Method and Case of Constraints: Relieving Residual Stress in Machined Parts

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  Key takeaways
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  Why residual stress shows up after machining
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  Natural aging and thermal aging: the two extremes
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  Vibration aging and sub-resonance aging
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  Cold work stretching and ultrasonic impact
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  How to choose when two methods both fit
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  Step by step: choosing and applying a relief method
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  Method and case of constraints: method comparison

Residual stress is locked into a part during welding, casting, forging and machining. Cooling rates differ across a section, so one region wants to shrink while the next one holds it back. The part looks fine on the machine. It moves once the clamps come off or once it sees a few thermal cycles in service.

In CNC work the biggest drivers are bulk stock removal and asymmetric material removal. Take 6 mm off one side of a 4140 plate and 1 mm off the other, and the balance of internal stress shifts. Thin walls and long slender parts bend first. A 4,000 mm weldment can twist by millimetres over a single roughing pass.

The symptoms are familiar. A bore measures 25.001 mm on the CMM and 25.018 mm after 48 hours. A flat plate rocks 0.05 mm on a surface plate. Cracks appear near a weld toe after a few hundred cycles. Fatigue life drops and corrosion starts earlier. None of this shows up in a first-article report taken straight off the machine.

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  Stock removal ratioRemoving more than 60% of the original section raises movement risk sharply.
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  Wall thicknessWalls under 1.5 mm distort during and after cutting.
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  Welded assembliesWeld toes concentrate stress and need relief before final machining.

Natural aging places the part in a yard or an open rack and lets time do the work. Stress relaxes by creep at ambient temperature. It needs more than a year to reach a 2–10% reduction, which is why almost nobody runs it in production. Its only real case is large castings that sit in storage anyway, such as machine beds or pump housings.

Thermal aging is the opposite approach. The part is heated to 500–650 °C, held long enough for the whole section to reach temperature, then cooled slowly. That range works for carbon and low-alloy steels. Aluminum gets a different window, usually 150–250 °C, because the alloy loses strength above that.

The trade-offs are real. Furnace time costs money, thin parts can sag on the rack, and the surface oxidizes, so you need a scale-removal step afterward. Hardened or precipitation-strengthened alloys may drop below their target hardness. For a casting that will be machined all over, that is acceptable. For a finished 17-4PH shaft at 40 HRC, it is not.

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  Hold timeRoughly 1 hour per 25 mm of section thickness, then furnace cool.
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  AtmosphereNitrogen or vacuum purge avoids oxide scale on finished surfaces.
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  Rack supportSupport long parts at multiple points so they cannot creep while hot.

Vibration aging clamps an exciter to the part and drives it through a sweep of frequencies. The part sees cyclic strain, and the stress peaks yield locally by tiny amounts. Reduction typically lands between 30% and 90%. A run takes 15–60 minutes. It works on a 4,000 mm weldment and on a 200 g aluminum bracket, which is why it is the default in most machine shops.

Sub-resonance aging is a gentler variant. The exciter stays below the first resonance of the part instead of sweeping through it. That avoids the loud, high-amplitude pass that worries operators of thin-walled parts. It uses less energy and is quieter, but it removes stress more slowly and only partially. It suits parts where full resonance could crack a weld or move a delicate feature.

Both methods leave the surface untouched, so they can run after finishing. Neither changes hardness or dimensions measurably. The limits are geometric: a part with no good mounting point for the exciter, or one with a natural frequency below the exciter range, is a poor fit. Log the sweep, the amplitude and the run time, because the record is what proves the process ran.

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  Sweep rateKeep the frequency change slow enough that the part responds at each step.
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  MountingClamp the exciter rigidly; a loose mount absorbs the energy you want in the part.
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  RecordSave the amplitude-versus-frequency trace with the part serial number.

Cold work stretching pulls the part past its yield point in tension, then releases it. The material yields in the high-stress regions and the locked-in stress redistributes. Aircraft stringers and long extrusions are the classic case, because a stretcher handles a 10 m section that no furnace will take. The catch is that you need a dedicated stretcher and a grip that will not mark the ends.

Ultrasonic impact treatment hammers the surface with a vibrating tool at 20–40 kHz. It introduces compressive stress in a shallow layer, typically 1–3 mm deep, and smooths the weld toe geometry at the same time. That combination is why it is used on weld repairs and on fatigue-critical joints. It does not relieve stress deep in the bulk of a thick section.

Both methods change the part in ways you have to plan for. Stretching alters the length, so leave machining allowance. Ultrasonic impact changes surface roughness and can leave a visible track pattern, so run it before final finishing or mask the cosmetic faces.

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  Stretch strainTypically 1–3% permanent elongation, applied in one controlled pull.
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  Impact depthCompressive layer of 1–3 mm; bulk stress is largely unchanged.
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  TimingDo impact treatment before the last finishing pass on visible surfaces.

Start with the constraint that cannot move. If the part must keep its hardness, thermal aging at 500–650 °C is out for hardened steel. If it must keep a cosmetic surface, skip anything that scales, marks or roughens the face you care about, which rules out thermal aging without a purge and pushes ultrasonic impact to before the last pass.

Next look at size. Vibration aging and cold work stretching scale to long parts. Furnaces do not. A 4,000 mm weldment goes on a vibration table or a stretcher, and that decision is made by the part envelope, not by the stress numbers.

Finally, ask what the part will do in service. A static bracket with generous walls may need no relief beyond good roughing practice. A weld toe on a part that sees 10 million cycles is a different problem, and ultrasonic impact earns its place there because it adds compressive stress exactly where cracks start. If you are unsure, send the drawing and we will tell you which method applies and which does not.

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  Hardness lockedAvoid thermal aging above the tempering temperature.
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  Cosmetic faceChoose vibration or impact before final finishing.
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  Fatigue-critical weldUltrasonic impact targets the toe where cracks initiate.