What does AWS D1.1:2025 require if welding stops and the joint cools below minimum preheat temperature?

AWS D1.1:2025 does not explicitly prohibit restarting after a cooling interruption, but it requires that minimum preheat be reestablished before welding resumes (Section 6.2). For high-restraint or high-strength steel joints, the WPS should specify whether intermediate NDE is required before restart, since hydrogen cracking can initiate in the deposited passes during the cool-down period.

How long can welding be interrupted before hydrogen cracking risk becomes significant?

There is no universal time limit in AWS D1.1:2025. Risk depends on diffusible hydrogen level (filler metal H-designation), preheat achieved, base metal carbon equivalent, and joint restraint. High-strength steels (Fy ≥ 70 ksi) with high CE are most susceptible. In practice, any interruption that allows the weld zone to drop below minimum preheat on a restrained joint should be treated as a potential risk event and documented by the CWI.

Can the welder simply reheat and continue after a preheat interruption?

Only if the WPS permits it and the CWI has verified the weld face is free of cracks before restart. For partial groove welds in progress, visual and MT or PT inspection of the deposited weld passes is good practice before applying reheat and continuing. For low-carbon structural steels (A36, A572 Gr. 50) in low-restraint joints, reheat-and-continue is generally acceptable with CWI documentation.

Does a preheat interruption require a new PQR?

No. A PQR documents the welding process variables; preheat is a production quality control step. However, if a shop changes its WPS to add a mandatory minimum preheat soak time or an interim NDE hold point after interruptions, that revision should be documented on the WPS and tracked as a non-essential variable change (unless the process or base metal also changes).

Preheat Interruption Mid-Weld: AWS D1.1:2025 Requirements

What AWS D1.1:2025 requires when welding is interrupted and the joint cools below minimum preheat—hydrogen cracking risk, CWI inspection steps, and restart procedures.

Interruptions happen on every welding project. A welder finishes their shift. The preheat torch runs out of fuel. A fire watch call pulls the helper away. By the time welding resumes, the joint may have cooled to ambient temperature—well below the minimum preheat specified on the WPS. What AWS D1.1:2025 requires in this situation, and what a competent CWI does before the next arc is struck, is not always spelled out in the code with the clarity shops need.

This article addresses the technical requirements, the hydrogen cracking risk window, and the practical inspection protocol that protects both the structure and the fabricator.

Why Preheat Exists and What Cooling Destroys

Preheat serves three purposes in structural welding under AWS D1.1:2025:

Slows the cooling rate of the heat-affected zone (HAZ), reducing hardness and susceptibility to hydrogen-assisted cracking (HAC).
Drives off surface moisture that would otherwise contribute to diffusible hydrogen in the weld metal.
Reduces thermal gradient stresses in the joint, especially in thick plate and high-restraint configurations.

When welding is interrupted and the joint cools, all three benefits are lost before the interrupted weld is completed. The deposited weld metal and HAZ from previous passes sit at or near ambient temperature, potentially trapping residual hydrogen at levels that can drive cold cracking over hours or days. The next weld pass laid on a cold joint sees a steep thermal gradient and a hard, hydrogen-charged substrate—exactly the conditions that create underbead cracks.

For more on why hydrogen cracking mechanics matter to your WPS, see Hydrogen-Induced Cracking Prevention in Structural Welding.

AWS D1.1:2025 Requirements for Interrupted Welds

AWS D1.1:2025 Section 6.2 establishes minimum preheat and interpass temperature requirements. The code specifies that welding shall not be performed when the base metal temperature is below the minimum preheat. It does not explicitly address what to do after an unplanned interruption results in cooling.

The practical implication: minimum preheat must be reestablished before welding resumes, regardless of how many passes have already been deposited. The restart is treated as if a fresh arc is being struck on a cold joint.

Rule library based on AWS D1.1:2025; verify against your governing edition (the AHJ or contract may specify 2020 or earlier).

Additionally, the code's general quality provisions require that weld joints be inspected and found acceptable before being covered by subsequent passes. If an interruption raises a question about whether the previously deposited material is sound, the CWI has authority under AWS D1.1:2025 Clause 4.3 to require inspection before authorizing continuation.

The Hydrogen Cracking Risk Window

Cold cracking in structural weld joints is time-dependent. Unlike hot cracks that form during solidification, hydrogen-assisted cracking typically develops 4–72 hours after welding—or longer in high-restraint joints with high-CE base metals. This means a joint that looks fine immediately after a cooling interruption may not show cracks for a day or more.

Risk factors that raise the stakes on an interrupted weld:

Base metal carbon equivalent (CE) above 0.40: Higher CE means a harder HAZ and greater susceptibility. A572 Gr. 50 is generally CE 0.38–0.45 depending on heat chemistry; A514 can exceed 0.70.
High restraint: Column-to-beam CJP groove welds, highly constrained box column connections, and thick-plate T-joints all prevent the weld zone from freely contracting during cooling, increasing residual stress.
Filler metal H-designation: E7018-H4 (max 4 mL/100g diffusible hydrogen) provides substantially more protection than E7018-H16 or non-hydrogen-controlled electrodes. For joints at risk, the WPS should specify an H4 or H8 electrode. See Diffusible Hydrogen Filler Metal Requirements Under AWS D1.1.
Incomplete preheat soak: The joint mass matters. A thick column splice requires the full section to reach minimum temperature, not just the weld zone surface. An interrupted weld where preheat was marginal before stopping is at higher risk.

CWI Inspection Protocol After a Cooling Interruption

A conservative but defensible CWI protocol for interrupted welds on high-risk joints:

Step 1: Document the interruption

Record in the inspection log: the time welding stopped, the measured temperature at the weld zone when it stopped, the number of passes deposited, and the reason for interruption. This documentation protects the fabricator and creates an audit trail.

Step 2: Allow adequate delay before inspection

For high-strength steels (Fy ≥ 70 ksi) or high-restraint joints, AWS D1.1:2025 Table 8.1 suggests a minimum waiting period before post-weld NDE when hydrogen cracking is a concern. For completed welds, 24–48 hours is standard practice on A514 and similar steels. Apply the same logic to inspecting partially completed welds after a significant cool-down.

Step 3: Visual and surface NDE of deposited passes

Before authorizing reheat, visually inspect the deposited weld metal and the adjacent HAZ. For high-risk conditions, MT (magnetic particle testing) per AWS D1.1:2025 Clause 8.9 or PT (liquid penetrant testing) per Clause 8.10 may be warranted. Look for:

Transverse cracks across the weld bead (most common HAC pattern in restrained joints)
Toe cracks at the junction of the weld and base metal
Underbead cracks (visible in access-limited areas only with PT or RT)

For more on MT and PT protocols and their acceptance criteria, see Magnetic Particle Testing Acceptance Criteria Under AWS D1.1.

Step 4: Establish preheat before restart

Per Section 6.2, remeasure and verify minimum preheat before welding resumes. For thick plate or high-CE material, consider raising the preheat 25–50°F above the minimum as a precaution on restart, since the partially completed weld creates additional restraint compared to an unwelded joint. Document measured temperature, thermocouple or contact thermometer calibration status, and measurement location.

Step 5: Inspect after restart

The first pass after an interruption is the highest-risk pass in the weld sequence. After completing a pass on restart, visually inspect for cracks before depositing the next pass. Per AWS D1.1:2025 Clause 4.17, visible cracks of any size are not acceptable.

Low-Risk vs. High-Risk Interruptions

Not every interruption is a crisis. The table below gives a practical risk triage for CWI decision-making:

Situation	Risk Level	Recommended Action
A36/A572 Gr. 50, low restraint fillet weld, E7018-H4 used	Low	Reheat to minimum preheat, resume, document
A572 Gr. 65 column splice, interrupted after root pass, H8 electrode	Moderate	Visual inspection + MT before reheat, raise preheat 25°F above minimum
A514 CJP joint, multiple interruptions, H16 electrode	High	24-hr hold, MT inspection, consider adding a hydrogen bake-out step before restart
Any joint in freezing weather, moisture condensed on weld surface	High	Remove moisture completely, verify dry base metal, reestablish preheat, document

WPS and Procedure Documentation for Interruption Scenarios

Many WPS documents are silent on interruption procedures, leaving welders and inspectors to make ad hoc decisions. Better practice is to include the following in the WPS notes or in the shop's QC plan:

Maximum allowable interruption time before inspection is required (e.g., "If welding is interrupted and the joint cools below minimum preheat, visual + MT inspection is required on all deposited passes before restart for joints in Category HIGH-RESTRAINT per the weld map")
Preheat soak time required before restart (minimum 15–30 minutes for heavy section joints to achieve through-thickness temperature equilibrium)
Identification of high-risk joint types where interruptions trigger automatic NDE holds

Including these provisions in the WPS doesn't add PQR-level requalification requirements; preheat soak time is a non-essential variable in the AWS D1.1:2025 framework. But it does give the CWI a standing order to enforce and gives the welder clear guidance.

For cold-weather welding, which compounds interruption risk significantly, see Cold-Weather Welding Requirements Under AWS D1.1.

Summary: What to Do When Welding Stops and the Metal Cools

Document the event (time, temperature, passes deposited, reason).
Assess the risk based on base metal CE, joint restraint, filler metal H-designation, and current temperature.
Inspect deposited passes before reheat on any moderate- or high-risk joint; for high-risk joints, include MT or PT and allow adequate delay.
Reestablish minimum preheat per the WPS before striking the next arc.
Inspect the restart pass before continuing.

A preheat interruption is manageable when it is documented and addressed systematically. A silent interruption that is never recorded and never inspected is a structural liability that may not become visible until the building is occupied or the crane is under full load.

If your shop needs a WPS that explicitly addresses interruption scenarios for high-strength or high-restraint structural applications, WPS Welding's generator can build a documented, AWS D1.1:2025-compliant procedure with notes for your specific joint and material combination.