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1、 Common welding defects can be divided into four categories:
(1) The weld size does not meet the requirements: for example, the weld is too high, too wide, too narrow, the height difference is too large, and the weld transition to the base metal is not smooth.
(2) Welding surface defects: such as undercut, overlap, concave, overflow, incomplete penetration, surface porosity, surface crack, etc.
(3) Internal defects of weld: such as air hole, slag inclusion, crack, incomplete fusion, tungsten inclusion, incomplete penetration of double-sided welding, etc.
(4) The performance of the welded joint does not meet the requirements: the mechanical properties and corrosion resistance of the welded joint are reduced due to overheating, overburning and other reasons`
2、 Welding defects
（1） Welding deformation
Generally, the workpiece will be deformed after welding. If the deformation exceeds the allowable value, it will affect the use.
The main reason is uneven local heating and cooling of weldments. During welding, the weldment is heated to high temperature only in local areas. The closer it is to the weld, the higher the temperature and the greater the expansion.
However, the metal in the heating area can not expand freely due to the metal with lower ambient temperature; When cooling, it can not shrink freely due to the restriction of surrounding metal.
As a result, the metal heated in this part has tensile stress, while the metal in other parts has compressive stress in equilibrium with it.
When these stresses exceed the yield limit of metal, welding deformation will occur; When the strength limit of metal is exceeded, cracks will appear.
（2） External defects of welds
1. weld reinforcement is too high
This phenomenon occurs when the angle of the welding groove is too small or the welding current is too small. Therefore, in order to improve the fatigue life of pressure vessels, it is required to flatten the weld reinforcement.
2. the weld is too concave
The strength of the joint is reduced due to the reduction of the working section of the weld.
3. weld undercut
The depression formed along the weld edge on the workpiece is called undercut. It not only reduces the working section of the joint, but also causes serious stress concentration at the undercut.
4. weld beading
The molten metal flows to the unmelted workpiece at the edge of the molten pool and accumulates to form a weld bead, which is not fused with the workpiece. The weld bead has no effect on the static load strength, but it will cause stress concentration and reduce the dynamic load strength.
5. burn through
Burn through refers to the leakage of some molten metal from the reverse side of the weld, or even burn through into holes, which reduces the strength of the joint.
The above five defects exist in the appearance of the weld, which can be found by naked eyes and can be repaired in time.
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