Discontinuity
Cold crack (hydrogen-induced cracking)
A crack that appears after the weld has cooled, sometimes many hours later, when dissolved hydrogen collects at defects in a hard microstructure and residual stress finishes the job.
TitleCold crack (hydrogen-induced cracking)
ClassCrack
ViewIndicative
What it looks like
- Bright, flat, relatively straight fracture faces without the oxidation of a hot crack
- Can appear in weld metal or heat-affected zone, at the toe, root, or under the bead
- Frequently discovered on a joint that was inspected and accepted immediately after welding
- Often audible as a sharp crack from a cooling assembly with nobody working on it
Why it happens, and how it is stopped
Causes
- Hydrogen from moisture in coverings and flux, from oil and paint on the joint, or from humid air
- A hard, low-toughness microstructure formed by rapid cooling in hardenable steel
- High tensile residual stress from restraint, thickness, or aggressive weld sequencing
- Low ambient temperature and heavy sections drawing heat out of the weld far too quickly
- Skipping preheat on material whose carbon and alloy content genuinely requires it
Prevention
- Combine low-hydrogen consumables with strict storage and exposure control so they stay low-hydrogen
- Preheat and hold interpass temperature to slow cooling and allow hydrogen to escape the joint
- Use a post-weld hydrogen soak on thick, restrained, or high-strength joints
- Remove all moisture, oil, and coatings from the joint area and keep the work out of the weather
How it is found
Which examination methods can see it, and what it is about the discontinuity that makes them able to.
- MTSurface-breaking cold cracks in steel are found readily by magnetic particle inspection, which is why it is the usual delayed-inspection method.
- UTReaches subsurface cracks that surface methods cannot see, and can size the crack through the thickness.
- PTApplies where the material is not ferromagnetic, provided the crack has reached a surface.
- VTA useful first look on a re-examined joint, since a cold crack that has grown can become visible without any equipment.
How inspectors think about it
- The delayed nature of this mechanism is the practical problem: inspectors on susceptible material deliberately wait before final examination rather than clearing a joint while it is warm.
- Any crack is treated as a rejectable planar discontinuity, and the presence of one usually calls the whole heat-input and consumable-handling regime into question.
- Because the three ingredients are hydrogen, hard microstructure, and stress, an inspector reasons about which of the three is easiest to remove on that particular job.
General reasoning only. The accept/reject limits for your work come from the code or specification governing the job, which this site does not reproduce.
Related discontinuities
Hot crack (solidification cracking)A crack that forms while the weld is still partly liquid, when low-melting films between growing grains are pulled apart by shrinkage before the metal has any strength to resist it.Underbead crackA crack lying in the heat-affected zone beneath a sound-looking weld bead, produced by hydrogen trapped in hard martensite that cracks under residual stress after the joint has cooled.Transverse crackA crack running across the weld axis, at right angles to the direction of travel, caused by longitudinal shrinkage stress in a weld that is stronger or harder than the metal around it.Toe crackA crack starting at the junction of the weld face and the base metal, where the abrupt change in section concentrates stress and the heat-affected zone is at its hardest and least forgiving.