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Defining Postweld Heat Treatment
Terminology related to postweld heat treatment (PWHT) is inconsistent across AWS standards. In this article, we examine the use of PWHT and related terms in AWS D1.1/D1.1M, Structural Welding Code — Steel; AASHTO/AWS D1.5M/D1.5, Bridge Welding Code; and AWS A3.0M/A3.0, Standard Welding Terms and Definitions, and make recommendations for future code editions.
The Definitions
Both D1.1 and D1.5 have undergone a series of revisions to differentiate PWHT, stress relief, and hydrogen diffusion postheat.
In 2025, A3.0 added a definition of PWHT: “Any heat treatment applied to a weldment subsequent to welding. See also hydrogen bakeout and stress relief heat treatment.” This definition is similar to the PWHT definition in the 2025 edition of D1.1: “Any heat treatment after welding.”
D1.5 was revised in 2025 to define PWHT as the following: “Any heat treatment after welding for the purpose of stress relief or for modifying the metallurgical structure or mechanical properties of the metal. Hydrogen diffusion postheat (see 12.15) and heat-assisted geometry modifications (straightening, cambering, curving) are not considered postweld heat treatment.”
The Processes
In this section, we review processes that potentially fall under the A3.0 definition of PWHT.
Stress Relief Heat Treatment
Stress relief is typically used to reduce residual stresses and improve dimensional stability. A3.0 was revised in 2025 to define stress relief heat treatment as “A type of postweld heat treatment used primarily for the reduction of residual stresses.” This process involves controlled heating of the weldment to a specific temperature and exposure duration, allowing the metallurgical structure to reorganize internally before the weldment is cooled slowly. The stress relief temperature for structural steels is typically around 1100° to 1200°F (590° to 650°C), warm enough to suppress yield strength and permit stress reduction while remaining comfortably below the lower transformation temperature of the base and filler metals. Heating can be accomplished using a furnace, induction heating equipment (Fig. 1), resistance heating equipment, or other local methods. D1.1 and D1.5 each specify holding temperatures, durations, and heating and cooling rates.
PWHT is often used synonymously with stress relief heat treatment, and stress relief is the most common form of PWHT used on structural welds. However, there are other forms of PWHT, and the two terms should not be used interchangeably.
Hydrogen Bakeout or Hydrogen Diffusion Postheat
In 2025, A3.0 added a definition of hydrogen bakeout as “Heating a base metal or weldment at elevated temperature to release residual hydrogen.” Also in 2025, D1.5 added a definition of hydrogen diffusion postheat as “A thermal treatment applied to an assembly after welding for the purpose of releasing diffusible hydrogen.” The two definitions are similar but not the same; one references residual hydrogen, while the other references diffusible hydrogen.
Atomic hydrogen can migrate into the metal matrix during welding and diffuse outward into the heat-
affected zone and atmosphere at room temperature. The diffusion process is accelerated at elevated temperatures. Per the Welding Handbook, Ninth Edition, Volume 4, Materials and Applications — Volume 1, sources of hydrogen include “the filler metal, moisture in the electrode covering, welding flux, shielding gas, or surface contaminants.” Elevated hydrogen levels, combined with material susceptibility and stress, can result in hydrogen-assisted cracking.
Both D1.1 and D1.5 contain many provisions intended to reduce hydrogen in the weld without the need for either hydrogen diffusion postheat or hydrogen bakeout as defined in A3.0. D1.5 also includes requirements for hydrogen diffusion postheat under certain circumstances.
In hydrogen diffusion postheat, the weld is raised to a temperature higher than typical preheat and interpass temperatures immediately after the weld is completed, without allowing it to cool first, and is held at that temperature for a prescribed duration. D1.5 specifies postheat at 450° to 550°F (230° to 290°C) for one hour per inch (25 mm) of weld thickness. Postheat is typically accomplished with a rosebud torch, similar to preheating (Fig. 2), although other methods used for stress relief, such as induction or resistance heating, can also be employed.
Hydrogen bakeout (A3.0) is often used synonymously with hydrogen diffusion postheat, although there is some confusion about whether they’re the same. For hydrogen bakeout (to address residual hydrogen), the temperatures are relatively high (above 1100°F [590°C]). Where D1.5 mentions “hydrogen diffusion postheat” (to address diffusible hydrogen), the temperatures are relatively low (450° to 550°F [230° to 290°C]).
Earlier editions of D1.5 used PWHT synonymously with both hydrogen diffusion postheat and stress relief, which was confusing.
Heat-Assisted Forming
Heat-assisted forming, although not defined in A3.0, encompasses straightening, curving, and cambering. The controlled application of heat, sometimes in conjunction with loading or restraint, is used to achieve the desired geometry. Specified or recommended maximum temperatures for carbon and low-alloy steels in the AWS D1 codes range from 1000° to 1200°F (540° to 650°C). D1.5 includes more detail about heat-assisted forming than D1.1, which mentions these processes only in the context of strengthening and repair. Many of the heat application provisions of D1.5 now appear in AASHTO LRFD Steel Bridge Fabrication Specifications and will not be included in the 2030 edition of D1.5.
Other Heat Treatments
Other heat treatments include annealing, normalizing, and quenching and tempering. These alter the metallurgical structure and affect strength, ductility, and toughness. None are typically used on structural welds, and neither D1.1 nor D1.5 has provisions for performing these treatments.
Subsequent Welding
Because heat treatment is not defined, any application of heat after welding could be considered PWHT, including heat from repair welding.
What Is Considered PWHT?
The scope of PWHT is relevant to AWS codes because its use is an essential variable for welding procedure specification (WPS) qualification in several codes, including D1.1 and D1.5. The D1.5 definition was written to distinguish between heating applications that affect WPS qualification and those that do not.
Stress relief heat treatment is addressed in both codes as PWHT.
Annealing, normalizing, and quenching and tempering are not mentioned in D1.1 but are discussed in the D1.5 commentary and fall under the D1.5 definition of PWHT.
Heat-assisted forming typically does not affect WPS qualification and is excluded by the D1.5 definition. D1.1 and A3.0 are silent, but D1.1 users generally do not treat this as PWHT.
Arguably, the A3.0 definition of PWHT is not intended to include heat from subsequent welding. By this logic, even the heat from subsequent passes would fall under the definition. Nonetheless, A3.0 does not rule it out. D1.5 does not exclude it explicitly, but it is excluded by the purpose statement in that definition.
Hydrogen diffusion postheat does not affect WPS qualification in D1.5 and is explicitly excluded from the definition. D1.1 does not mention postheat. Neither code mentions hydrogen bakeout as defined in A3.0, but A3.0 includes “See also hydrogen bakeout” in its PWHT definition. Considering the high temperature typically associated with hydrogen bakeout as defined in A3.0, and the purposes for which it is used, it could be included within the purpose statement in the D1.5 definition.
Recommendations for Future Editions
- The A3.0 PWHT definition should be refined to reflect the practical implications of what should be considered PWHT. If not, D1.1 should adopt its own definition, similar to the definition in D1.5.
- A3.0 should either add a definition of hydrogen diffusion postheat similar to the definition in D1.5 or clarify its hydrogen bakeout definition. If not, D1.1 and D1.5 may need to clarify hydrogen bakeout and hydrogen diffusion postheat.
- Because heat-assisted forming can be a fabrication process (including correction of welding distortion) as well as a repair process, D1.1 should include relevant provisions in its Fabrication clause.
This article was written by Travis Green (Wiss, Janney, Elstner Associates Inc.) and Heather Gilmer (Pennoni) for the American Welding Society.