For the SA-516 70 head described here, do not remove the vendor’s forming-related heat-treatment step merely because the completed vessel is scheduled for PWHT. First identify the treatment, its purpose, and the applicable construction-code edition; then decide whether the final vessel cycle can satisfy that requirement.
Separate the forming treatment from weld PWHT
The vendor’s terminology changed during the review: the offer was first understood to include “PWHT” or stress relief, then the vendor described the operation as PFHT after forming the crown and before forming the knuckle. Those names do not establish that the operations are interchangeable. A weld PWHT is a controlled temperature-rise, hold, and cooldown cycle applied to address requirements associated with welding. A forming-related heat treatment may instead address material condition or forming effects, and its temperature and sequence can differ.
Ask the head vendor to identify the actual operation in writing. Request its purpose, procedure, temperature range, hold and cooldown controls, point in the forming sequence, and the material or code requirement it is intended to satisfy. Ask whether the operation is heat treatment required by the plate specification, a forming treatment intended to reduce cracking risk, or postweld heat treatment. Do not approve or delete a cost line based on the label “PWHT” alone.
| Observed statement or condition | Question to resolve | Why it matters |
|---|---|---|
| Offer says “stress relief” or “PWHT” | What cycle is actually planned, and for what purpose? | Different heat treatments serve different purposes and may not share the same cycle. |
| Vendor says PFHT between crown and knuckle forming | What material, forming, or specification requirement calls for it? | A cycle before completion of forming is not automatically replaced by final vessel PWHT. |
| Completed vessel is scheduled for PWHT | Does the applicable requirement permit that later cycle to satisfy the head requirement? | Some requirements may be met after welding, but this depends on the governing code and condition. |
| Vendor cites crack prevention | What forming risk or procedure supports that claim? | Crack prevention during subsequent forming is a different objective from treating completed welds. |
Confirm the head condition and forming sequence
The case describes a head made from SA-516 70 plate, 50 mm thick before forming, for a shell with a 6950 mm inside diameter. The crown radius is 6255 mm and the knuckle radius is 1195 mm. The head vendor said it wanted treatment after forming the crown radius and before forming the knuckle radius to prevent cracks in the knuckle region. Preserve these values and sequence in the technical review; they are not a substitute for the actual forming procedure or code check.
The purchaser’s review stated that UG-79 and UCS-79 did not require PFHT for the stated dimensions because calculated fiber elongation was 3 percent. Treat that as the calculation to verify, not as a universal exemption: confirm the applicable code edition, the method and inputs used to determine forming strain, material condition, and the relevant acceptance criteria. Separately resolve the vendor’s stated crack-prevention concern. A conclusion about a code-triggered treatment does not by itself disprove a forming-process need.
Ask whether the plate received the heat treatment required by its material specification before forming. The discussion raises the possibility of “green” plate, meaning plate supplied without a required heat treatment, and of plate being formed at a heat-treatment temperature or treated after forming. Verify the material test documentation and the head manufacturer’s forming/heat-treatment records; do not infer plate condition from the head’s appearance.
Check whether the final PWHT can satisfy a code requirement
Do not assume that every required head treatment must occur immediately after forming, or that any later vessel PWHT automatically replaces it. The evidence includes a Section VIII, Division 1 interpretation identified as the 1992 Edition with 1993 Addenda, UCS-79(d), dated 03/13/1995. It answers that “heat treated subsequently” does not mean only immediately after forming and before other operations, and that PWHT in accordance with UCS-56 following welding of the head to the vessel satisfies the UCS-79(d) requirement in that interpretation.
Use that interpretation only within its stated edition and paragraph context. Before applying the same conclusion to a current project, check the project’s governing edition/addenda and the exact applicable provisions with the responsible pressure-vessel design authority or an independent metallurgical/code specialist. Also establish that the completed vessel cycle is actually performed in accordance with the applicable requirement and covers the relevant head condition. A planned treatment is not proof that the treatment was completed or qualified.
The discussion also distinguishes the case where a cold-formed head requires treatment because of fiber elongation: if the completed vessel receives the required PWHT, duplicating that heat treatment on the head may not be necessary. But if the head manufacturer does not perform a required treatment, it may be unable to certify the head as meeting the cited forming requirements. Confirm certification responsibility before changing the purchase order.
Resolve the mechanical-property concern before authorizing a cycle
The purchaser raised concern that exposure to PWHT-range temperatures could deteriorate mechanical properties and argued that heat treatment should be minimized. Do not treat that concern as a reason to omit a required operation, and do not treat a proposed cycle as harmless without review. The relevant question is whether the specified material condition and mechanical-property requirements remain satisfied after the complete sequence of plate treatment, forming, any intermediate treatment, welding, and final PWHT.
Have the responsible materials or metallurgical reviewer compare the proposed cycle with the material specification and project requirements. Review the actual temperature, hold duration, heating and cooling controls, number and sequence of cycles, and required post-treatment mechanical-property evidence. Obtain the vendor’s procedure and certification records rather than estimating effects from the term “PWHT.” If the planned cycle or property acceptance basis is unclear, stop the change and get an independent technical review before production proceeds.
Document the treatment decision before fit-up
- Collect the head drawing, material documentation, forming sequence, vendor offer, and proposed heat-treatment procedure. Resolve whether the disputed operation is plate-specification heat treatment, forming-related PFHT/stress relief, or weld PWHT.
- Verify the code edition/addenda and the forming-strain calculation. Record the measured/design inputs, the applicable paragraph, and whether the requirement is mandatory for this head.
- Ask the head manufacturer to state the purpose of its proposed intermediate cycle and the technical basis for crack prevention. If that basis remains disputed, obtain an independent metallurgical or pressure-vessel code review.
- Determine whether final vessel PWHT will be a suitable substitute for any required head treatment. Confirm the applicable code basis, treatment scope, and which organization will certify the head and completed vessel.
- Review the complete thermal history for material-property effects and confirm what inspection, records, and mechanical-property evidence the project requires.
- Revise the purchase order and inspection/test plan to identify each required operation, its sequence, procedure, records, and acceptance responsibility. Do not accept an ambiguous “PWHT” line item as the technical specification.
Before authorizing fit-up, check that the vendor, fabricator, and design authority agree on the named treatment and its compliance basis. The hold point is cleared only when the required cycle and certification path are documented.
Plan furnace and local-heating limits before fabrication
A final vessel PWHT and a whole-head stress-relief treatment may have different practical heating arrangements. The discussion warns that a secondary fabrication shop may not be able to place the entire vessel in a furnace for a controlled whole-vessel preheat, heat-up, hold, and cooldown sequence. The shop may instead use heat blankets or local heaters at individual joints. Such local joint treatment does not, by itself, show that the whole head received the same controlled stress-relief cycle.
Ask the fabricator how the vessel and head will be heated, what region the procedure treats, and how temperature control and records will demonstrate coverage. If the head needs a whole-head treatment, determine whether the original head shop can process it in one furnace under controlled rates. Do not assume local weld heating can replace a whole-head cycle; likewise, do not prescribe whole-vessel furnace treatment without confirming the applicable requirement and the fabricator’s approved procedure.
Verify the completed treatment and certification
Before closing the fabrication record, compare the completed treatment records with the approved procedure and the code basis selected for the job. Confirm the actual sequence, covered component or weld regions, temperature history, hold, and controlled cooldown. Check that material and head certifications reflect the treatment actually performed and that any required post-treatment property or inspection results meet the project acceptance criteria.
For the specific case described, the end-to-end decision is not simply “the vessel gets PWHT, so omit the head treatment.” It is: establish what the head vendor means by PFHT/stress relief; verify the 3 percent forming-strain calculation and any independent forming-risk basis; confirm whether the applicable code allows subsequent UCS-56 PWHT to satisfy a UCS-79(d) requirement for the project’s edition; and verify that the fabricator can perform and document the required whole-head or vessel cycle. Release fit-up only after the treatment and certification plan agree.
FAQ
What happens if the whole vessel will receive PWHT?
A later vessel PWHT may satisfy a cold-formed-head heat-treatment requirement in the cited 1992 Edition/1993 Addenda interpretation when performed in accordance with UCS-56 after welding. Verify the governing project edition, applicable paragraph, cycle, and certification responsibility before relying on that interpretation.
What happens if the vendor calls the treatment PFHT instead of PWHT?
Get the actual procedure, purpose, and required sequence. The label alone does not tell you whether the cycle treats plate condition, forming effects, cracking risk, or completed welds.
What happens if fiber elongation is only 3 percent?
Verify the calculation inputs and applicable code edition against the head geometry and forming sequence. The stated 3 percent assessment addresses the purchaser’s code-trigger question, but it does not alone resolve the vendor’s separate concern about cracking during knuckle forming.
What happens if the shop uses local heaters instead of a furnace?
Confirm the treatment scope in the approved procedure and temperature records. Local heating of joints does not demonstrate that the whole head received a controlled whole-head stress-relief cycle.
Stop the heat-treatment change if the required cycle, code basis, property acceptance, or certification path remains unclear. Escalate the package to the responsible pressure-vessel design authority and an independent metallurgical/code specialist; contact official code or manufacturer support through the project’s authorized channels when needed.