Stress Relief for Invar Machined Parts: When and How Shops Do It

By Invar Machining editorial team· Updated September 26, 2026 · How we write these guides

Invar parts that skip stress relief can look perfect off the machine and then drift out of tolerance days or weeks later. The cause is residual stress and cold work left behind by roughing. The fix is a furnace cycle, typically 315 to 345 °C for 1 to 2 hours, that shops build into the machining sequence between roughing and finishing rather than treating as an afterthought.

Why Invar moves after machining

Roughing removes most of a part’s stock, and every cut leaves residual stress in the material near the surface along with a thin layer of cold-worked metal. In most alloys that stress relaxes slowly enough that it never shows up as a real problem. Invar is different for two reasons.

First, the whole reason to specify Invar 36 is that the part has to hold its shape and size to a degree other alloys cannot match. A part that moves 0.02 mm after machining is a rounding error in a stainless bracket. In an Invar mirror mount it can be the entire tolerance band.

Second, cold work changes the alloy itself. The low expansion of Invar 36 comes from its magnetic ordering, and plastic deformation near a machined surface disturbs that ordering enough to raise the local CTE above the annealed value of 1.2 to 1.6 ppm/°C on the mill cert. So an unrelieved part is not only stressed, it is also slightly less Invar than the material you paid for. The Wikipedia article on Invar explains the mechanism, and Invar work hardening covers how machining causes it.

The movement is not random. The part relaxes toward its unstressed shape as the roughing stress releases, whether that happens over weeks on your bench or in a few hours in a furnace.

Typical cycles and what each one does

A stress relief cycle heats the part to a temperature well below the alloy’s transformation range, holds it there long enough for internal stress to relax, then cools it slowly and evenly. The point is not to change the material’s properties. It is to let the stress that roughing put in come out under controlled, even conditions instead of releasing unevenly over time in the field.

Even heating and cooling matter as much as the target temperature. A part that heats unevenly, or rests in the furnace on a point that constrains it, can come out with new stress instead of less. Shops support flat parts on a plate or evenly spaced blocks and stand rings on their ends.

Two families of cycle are in common use for Invar 36. The table gives typical parameters. Every number is typical, drawn from AMS 2454 style cycles and common shop practice, and the drawing, the material spec or the customer’s own procedure governs.

Cycle	Typical steps	What it does to stress	What it does to CTE and long-term stability	Typical use
Stress relief only	315 to 345 °C, 1 to 2 hours, slow cool in the furnace or in still air	Relaxes most roughing stress	Brings surface CTE back close to the annealed value. Good stability for months to a few years	Most precision mounts, tooling faces, fixtures
Full anneal, quench, temper and stabilize	Anneal about 830 °C, water quench, temper 315 °C for about 1 hour, then stabilize at about 95 °C for 24 to 48 hours	Removes all prior stress and cold work, then relieves quench stress	Resets the structure, sets the lowest and most repeatable CTE, settles the part for years	Optical benches, reference frames, space and lithography parts

A few points on reading the table:

• The stress relief only cycle is what most drawings mean by “stress relieve after rough machining”. It is cheaper, it fits in a furnace load with other work and it is enough for a part that has to hold a tolerance through normal handling and a modest temperature band.
• The full cycle is a material treatment, not just a stress treatment. The water quench fixes the low CTE, and the long soak near 95 °C stops the part creeping afterward. It is usually run once on the rough machined blank.
• Neither cycle helps if the finishing cut afterward puts a fresh cold-worked layer back in. Light final passes with sharp tools are part of the treatment.

The full cycle and its variants under ASTM F1684 are covered in Invar 36 heat treatment to ASTM F1684. Definitions of anneal, temper and stabilize are in the glossary.

Where it sits in the machining sequence

Most shops that cut Invar regularly follow the same basic sequence, explained in more depth in the Invar machining buyer’s guide:

1. Rough. Remove the bulk of the stock, leaving a consistent allowance on every face that matters, often 0.5 to 1 mm.
2. Stress relieve. Heat the part, hold it, cool it slowly. This is where the bulk of the roughing stress comes out.
3. Finish. Light cuts with sharp tools bring the part to final size on material that has already stopped moving.
4. Stabilize and inspect. For the most demanding optical or metrology parts a further stabilization cycle follows, and the part is measured only after it has soaked to 20 °C.

Doing the stress relief before finishing, not after, is the detail that matters most. Finish first and the part moves during the cycle with no second pass to correct it.

Worked example: a hypothetical ring finished before and after relief

Take a hypothetical Invar 36 lens cell, 200 mm outside diameter, 12 mm wall, with a bore toleranced to ±0.01 mm and a mounting face flat to 0.01 mm. Two shops machine it.

Shop A roughs the ring, leaves 0.8 mm on the bore and the face, stress relieves at 330 °C for 2 hours and then finishes. During the furnace cycle the bore goes out of round by 0.04 mm and the face bows by 0.03 mm. None of that matters, because the finishing pass removes 0.8 mm and cuts the bore and the face true on a part that has stopped moving. The delivered part measures inside tolerance at 20 °C and stays there.

Shop B finishes the ring to size in one setup, then sends it for the same stress relief because the drawing asks for it. The same movement happens, but now to the finished surfaces. The bore is four times outside its ±0.01 mm tolerance, there is no stock left to re-cut and the part is scrap.

Both shops ran the same furnace cycle. The numbers are illustrative and the actual movement depends on geometry and stock removal, but the shape of the outcome is typical. The sequence position is worth more than the cycle parameters.

What this costs and why it is worth asking for

A stress relief cycle adds a furnace run, handling in and out of the furnace and a second machine setup for finishing. It is one of the larger line items on an Invar quote, covered in why Invar 36 parts are costly. Shops batch parts into a single furnace load where they can, since a run for ten parts costs little more than a run for one.

Skipping stress relief on a part that does not need it is a legitimate way to save money, and a shop with Invar experience can tell you when that applies. The risk is skipping it on a part that does need it, because the failure is not visible at inspection. The part passes CMM the day it ships and moves out of tolerance in the customer’s hands. How that drift shows up over years is in long-term dimensional stability of Invar.

How to write the heat treatment note on a drawing

The note should say what cycle, where in the sequence and what inspection follows. Three typical forms, from least to most prescriptive:

• Shop to recommend. “Stress relieve after rough machining, cycle per shop practice for Invar 36, record furnace chart.” Use this when you know the part is thermally critical but have no procedure of your own. A shop that runs Invar can match the cycle to the tolerance.
• Stress relief specified. “After rough machining with 0.5 mm minimum stock on all toleranced features, stress relieve at 315 to 345 °C for 1 to 2 hours, furnace cool. Finish machine after stress relief. Inspect toleranced features at 20 ± 1 °C after a minimum 4 hour soak.”
• Full cycle specified. “Heat treat rough machined blank per [customer procedure or AMS 2454 style cycle]: anneal, water quench, temper, stabilize. Finish machine after heat treatment. No cold working of toleranced surfaces after stabilization. Supply furnace charts with the parts.”

Whichever form you use, state the stock allowance before the furnace, state that finishing comes after and state the inspection temperature. Those three lines turn a heat treatment callout into a machining sequence. If the part is thin or asymmetric, add “support flat during heat treatment”. Do not write a cycle you have no way to verify unless a program document requires it. A shop that has run the alloy will usually propose a closer fit than a number copied from a handbook.

What a shop needs to have or arrange

Stress relief for Invar does not need exotic equipment, but it does need the right pieces in place. The table lists what a typical precision job requires and whether a shop usually has it in house or arranges it.

Item	Why it matters	Typically in house or arranged
Furnace with even temperature and a chart recorder	A 315 to 345 °C hold with uneven heating puts stress back in. The chart is your evidence	Small shops arrange it, larger shops own one
Controlled cooling	Slow, even cooling is part of the cycle. Pulling the part hot undoes the work	Same furnace, or a still-air rack
Water quench tank and higher temperature furnace	Needed only for the full anneal, quench, temper cycle at about 830 °C	Usually a heat treater
Low temperature oven for the 95 °C stabilize	24 to 48 hours of hold. Ties up equipment, so it is often batched	Either, often a lab oven in house
Flat support plates and racks	Parts must be supported evenly so they do not sag or bend during the hold	In house
Low-stress fixturing for finishing	The second setup must not clamp new stress into a relieved part	In house
CMM in a 20 °C room	Verifies the tight features after soak. Without it the sequence cannot be checked	In house for precision shops, otherwise arranged

When a shop arranges the furnace work outside, ask who owns the cycle record and how the part is packed for transport. A relieved part that is dropped or clamped hard on the way back has been partly undone. How a good shop documents this is in Invar CMM inspection reports.

What to put on the RFQ

If your part is thermally critical, say so explicitly rather than assuming the shop will infer it from the alloy callout. Include:

• Whether the drawing calls for stress relief only, a full stabilization cycle or “shop to recommend”
• The stock allowance you expect before the furnace, or ask the shop to state it
• Any features where post-machining movement would be unacceptable
• The inspection temperature and soak, 20 °C after several hours for tight features
• Whether you need furnace charts with the parts

Send your drawing through our RFQ form and note the tolerance and any thermal requirements. We match you with shops that already run this sequence, so the quote you get includes the step your part needs.

For how stress relief fits with testing, aging and joint design, read our guide to Invar dimensional stability.

See also: Invar stress relief and stabilization cycles, published cycles compared side by side.

Frequently asked questions

Reviewed against

• ASTM F1684, Standard Specification for Iron-Nickel and Iron-Nickel-Cobalt Alloys for Low Thermal Expansion Applications
• Wikipedia, Invar

Values in this guide are typical handbook figures for planning. The mill certificate for your heat lot and the shop's quote for your geometry govern.

Keep reading

• Heat Treating Invar 36: What Each Step Does and How to Specify It · Process
• Invar Machining: What Buyers Need to Know Before Sending a Drawing · Materials
• Long-Term Dimensional Stability: Does Invar Age After Machining? · Materials
• Invar Machining Tolerances: What a Shop Can Hold and What It Costs · Process

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