A building fire or industrial incident that exposes structural steel to elevated temperatures creates a compressed timeline, structural uncertainty, and a documentation challenge that many fabricators and CWIs have never encountered. The pressure to restore occupancy or production quickly pushes toward starting repairs before the welding procedure questions are fully resolved. That shortcut creates risk — both structural and compliance.
AWS D1.1:2025 does not contain a chapter dedicated to post-fire repair. Its provisions for welding on existing structures and for repair welds provide the framework, but the correct application depends on an assessment of the base metal condition that must precede any WPS selection. This article walks through that sequence: assessment first, WPS selection second, documentation third.
Understanding what fire does to structural steel
Not all fire-exposed steel is equally compromised. Mild carbon steels — A36, A572 Grade 50, A992 — have a forgiving metallurgy. When heated and allowed to cool naturally, these steels typically recover most of their original room-temperature mechanical properties, provided the peak temperature did not exceed the Ac1 transformation temperature (roughly 1,340°F [727°C] for plain carbon steels), where austenite begins to form. Below that threshold, what you mainly see is oxidation scale and color change, not structural property degradation.
Above Ac1, and especially above Ac3 (full austenization, roughly 1,600°F [870°C] for many carbon steels), the steel undergoes phase transformation on cooling. The resulting microstructure depends on the cooling rate. Slow air cooling after significant austenization in A36 or A572 generally produces a fine-grained pearlitic/ferritic structure that may actually improve yield strength slightly — but also increases hardness and may reduce notch toughness. Heat-treatable grades such as ASTM A514 (quenched and tempered high-strength plate, Fy = 100 ksi) and ASTM A913 (quenched and self-tempered) can lose significant strength if they were tempered in service by the fire and cooled slowly.
For the welding procedure engineer, the key question is: has the steel's chemistry and mechanical condition changed enough that the original ASTM specification is no longer met?
Step 1: Structural engineering assessment
AWS D1.1:2025 does not replace the structural engineer's role. Before any WPS is selected, a structural engineer must assess:
- Peak temperature reached by the member (estimated from paint char, sprinkler actuation, fire duration, and thermal modeling or survey)
- Member type and steel grade (from original construction documents or field sampling)
- Post-fire distortion, section loss from oxidation, or visible cracking
- Whether the member's role in the load path requires it to be replaced outright, reinforced, or repaired
AISC Design Guide 3, Fire Damage to Steel Structures (third edition, 2016), provides the engineering framework for this assessment. It includes temperature-estimation methods based on physical evidence and guidance on when steel can be left in place versus replaced. The CWI's job is to understand and document the conclusions — not to make the structural determination.
Step 2: Base metal verification for WPS selection
Once the structural engineer has determined that repair welding is appropriate, WPS selection depends on whether the base metal can be positively identified:
Known and verified base metal: If original mill certifications are available, the steel grade is unambiguous from design documents, and field hardness checks fall within the range expected for that grade, the steel can be treated as a listed prequalified base metal per AWS D1.1:2025 Table 6.1. An existing prequalified or previously qualified WPS for that base metal applies, subject to the considerations below about preheat.
Unknown or suspect base metal: If the original steel grade cannot be verified — common in older buildings with pre-CMTR documentation practices — or if field hardness testing shows anomalous results inconsistent with the expected grade, the steel must be treated as an unlisted base metal. AWS D1.1:2025 provisions for unlisted base metals require full PQR qualification rather than a prequalified WPS. Test samples must be taken from the actual material, typically cut from the member being repaired or from a companion element of the same heat.
Hardness testing is the most practical field tool. A Brinell hardness reading can be correlated to approximate tensile strength, confirming that the steel has not been significantly altered. ASTM A36 plate typically reads 120–160 HBW; significant deviation warrants further investigation. For hardness testing requirements in the PQR context, see the PQR qualification guidance.
Step 3: Preheat selection for fire-affected base metal
Even when the base metal is verified to meet its original specification, post-fire repair welding warrants conservative preheat selection. Three factors drive this recommendation:
Surface contamination and hydrogen uptake. Fire suppression systems (water, foam) can introduce moisture into the base metal surface and scale. Scale and oxidized surfaces hold moisture that converts to hydrogen during welding. Until the surface is thoroughly cleaned and dried, hydrogen-assisted cold cracking risk is elevated. Preheat drives off moisture before and during welding.
Altered microstructure from heat cycling. Even below the Ac1 transformation temperature, thermal cycling can alter grain boundary carbide distributions in carbon steels. Combined with constraint (fire damage often warps and locks up members in high-restraint configurations), this creates a more crack-susceptible joint than a standard new-steel repair.
Uncertain residual stress. Fire causes differential thermal expansion and contraction. The resulting residual stress field in the repaired member may be high and of uncertain orientation — adding restraint stress on top of weld residual stress increases crack initiation risk.
The conservative approach: use the preheat requirement for the next heavier carbon equivalent group above what the base metal chemistry alone would indicate, as permitted by AWS D1.1:2025. Document the basis in the repair procedure.
Rule library based on AWS D1.1:2025; verify against your governing edition (the AHJ or contract may specify 2020 or earlier).
Step 4: Repair WPS content and documentation
A post-fire repair WPS must address all standard WPS elements under AWS D1.1:2025 Annex M format (or equivalent), plus the following repair-specific documentation:
Base metal characterization. The WPS or its supporting documentation should reference the structural engineer's assessment report and the field hardness test results. If CMTRs were obtained and verified, note their heat numbers and the specific members to which they apply.
Extent of weld removal. If the repair involves removing cracked or degraded weld metal from an existing joint, the removal method (air carbon arc gouging, grinding) and the inspection required after removal (MT or PT to confirm clean substrate) must be specified. See repair weld procedure qualification under AWS D1.1 for the general framework.
Weld sequence for high-restraint joints. Fire-distorted members often cannot be moved to flat position. The WPS must address the welding position, any use of strongbacks or fixtures, and the sequence to minimize additional distortion.
NDE plan. Post-repair inspection should include visual examination per AWS D1.1:2025 acceptance criteria, followed by MT or PT of the weld and heat-affected zone. For structural members in tension zones or fatigue-category welds, UT is appropriate. NDE method selection for structural welds provides guidance on when each method applies.
Documentation retention. The repair record should become a permanent part of the building file: structural engineer assessment, base metal verification data, WPS with heat input records, NDE reports, and welder identification. Insurance carriers and future AHJs will need this documentation if the building changes hands or is reviewed after a subsequent event.
Weld repair in occupied or partially occupied structures
AWS D1.1:2025 contains provisions relevant to welding on existing structures, addressing continuity of load path, access limitations, and the prohibition on welding when conditions exceed the standard's environmental limits. When fire-damaged areas are adjacent to occupied portions of a building, additional coordination with the structural engineer and authority having jurisdiction is required before welding begins — fume, heat, and ignition risk during repair welding must be assessed.
This is not a niche situation. Post-earthquake assessment in seismic regions routinely involves welding on occupied structures; post-fire repair in industrial facilities with continued adjacent operations is common. The welding engineer and CWI must document that the repair was performed under controlled conditions meeting the AHJ's requirements, not just that the weld technically meets D1.1 criteria.
Common documentation gaps in post-fire repair projects
The three gaps most likely to surface in third-party review of a fire-damage repair weld package:
No base metal verification record. The shop used its standard A572 Gr. 50 WPS on the repair without documenting that the fire-exposed steel still met that specification. If the repair is ever challenged, there is no traceability back to the base metal condition.
Preheat not adjusted for field conditions. The repair was performed in an open structure days after the fire, with residual moisture in the member from suppression efforts, but the standard preheat from the shop's WPS — developed for clean new steel — was applied without adjustment.
Repair record not preserved in building file. The NDE reports went to the GC's project closeout binder but were not formally integrated into the building's structural documentation. When the building is sold five years later, the new owner's structural engineer finds no record of the repair.
For a systematic approach to ensuring your weld documentation survives ownership transitions and audits, the audit-ready WPS and weld procedure library is built around exactly these documentation requirements.