Hydrogen cracking (HIC)

Hydrogen cracking — also called cold cracking or delayed cracking, all three names for the same mechanism — is defined by three conditions that must be present together: diffusible hydrogen in the weld metal, a susceptible microstructure able to trap it, and tensile stress to drive the crack once it starts. Remove any one of the three and the mechanism stops, which is exactly why prevention attacks all three independently rather than relying on a single control.

The delay is what makes it dangerous. Hydrogen diffuses through the metal lattice over hours, concentrating at microstructural traps — inclusions, grain boundaries, and above all untempered martensite in a hardened heat-affected zone — faster than it can escape to the surface. A joint can pass visual inspection immediately after welding and still crack later, once enough hydrogen has migrated to a susceptible location under sustained residual stress. This is also why an arc strike outside the intended joint is a genuine risk on its own: a stray arc creates a small, fast-quenched hardened spot with no filler and no preheat, and on material with an elevated carbon equivalent that spot can crack by the same delayed mechanism as a full weld — see arc strike for how that specific case is assessed and treated.

Prevention works on each of the three conditions separately. Hydrogen: low-hydrogen consumables, correctly baked and stored, and shielding practice that keeps atmospheric moisture out of the arc. Microstructure: preheat and interpass temperature high enough, for a given carbon equivalent and section thickness, to slow cooling below the rate that forms untempered martensite — sometimes combined with a post-heat or dehydrogenation treatment immediately after welding, before the joint is allowed to cool fully to ambient. Stress: joint design, restraint control and welding sequence that limit the tensile residual stress locked into the joint as it cools. Because the crack can take hours to appear, verifying that all three controls were actually followed — not just specified — is the more reliable path than waiting to see whether a crack shows up on the next inspection round.