Hydrogen cracking in HAZ (heat affected zone) of a steel occurs when four conditions exists at the same time:
- Hydrogen level : >15ml/100g of weld metal deposited
- Stress : 0.5 of the yield stress
- Temperature : <300oC
- Susceptible Microstructure: >400HV hardness
These four factors are mutually interdependent so that the influence of one factor depends on how active the other three factors are.
Hydrogen (H) can enter the molten weld metal when hydrogen containing molecules are broken down into H atoms in the welding arc. As H atoms are small they can easily move (diffuse) in solid metal and when weld metal is hot then they diffuse to the surface of the metal and escapes into the atmosphere. However as weld solidifies at very faster rates there is tendency of H atoms to get trapped.
If HAZ has susceptible microstructure and also if tensile stresses in the weld are more then H cracking can occur.
It is believed that H causes embrittlement of regions of the HAZ so that high localized stresses cause cracking rather than plastic straining.
To avoid hydrogen cracking in HAZ only one thing is to be ensured that at least one of the four factors mentioned above is not active during welding.
Methods used to minimize the influence of each of the four factors will be summarized in next article, till then
Happy welding, Keep reading…