Guides / Process

Post Weld Heat Treatment on Large Welded Fabrications

September 30, 2026

Post weld heat treatment gets talked about as if it is one thing, a trip through an oven that a welded part takes before it ships. In practice it covers two different reasons to heat a fabrication after welding, and confusing them leads to either an underspecified part or an overpriced one.

Two different reasons for the same word

General stress relief. Aimed at reducing the residual stress that welding leaves behind, mainly so the part machines predictably and holds its dimensions afterward. This is the stress relief covered in stress relief before finish machining, and it is chosen based on what the tolerance needs, not because a code demands it.

Code-mandated PWHT. A specific thermal cycle required by a design or fabrication code such as ASME or API, for reasons that go beyond machining accuracy. Depending on the material and service, this can be about toughness at low temperature, resistance to hydrogen-induced cracking, or restoring properties in the heat-affected zone next to a weld. When a code calls for it, the temperature, hold time and cooling rate are usually specified precisely, and it is not optional or negotiable the way a general stress relief sometimes is.

The practical difference matters because a code-mandated cycle can be more demanding, and often better documented with a full time-temperature record, than a general stress relief aimed only at dimensional stability. If your specification references a code, find out exactly what that code requires before assuming a generic thermal cycle satisfies it.

Furnace size is usually the real constraint

On a large fabrication, the question is rarely whether PWHT is a good idea. It is whether a furnace exists that the part actually fits into.

A furnace has a fixed internal envelope, and a large frame, a long shaft housing or a wide baseplate can simply exceed what is available locally. This is often the detail that decides which shops can even be considered for a job, ahead of table size or spindle power, because a shop with an oversized machining envelope but no matching furnace access cannot complete the sequence on its own.

Shops handle this limit a few different ways.

Sourcing a large enough furnace. Some shops or their subcontractors maintain or have access to furnaces sized for heavy fabrication work, which is usually the most straightforward route when the part fits.

Splitting the treatment. Complex assemblies are sometimes stress relieved in sub-sections before final assembly welding, then given a smaller final treatment, when the code and the design allow it.

Local heating. Heating bands or torches applied around a specific weld rather than the whole part. This is a real technique with real applications, but it does not replace a code-mandated furnace cycle when one is required, so it is not a substitute to reach for without checking the specification first.

If your part is close to the largest furnace size in your supply chain, say so early. It is a scheduling and shop-selection question, not a last-minute detail.

Distortion after PWHT, and why final machining follows it

A long, high-temperature cycle does more to a large fabrication than relieve stress evenly. Uneven heating and cooling, the part’s own weight sagging slightly at temperature, and the release of stress itself can all leave the part with new distortion that did not exist before it went into the furnace.

This is exactly why the sequence matters: rough machine first, leaving allowance on every surface that finishes later, then heat treat, then finish machine. Finishing before PWHT wastes the finishing operation, because the part is likely to move during the thermal cycle and undo some of what was just cut. The same logic and the same allowance planning described in stress relief before finish machining applies here, whether the cycle is a general stress relief or a full code PWHT.

How this differs from vibratory stress relief

Vibratory stress relief, covered on this site as one of the standard methods for relieving stress in a weldment, works by exciting the part at resonance rather than heating it, and it has no furnace size limit at all. That makes it attractive on very large parts specifically because it sidesteps the constraint this article is about.

The tradeoff is that vibratory treatment is generally not accepted as a substitute where a code specifically calls for a thermal PWHT cycle. It is a legitimate option for dimensional stability on parts that no furnace can hold, but it answers a different question than a code requirement does, and the two should not be assumed interchangeable without checking what your specification actually asks for.

What to put on the RFQ

• Whether PWHT is required by a named code, or is a general stress relief for machining accuracy
• Material grade, thickness and the specific cycle parameters if the code specifies them
• Maximum part envelope, so the shop can confirm furnace access before quoting
• Machining allowance planned before the thermal cycle, and the sequence expected: rough, treat, finish
• Documentation required, such as a time-temperature chart, and to what standard

What to do next

If your specification references a code for PWHT, get the exact clause before sending the RFQ, since it changes both the process and the paperwork a shop has to plan for. Also decide early whether the blank will be a weldment at all, which is its own question covered in weldment or casting for large parts.

Related reading: stress relief before finish machining, weldment or casting for large parts, machining allowance on large blanks.

Frequently asked questions

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