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.
Gulf Coast Combustion has been executing ASME-compliant pressure vessel PWHT via direct gas fire combustion since 2014. Every job includes full documentation before the crew leaves the yard.
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Technical Resource
The Fabricator’s Complete Guide to Pressure Vessel PWHT
Everything you need to know about on-site pressure vessel PWHT — code requirements, execution standards, thermocouple methodology, documentation, and how to evaluate a heat treating contractor.
When PWHT Is Required Under ASME Section VIII
ASME Section VIII mandates PWHT based on a combination of factors: the thickness of the weld, the service conditions, and any supplemental requirements from the Welding Procedure Specification (WPS). For carbon steel pressure vessels, PWHT is generally required when the nominal thickness exceeds 1.5 inches — but that threshold drops for materials with higher carbon equivalents or for vessels destined for lethal service.
| Trigger Factor | Condition | PWHT Required? |
|---|---|---|
| Wall thickness (carbon steel) | Nominal thickness exceeds 1.5″ | Yes |
| Lethal service | Vessel handles lethal substances | Yes — regardless of thickness |
| High temperature service | Specific high temp operating conditions | Yes — additional requirements apply |
| High carbon equivalent material | Carbon equivalent exceeds code threshold | Yes — lower thickness threshold |
| Client/WPS specification | Customer specifies PWHT in WPS or contract | Yes — regardless of code minimums |
| Corrosive service | Specific corrosive environments | Yes — stress corrosion cracking risk |
Temperature and Hold Time Requirements
The hold temperature and time at temperature are specified by the code. For carbon steel pressure vessels, the required temperature range is 1,100°F to 1,200°F, with hold time calculated based on thickness — one hour per inch for the first two inches, then 15 minutes per inch for each additional inch, 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. Gulf Coast Combustion’s standard is 1,150°F ±50°F — held precisely, monitored continuously, and documented in full.
ASME Section VIII (UCS-56) — GCC Standard Parameters
By the Numbers
1,150°F
GCC Standard Soak Temp ±50°F
400°F/hr
Max Heat-Up Rate ÷ Wall Thickness
500°F/hr
Max Cool-Down Rate ÷ Wall Thickness
250°F
Max Temp Differential During Soak
300°F
Monitoring Begins
800°F
Free Air Cooling Below
| Wall Thickness | Minimum Hold Time | Calculation |
|---|---|---|
| Up to 1″ | 1 hour minimum | Minimum regardless of thickness |
| 1″ | 1 hour | 1 hr/inch |
| 2″ | 2 hours | 1 hr/inch |
| 3″ | 2 hours 15 minutes | 2 hrs + 15 min for 3rd inch |
| 4″ | 2 hours 30 minutes | 2 hrs + 15 min each additional inch |
| 5″ | 2 hours 45 minutes | 2 hrs + 15 min each additional inch |
Documentation Is Part of the Code Requirement
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.
What GCC Provides on Every Job
Heat treat record (HTR) — signed by GCC and the client representative
Strip chart recorder trace — complete time-temperature history for every thermocouple channel
NIST traceable calibration certificate — confirming recorder accuracy
Execution plan — pre-approved technical plan specific to that vessel, submitted before work begins
On-Site PWHT vs. Shipping to a Furnace
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. We’ve performed PWHT on vessels that simply couldn’t have been moved without cutting them apart.
No freight. No transport risk. No waiting on another shop’s schedule. If you’re shipping a large vessel to a furnace and something about it feels like a lot of exposure, read this before you book the truck.
| Factor | Shop Furnace | On-Site Direct Gas Fire |
|---|---|---|
| Vessel transport | Required — permits, rigging, transit risk | None. Equipment comes to you. |
| Size limits | Limited by furnace dimensions | No limit. Vessels to 120’+ treated on-site. |
| Schedule | Furnace queue dependent | Scheduled around your timeline. |
| QC witnessing | Off-site — limited visibility | Your QC team on your floor. |
| ASME compliance | Yes — when executed correctly | Yes — same code, full documentation. |
Before You Choose a Heat Treating Method
On-Site vs. Shop Furnace PWHT — The Full Comparison
Cost · Transport Risk · Size Limits · Schedule Control · Documentation
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This Is What ASME-Compliant On-Site PWHT Actually Looks Like
James Benefield is involved in every job. Direct gas fire combustion, on-site, right where the vessel sits. No furnace. No transport.
Time lapse: 621,000 lb vessel, Central Texas. Direct gas fire combustion, on-site.
What Gulf Coast Combustion Offers
On-Site Heat Treating Services
Pressure Vessel PWHT
On-site direct gas fire combustion. No furnace, no transport risk. Vessels from 16ft to 120ft+.
Localized PWHT
For nozzle welds, pipe spools, and vessel repairs where full vessel treatment isn’t required.
Refractory Dry Outs
Controlled cure cycles for new and repaired refractory linings. Full documentation included.
In-House Furnace
30’×10’×10′ furnace in Spring, TX. Up to 25,000 lbs. For components that fit.
Frequently Asked Questions
What materials require PWHT under ASME Section VIII?+
Carbon steel is the most common material requiring PWHT — generally when nominal thickness exceeds 1.5″. Chrome-moly alloys like P91 require higher soak temperatures and longer hold times than carbon steel. Always verify requirements against the applicable code edition and your WPS.
Can PWHT be waived or substituted?+
In some cases, ASME allows alternative measures such as controlled deposition welding or temper bead techniques in lieu of PWHT — but these require specific approval and documentation. When code requires PWHT and no alternative is approved, there is no substitute. Gulf Coast Combustion executes to whatever the governing specification requires.
What documentation does ASME require for PWHT?+
ASME requires a time-temperature record for every PWHT cycle documenting heat-up rate, hold temperature, hold time, and cool-down rate. This record becomes part of the vessel’s permanent documentation. GCC provides a complete package on every job: heat treat record, strip chart recorder trace, NIST traceable calibration certificate, and written execution plan — all delivered before our crew leaves your yard.
Does on-site PWHT meet the same ASME requirements as furnace PWHT?+
Yes. ASME Section VIII specifies parameters — not method. Whether PWHT is performed in a furnace or on-site via direct gas fire combustion, the code requirements are identical. GCC executes to ASME UCS-56 standards on every job with full documentation.
What is the maximum temperature differential allowed during soak?+
ASME Section VIII limits the temperature differential between the highest and lowest thermocouple readings to 250°F during the soak period and above 800°F. GCC monitors this continuously via multiple Type K thermocouples placed at intervals not exceeding 15 feet along the vessel length.
Do you have questions about your specific vessel’s PWHT requirements?+
Call James directly at 832-797-3428. We’ve been navigating ASME requirements on large pressure vessels since 2014 and we’re glad to talk through what your specific project needs before it starts.
Gulf Coast Combustion performs ASME-compliant pressure vessel PWHT across Texas and the Gulf Coast, including Houston, Beaumont/Port Arthur, Corpus Christi, Midland/Odessa, San Antonio, Dallas/North Texas, and Baton Rouge. For the full list see our service areas page. GCC mobilizes anywhere in the contiguous United States.
Where We Work
Serving Fabricators Across Texas and the Gulf Coast
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Talk to James About Your Next Project
Call or text the owner directly at 832-797-3428 — or reach the office at 713-425-3773.
