Stress Relief for Welded Machine Bases: Thermal vs Vibratory

A welded machine base comes out of the shop flat, gets shipped, gets installed, and six weeks later the alignment has drifted. The welds did not fail. The steel simply finished doing what the welding started.

This article covers why that happens, what stress relief does about it, and how to specify it so the shop and your QA are talking about the same thing.

Why a weldment moves

Welding is localised melting. The weld pool and the metal around it expand as they heat and contract as they cool, but the surrounding cold steel restrains that contraction. The result is locked-in residual stress, frequently approaching the yield strength of the material at the weld.

The piece is in equilibrium, so it looks fine. Then you remove metal from one face during machining, the equilibrium breaks, and the piece relaxes into a new shape. A 3 m base can move a few tenths of a millimetre, enough to put a bearing alignment out of tolerance.

Two secondary effects matter on heavy plate:

• Service loads add to residual stress. A frame already at 80% of yield from welding has little margin left.
• Time and temperature cycling release stress slowly. This is why a piece can pass FAI and drift months later in a warm plant.

What thermal stress relief does

Thermal (or post-weld heat treatment) works by taking the steel to a temperature where its yield strength drops far enough that the residual stresses exceed it and relax plastically, while the part is free to move, in an oven, rather than later on your foundation.

For carbon and low-alloy structural steel, the sub-critical cycle is broadly:

Stage	Typical figures
Heating rate	50–100 °C per hour above 300 °C, controlled to limit thermal gradient
Soak temperature	565–620 °C (roughly 1,050–1,150 °F)
Soak time	About 1 hour per 25 mm of thickness, 1 hour minimum
Cooling	Controlled in the oven to below 300 °C, then still air

Both the rate and the uniformity matter. Heating one end of a long frame faster than the other introduces new stress while removing old. A shop with a properly instrumented oven will give you a chart with thermocouple traces; one without will give you an assurance.

Ask for the chart. It is the only evidence the cycle happened as specified.

Where thermal relief sits in the sequence

This is the part most often got wrong. The correct order for a machine base that has to hold tolerance:

1. Fabricate and weld complete
2. Stress relieve
3. Rough machine
4. (On heavily stressed or thick fabrications) second stress relief
5. Finish machine to final tolerance
6. Final dimensional inspection

Machining before relief wastes the machining. Relieving after finish machining undoes the finish. The second relief between rough and finish passes is not always necessary, but on thick sections with a lot of metal coming off, it is the difference between holding tolerance and not.

Vibratory stress relief: when it is legitimate

Vibratory stress relief (VSR) puts the weldment on isolators, attaches an eccentric-mass exciter, and sweeps through frequencies to find and dwell at resonances. The cyclic stress it induces, added to the residual stress, exceeds local yield at the peaks and allows some redistribution.

What it genuinely offers:

• No oven size limit, the main reason it exists for large frames
• No oxidation or scaling, so machined surfaces survive
• Hours rather than days, and far less energy
• No risk of distortion from uneven heating

What it does not do:

• It redistributes and reduces peak residual stress; it does not remove stress the way a thermal soak does
• It has no metallurgical effect, no tempering of the heat-affected zone, no hydrogen removal
• Its effectiveness depends on the part responding at the frequencies the equipment can reach; stiff, compact parts respond poorly

The honest position: VSR is a reasonable choice for dimensional stability on large, relatively flexible frames where the oven is genuinely the constraint. It is not an equivalent substitute where the specification requires PWHT for metallurgical reasons, and it should never be quietly swapped in without telling you.

How to specify it

Write four things on the drawing:

1. Whether relief is required at all. Not every frame needs it. A bolted support structure with no machined datums probably does not.
2. Method, or an explicit permission. Either “Thermal stress relief per AWS D1.1” or “Thermal stress relief; vibratory acceptable with written approval”. Silence invites substitution.
3. Where it falls in the sequence. “Stress relieve after welding, before machining” removes the most common ambiguity.
4. What documentation you want. For thermal: the time-temperature chart with thermocouple locations. For vibratory: the pre- and post-treatment frequency response scans.

Common failure modes

“We stress relieved it” with no chart. Possibly true. Unverifiable.

Relief done in a kiln built for something else. Brick kilns and galvanising-adjacent furnaces sometimes get pressed into service. Without zone control and thermocouples on the part, the temperature at the centre of a thick section is a guess.

Galvanising treated as stress relief. The hot-dip kettle runs around 450 °C, well below the soak range, and the immersion is minutes not hours. It is not stress relief. It can, however, cause its own distortion on thin sections, which is covered in surface finish systems for structural steel.

Relief skipped because the piece “looked fine”. It will look fine. That is the problem.

The cost question

Thermal relief is a small line on most quotes, commonly 3–8% of the job. What makes it expensive is a piece longer than the available oven, which forces a subcontract move with transport, handling and schedule risk attached.

This is why oven length belongs in your first conversation with a fabricator, alongside crane capacity and machining envelope, see how to choose a heavy fabrication supplier. It is also one more argument for split-and-bolt design, where each section fits an ordinary oven.

Not sure whether your frame needs relief, or whether the shop quoting it has an oven long enough? Send the drawing. We will tell you within 24 business hours.