A structural steel connection near a furnace, exhaust stack, or boiler support framing raises a question that does not appear in most WPS programs: which code actually governs? AWS D1.1 is written for structural steel fabrication and erection. ASME codes govern pressure-containing systems. Between those two domains lies a zone of structural members that carry load in high-temperature environments — and the code boundary is not always obvious from the drawing.
Understanding where AWS D1.1:2025 ends and ASME (or another code) begins is not just a compliance exercise. It determines which WPS is valid, which filler metals are required, whether PWHT is mandatory, and which inspector is qualified to sign off on the work.
The Scope Question: Structural vs. Pressure-Containing
AWS D1.1:2025 Clause 1.1 states its scope as structural steel fabrication and erection. The code does not enumerate specific excluded applications in a temperature-based way — it does not say "above 800°F, use ASME." Instead, the scope is defined by the nature of the structural member and the applicable contract specification.
The practical decision rule used by most CWIs and engineers of record:
- Pressure-containing component (pipe, vessel wall, header, flange): governed by ASME B31.1 (Power Piping), ASME B31.3 (Process Piping), or ASME Section VIII (Pressure Vessels), regardless of temperature.
- Structural support member (base frame, access platform, structural brace, support rack for a pressure vessel): potentially governed by AWS D1.1 if designed as structural steel and specified as such.
- Hybrid connection (structural beam welded directly to a pressure vessel nozzle flange): governed by the more stringent code at the joint, per the engineer of record's decision.
The answer is in the contract specification and the engineer of record's designation — not in AWS D1.1 itself. If the specification calls out AWS D1.1 for structural members on a boiler skid, that is the governing document. If it says ASME IX for all welds on the skid, that overrides. When the specification is silent, escalate to the EOR before welding.
Rule library based on AWS D1.1:2025; verify against your governing edition.
When AWS D1.1 Applies to High-Temperature Structural Members
Typical AWS D1.1-governed work in high-temperature environments includes:
Boiler room structural frames and platforms: The structural steel building frame that houses a boiler is AWS D1.1-governed structural work. The boiler itself is ASME. The boundary is the boiler's pressure-boundary welds.
Exhaust stack support structures: A structural steel monopole or guyed frame supporting a stack is AWS D1.1 structural work. The stack shell itself may be AWS D1.1 or ASME B31.1 depending on design pressure and temperature.
Structural attachment welds on pressure vessels: Stiffener rings, lifting lugs, and saddle support welds on the exterior of a pressure vessel are sometimes specified under ASME Section VIII Appendix 2 (nozzle design) and sometimes as structural AWS D1.1 welds on external attachments. The vessel fabricator's engineering document controls.
Furnace structural supports and skids: Process industry modules where structural steel frames operate in ambient temperatures elevated by proximity to furnaces — typically 200–500°F — are routinely governed by AWS D1.1. In this range, base metal properties are not significantly degraded and no special filler metal is required.
Base Metal Selection for Elevated Service Temperature
Standard structural steels (A36, A572, A992, A500) are suitable for continuous service to approximately 650°F (343°C) per AISC allowable stress tables. Above this range, elevated-temperature design values from AISC 360 Appendix or ASTM published data must be used.
For sustained service above 700°F:
ASTM A387 (Cr-Mo alloy): Grades 11, 22, and 91 are designed for elevated-temperature pressure service. A387 is not listed as a prequalified base metal under AWS D1.1:2025. If structural members are specified in A387, a tested WPS with a PQR is required. Most A387 work is also under ASME IX or a project-specific code, because the chromium-moly composition requires controlled preheat, interpass temperature, and PWHT to prevent HAZ cracking and temper embrittlement. See unlisted base metal WPS qualification under AWS D1.1 for the tested-WPS path when your base metal is not in Table 6.9.
ASTM A516 Gr. 70 (carbon steel, pressure vessel plate): Commonly used in structural roles on pressure-containing equipment skids. A516 is listed in AWS D1.1:2025 Table 6.9 and can be covered by a tested WPS using typical structural electrodes. However, PWHT is commonly required by ASME VIII for thicknesses above 1.5 inches (38 mm), which may extend to the structural welds per the contract specification.
For connections between structural A572 and pressure-vessel A516, dissimilar base metal welding under AWS D1.1 covers filler metal selection and qualification coverage.
Filler Metal Considerations
For structural steel operating in the 300–650°F range (elevated but within standard structural design limits), standard low-hydrogen carbon steel fillers remain appropriate:
- E7018, E71T-1C/M, ER70S-6: adequate strength throughout this range
- No special elevated-temperature filler designation is required under AWS D1.1
For members intended for service above 700°F under an ASME code, Cr-Mo electrodes (E8018-B2 for 1.25Cr-0.5Mo, E9018-B3 for 2.25Cr-1Mo) or their FCAW/GMAW equivalents are specified. These must be matched to the base metal composition. Mixing a Cr-Mo base metal with a carbon-steel filler in elevated-temperature service can cause carbon migration at the fusion line — a form of long-term degradation not visible on initial NDE.
PWHT: When Does It Apply?
Under AWS D1.1:2025, PWHT for structural welds is addressed in Clause 5 (prequalified) and the PQR-specific requirements for tested WPS. The trigger for mandatory PWHT is driven by base metal type and thickness, not by service temperature. A36 or A572 structural welds below the thickness thresholds specified in Clause 5 do not require PWHT regardless of service temperature.
If the engineer of record imposes PWHT for residual stress relief in a high-temperature fatigue application (for example, a cyclically loaded support frame at a reciprocating compressor), that requirement appears in the contract documents as a project-specific supplement to AWS D1.1. Your WPS and PQR must include the PWHT cycle. See PWHT on a WPS for how to document the heating/cooling rates and soak time as essential variables.
For A387 Cr-Mo work, PWHT is virtually always required and the parameters (temperature range, hold time, controlled cooling rate) are critical variables that affect both HAZ hardness and long-term creep resistance.
ASME B31.1 at the Scope Boundary
When your structural work is adjacent to or connected to ASME B31.1 Power Piping, ASME B31.1 power piping weld procedure requirements covers the PQR and WPS requirements on the ASME side. The boundary weld — where structural steel attaches to a pipe nozzle or equipment item — requires a decision from the engineer of record on which code governs that specific joint.
Document that decision. A CWI signing off on a structural weld at the code boundary needs written confirmation that AWS D1.1 (and not ASME) governs the joint — especially if an owner's QA engineer or third-party inspector later questions the qualification basis.
Practical Steps for a High-Temperature Structural Project
- Obtain the contract specification before writing any WPS. Identify every base metal specification, every code reference, and any project-specific supplements. If the spec is silent on temperature service requirements, issue an RFI to the EOR.
- Check base metal specification against AWS D1.1:2025 Table 6.9. If the material is not listed, you are automatically on a tested-WPS path, which takes time — schedule accordingly.
- Confirm PWHT requirements from both the code (D1.1 thickness triggers) and the contract specification (project-imposed PWHT for service conditions).
- Brief your NDE personnel on service environment. High-temperature service often means future inspection access is limited by insulation — the initial fabrication NDE record becomes the baseline. Make it complete.
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