Welded steel pipes may encounter various defects during production or when welding during use. The following are some common defects in welded steel pipes:
I. Surface Defects
- Non-conforming Weld Dimensions: The weld reinforcement and its difference, weld width and its difference do not meet standard requirements, resulting in uneven weld heights and widths. Uneven weld dimensions not only affect the aesthetic appearance of the weld but also weaken the bond strength between the weld and the base material. Excessive weld reinforcement can cause stress concentration, while a weld lower than the base material fails to achieve sufficient joint strength.
- Undercut: A V-shaped groove appears along the centerline of the weld on the edge of the weld seam. The main reasons are inappropriate welding speed, current, and voltage conditions, with welding speed being too high being more likely to cause undercut defects than unsuitable current. Undercut weakens the weld joint strength of the welded pipe and can easily initiate cracks due to stress concentration.
- Offset: The outer walls of the weld seam on both sides are not on the same plane, with the offset amount exceeding 10% of the base material thickness or more than 4mm. This occurs when the welding parts are not properly fitted, and welders perform tack welding and welding under unsuitable conditions.
II. Internal Defects
- Porosity: It mostly occurs in the center of the weld seam, where hydrogen remains hidden in the form of bubbles within the weld metal. The main causes include wetness on the surface of welding wire and flux, which are used without drying; also, accelerating the solidification of metal can occur when the welding current is too low and the welding speed is too fast.
- Slag Inclusion: The weld seam contains iron oxide and other molten slag, with interlayer slag inclusion being a common type. It affects the toughness of the weld bead. The main causes include: small groove angles or gaps, excessive root face leading to insufficient slag melting; inadequate melting of slag formed by welding wire and electrode; and incomplete removal of slag between layers, causing slag to be buried in the weld bead. Slag inclusion weakens the effective cross-section of the weld, thereby reducing its mechanical properties and causing stress concentration, which can easily lead to failure of the welded structure under load.
- Incomplete Penetration: The phenomenon where the root of the welded joint is not completely melted. Incomplete penetration causes stress concentration and can easily lead to cracks. Important welded joints are not allowed to have incomplete penetration. The main causes include: too small groove angle or gap, excessive root face, poor assembly; improper selection of welding process parameters, such as too low welding current and too fast welding speed; and poor welding operator technique.
- Lack of Fusion: The partially unmelted and unbonded area between the weld bead and the base material or between weld beads. Lack of fusion directly reduces the mechanical properties of the joint and can cause the welded structure to be unable to bear loads in severe cases. The main causes include: high welding speed with low welding current, resulting in low welding heat input; eccentric welding wire and improper angle between the welding wire and the workpiece, causing the arc to deviate; rust, dirt, and contaminants on the groove sidewalls; and incomplete slag removal between layers.
- Cracking: The welding crack formed in the weld seam and the heat-affected zone metal when cooled to the high temperature range near the solidus line is called a hot crack, which is a dangerous welding defect that is not allowed. The main cause is that the low-melting eutectic and impurities in the molten pool form severe intragranular and intergranular segregation during crystallization, and are pulled open along the grain boundaries under welding stress, forming hot cracks.
III. Other Defects
- Overlap: During welding, molten metal flows outside the weld seam and onto the unmelted base material, forming a metal lump. This not only affects the appearance of the weld but may also cause stress concentration, reducing the strength and toughness of the weld.
- Burn-through: Molten metal flows out from the back of the groove, forming a perforated defect. This is usually caused by excessive welding current, too slow welding speed, or too large a groove gap.
These defects have a serious impact on the quality and performance of welded steel pipes. Therefore, during the production and use of welded pipes, it is necessary to strictly control the welding process parameters and operating procedures to minimize the occurrence of these defects. At the same time, regular quality inspection and maintenance of welded pipes are required to ensure their long-term stable operation.
