Why does hydrogen cause cracking in welding?

Hydrogen-induced cracking or cold cracking is one of the most dangerous weld defects because it can appear hours or days after welding.

Mechanism:

1. Hydrogen atoms (H) diffuse into the HAZ and weld during welding
2. Hydrogen sources: moisture in the electrode, rust, oil, paint, dirt
3. Hydrogen concentrates in high-stress zones (undercut, notch)
4. When the temperature drops, hydrogen becomes less soluble and forms H₂ molecules under pressure, creating microcracks — it is the cooling, more than holding at a given temperature, that triggers the phenomenon

Aggravating factors:

• High-strength steels (high yield strength)
• Large thickness
• Fast cooling (no preheating)
• High residual stresses

Prevention:

• Use low hydrogen electrodes (E7018, E8018) and follow the manufacturer's data sheet for storage and baking (oven temperature, duration, permitted number of re-bakes)
• Clean the joint: remove rust, oil, paint, moisture
• Preheating and, where prescribed, post-heating at the WPS temperatures, to slow cooling and let hydrogen diffuse out of the weld