Do you know common Welding Defects?
There are four categories common welding defects.
1. Common welding defects can be divided into four categories:
(1) The size of the weld does not meet the requirements: over height weld, over width, narrow weld , big height difference, rough weld transition to the base metal, etc.
(2) Weld surface defects: undercut, weld beading, indentation, overflow, incomplete penetration, surface pores, surface cracks, etc.
(3) Weld internal defects: pores, slag inclusion, cracks, lack of 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. Weld 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 the uneven local heating and cooling of the welding workpiece. Because the workpiece is only heated to high temperature in a local area during welding. The closer it is to the welding seam, the higher the temperature and the greater the expansion.
However, the metal in the heating area cannot expand freely because it is blocked by the metal with lower surrounding temperature, and it cannot shrink freely due to the restraint of the surrounding metal during cooling.
As a result, this part of the heated metal has tensile stress, while the other part of the metal has a balanced compressive stress.
When these stresses exceed the yield limit of the metal, welding deformation will occur. When the strength limit of the metal is exceeded, cracks will appear.
(2) Weld External Defects
1. Too high welding reinforcement
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 the workpiece, the reinforcement of the weld is required to be flattened.
2. More Concave Weld
The strength of the joint is reduced due to the reduction of the working section of the weld.
3. Weld undercut
The depression formed on the workpiece along the edge of the weld is called undercut. It not only reduces the working section of the joint, but also causes serious stress concentration at the undercut.
4. Weld Tumor
The molten metal flows to the unmelted workpiece at the edge of the molten pool, and it accumulates to form a weld bump, 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 means that part of the molten metal leaks from the opposite side of the weld, or even burns through into a hole, which reduces the strength of the joint.
The above five kinds of defects exist on the surface of the weld, which can be found by naked eyes and can be repaired in time.
(3) Internal Defects of the Weld
1. Not penetrated
Incomplete penetration refers to a defect in which the workpiece and the weld metal or weld layer are not locally fused. Less penetration will weaken the working section of the weld, cause serious stress concentration, and greatly reduce the strength of the joint. It often becomes the source of weld cracking.
2. Slag Inclusion
There is non-metallic slag in the weld, which is called slag inclusion. Slag inclusion reduces the working cross-section of the weld, causing stress concentration, and reducing the strength and impact toughness of the weld.
3. Pores
When the weld metal is at a high temperature, it absorbs too much gas (such as H2) or the gas generated by the metallurgical reaction in the molten pool (such as CO). It is too late to be discharged when the molten pool is cooled and solidified, and cavities are formed inside or on the surface of the weld, which is the pore.
The existence of pores reduces the effective working section of the weld and reduces the mechanical strength of the joint. If there are penetrating or continuous pores, it will seriously affect the tightness of the workpiece.
(4) Crack
During or after welding, the partial fracture of the metal in the area of the welded joint is called crack. Cracks may occur in the welding seam or in the heat-affected zone on both sides of the weld. Sometimes it occurs on the surface of the metal, and sometimes it occurs inside the metal.
Usually, it can be divided into two types according to the different mechanism of crack generation,: hot crack and cold crack.
4.1 Hot Crack
Hot cracks are generated during the crystallization process from liquid to solid in the weld metal, and most of them occur in the weld metal.
The main reason for this is the presence of low melting point substances (such as FeS, melting point 1193°C) in the weld, which weakens the relationship between the crystal grains. When subjected to greater welding stress, it is easy to cause cracks between the crystal grains.
When the workpiece and welding wire contain a lot of impurities such as S and Cu, thermal crack is easy to occur.
Hot cracks are characterized by distribution along grain boundaries. When the crack penetrates the surface and communicates with the outside, it has an obvious hydrogenation tendency.
4.2 Cold Crack
Cold cracks are produced during the cooling process after welding, mostly on the fusion line of the base metal or the junction of the base metal and the weld.
The main reason is that the quenched structure is formed in the heat-affected zone or the weld. Under the action of high stress, it causes cracks in the grains. When welding easy-quenchable steels with higher carbon content or more alloying elements, it is more easy to produce cold cracks.
Too much hydrogen melted into the weld can also cause cold cracks.
Crack is a very dangerous defect. In addition to reducing the load-bearing section, it also produces severe stress concentration. The crack will gradually expand during use, and may cause damage to the component. Therefore, such defects are generally not allowed in the welded structure. Once found, it must be removed and re-welded.
Purchase Top Quality CNC welding machine, please contact GloryTech .