Important update: AWS D1.1 2025 edition changes the rules and corrects errors in post-weld heat treatment

The new edition of AWS D1.1/D1.1M:2025 brought relevant and very welcome changes to stress-relief heat treatment (PWHT). As well as organising the requirements better and demanding greater technical rigour in procedures, the 2025 edition finally corrects a calculation error that had been carried over since the 2015 version, directly affecting application of the code in metric units.

In this article I share the main changes and their practical impact for engineers, inspectors and fabricators.

The first change is conceptual and structural. The 2020 edition refers to the process as “Stress-Relief Heat Treatment”. The 2025 edition adopts the more specific term “Postweld Heat Treatment for Stress Relief (PWHT)”, aligning with modern industry terminology.

Beyond terminology, clause 7.8, dedicated to PWHT, has been restructured into three main topics:

• 7.8.1 - Steels not recommended for PWHT
• 7.8.2 - Procedure requirements
• 7.8.3 - Holding temperatures and times

This new organisation makes the requirements easier to understand and apply, and aligns the format with other widely used codes such as ASME Section IX.

Item 7.8.1 - Generalising the steels not recommended for PWHT

The treatment of which steels are not recommended for PWHT has also been modernised.

• 2020 edition: the code listed specific steels by their ASTM designations (such as ASTM A514, A517, A709 and others) for which PWHT was not recommended, and warned about possible embrittlement and cracking in those steels.
• 2025 edition: the new edition generalises the steel types by their manufacturing route, citing the categories quenched and tempered (Q&T), thermomechanically controlled processed (TMCP), quenched and self-tempered (QST) and precipitation hardened (PH). This makes the code broader and more adaptable to new materials. The code also specifies that when PWHT is required for these materials, the treatment shall follow the parameters recommended by the material manufacturer and approved by the responsible engineer.

Item 7.8.2 - Among the most important new requirements is the demand for a written PWHT procedure, containing minimum data such as:

• Initial furnace temperature
• Heating rate
• Treatment (holding) temperature
• Treatment (holding) time
• Cooling rate
• Final recording temperature
• Location and number of thermocouples, or another temperature control method
• Heating method (for example furnace, induction, electrical resistance blanket, gas torch)

Formalising this improves quality control and process traceability, which is essential in critical applications.

Item 7.8.2.3 - Raises the maximum furnace temperature at the moment the welded assembly is placed in it, from 600 °F (315 °C) to 800 °F (430 °C).

Item 7.8.2.4 - Clarified the calculation method in metric units, grouping the explanation of 560 °C divided by thickness in centimetres.

Item 7.8.2.5 - Increased the maximum permitted difference between thermocouples during treatment, from 65 °C to 85 °C. Curiously, this change went unmarked in the text (no revision highlighting), suggesting it was revised but possibly overlooked by the editors.

Item 7.8.2.6 - Correction of a historical error in the cooling rate calculation

An important technical flaw present since the 2015 edition has finally been corrected in AWS D1.1:2025. Earlier versions of the code stated, for metric units, that the cooling rate should be:

“260 °C/h divided by the thickness in millimetres.”

Although that formulation looked equivalent to the inch rule (500 °F/h ÷ in), it caused a significant technical problem: the unit “°C/h ÷ mm” produces numerically very low results, especially on thicker parts.

In practice the calculation almost always gave cooling rates below 10 °C/h, forcing users to apply the minimum permitted by the code: 55 °C/h (or 100 °F/h). The consequence was a systematic inconsistency between the metric and imperial calculations, plus confusion in procedures and audits.

The 2025 edition resolves this by correctly redefining the reference unit:

700 °C/h divided by the thickness in centimetres

That single change - from millimetres to centimetres - restores equivalence between the unit systems and makes the calculation technically coherent regardless of the country or industry applying it.

The same item also raised the maximum required cooling rate from 260 °C/hour to 280 °C/hour, and the final recording temperature from 315 °C to 430 °C.

Item 7.8.3.2 - Removed the maximum temperature requirement for quenched and tempered steels.

Conclusion

The revisions in AWS D1.1:2025 make the code clearer, more objective and better aligned with industrial practice. As well as correcting historical inconsistencies, it now presents less restrictive parameters, allowing greater operational flexibility, lower cost and productivity gains in post-weld heat treatment.

These updates deserve attention: companies, inspectors and engineers should review their procedures, WPSs and thermal control routines to ensure full compliance with the new edition, avoiding non-conformities, rework and technical risk in critical applications.