PWHT Requirements Under ASME Section VIII

What Pressure Vessel Fabricators Need to Know:

Understanding the Code Requirements That Drive Your PWHT Decisions

If you fabricate pressure vessels, post weld heat treatment (PWHT) isn’t optional — it’s a code requirement driven by ASME Section VIII, Division 1. But understanding exactly when it’s required, and what the parameters look like, is where things can get complicated. Here’s a practical breakdown.

ASME Section VIII mandates PWHT based on a combination of factors: the P-Number of the base material, the thickness of the weld, the service conditions, and any supplemental requirements from the Welding Procedure Specification (WPS). For carbon steels in Group 1 (P-No. 1), PWHT is generally required when the nominal thickness exceeds 1.5 inches — but that threshold drops significantly for materials with higher carbon equivalents or for vessels destined for lethal service.

The hold temperature and time at temperature are also specified by the code. For most P-No. 1 carbon steels, the required temperature range is 1100°F to 1200°F, with hold time calculated based on thickness — typically one hour per inch of thickness, with a one-hour minimum. Getting this right matters: too low and you don’t achieve the stress relief; too high and you risk degrading the material properties you worked to achieve.

One thing fabricators sometimes overlook is documentation. ASME requires detailed time-temperature records for every PWHT cycle. These records — showing heat-up rate, hold time, hold temperature, and cool-down rate — become part of the permanent record for the vessel. At Gulf Coast Combustion, we provide complete chart recorder documentation for every job as a standard deliverable.

The other common question is whether PWHT can realistically be done on-site at the fabrication shop rather than trucking the vessel to an outside furnace. For large vessels, this isn’t just possible — it’s often the better option. Our mobile direct gas fire units can be set up and operational at your facility, with no vessel movement required.

If you have a vessel coming up that requires PWHT and you’re not sure about the applicable requirements, give us a call. We’ve been doing this since 2013 and we’re happy to talk through the code requirements before the project even starts.

Gulf Coast Combustion serves fabricators across the Gulf Coast and beyond, executing every job to ASME Section VIII requirements with full documentation on every cycle.

If you’ve confirmed PWHT is required for your project and you’re weighing how to get it done, read: Post Weld Heat Treating: Can It Be Done On-Site?