Hydrogen trapping effect
The reversible or irreversible trapping of diffusing hydrogen at lattice defects, precipitates, or grain boundaries. This reduces the amount of freely mobile hydrogen atoms and mitigates cracking risk.
Hydrogen can settle in the weld metal and in the heat-affected zone and cause damage there. If it gets stuck at lattice defects or inclusions, that's even good, because then it can no longer migrate and cause damage. The problem is the free, mobile hydrogen that leads to cracks, especially in hard microstructures. In practice, this means: you have to get the hydrogen out of the weld before it gets too cold. Slow cooling or annealing immediately after welding helps the hydrogen to escape. If you cool too quickly, you get a hard, brittle microstructure and the hydrogen stays in – then the weld will crack. So better keep the weld warm and let it cool slowly, that's the safest way.