If a WPS was qualified without PWHT, can it be used on a job that requires PWHT?

No. QW-407.1 makes PWHT an essential variable — adding postweld heat treatment after the procedure was qualified without it requires a new PQR. The test coupon must be given the same PWHT condition (or lack thereof) as the intended production weld to be within the qualified range.

What specific QW-407 variables can trigger requalification?

QW-407.1 covers addition or deletion of PWHT: any change from a qualified PWHT condition to no PWHT, or vice versa, requires requalification. QW-407.2 makes a change in the PWHT temperature range (beyond stated limits) an essential variable. QW-407.4 addresses an increase in time at temperature beyond what was tested. Each subparagraph must be tracked separately on the PQR.

Can one PQR support both PWHT and non-PWHT production WPS procedures?

No. Because PWHT is essential under QW-407.1, a PQR qualified with PWHT can only support WPS procedures that will receive PWHT in production; a separate PQR without PWHT is required to support procedures where PWHT is not applied. The only way to cover both production conditions is to run two PQRs — one with and one without heat treatment.

Does ASME IX require PWHT, or does the construction code impose it?

ASME IX sets the qualification rules — it governs whether a WPS is qualified for the PWHT condition used in production. The construction code (ASME B31.3 for process piping, ASME Section VIII for pressure vessels, or B31.1 for power piping) determines whether PWHT is mandatory for a given P-number group and base metal thickness in production. The fabricator must satisfy both: the construction code tells you when PWHT is required; ASME IX tells you whether your WPS is qualified for the PWHT condition.

ASME IX QW-407: PWHT as an Essential Variable

Adding or removing postweld heat treatment after PQR qualification is an essential variable under ASME IX QW-407. Here's what triggers requalification and why.

Postweld heat treatment (PWHT) changes the mechanical properties of a weld joint in ways that affect what the procedure qualification demonstrates. Tempering at elevated temperature can relieve residual stresses, soften the heat-affected zone in hardenable steels, and alter tensile and impact properties — which is exactly why ASME IX QW-407 makes PWHT an essential variable. A PQR run with PWHT and a PQR run without PWHT do not qualify the same thing.

For a QC manager or CWI who manages pressure vessel or piping WPS libraries, understanding QW-407 means knowing which PWHT condition each PQR was tested in, tracking when construction code requirements change the PWHT obligation for a given job, and maintaining enough PQRs to cover both PWHT and non-PWHT production conditions when both arise.

Where QW-407 sits in the ASME IX essential variable structure

ASME IX Section IX divides welding variables into three categories: essential variables (qualification is required for any change), supplementary essential variables (essential only when impact testing is required by the construction code), and nonessential variables (may be changed by a WPS revision without new PQR). This structure is established in QW-250 (for arc welding processes) through the process-specific variable tables.

QW-407 is the variable grouping that covers heat treatment conditions. Its subparagraphs apply across most arc welding processes (SMAW, SAW, GMAW/FCAW, GTAW, and others), so the PWHT essential variable rules discussed here are not process-specific — they apply to the full WPS/PQR set regardless of which process deposited the weld.

See ASME IX essential, supplementary, and nonessential variable types for the full framework before working through process-specific tables.

QW-407.1: Addition or deletion of PWHT

QW-407.1 is the core rule. It makes addition or deletion of PWHT an essential variable. The implications in practice:

A WPS qualified with PWHT may only be used on production welds that will receive PWHT. Using the same WPS on a joint that is not given PWHT is an essential variable change and requires a new PQR (or the existing PQR must have also been tested without PWHT, which is uncommon — one PQR reflects one PWHT condition).
A WPS qualified without PWHT may only be used where PWHT is not performed. If the construction code later requires PWHT for the material or thickness on a new job, that procedure is no longer qualified for the production condition.

The most common practical trap: a shop qualifies all its procedures without PWHT during initial WPS development (because most carbon steel structural and low-pressure applications don't need it), then takes on a job using P-No. 4 (Cr-Mo alloy steel) where ASME B31.3 Table 331.1.1 requires mandatory PWHT above a certain base metal thickness. The existing WPS is not qualified for the PWHT condition, and a new PQR is required.

QW-407.2: PWHT temperature range

QW-407.2 addresses the PWHT temperature range applied to the test coupon. Changing the PWHT temperature outside the qualified range is an essential variable change. The PQR must record the actual temperature and hold time used during heat treatment of the test coupon; the qualified production WPS may then specify a temperature range within the limits established by the PQR.

For most carbon and low-alloy steels, PWHT temperatures fall in the subcritical range (below the lower critical transformation temperature, Ac1) — typically in the range of 1100°F to 1250°F [595°C to 675°C] for carbon steel, lower for some alloys. The PQR records the actual soak temperature; the construction code often sets minimum PWHT temperatures; the fabricator must ensure the production temperature falls within both the code minimum and the range demonstrated by the PQR.

One nuance: PWHT above the upper transformation temperature (Ac3), though rare in fabrication contexts, would change the weld metal and HAZ microstructure in ways that are very different from subcritical tempering. If for any reason a different heat treatment cycle is applied, it almost certainly falls outside the scope of what the original PQR demonstrated.

QW-407.4: Increase in time at temperature

QW-407.4 makes an increase in time at temperature beyond the range tested an essential variable. Time at temperature matters because prolonged exposure causes further tempering, potential grain growth in the HAZ, and changes to carbide morphology in some alloys.

The PQR should record the total time the test coupon was held at PWHT temperature. The qualified WPS may allow a range of hold times, but extending well beyond what the test coupon experienced goes outside the qualified condition.

Practically, this variable most often matters on repair situations (where a joint that has already been PWHT'd is rewelded and must be PWHT'd again, accumulating total heat treatment time) and on vessels with multiple PWHT cycles due to sequential fabrication stages. ASME VIII Division 1 Appendix 6 requires that the total PWHT time applied to test plates represent the total expected PWHT exposure of the production weld — meaning if production involves two PWHT cycles, the PQR test coupon should be given two equivalent cycles before testing.

Construction code PWHT vs. ASME IX qualification PWHT

A consistent point of confusion: ASME IX does not mandate that PWHT be performed. ASME IX only governs whether a WPS is qualified for the PWHT condition used in production. The obligation to perform PWHT comes from the construction code or owner specification.

For pressure piping, ASME B31.3 Table 331.1.1 sets minimum preheat and PWHT requirements by P-number and nominal thickness. For pressure vessels, ASME BPVC Section VIII Division 1 UCS-56 establishes PWHT requirements for carbon and low-alloy steels by P-number group and thickness. For power piping, ASME B31.1 Appendix A does the same.

The fabricator's WPS library management task is:

Identify which construction code governs the job.
Determine whether the code requires PWHT for the P-number and thickness in production.
Confirm that the supporting PQR was tested under the same PWHT condition (PWHT if PWHT is required, no PWHT if not required).
If no qualified PQR exists for the required PWHT condition, run a new PQR before production begins.

This check belongs in the pre-job WPS review process, not after the welds are made. See ASME IX QW-482 and QW-483 WPS and PQR form requirements for what each form must record to support this traceability.

Common P-number groups where PWHT is frequently required

The following P-number groupings most often trigger construction-code-mandatory PWHT in ASME pressure fabrication:

P-No. 1 (carbon steel, Groups 1–4): PWHT is not required by ASME VIII Division 1 UCS-56 for P-No. 1 below 1.5 in [38 mm] nominal thickness in most cases. Above that threshold and for certain impact-tested applications, PWHT is required — which means if you've only qualified P-No. 1 procedures without PWHT, you may lack coverage for thick-section pressure vessels.

P-No. 3 (Cr-Mo low alloy, such as ASTM A387 Gr. 11 and 22): PWHT is nearly always required by the construction code. A shop moving from carbon steel work to Cr-Mo vessel fabrication must ensure its PQRs for P-No. 3 base metals were heat treated. The same applies to P-No. 4 (1-1/4 Cr, 1/2 Mo and similar).

P-No. 5A and 5B (5Cr-Mo and higher chromium alloys): Mandatory PWHT under most construction codes; qualification without PWHT is essentially not useful for production.

P-No. 11A (precipitation-hardening stainless): The aging treatment may constitute PWHT in the ASME IX sense; coordinate with the applicable construction code.

Documenting PWHT on the PQR and WPS

The PQR (ASME QW-483 form) must record:

Whether PWHT was applied (yes/no)
Temperature range and hold time actually used
Heating and cooling rates if specifically controlled
The heat treatment cycle description (furnace, local, etc.)

The WPS (QW-482 form) must document the PWHT condition for production:

Required temperature range (within the PQR-qualified range)
Minimum hold time
Heating and cooling rate requirements if applicable

If the WPS allows either PWHT or no PWHT as alternatives in production, that WPS is not properly qualified — each condition needs a separate supporting PQR. The WPS should state clearly whether PWHT is required, prohibited, or optional, and if optional, it should cite separate supporting PQRs for each condition.

For shops managing multiple PQRs across carbon steel and alloy applications, maintaining a matrix that links each PQR to its PWHT condition and the P-number groups and thickness ranges it covers is the fastest way to confirm qualification status on a new job. A WPS software system that stores PQR data and filters by PWHT condition eliminates the spreadsheet cross-referencing that slows this review down.

See also PWHT documentation on WPS forms for how the WPS itself must reflect the heat treatment requirement, and ASME IX impact testing requirements under QW-170 for how PWHT interacts with CVN qualification when supplementary essential variables are in play.