Hydrogen-induced cracking — also called cold cracking, delayed cracking, or hydrogen-assisted cracking — is one of the most common causes of structural weld failure, and almost entirely preventable. The three ingredients that must all be present for HIC to occur are: diffusible hydrogen in the weld, a susceptible microstructure (hard heat-affected zone), and tensile stress. Remove any one of those, and cracking does not happen. The WPS is where you document the specific controls that remove the hydrogen ingredient and limit microstructure susceptibility.
This article focuses on what AWS D1.1:2025 requires, what project specifications commonly add, and how to write those controls into a WPS that a CWI can verify during production.
The three levers on your WPS
Diffusible hydrogen level
Every electrode or filler metal classified by AWS has an optional diffusible hydrogen designator. Under AWS A5.1, E7018 electrodes can carry H16, H8, or H4. Under AWS A5.20, FCAW-S wires can carry H8 or H4. The designator tells you the maximum milliliters of diffusible hydrogen per 100 grams of deposited weld metal — tested at 316°C — for that electrode lot.
When your WPS specifies the electrode classification, include the hydrogen designator. "E7018-H8" is not the same as "E7018" for HIC-critical work. The PQR should record the designator used during qualification, and production records should match it.
For joints in plate thicker than one inch, high-strength steels, or any high-restraint configuration, H8 is a reasonable baseline and H4 is better. H16 electrodes are appropriate only for low-restraint joints on low-carbon steel. Some project specifications on fracture-critical bridge work specify H4 as the minimum across all processes.
Preheat and interpass temperature
Preheat slows the cooling rate of the weld and the heat-affected zone. Slower cooling reduces hardness in the HAZ martensite and gives diffusible hydrogen time to escape before the material is cold enough for cracking to initiate.
AWS D1.1:2025 Table 5.6 establishes minimum preheat for prequalified WPS based on base metal group, thickness, and welding process. The minimum preheat is a floor — not a target. Many fabricators add 25°F to 50°F margin over the Table 5.6 minimum on thick plate and high-restraint joints.
The WPS must state:
- Minimum preheat temperature before the first arc strike
- Minimum interpass temperature (often the same as preheat, or higher for thick sections)
- Maximum interpass temperature (where specified — typically for low-alloy and quenched-and-tempered steels)
- Method of preheat application (induction, oven, propane torch, resistance mat) — relevant because method affects soak depth
When the project specification or engineer's note adds a preheat requirement beyond Table 5.6, the stricter value governs and must appear on the WPS.
Heat input control
Heat input affects both the HAZ hardness and the cooling rate independently of preheat. Higher heat input generally reduces HAZ hardness by slowing cooling through the critical temperature range. But for quenched-and-tempered steels (A514, A517) and some TMCP steels (A913), excessive heat input can reduce toughness or anneal the base material, reversing the benefit.
AWS D1.1:2025 Table 6.6 lists heat input as an essential variable. The PQR documents the heat input range during qualification testing. Production welds must stay within that range. A WPS that specifies voltage, amperage, and travel speed implicitly controls heat input — but only if the welder maintains those parameters in production. Recording actual parameters on weld traveler tickets and checking heat input calculations on completed joints is the verification side of that control.
Electrode conditioning: the step most shops skip
Low-hydrogen electrodes absorb moisture from the atmosphere. Once absorbed, that moisture is the source of diffusible hydrogen in the weld. The conditioning chain has three links: rod oven storage, time out of the oven, and rebaking when time limits are exceeded.
AWS A5.1 and A5.5 specify the maximum time low-hydrogen electrodes may be exposed to ambient conditions before reconditioning is required. For H8 electrodes, the limit is typically four hours at an ambient humidity above 50%. For H4, the limit is shorter.
The WPS or the accompanying quality plan should state:
- Storage oven temperature range (typically 250°F–400°F for SMAW low-hydrogen)
- Maximum exposure time at ambient before rebaking
- Rebake temperature and duration
- Maximum number of rebake cycles
These parameters are not always listed directly on the WPS form, but they must appear somewhere in the project quality documentation and be verified by the CWI. For more on electrode conditioning requirements by designator, see the low-hydrogen electrode conditioning H4, H8, H16 guide.
Carbon equivalent and the preheat calculation
The minimum preheat needed to prevent HIC is not arbitrary — it can be calculated from the base metal chemistry using carbon equivalent (CE) formulas. The International Institute of Welding (IIW) formula is widely used:
CE = C + Mn/6 + (Cr+Mo+V)/5 + (Ni+Cu)/15
This CE value, combined with the material thickness, gives the cold cracking susceptibility index. The resulting minimum preheat is the temperature at which hydrogen can diffuse out of the HAZ before cracking initiates. The material test report (MTR) for the base metal provides the ladle chemistry inputs. See MTR base metal traceability requirements for what the MTR must contain and how to verify it.
High-restraint joints: what to add beyond the basics
For thick plate (over 1.5 inches), T-joints with full-penetration welds into a restrained flange, column base plates, and similar high-restraint conditions, the standard HIC controls may not be enough on their own. Additional measures to document on the WPS or quality plan include:
Controlled preheat soak depth. Measuring surface temperature is not enough if the heat is only skin-deep. Soak time or use of induction heating that produces through-thickness thermal uniformity should be specified when section thickness exceeds what a hand-held contact pyrometer can verify accurately.
Post-weld heat maintenance. Specifying a minimum temperature to maintain after the final weld pass — before the joint is allowed to cool — is a practice borrowed from pressure vessel welding. AWS D1.1 does not require it for standard structural work, but some bridge and fracture-critical specifications add a 300°F minimum hold for one hour after completion before controlled cooling.
Delayed inspection hold. Hydrogen-induced cracking can initiate hours or days after welding is complete, particularly in high-restraint thick joints. Specifying a 24–48 hour delay before final ultrasonic testing is conducted gives time for any latent cracks to grow to detectable size or for hydrogen to diffuse to innocuous levels. When the project specification requires a hold, document it in the inspection plan tied to the WPS.
CWI verification during production
The WPS sets the requirements; the CWI verifies them before and during welding. The pre-weld checklist for HIC-controlled joints:
- Confirm the electrode classification and hydrogen designator match the WPS.
- Check the electrode storage oven temperature log and the out-of-oven time for this issue.
- Verify the preheat using a calibrated contact or digital pyrometer at the specified distance from the joint — both sides, both ends.
- Confirm the preheat has soaked into the material (not just warm surface) for thick plate.
- Check the welding parameters (amps, volts, travel speed) for the first pass against the WPS range.
During welding, monitor interpass temperature with a pyrometer between passes. Record values on the weld traveler. After the final pass, verify the material has not dropped below minimum interpass temperature before the inspection hold begins.
Shops that track WPS parameters digitally and flag deviations automatically reduce the inspector's cognitive load and create a timestamped record that stands up to audit review. For an overview of how digital WPS management integrates with inspection documentation, see NDE documentation and the audit packet and welding procedure library for audit-ready shops.
Rule library based on AWS D1.1:2025; verify against your governing edition.
If your WPS library is missing hydrogen designators, preheat calculation records, or electrode conditioning controls, WPS software with built-in HIC control documentation ensures every procedure covers these requirements before a weld is made.