When a Welded Frame Needs Stress Relief, and When It Does Not
Welding puts heat into a small volume of a large part. That volume expands, is restrained by the cold metal around it, then shrinks as it cools. What is left behind is residual stress, locked into the structure at close to the yield strength of the material in places.
The frame looks fine. It measures fine. Then a cutter opens a face, removes the material that was holding that stress in balance, and the part moves.
What stress relief actually does
Stress relief does not make the part stronger and it does not fix bad fit-up. It lowers the residual stress locked in by welding so that the part is stable when metal is removed from it.
In a thermal cycle the steel is heated to a temperature where its yield strength drops well below the locked-in stress. The stress can no longer be supported, so it relaxes through small plastic movement while the part is hot and evenly supported. Cool it slowly and evenly and the part comes out at a much lower stress level than it went in.
The important word is evenly. A fast or uneven cool puts new stress straight back in, which is why the ramp rates matter as much as the soak temperature.
When the part needs it
Ask four questions about the part.
How much weld metal went in? A frame with hundreds of metres of weld carries far more stored energy than a bracket with four fillets.
How restrained are the joints? Welds made into a stiff, boxed structure cannot shrink freely, so the stress has nowhere to go. Open, unrestrained joints relieve themselves as they cool.
How much material comes off in machining? Removing 5 mm from a face is an invitation for the section underneath to relax. Skimming 0.5 mm off a small pad is not.
How tight is the feature that has to survive? Bore alignment across a long frame, a flatness callout on a mounting face, parallelism between rails. If those numbers are on the drawing, stress relief is normally part of the answer.
If three or four of those point the same way, budget for it.
A typical thermal cycle
For carbon and low alloy steel a common cycle is:
| Stage | Typical figure |
|---|---|
| Heating rate above 300 degrees C | Controlled, often 100 to 200 degrees C per hour depending on thickness |
| Soak temperature | 580 to 650 degrees C |
| Soak time | About one hour per 25 mm of the thickest section, with a stated minimum |
| Cooling in the oven | Controlled down to around 300 degrees C, then still air |
These are published practice, and the applicable code or specification is what actually governs. Two material families need a separate conversation. Quenched and tempered steels such as A514 or S690QL were given their properties by a heat treatment, and a stress relief cycle run too hot or too long will take some of it away. Austenitic stainless has its own sensitisation range, so the cycle used for carbon steel is not transferable.
Vibratory stress relief
Vibratory stress relief clamps a motor to the part, sweeps through the frequency range to find resonances and holds the part there for a period. It is fast, it costs a fraction of oven time, it does not need a furnace big enough for a 6 m frame and it does not put any scale on the surface.
It is also not the same process. It does not change the metallurgy, it acts mainly on the peak stresses rather than on the whole field, and codes that require post-weld heat treatment for a metallurgical reason will not accept it as a substitute.
In practice it earns its place on large machine frames where the goal is dimensional stability through machining rather than code compliance, and where the alternative is trucking a 4 tonne part to an oven two hundred kilometres away. Ask the shop what it has seen work on parts like yours and how it verifies the result.
Where it sits in the sequence
The order that works for most machined weldments is:
- Cut and prepare
- Fit and tack
- Weld to a planned sequence
- Inspect the welds
- Stress relieve
- Rough machine if there is a lot of stock
- Finish machine
- Coat, with machined faces masked
Two variations show up on demanding parts. On a heavily welded frame with very tight geometry some shops rough machine, stress relieve a second time, then finish. That costs another oven cycle and it is worth it when the part has already shown it will move.
The other variation is the one to avoid. Moving stress relief after machining does not save money. It delivers a part that measured correct at the shop and measures wrong on your floor.
What to put on the drawing
Say what you want in words the shop can price:
- “Stress relieve after welding, before machining.” Not “stress relieve” on its own.
- The method if you care, thermal or vibratory. If you do not care, say that too and let the shop propose.
- The condition each geometric tolerance applies to, for example “flatness 0.1 mm after stress relief and finish machining”.
- Whether a record is required. A time and temperature chart from the oven is normal and cheap to ask for at quote time, and awkward to ask for afterwards.
If you are not sure which way to go, put the question in the RFQ. A shop that welds and machines the same part every week has an opinion worth more than a rule of thumb, and it costs nothing to ask before the quote rather than after the first part.
Have a frame that has to hold position after machining? Send the drawing and we will match it with shops that own the oven and the machine.
Frequently asked questions
Does every weldment need stress relief?
No. A lightly welded bracket with loose tolerances does not. A heavily welded frame with bores or mounting faces that must hold position usually does, because the residual stress released during cutting is what moves those features.
What temperature is used for stress relieving carbon steel?
Typically 580 to 650 degrees C with a soak of about one hour per 25 mm of thickness, plus a controlled heating and cooling rate. The exact cycle comes from the applicable code and the material. Quenched and tempered steels have their own limits because the cycle can affect the temper.
Is vibratory stress relief a real alternative to an oven?
It is used in industry, it is much faster and it costs a fraction of oven time. It works differently from a thermal cycle and it is not accepted everywhere a code calls for post-weld heat treatment. Treat it as a shop decision to discuss rather than a drop-in substitute.
Need a quote for this part?
Send the drawing. We match you with up to 3 shops that weld and machine it in house, and we reply within 24 business hours.
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