Welding Defect Crack

 

Welding Defect Crack 

Cracks are fractures of metals that can occur in different parts of the weld metal and also in the base metal, the types and causes of these are given below... 

 Crack

 Cracks can be classified based on their direction and positions as follows:

1. Longitudinal crack, 

2. Transverse crack, 

3. Crater crack, 

4. Throat crack, 

5. Toe crack, 

6. Root crack, 

7. Under- Bead crack,

8. Heat Affected Zone crack, 

9. Hot crack, 

10. Cold or Delayed crack

Longitudinal crack

When a crack occurs in the direction of the weld axis, called longitudinal crack. It may occur in both the weld metal and base metal. Basically, this type of crack occurs due to the fast cooling of the weld metal and shrinkage stresses in the high constraint parts of the metal. This type of crack may be eliminated by preheating of the weld joint and weld towards less constraint.

To repair this type of crack generally needs to remove the affected part and reweld.

Transverse crack

This type of crack occurs in the weld metal, transverse to the weld line direction, due to the hardness of the weld metal and excess heat input. This type of crack can be eliminated by reducing the heat input and using preheat action. 

For repair, these cracks should remove the affective part thoroughly and reweld.

 Crater crack

This type of crack is commonly found in the crater in the "X" shape. This defect is caused by the freezing of the centre of a weld before the outside of the weld pool. this type of crack is a hot crack and it can be prevented by a short pause or slightly step back at the weld termination.

Throat crack

this type of defect occurs at the throat of the joint in longitudinal due to transverse stresses. It may occur if using inadequate filler metal and can be prevented by preheating and use correct filler metals.

Toe crack

this type of cracks occurs at the welding toe area due to transverse shrinkage stress by HAZ brittleness.

 Root crack

This type of cracks occur at the root of the joint due to transverse stress and is prevented by preheating the joint and using more ductile filler metals. Sometimes a crack in the root occurs when the chemical composition of the filler metal is different from the base metal.

Under- Bead crack

This type of crack occurs in the combined base metal of the heat-affected zone when the area hardens and becomes brittle after the weld metal is deposited and more hydrogen is absorbed by the weld metal. To prevent this defect less hydrogen filler metal is required to be used and the pre-heating method should be applied.

Solidification/Hot crack

This defect occurs when weld metal solidifies and the weld metal shrinkage causes a micro-stress in the weld metal as it cools from the liquid to the solid-state. and crack can occur in such a situation.usually this defect occurs when the depth/width ratio of the weld bead is large. the base metal composition is also responsible for this type of crack. This defect can be eliminated by using low tensile filler material and using preheat.

Cold or Delayed crack

Cold crack is a type of internal crack. This defect can occur due to the non-release of stresses generated in the metal in the solidification process, this cracks sometimes show immediately but may also occur a few days later. This can be large due to external stress and one reason is the inclusion of a sufficient amount of hydrogen at the time of solidification also.

Conclusion

These cracks are also classified as internal and external cracks. The above description makes it clear that the Fracture of the metal is called crack. And the main reasons for its occurrence that the chemical composition of filler material does not match the chemical composition of base metal insufficient Pre-heating, using inappropriate welding procedure specification Hydrogen ingestion. These all can be minimized by using appropriate welding procedures,pre-heating post-heating,post-weld heat treatment, etc.

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Common welding defects