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Key takeaways
- PWHT heats a welded joint to a controlled temperature, holds it and cools it slowly to reduce residual stress and hardness.
- B31.3 requires PWHT of carbon steel (P-No. 1) above 19 mm nominal thickness, typically at 595 to 650 °C.
- Service conditions such as sour, caustic and amine services often require PWHT at any thickness.
What PWHT does
Welding leaves residual stresses close to yield strength, and the heat-affected zone can be hard, especially in thicker sections or low-alloy steels. PWHT:
- reduces residual stress,
- tempers hard microstructures,
- improves resistance to some forms of stress corrosion cracking and hydrogen cracking,
- improves toughness in some steels.
When the code requires it
B31.3 Table 331.1.1 sets PWHT by P-number and thickness. For carbon steel (P-No. 1), PWHT is required when the nominal thickness of the weld exceeds 19 mm (3/4 in). The usual temperature range is 595 to 650 °C, with a holding time based on thickness (about 1 hour per 25 mm, with a minimum).
Low-alloy chrome-moly steels (P-No. 4 and 5A) have their own thresholds and higher temperatures. Austenitic stainless steel generally doesn’t need PWHT, and duplex steels must not be given a conventional PWHT because it would damage their structure.
When the service requires it
Even below the code thickness, PWHT is often required by the owner’s specification for:
- sour service, to keep weld hardness within limits,
- caustic service above certain temperatures,
- amine service, to prevent stress corrosion cracking,
- hydrogen service for some materials.
How it is done
- Furnace PWHT for spools that fit in the furnace.
- Local PWHT for field welds, using electric resistance pads or induction coils around the joint, with insulation and thermocouples.
The heating rate, holding temperature, holding time and cooling rate are recorded on charts. Hardness tests after PWHT confirm it worked.
Practical consequences
- Weld everything that needs PWHT before the treatment. Welding attachments to the pipe afterwards means repeating PWHT.
- Flange faces and threaded parts need protection during PWHT.
- PWHT adds time to the fabrication schedule, so keep it to the lines that genuinely need it.
Code reference
ASME B31.3, para. 331 and Table 331.1.1 Heat treatment requirements.
NACE SP0472 Methods and controls to prevent in-service environmental cracking of carbon steel weldments in corrosive petroleum refining environments.
API RP 945 Avoiding environmental cracking in amine units.
What to take back to your desk
Check the PWHT column in each piping class and the line list. If PWHT is required, make sure the isometric says so, because fabricators work from the drawing, not from the specification.
Educational content. Always design to your project specification.
