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When Is PWHT Required Under D1.1:2025

PWHT is not automatically required by D1.1. It must be specified in the contract documents. When specified, it may be prequalified under Clause 5.9 or it may be a Clause 7.8 stress-relief requirement applied outside the prequalified WPS framework.

The Engineer specifies PWHT when residual stresses from welding must be reduced — typically for applications where dimensional stability, stress corrosion cracking susceptibility, or material service requirements demand it. D1.1 does not define the service conditions that trigger PWHT; that judgment belongs to the Engineer of Record. D1.1 defines the procedure when PWHT is required.

Clause 5.9 — PWHT Prequalification

A WPS using PWHT is prequalified under Clause 5.9 only when all five conditions are met:

(1) Yield strength — The minimum specified yield strength of the base metal does not exceed 50 ksi [345 MPa].

(2) Steel type — The steel is not quenched and tempered (Q&T), quenched and self-tempered (QST), thermomechanically controlled process (TMCP), or cold-worked to produce enhanced mechanical properties. These metallurgical conditions are sensitive to elevated temperature and PWHT may degrade them.

(3) CVN requirements — The WPS is not required to meet Charpy V-notch (CVN) notch toughness requirements. PWHT may alter the toughness of the weld metal in ways that require specific qualification testing.

(4) Weld metal data — Weld metal mechanical property data is available for the post-PWHT condition from the filler metal manufacturer or from prior qualification testing.

(5) Procedure compliance — PWHT is conducted in accordance with Clause 7.8.

If any of these five conditions is not met, the WPS must be qualified by testing per Clause 4.

Clause 7.8.1 — Steels Not Recommended for PWHT

Q&T, QST, precipitation-hardened (PH), and TMCP steels are not recommended for PWHT under Clause 7.8.1. The elevated temperatures of PWHT can alter the metallurgical condition these steels were manufactured to achieve, reducing yield strength, tensile strength, or impact toughness below minimum specified values.

If PWHT is required on any of these steel types, the procedure must be developed in accordance with the steel manufacturer's recommendations and is subject to approval by the Engineer. The prequalified Clause 5.9 route is not available for these steels — a qualification test is required.

Clause 7.8.2 — PWHT Procedure Requirements

The PWHT procedure must be written and shall include the following elements per Clause 7.8.2:

Alternative PWHT (Clause 7.8.3.3)

Lower temperatures with longer holding times are permitted per Table 7.3. This provides flexibility when the standard 1100°F [600°C] holding temperature is not achievable or would be detrimental to the material. The alternative temperature-time combinations in Table 7.3 must be followed exactly — the holding time increases as the temperature decreases below the standard range. This route may be useful for field heat treatment where furnace temperature precision is limited.

Multi-Standard PWHT Comparison

PWHT requirements vary significantly across welding codes. The temperature ranges, holding times, and mandatory vs. optional nature differ based on the code’s scope and the materials covered.

ASME VIII PWHT (UCS-56)

Unlike D1.1, ASME Section VIII Division 1 makes PWHT mandatory for many material/thickness combinations — the engineer does not need to specify it separately. UCS-56 provides a table that determines PWHT requirements based on the P-Number, nominal thickness, and minimum preheat temperature. For P-Number 1 carbon steels, PWHT is generally required for thicknesses exceeding 1.5 inches (38 mm) unless exempt per UCS-56(b). For P-Number 3 and higher alloy steels, PWHT thresholds are lower — often required for any thickness.

ASME VIII also allows exemptions from PWHT when specific conditions are met, including: maximum carbon content limits, preheat maintenance above minimum temperatures, and use of specific filler metals that produce lower-hydrogen weld deposits. These exemptions are carefully defined in UCS-56 and must be documented in the fabrication records.

PWHT for Cr-Mo and P91 Steels

Chromium-molybdenum steels (P-Number 4, 5A, 5B) and 9Cr-1Mo-V steels (P-Number 15E) require PWHT under virtually all construction codes. The PWHT parameters are material-specific and must be precisely controlled:

• P-Number 4 (1-2% Cr): Code minimum 1,200°F (650°C) per UCS-56-3, 1 hr/inch. Mandatory for all thicknesses (with limited exemptions for thin-wall pipe ≤5/8 in).
• P-Number 5A/5B/5C (2.25-9% Cr): Code minimum 1,250°F (675°C) per UCS-56-4, 1 hr/inch. Mandatory for all thicknesses (with limited exemptions for thin-wall pipe ≤5/8 in with ≤3% Cr).
• P-Number 15E (P91): Code minimum 1,300°F (705°C) per ASME VIII UCS-56-11. Industry best practice narrows to 1,350–1,425°F for optimal toughness. Maximum depends on filler Ni+Mn content (1,455°F max for Ni+Mn ≤1.0%). Hold 2 hr minimum. The 1,300–1,350°F range is code-legal but produces inferior toughness.

Key Takeaways

• D1.1 PWHT is optional — only required when specified in contract documents. ASME VIII makes it mandatory based on material and thickness.
• D1.1 specifies both heating and cooling rate limits (400/500°F per hour divided by thickness) — ASME VIII does not mandate specific rates.
• P91 PWHT is critical — code minimum is 1,300°F (UCS-56-11), best practice is 1,350–1,425°F. Errors in either direction cause in-service failures that may not be detectable by NDE.
• Alternative lower-temperature PWHT is allowed by both D1.1 (Table 7.3) and ASME VIII (UCS-56) with longer holding times.

"PWHT is the most commonly over-specified and under-understood requirement in structural fabrication. Under D1.1, if the contract documents don’t call for it, it’s not required — regardless of material thickness."

D1.1:2025 Clause 5.9 defines PWHT as a prequalified condition only when specified by the Engineer in the contract documents

Reference data from AWS D1.1/D1.1M:2025 and ASME BPVC VIII-1. Not affiliated with AWS or ASME.