Common External Welding Defects of Large Diameter Elbows
Posted: 12/30/2020 09:55:28 Hits: 25
Generally, large diameter elbows will be deformed after welding. If the deformation exceeds the permissible value, the use of the elbows will be affected. The main reason for the deformation of the elbows is local heating and cooling of weldments. During welding, only the partial area of the weldment is heated to high temperature, and the closer the part is to the weld, the higher the temperature and the greater the expansion becomes.
However, the heated metal can not expand freely because of the surrounding metal with lower temperature. As the metal cools, it cannot contract freely due to the containment of surrounding metal. As a result, the heated metal has tensile stress, while the surrounding metal has compressed stress to balance the tensile stress. When these stresses exceed the yield limit of metal, welding deformation will occur. When these stresses are in excess of the strength limit of metal, the cracks will appear in the elbow.
The external defects of the weld
Excessive weld reinforcement: This phenomenon will occur when the angle of the welding groove is too small or the welding current is too low. Because stress concentration is easy to damage the fitting, it is required to remove the extra weld metal to improve the fatigue life of the pressure vessel.
Undercut: Undercutting is when the weld reduces the cross-sectional thickness of the base metal. And serious stress concentration will occur at the undercut.
Weld beads: The molten metal flows to the unfused welding elbow at the edge of the (molten) bath and accumulates to form the weld bead, which is not fused with the welding elbow. The bead has no effect on the static load capacity, but it will cause stress concentration and reduce the dynamic load.
Burn-through: A localised collapse of the molten pool due to excessive penetration or loss of control, resulting in a hole in the weld root run, which reduces the strength of the joint.
The above defects exist on the surface of the weld, and they can be found by naked eyes and repaired in time.
However, the heated metal can not expand freely because of the surrounding metal with lower temperature. As the metal cools, it cannot contract freely due to the containment of surrounding metal. As a result, the heated metal has tensile stress, while the surrounding metal has compressed stress to balance the tensile stress. When these stresses exceed the yield limit of metal, welding deformation will occur. When these stresses are in excess of the strength limit of metal, the cracks will appear in the elbow.
The external defects of the weld
Excessive weld reinforcement: This phenomenon will occur when the angle of the welding groove is too small or the welding current is too low. Because stress concentration is easy to damage the fitting, it is required to remove the extra weld metal to improve the fatigue life of the pressure vessel.
Undercut: Undercutting is when the weld reduces the cross-sectional thickness of the base metal. And serious stress concentration will occur at the undercut.
Weld beads: The molten metal flows to the unfused welding elbow at the edge of the (molten) bath and accumulates to form the weld bead, which is not fused with the welding elbow. The bead has no effect on the static load capacity, but it will cause stress concentration and reduce the dynamic load.
Burn-through: A localised collapse of the molten pool due to excessive penetration or loss of control, resulting in a hole in the weld root run, which reduces the strength of the joint.
The above defects exist on the surface of the weld, and they can be found by naked eyes and repaired in time.
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