Local vs Furnace PWHT: AWS D1.1 WPS Requirements

When Structural Welds Need Post-Weld Heat Treatment

For most structural steel weldments under AWS D1.1, PWHT is optional — the code does not mandate it for standard A36, A572, or A992 steel connections. PWHT enters the picture when:

• The project specification or engineer of record requires it (common for heavy equipment supports, lifting beams, or critical load-transfer components)
• Welding quenched-and-tempered steels (A514, A517, A709 HPS 70W) where the steel producer recommends controlled thermal treatment to restore base metal properties in the heat-affected zone
• Owner specifications for critical structures (nuclear facility anchorages, offshore platform supports, bridges with CVN toughness requirements)
• Repair welds on high-restraint joints in thick plate where hydrogen cracking risk drives a bake-out requirement per the WPS

When the procedure does require PWHT, how you apply it — furnace or local — affects both the WPS documentation and the CWI's inspection responsibilities during the heat treatment operation.

Furnace PWHT: The Baseline Method

Furnace PWHT places the complete weldment or a major assembly section in a controlled-atmosphere furnace for a defined temperature-soak cycle. This is the simplest method to document and control:

• Temperature uniformity across the load is maintained by the furnace controls, verified by multiple thermocouples attached to the parts — not just the furnace air temperature.
• Heating and cooling rates apply to the entire assembly simultaneously, eliminating steep thermal gradients between weld and base metal.
• Chart recorder output (temperature vs. time for each TC channel) is the primary QC document. The chart must show that every monitored point held within the required temperature band for the full soak time.

On the WPS, furnace PWHT documentation requires: temperature range (minimum and maximum), soak time at temperature per inch of governing thickness, heating and cooling rate limits, and a note that the method is furnace-applied. The fabricator typically provides furnace calibration records and TC calibration documentation in the audit packet.

The limitation of furnace PWHT is obvious: it requires the weldment to fit in a furnace, which excludes large structural assemblies, field connections, and repairs to in-service structures. That is where local PWHT becomes necessary.

Local PWHT: Methods and WPS Documentation

Local PWHT applies heat to a defined band around the weld rather than the entire component. Two principal methods are used in structural fabrication:

Electric resistance heating blankets are the standard for piping and pressure vessel work that crosses into structural applications. Resistance heating elements are wrapped around the joint, insulated with refractory blanket, and controlled by proportional temperature controllers wired to attached thermocouples. This method provides precise temperature control and an electronic record comparable to furnace chart output.

Flame (torch) heating is used for field repairs and smaller structural components where blanket setup is impractical. It requires a more disciplined operator and careful thermocouple placement. Flame heating is harder to control uniformly and demands more frequent CWI monitoring during the thermal cycle.

For the WPS, local PWHT documentation must specify:

1. Heat band width — typically 3× the weld thickness on each side of the weld centerline, or per the project specification (use the more conservative)
2. Soak band — the zone required to reach and hold temperature, which may be narrower than the heated zone
3. Temperature range — document as a minimum/maximum band, not a single target point
4. Soak time — minutes per inch of governing thickness, with a stated minimum for thin sections
5. Heating and cooling rate limits — above 600°F (315°C) for low-alloy structural steels; below 600°F the part can typically be cooled freely
6. Thermocouple attachment method — welded, mechanically clamped, or spring-loaded contact; peened-on or taped TCs are not reliable at PWHT temperature ranges
7. Insulation requirements — minimum insulated zone length to prevent steep thermal gradients at the boundary of the heated zone

Rule library based on AWS D1.1:2025; verify against your governing edition.

Essential Variable Implications Under Table 6.6

AWS D1.1:2025 Table 6.6 lists PWHT as an essential variable for qualified procedures: adding PWHT to a procedure qualified without it — or removing it from a procedure qualified with it — requires a new PQR test weld and mechanical testing. See PWHT on a WPS for a complete walkthrough of how PWHT affects WPS scope and PQR requirements.

This matters practically when a fabricator qualifies a WPS without PWHT, then receives a job requiring PWHT for the same joint configuration. The options are:

1. Qualify a new WPS: PQR coupon welded and PWHT-treated before mechanical testing — the cleanest, most defensible path.
2. Obtain a Standard Welding Procedure Specification (SWPS): AWS publishes SWPSs for some process/base metal/PWHT combinations. Verify the SWPS table covers your base metal group and PWHT parameters before use.
3. Confirm with the EOR that no requalification is needed: rare in practice; most EORs require the PQR to reflect the production thermal cycle.

For the PQR, the coupon heat treatment must match the production PWHT parameters. If production PWHT is 1100–1150°F for 1 hr/in, the PQR coupon must be heat-treated at those same parameters before guided bend and tensile tests are performed. See PWHT heating and cooling rates in WPS documentation for detailed parameter recording requirements on the PQR test report.

Thermocouple Placement and CWI Verification

The most common audit finding on local PWHT is inadequate thermocouple coverage. A single TC at the top of a plate butt weld does not confirm temperature at the weld root, at the weld toes, or at the heat-affected zone boundary on the far side of the insulated zone.

CWI verification during local PWHT should confirm:

• TC locations match the PWHT procedure, not improvised on-site day-of
• Attached TCs are welded or clamped per the WPS specification
• Chart recorder is running with a readable date/time scale and each TC channel is labeled
• Insulation covers the full specified band; mineral wool or ceramic blanket must be fully closed at edges with no air gaps at the joint
• After cool-down, TC attachment weld spots on the base metal are ground smooth and visually inspected per the WPS

For quenched-and-tempered steels like A514, local PWHT temperature control is especially critical. Exceeding the maximum temperature limit (which is below the original austenitizing temperature) softens the Q+T base metal irreversibly without re-quenching and tempering. See high-strength steel WPS for A514 for the heat input and PWHT limits that apply to Q+T structural steels.

Furnace vs Local PWHT: Documentation Comparison

Both methods require the same fundamental WPS documentation fields. The key difference is that local PWHT adds the heat band width, soak band definition, and TC placement requirements as explicit WPS variables, because these are not inherently controlled by furnace geometry.

Documenting PWHT in the Closeout Package

For projects requiring PWHT documentation in the owner turnover package, the standard deliverable set includes:

• WPS face sheet showing the PWHT parameters
• PWHT procedure (separate from the WPS if required by the project specification)
• TC calibration certificates current at the time of heat treatment
• Time-temperature chart recorder output, annotated with joint ID and applicable weld numbers
• Furnace calibration (furnace PWHT) or resistance heating equipment calibration records
• Post-PWHT hardness test results if specified by the project or the steel producer

Keeping these records organized per joint ID rather than per heat date makes the audit significantly faster. When managing a library of WPS procedures and PWHT records across a complex structural project, purpose-built WPS management software handles the linkage automatically. See wpswelding.com/pricing for plan details.