Guides / Process
Why Large Parts Are Stress Relieved Between Roughing and Finishing
September 16, 2026
A steel structure that has been welded, flame cut or heavily roughed is not in equilibrium. There is locked-in stress in it, and the moment you remove metal from one side, the remaining material relaxes and the part changes shape.
On a 300 mm bracket that movement is inside the tolerance and nobody notices. On a three metre frame it is the reason two bores that were in position after roughing are out of position after finishing.
Where the stress comes from
Welding. Each weld shrinks as it cools and pulls the surrounding metal with it. A fabrication with many welds carries a complicated internal balance of tension and compression.
Flame and plasma cutting. The cut edge is heated and quenched by the surrounding plate, leaving a hard, stressed skin.
Rolling. Plate carries residual stress from the mill. This is why large aluminium plate in particular moves as soon as you start cutting into it.
Roughing itself. Removing a large volume of material unbalances whatever was there before. This is the reason stress relief goes between roughing and finishing rather than before both.
The standard sequence
The honest sequence for a heavy weldment is:
- Weld and inspect the blank
- Rough machine, leaving an allowance on every surface that will be finished
- Stress relieve
- Finish machine
Step two matters as much as step three. Roughing first releases most of the stress that machining is going to release at all, so the thermal cycle then settles a part that has already been disturbed. Stress relieving a raw weldment and then machining it heavily gets you much less.
The allowance left at step two is usually a few millimetres, enough to clean up whatever movement the oven produces. More detail in machining allowance on large blanks.
Methods, and what each costs you in time
Thermal stress relief. The part is heated to a soak temperature below the transformation range, held, then cooled slowly under control. For carbon and low alloy steel that is typically in the region of 580 to 650 degrees Celsius, with soak time scaled to section thickness.
What it costs: a transport cycle if the oven is off site, a day or more of oven and cooling time, and a handling cycle at each end. Two to five working days is a realistic planning number, and more if the oven is busy.
The practical limit is oven size. Many shops can machine a part they cannot heat treat, and on very large fabrications the oven, not the machine, sets what is possible.
Vibratory stress relief. The part is excited at resonance for a period, which redistributes peak stresses. It is quick, has no size limit and needs no oven.
What it costs: hours rather than days. It is usually treated as a partial measure on heavily welded steel, so it fits moderate tolerances or parts nothing can fit in an oven.
Natural ageing. Leaving the part outside for weeks or months. Still used for cast machine bases in some traditions. It costs nothing but time, and time is usually the thing you do not have.
How to read a quote for this
This is the most common reason two heavy quotes differ by a large margin. One shop has planned rough, stress relieve, finish, and priced the extra handling cycle and the oven. The other has planned to machine it once.
Neither is dishonest. The second shop may be right if your tolerances are loose. But you should know which one you are buying, so ask every quote the same question: is stress relief in the sequence, and if not, what happens if the part moves.
More on that in comparing heavy machining quotes.
What to write on the drawing
Do not leave it to the shop to guess. A note like this removes all the ambiguity:
Rough machine with 3 mm allowance on machined surfaces. Thermal stress relief per the material specification. Finish machine after stress relief. Bores to be machined in the final operation.
If you do not care how it is achieved, say that instead:
Supplier to select the machining sequence. Bore positions to hold 0.08 mm at room temperature after all thermal operations.
The second version is often better, because it states the outcome you need and lets the shop pick the cheapest route to it.
When you can skip it
- Castings with a known ageing or heat treatment history
- Parts where the machining allowance is small and symmetric
- Tolerances loose enough to absorb a few tenths over the length
- Solid stock rather than a fabrication, machined evenly on all sides
What to do next
Decide whether your tolerance can absorb movement, and write the answer on the drawing rather than in an email. Then read tolerance across a large part, because the two questions are really one question.
Frequently asked questions
How much does a large part move if stress relief is skipped?
It depends on the blank and how much metal comes off, but movement of a few tenths of a millimetre over a metre is common on a welded steel structure, and more on a heavily flame cut one. On a loose part that is invisible. On a bore position held to 0.08 mm it is the whole tolerance.
Can vibratory stress relief replace a thermal cycle?
Sometimes. It is faster and cheaper and it has no oven size limit, which matters on very large parts. It is generally seen as less complete than a thermal cycle on heavily welded steel, so shops tend to use it where the tolerance is moderate or the part is too big for any oven.
Do castings need stress relief too?
Often less than weldments, because many foundries age or heat treat castings before shipping. Ask for the casting's thermal history. If it has none, treat it like a weldment.
Need a quote for a part like this?
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