Weld Imperfections : Hydrogen Induced Cracks

Advertisement

Hydrogen induced cracking occurs primarily in the grain-coarsened region of the HAZ, and is also known as cold cracking, delayed cracking or under-bead / toe cracking.

Under-bead cracking lies parallel to the fusion boundary, and its path is usually a combination of inter-granular and trans-granular cracking. The direction of the principal residual tensile stress can, for toe cracks, cause the crack path to grow progressively away from the fusion boundary towards a region of lower sensitivity to hydrogen cracking. When this happens, the crack growth rate decreases and eventually arrests.

Hydrogen Induced Cracks

Hydrogen Induced Cracks

A combination of four factors is necessary to cause HAZ hydrogen cracking:

  1. Hydrogen level > 15 ml/100 g of weld metal deposited.
  2. Stress > 0.5 of the yield stress
  3. Temperature < 300 Degree Celcius.
  4. Susceptible micro-structure > 400 HV hardness.
Hydrogen Induced Cracks

Hydrogen Induced Cracks

If any one factor is not satisfied, cracking is prevented. Therefore, cracking can be avoided through control of one or more of these factors.

  1. Apply preheat (to slow down the cooling rate and thus avoid the formation of susceptible micro-structures).
  2. Maintain a specific inter-pass temperature (same effect as preheat).
  3. Post heat on completion of welding (to reduce the hydrogen content by allowing hydrogen to effuse from the weld area).
  4. Apply PWHT (to reduce residual stress and eliminate susceptible micro-structures).
  5. Reduce weld metal hydrogen by proper selection of welding process/ consumable (eg use TIG welding instead MMA, use basic covered electrodes instead cellulose ones).
  6. Use multi-run instead single-run technique (eliminate susceptible micro-structures by means of self tempering effect, reduce the hydrogen content by allowing hydrogen to effuse from the weld area).
  7. Use a temper bead or hot pass technique (same effect as above).
  8. Use austenitic or nickel filler (avoid susceptible micro-structure formation and allow hydrogen diffusion out of critical areas).
  9. Use dry shielding gases (reduce hydrogen content).
  10. Clean joint from rust (avoid hydrogen contamination from moisture present in the rust).
  11. Reduce residual stress.
  12. Blend the weld profile (reduce stress concentration at the toes of the weld).

 

%d bloggers like this: