6.1 Hydrogen Cold Cracking

6.1 Hydrogen Cold Cracking

Quick Answer: Hydrogen cold cracking (also called hydrogen-induced cold cracking, HICC, or delayed cracking) needs four conditions at once: diffusible hydrogen, a hard/susceptible microstructure, tensile residual or applied stress, and low temperature after the weld cools. Remove any one condition and cracking risk collapses. Cracks often appear hours after welding, not while the pool is liquid.

Module WT2.5 expects IWI-S candidates to distinguish cold cracking from hot cracking, name the four conditions, list practical hydrogen sources, and know which procedure controls (low-H consumables, baking, preheat, interpass, restraint, PWHT) the inspector must verify. This section is the foundation for later work on preheat/interpass (Chapter 7) and crack imperfections under ISO 6520.

What “Cold” and “Delayed” Mean

Cold cracking means the crack forms after the weld metal and HAZ have cooled well below solidification temperature—typically near ambient or in the lower part of the cooling curve—not during the liquid-to-solid transition.

Delayed cracking emphasises timing: hydrogen can remain mobile in the steel for a period after welding. Cracks may open hours later (sometimes longer under severe conditions) under residual stress once hydrogen has diffused to susceptible sites. That is why some specifications impose a delay before final NDT on crack-sensitive fabrications: early inspection can miss cracks that have not yet formed.

Typical locations:

• HAZ of hardenable carbon and low-alloy steels (most classic cases)—often along the fusion boundary or in the coarse-grain HAZ
• Weld metal when weld chemistry, hydrogen, and stress are unfavourable (e.g. under-matched toughness or high-strength deposited metal)
• Toe and root regions where residual tensile stress and geometric stress concentration combine

Orientation is commonly transverse or longitudinal depending on stress field and joint type; the inspector’s job is recognition and process-root-cause thinking, not full fracture-mechanics analysis.

The Four Conditions (Memorise as a Checklist)

HICC is a threshold phenomenon. All four must be present simultaneously:

Exam logic: If any condition is removed by design or procedure, cold cracking risk drops sharply.

• No hydrogen → low risk even on hard HAZ
• Soft, non-hardenable microstructure → low risk even with some hydrogen
• Compressive or low tensile residual stress (good design, peening where allowed, PWHT) → reduced driving force
• Keeping the joint warm longer (preheat, interpass, post-heat) slows cooling, softens HAZ development, and allows hydrogen to diffuse out before the structure becomes fully “cold” and embrittled

Sources of Diffusible Hydrogen

Hydrogen enters the arc atmosphere and dissolves into the liquid weld metal. On solidification and cooling it becomes diffusible hydrogen in the solid. Practical sources the IWI-S must recognise:

1. Moisture in consumables — damp MMA coatings (especially basic low-hydrogen electrodes left out of dry storage), wet flux-cored wires, damp SAW fluxes
2. Consumable chemistry — cellulosic electrodes generate high hydrogen in the arc; they are unsuitable for many hardenable, highly restrained joints without special procedure justification
3. Surface contamination — oil, grease, paint, primers, cutting fluids, rust with moisture, condensation on cold plate
4. Coatings and shop primers — organic coatings and some shop primers can contribute hydrogenous species if not removed from the weld zone as required by the WPS
5. Environment — high humidity, rain, condensation on incompletely protected joints; wet gloves and contaminated tools
6. Shielding problems — less often primary for “classic” HICC than for porosity, but moisture in shielding gas or leaks that admit humid air can still add hydrogen

Storage discipline for low-hydrogen MMA is a classic inspection topic: sealed packs, holding ovens/quivers at manufacturer temperatures, controlled issue time on the floor, and re-drying only when the manufacturer and WPS allow it. Do not bake cellulosic electrodes as if they were basic low-hydrogen types.

Susceptible Microstructure and Steel Selection

Susceptibility rises with hardenability and cooling rate into hard microstructures. Carbon equivalent concepts and ISO/TR 15608 grouping (Chapter 5 / 7) help engineering decide preheat and consumable strategy. Thick sections, high carbon or alloy content, and low heat input produce harder HAZ regions.

Inspectors do not redesign alloys on the floor, but they must recognise when the combination of material group, thickness, and process is in a high-risk regime—and then verify that the WPS controls (preheat, heat input window, low-H consumable) are actually applied.

Stress and Restraint

Welding residual stresses are often of yield magnitude in restrained joints. High restraint examples: thick-section butts, stiff node joints, repair welds in rigid structures, and heavily clamped assemblies. Applied loads during welding (lifting, fit-up force, residual assembly stress) add to the tensile field.

Prevention angles:

• Joint design that reduces through-thickness and high residual tension where possible
• Balanced welding sequences and back-step techniques where specified
• Reduced clamp force once fit-up is achieved (as procedure allows)
• PWHT where the specification requires residual-stress relief or tempering of hard HAZ

Temperature, Preheat, Interpass, and Hydrogen Diffusion

Preheat and minimum interpass temperature:

• Slow cooling → less hard martensite in many steels
• Keep the joint warmer longer → hydrogen can diffuse out of the weld and HAZ before the structure sits cold under full residual stress

Some procedures specify post-heat (holding at elevated temperature after welding) for hydrogen bake-out on critical joints. PWHT (full post-weld heat treatment) is a different, usually higher-temperature operation for residual stress and microstructure control—use only as specified.

Prevention Summary (Procedure Toolbox)

Inspector Checks (Pre / During / Post)

Before welding

1. Confirm material, thickness, and WPS hydrogen-control strategy (consumable class, preheat).
2. Check electrode/flux/wire condition, oven/quiver logs, and exposure times.
3. Verify joint cleanliness and coating removal in the weld zone.
4. Verify preheat method and measurement locations match the procedure.

During welding

1. Monitor preheat and interpass temperatures with approved methods (contact thermometer, tempil sticks as allowed—not guesswork).
2. Watch for parameter drift that drops heat input excessively on hardenable steels.
3. Confirm stop/start practice and repair of defects do not create local hard, high-hydrogen spots without re-control.

After welding

1. Enforce any delay before final NDT required by code, client, or WPS.
2. Ensure VT/MT/PT (as specified) includes toes, roots (when accessible), and HAZ-adjacent surfaces.
3. Record non-conformances (damp consumables, missed preheat) immediately—do not “wait to see if it cracks.”

Link Forward

Hydrogen cold cracking is the dominant “delayed” crack story in carbon and low-alloy fabrication. Section 6.2 contrasts it with hot cracking during solidification. Chapter 7 expands preheat, interpass, and PWHT as systematic controls. When cracks appear in production, the IWI-S should structure the investigation around the four conditions, not only the final NDT report.