Welding Inspector

3.1 Crack is an imperfection caused by a local rupture in the solid state, which may arise from the effect of cooling or stresses. Cracks are very significant than other type of imperfections, as crack will produce large stress concentration at the crack tip, making them more likely to cause fracture.

Type of cracks:

• Longitudinal cracks
• transverse cracks
• crater cracks
• branching cracks

Crack can be located:

• in the weld metal
• in the HAZ
• in the parent metal

Note: Crater crack only be found in the weld metal.

Base on the Nature of crack, cracks can be:

• Hot Crack
• precipitation induce crack
  
• cold cracks
  
• lamelar tearing

3.1-1 HOT CRACK
Hot crack can be solidification crack and liquation cracks.

• Solidification crack is a crack that occur in weld metal, usually along centreline of weld, as a result of solidification process. When it is happen? 1. if the weld has a high carbon or impurity content, 2. the depth to width ratio of the solidifying weld bead is large (depth and narrow), 3. disruption of the heat flow condition occurs such as stop/start condition.
  
• Liquation crack, occur in the coarse grain HAZ, as a result of heating the material to an elevated temperature, high enough to produce liquation of the low melting point constituents placed on grain boundaries.

Hydrogen Induced Crack
This crack primarily occur in the grain-coarsened region of the HAZ, and also well known as cold cracking, delayed cracking or underbead / toe cracking. it is usually lies parallel to the fusion boundary, and usually a combination of intergranular and transgranular cracking.

There are three combination factors that will cause HAZ hydrogen cracking: Tensile stress, susceptible microstructure, high hydrogen concentration.

How to avoid Hidrogen Induced Crack?

1. Apply preheat, it's mean to slow down cooling rate and avoid the formation of susceptible microstructures.
   
2. maintain a specific interpass temperature
3. postheat on completion of welding.
   
4. apply PWHT
5. Proper selection of process / consumable.
   
6. clean joint from rust
7. reduce residual stress
8. blend the weld profile.