Monday, January 18, 2010
How Hydrogen Cracks Occur? Useful information for NDT Technicians
Whenever hydrogen cracks are generated, three things occur (are present) simultaneously:
1. Dissolved Hydrogen
2. Tensile Stresses
3. Low Ductility
This is the sequence of events leading to this deadly cracking.
a) Hydrogen enters the arc either by the shielding gas, flux or material contamination and dissolves into the weld metal itself.
b) As the weld cools, it becomes saturated in hydrogen, all of which diffuses out into the HAZ.
c) If cooled quickly (rapid), the hydrogen is retained in the 'austenite' instead of escaping.
d) The Hydrogen is trapped and seeks rifts and discontinuities in the material lattice and concentrates at these points.
e) The external stresses generated due to transformation act with the hydrogen to enlarge the 'discontinuities' to 'crack' size. This hydrogen may contribute to cracking by lowering the strength of the lattice and/or by adding to the stresses at the discontinuity level.
f) Crack growth takes the crack-root away for the hydrogen concentration and may be interrupted until the hydrogen diffuses to a new root location. This leads to the time-dependent behavior of the cracking phenomenon.
The Main Defence:
Preheat - The main reason for this heat is to assist in removing hydrogen from the weld by allowing the weld to cool slower; (diffusing hydrogen out) and remaining more ductile, thus resisting cracking.