by reducing the welding current and increasing the thickness of the blunt edge.

1.2 Undercut

The undercut is the groove and incomplete filling formed by the weld metal on the parent material adjacent to the weld toe. Undercuts are generated on both sides of the weld during low-current high-speed welding, which can be eliminated by increasing the heat input; undercuts caused by large offset of the weld point of thin-walled straight seam submerged arc welded pipes can be eliminated by adjusting the position of the weld point at the end of the pipe.

1.3 Porosity

The pores generated by the welding process of thin-walled straight seam submerged arc welded pipes are pores formed by the CO gas generated during the metallurgical reaction, which does not have time to escape during the crystallization process and remains inside the weld. The surface pores in the production process of welded steel pipes can be eliminated by increasing the three-wire current. The worm-like pores in the production process of welded steel pipes can be eliminated by reducing the current of the first and second wires and increasing the current of the third wire. No pores were found when the welded steel pipes were produced using the same welding process.

2. Factors affecting the straight seam submerged arc welded pipe

2.1 Influence of pipe diameter

The generation of welding defects is related to welding conditions. Changes in things related to the welding process of welded pipes may lead to welding defects. Due to different pipe diameters, the stress state of the molten pool metal in the weld of welded pipes, the influence of "pout" on the groove angle, and the size of the blunt edge angle are different from those of large-diameter welded pipes. The liquid metal in the molten pool flows under the action of various forces. In addition to the arc pressure, the liquid metal is also affected by gravity, electromagnetic force, surface tension, plasma flow force, and external force. Changes in the size or direction of any of these forces will cause changes in the flow range or flow direction of the liquid metal, resulting in changes in the shape of the molten pool.

2.2 Influence of wall thickness

The heat input of the straight seam submerged arc welded pipe is proportional to the wall thickness; as the wall thickness decreases, the bearing capacity of the bottom of the molten pool also decreases, and the weld is prone to burn-through, so the welding heat input also decreases accordingly with the decrease in wall thickness. Heat input is related to the surface tension of the liquid metal in the molten pool. The force along the surface of the liquid that makes the surface tend to shrink is called the surface tension of the liquid, and the surface tension decreases with the increase in temperature. The temperature of the molten pool is proportional to the heat input. The smaller the heat input, the faster the temperature of the molten pool decreases, and the greater the surface tension of the molten pool. The surface tension prevents the flow of the molten pool metal under the action of the arc force. The greater the surface tension, the smaller the flow range of the liquid metal in the molten pool, and it is easy to produce a narrow and high weld.

3. Analysis of the causes of defects of straight seam submerged arc welded pipes

3.1 Burn-through

During welding, the mass of the liquid metal in the molten pool exceeds the bearing capacity of the metal at the bottom of the molten pool, resulting in burn-through. The mass of the liquid metal in the molten pool is related to the heat input. The greater the heat input, the greater the mass of the molten pool metal. The bearing capacity of the metal at the bottom of the molten pool is affected by many factors: the larger the current, the higher the temperature at the bottom of the molten pool, the lower its strength, and the smaller its bearing capacity; the smaller the wall thickness at the bottom of the molten pool, the smaller its bearing capacity; the larger the groove gap, the smaller its bearing capacity; the larger the groove depth or angle on the back of the weld, the smaller it's bearing capacity; the thinner the pre-weld, the smaller its bearing capacity. When producing thin-walled straight seam submerged arc welded pipes, to avoid burn-through, appropriate heat input, welding current, and groove size should be selected.

3.2 Porosity

The weld heat input is small, the molten pool solidifies quickly, the time for CO gas to escape from the molten pool is shortened, and the weld is prone to pores; the welding penetration is large or the weld is high, the distance for CO gas to escape from the molten pool becomes longer, and the weld is also prone to pores. Factors that lead to reduced welding heat input, increased penetration, or increased weld excess height may lead to pores in the weld: welding current is too small or too large, groove depth is too large, blunt edge gap is too large, the "pout" outside the steel pipe is too large, and flux fluidity is poor. When producing thin-walled straight seam submerged arc welded pipes, under the influence of gravity, the increase in the height of the inner weld and the decrease in heat input are prone to surface pores; when the outer "pout" is large, the influence of gravity increases, and the possibility of pore defects increases. When welding thin-walled straight seam submerged arc welded pipes, to prevent the generation of CO pores in the weld, the weld "pout" value must be controlled, there must be sufficient heat input, and the weld height must also be controlled.

3.3 Undercut

The undercut defect is caused by a change in the magnitude or direction of the force on the molten pool during welding, which causes a change in the direction or range of the metal flow in the molten pool, and the edge of the molten pool is not filled with metal. When there is a weld point offset, the molten pool metal is affected by gravity to produce a bias flow, which makes it easy to produce an undercut above the weld. When the heat input is too small, the molten pool temperature is low, the cooling rate is fast, the surface tension of the molten pool increases sharply, the flow range of the molten pool metal is reduced, and it is easy to produce an undercut on both sides of the weld. When the wire feeding speed is unstable, the welding current or voltage fluctuates greatly, the arc pressure on the molten pool metal also fluctuates, the metal flow range fluctuates, and the weld edge is prone to undercutting. When producing thin-walled welded pipes, the weld offset should be controlled, the heat input should not be too small, and the wire feeding speed should be adjusted to be stable to prevent the weld from undercutting.

Recommended measures

When producing thin-walled straight seam submerged arc welded pipes, it is necessary to prevent the weld from having defects such as burn-through, pores, and undercuts. Factors that affect the generation of welding defects should be strictly controlled, such as controlling the welding heat input, groove size and processing accuracy, selecting a suitable blunt edge angle, controlling the "pout" after pre-welding, ensuring the straightness of the welding groove, increasing the width of the inner weld, selecting a flux with good fluidity, and reducing the current difference between the inner welding wires.