Heat Treating Invar 36: What Each Step Does and How to Specify It

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

Heat treating Invar 36 is what turns a low expansion alloy into a dimensionally stable part. A typical full cycle has four steps: anneal at about 830 °C, water quench, stress relieve at about 315 °C for 1 h and stabilize at about 95 °C for 24 to 48 h. Most machined parts do not need all four. What they do need is a stress relief between roughing and finishing plus a drawing that says which condition is required. ASTM F1684 covers the alloy; the drawing has to cover the condition.

Why Invar 36 needs heat treatment at all

The low expansion of Invar 36 comes from the magnetic state of the iron nickel lattice, and that state is sensitive to two things a machine shop controls: residual stress and cold work. A bar that arrives annealed is close to its ideal state. Roughing puts stress and a cold worked layer back in. Left alone, that stress relaxes over days or weeks and the part moves, and the cold worked surface expands more than the annealed core. Both effects are covered in how the CTE of Invar 36 changes with condition.

Heat treatment resets the material. The anneal removes cold work and homogenizes the structure. The quench locks in the low expansion state. The stress relief lets stress out under controlled conditions. The stabilization ages out the slow structural drift that would otherwise show up as creep in service. A part that gets the right subset of these steps, in the right order relative to the machining, holds its size. One that does not can pass inspection on the day and drift afterwards, as explained in long term dimensional stability of Invar.

The four steps and what each does

Anneal

The anneal is a solution treatment at typically 800 to 850 °C, with 830 °C a common target, held long enough for the section to reach temperature, often quoted as 30 min to 1 h per 25 mm of thickness. It dissolves carbides, removes the effect of prior cold work and gives a uniform structure. An air anneal scales the surface, so shops use a protective or vacuum atmosphere and leave stock because the part will move.

Quench

Water quenching from the anneal freezes the disordered state that gives the lowest CTE. Slow cooling from the anneal produces a slightly higher CTE, typically a few tenths of a ppm/°C. Some drawings accept that for parts where distortion in the quench is the bigger risk. The quench also puts stress back into the part, which is why a stress relief always follows it.

Stress relief

The stress relief, sometimes called a temper in older specifications, is a hold at typically 315 to 345 °C for 1 to 2 h followed by slow cooling, often in the furnace. It is well below the Curie temperature and far below the anneal, so it does not change the structure. It lets the stress from quenching, from roughing or from both relax while the part is unconstrained in the furnace, rather than on your bench. This is the step almost every precision Invar part gets, and it is the one shops schedule between roughing and finishing. The guide to stress relief in the machining sequence covers the shop floor side.

Stabilize

Stabilization is a long low hold, typically 95 °C for 24 to 48 h, done after the stress relief and usually after finish machining. It accelerates the slow structural change that Invar 36 undergoes at room temperature for months after quenching, so that the change happens in the furnace instead of in service. Optical benches, laser resonators and metrology frames commonly specify it. Brackets and tooling that only see a few degrees of swing often do not.

Typical cycles and where they are used

The table lists typical cycles. The drawing, the referenced process specification and the shop’s quote govern; these numbers are for orientation.

Cycle	Typical steps	Typical use
Intermediate stress relief only	315 to 345 °C, 1 to 2 h, slow cool, between roughing and finishing	Most machined parts from annealed bar or plate
Stress relief plus stabilize	As above, then 95 °C for 24 to 48 h after finishing	Optical mounts, metrology parts, reference frames
Full cycle, AMS 2454 style	Anneal about 830 °C, water quench, 315 °C 1 h, 95 °C 24 to 48 h	Aerospace and space programs, parts from stock of unknown history, lowest CTE
Anneal and slow cool	About 830 °C, furnace cool, then stress relief	Complex shapes where quench distortion is the larger risk

Times are hold times at temperature, not total furnace time; ramping a thick plate up and down adds hours. The full cycle is applied to the roughed part, not the bar, because the anneal and quench distort the workpiece and the finishing passes remove that distortion.

Where heat treatment fits in the machining sequence

The sequence that shops use for a precision Invar part looks like this:

1. Rough all features, leaving a typical 0.2 to 0.5 mm of stock on critical faces and bores, more if a full anneal and quench is called out.
2. Heat treat to the specified condition. For most parts that is the intermediate stress relief; for others it is the full cycle.
3. Finish with light cuts, sharp positive rake tools and flood coolant, so the finishing pass adds as little new stress as possible.
4. Stabilize at about 95 °C if the drawing requires it. Because the temperature is low and the hold is unconstrained, movement here is very small and the part is normally already at final size.
5. Inspect after the part has soaked at 20 °C.

Step 2 sits before step 3 because the part moves during any heat treatment. Roughing first, then relieving, then finishing means the shop cuts away the movement instead of shipping it.

Some drawings call for two stress reliefs, one after roughing and a second after semi finishing, on very thin or very tight parts. It costs another furnace run and another setup.

Why the drawing must state the condition

ASTM F1684 tells the mill what to ship and tells the shop what alloy it is receiving. It does not tell the shop what to do after roughing. If the drawing says only “Invar 36 per ASTM F1684”, three shops can deliver three parts: one with no heat treatment, one with a stress relief and one with the full cycle. All three meet the material specification. Only one of them behaves the way the designer expected.

The reference page of Invar specifications lists the standards and UNS numbers. What the drawing itself needs is:

• The alloy and material specification, for example Invar 36 to ASTM F1684, UNS K93600 or K93603 as your program requires.
• The condition required: either stress relieved only or the full anneal, quench, stress relieve and stabilize cycle.
• Where in the sequence the stress relief happens: after rough machining, before finish machining.
• Whether a heat treat certificate is required.

How to write the drawing note

A note that a shop and a heat treater can act on, written here as an example:

HEAT TREAT PER [PROCESS SPEC OR DRAWING NOTE]: ANNEAL 830 °C, WATER QUENCH. STRESS RELIEVE 315 °C, 1 H MIN, SLOW COOL, AFTER ROUGH MACHINING AND BEFORE FINISH MACHINING. STABILIZE 95 °C, 48 H MIN, AFTER FINISH MACHINING. FURNISH HEAT TREAT CERTIFICATION WITH FURNACE CHART.

For a part that only needs the intermediate step:

STRESS RELIEVE 315 TO 345 °C, 1 TO 2 H, SLOW COOL, AFTER ROUGH MACHINING AND BEFORE FINISH MACHINING. FURNISH HEAT TREAT CERTIFICATION.

If you are not sure which cycle the part needs, “STRESS RELIEVE BETWEEN ROUGH AND FINISH MACHINING, SHOP TO RECOMMEND CYCLE, OPERATING RANGE 10 TO 40 °C” is better than silence. It tells the shop the intent and the range. A shop that has done this before can then propose the cycle. State the operating range in every case; without it a shop cannot judge whether stabilization is worth the cost.

What a heat treat certificate should show

A certificate is the only evidence you will have that the cycle happened. It should include:

Item	Why it matters
Part number, drawing revision, quantity and heat lot	Ties the certificate to your parts and your mill cert
Specification or drawing note followed	Shows the heat treater worked to your requirement, not a default
Each step with set temperature, actual temperature, hold time and atmosphere	Confirms the cycle rather than just naming it
Quench medium and delay from furnace to quench	A slow transfer changes the result
Furnace chart or data logger trace with thermocouple location	The primary record; a load thermocouple is more meaningful than a chamber reading
Furnace identification and survey or calibration date	Shows the furnace reads true
Date, operator and signature	Traceability

These items are typical; your own quality system governs what you require. Ask for the chart, not just the summary. A summary says 315 °C for 1 h; the chart shows whether the load actually reached 315 °C and for how long. Our guide to Invar material certification covers how the heat treat certificate sits alongside the mill cert and the inspection report.

Worked example: scheduling a stabilized optical bench

Take a hypothetical 300 mm by 200 mm by 25 mm Invar 36 optical bench plate with flatness of 0.01 mm on the mounting face and a dozen ±0.01 mm bores, to be used at 15 to 30 °C in a laser system. The drawing calls for stress relief between roughing and finishing and stabilization at 95 °C for 48 h after finishing.

Step	Typical duration	Note
Saw plate, rough both faces and bores, leave 0.3 mm	1 day	Plate to ASTM F1684 arrives annealed
Stress relieve 330 °C 1.5 h, furnace cool	1 day including ramp and cool	Batched with other parts if the schedule allows
Finish faces and bores, light passes	1 day	Second fixture, low clamping force
Stabilize 95 °C 48 h	2 to 3 days	Part is already at size
Soak at 20 °C, CMM inspection	1 day	Flatness and bore positions measured after soak

The illustrative total is about a week of process time before queue time at the shop or the heat treater. Skipping the stabilization would save two or three days and one furnace run. For a bracket that is a sensible saving. For a laser bench that has to hold cavity alignment for years, the 48 h in the furnace is the cheapest part of the build. These durations are typical and illustrative; the shop’s quote governs.

What to put on the RFQ

The heat treatment is the line that changes most between quotes, so state it clearly:

• Alloy and spec. Invar 36 to ASTM F1684 with the UNS number if required.
• Condition and sequence. The drawing note above. Otherwise “shop to recommend” with the operating range.
• Certificate. Whether you need the heat treat certificate and furnace chart.
• Lead time. Furnace runs are batched, so a flexible date often means a lower price.

When you send the drawing through the RFQ form, we match it with up to three shops that cut Invar and can run the anneal, stress relief and stabilization the part calls for, either in house or through a heat treater they use regularly. Each shop quotes the same sequence, so the numbers you get back can be compared line by line.

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

• Stress Relief for Invar Machined Parts: When and How Shops Do It · Process
• How the CTE of Invar 36 Changes With Temperature and Condition · Materials
• Long-Term Dimensional Stability: Does Invar Age After Machining? · Materials
• Invar Mill Certifications: What a Certified Material Report Should Show · Choosing a Supplier

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