How the CTE of Invar 36 Changes With Temperature and Condition

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

The CTE of Invar 36 is not a single number. Annealed material expands about 1.2 to 1.6 ppm/°C between 20 and 100 °C, which is the value most datasheets quote. Above about 150 °C the mean value climbs, and once the alloy passes its Curie temperature near 230 °C it expands like an ordinary nickel steel. Below room temperature the expansion stays low all the way to cryogenic conditions. Heat treatment and cold work move the room temperature value as well, which is why the machining sequence matters as much as the alloy you order.

Why Invar 36 has a low CTE at all

Every metal expands when it warms because its atoms vibrate with more energy and sit slightly further apart. Invar 36, an iron alloy with 36 % nickel, adds a second effect that works in the opposite direction. Below its Curie temperature the alloy is ferromagnetic, and the magnetic ordering holds the lattice slightly expanded. As the temperature rises the magnetic order weakens, the lattice tries to shrink and that shrinkage cancels most of the normal thermal growth. The result is the near zero band engineers rely on. The Wikipedia entry on Invar covers the history and the physics in more depth.

Two consequences follow directly from this mechanism:

• The cancellation only works while the alloy is magnetic. Approach the Curie point and the low expansion fades.
• Anything that disturbs the magnetic and structural state of the surface, such as cold work or residual stress, disturbs the cancellation and raises the local CTE.

That is the whole story of Invar 36 machining in one paragraph. The rest of this guide puts numbers on it.

Typical mean CTE by temperature interval

Datasheets report mean CTE, the average slope of the expansion curve between two temperatures. The table lists typical mean values for annealed and stress relieved Invar 36. Values vary by heat lot and condition, so treat these as typical and let the mill certificate or your own dilatometer report govern.

Temperature interval	Typical mean CTE, ppm/°C	Note
-150 to 20 °C	about 1.0 to 2.0	Stays low, falls toward zero near cryogenic temperatures
20 to 100 °C	1.2 to 1.6	The datasheet value, annealed and quenched
20 to 150 °C	about 1.8 to 2.3	Slope starting to rise
20 to 200 °C	about 2.0 to 2.8	Well past the flat band
20 to 250 °C	about 3.5 to 4.5	Crossing the Curie temperature
20 to 300 °C	about 5 to 6	Mean pulled up by the high end
Above Curie, instantaneous	about 11 to 14	Similar to a plain nickel steel

Read the table as a curve, not as steps. Expansion is small and nearly linear up to roughly 100 °C, bends upward through 150 to 200 °C and then joins the normal metal line. The mean value over 20 to 300 °C looks moderate only because the first hundred degrees contribute almost nothing.

Instantaneous versus mean CTE

Instantaneous CTE is the slope of the curve at one temperature. Mean CTE is the chord between two temperatures. For a part that lives at 20 to 60 °C the two are close. For a part that cycles between 20 and 250 °C they are very different, and using the 20 to 100 °C datasheet value for that part will underestimate growth several times over. When you read a datasheet, check which one is quoted and over which interval. When you write a drawing, state the interval you care about. The glossary has short definitions of both terms.

What happens below room temperature

Invar 36 keeps its low expansion at sub zero temperatures, which makes it common in cryogenic optics, cold shields and liquefied gas containment. As the temperature falls the instantaneous CTE drops further and approaches zero near liquid nitrogen temperature. Mean values quoted from 20 °C down to -150 °C are typically in the 1 to 2 ppm/°C band, but published curves differ, so for a cryogenic instrument ask for dilatometer data over the real range rather than extrapolating room temperature numbers. Our guide to Invar in cryogenic and vacuum applications covers the practical side, including fixturing for parts that will be cooled hundreds of degrees.

One caution: Invar 36 is stable in this range, but Super Invar is not. Below roughly -60 °C Super Invar can partly transform and lose its low expansion. If your part sees deep cold, Invar 36 is often the safer choice. See when Invar 36 is not enough for the trade.

How heat treatment shifts the CTE

The same bar of Invar 36 can show a different room temperature CTE depending on how it was treated. Typical effects, in order of size:

Condition	Typical effect on 20 to 100 °C CTE	Why
Annealed about 830 °C and water quenched	Lowest value, about 1.2 to 1.4 ppm/°C	Quench locks in the disordered, low expansion state
Annealed and slow cooled	Slightly higher than quenched	Slow cooling allows some ordering
Quenched, then stress relieved 315 to 345 °C	Close to quenched value, more stable over time	Relieves quench stress without raising CTE much
Stabilized at about 95 °C for 24 to 48 h	Same CTE, better long term dimensional stability	Ages out slow structural change
Heavily cold worked, no anneal	Can exceed 2 ppm/°C in the worked layer	Strain disturbs the magnetic contribution

These are typical values. The drawing or the referenced specification sets the actual cycle, and the shop’s heat treat certificate records what was done. The full sequence and what each step does are covered in Invar 36 heat treatment: anneal, quench, stress relieve, stabilize.

The practical rule is simple. If the drawing does not state a condition, two shops can deliver two parts with different CTE and both will be within the alloy specification. State the condition.

How cold work from machining shifts the CTE

Cutting is cold work. A dull tool, a heavy rubbing pass or an aggressive clamp leaves a strained layer a few hundredths of a millimeter deep. In that layer the CTE can rise well above the annealed value, and the residual stress relaxes over days or weeks and moves the part.

Shops that cut Invar for optics manage this in three ways:

1. Rough the part, then stress relieve at a typical 315 to 345 °C for 1 to 2 h with a slow cool.
2. Finish with sharp positive rake tools, real chip loads and flood coolant so the tool cuts rather than rubs.
3. Inspect only after the part has soaked at 20 °C, the reference temperature in ISO 1.

Our guide to Invar work hardening explains the metallurgy behind step 2. A part machined without stress relief may pass inspection on the day and still drift out of tolerance a month later.

How to read a dilatometer report

If your program needs measured CTE rather than a handbook value, the test is normally a push rod dilatometer to ASTM E228. Small specimens are sometimes run on a thermomechanical analyzer to ASTM E831. A useful report shows:

• Specimen identity and condition. Heat lot plus the exact heat treatment the specimen received. A specimen annealed and quenched while your parts were only stress relieved does not represent your parts.
• Reference length at 20 °C and the specimen orientation relative to the bar or plate.
• The expansion curve, ΔL/L0 against temperature, over the full range tested, not just a summary.
• Mean CTE over stated intervals. Check that at least one interval matches your operating band.
• Heating rate and atmosphere. A slow rate gives a truer curve.
• Calibration reference, typically fused silica or a platinum standard, with its date.

Two things to check before you accept the number. First, the interval: a value quoted for 20 to 300 °C tells you little about a part used at 0 to 60 °C. Second, the direction: rolled plate can differ slightly between longitudinal and transverse, so for a large plate part ask for both. Our guide to Invar material certification covers what a mill cert includes and what a dilatometer test adds on top.

Worked example: an instrument used at 0 to 60 °C

Take a hypothetical optical instrument with a 400 mm Invar 36 spacer between two lens groups. The instrument is aligned at 20 °C in the lab and then used outdoors anywhere from 0 to 60 °C. The designer needs the worst case change in spacer length relative to the aligned state.

Step 1: pick the CTE. The operating band sits inside the flat 20 to 100 °C region, and the sub zero end is only 20 K below reference, so the datasheet mean of 1.2 to 1.6 ppm/°C applies. To be safe the designer uses 1.6 for a well stress relieved part and 2.0 as an allowance for a part finished without stress relief.

Step 2: find the largest excursion. From 20 °C the largest swing is +40 K to 60 °C.

Step 3: compute. Growth = length × CTE × ΔT.

Material of the 400 mm spacer, +40 K	Typical CTE, ppm/°C	Growth, mm
Invar 36, stress relieved	1.6	0.026
Invar 36, surface cold worked	2.0	0.032
316 stainless	16.5	0.264
6061 aluminum	23.5	0.376

Values in the table are typical and the calculation is illustrative. The spacer in Invar 36 moves about 0.026 mm across the whole operating range, which for many lens designs is within the depth of focus. The same spacer in aluminum moves fourteen times more and would need active compensation. The difference between the stress relieved and cold worked cases, 0.006 mm, is small in this example, but it comes with the drift over time that the cold worked part carries, which is the real reason to insist on the stress relief.

Step 4: write it down. The drawing states Invar 36 to ASTM F1684, annealed, quenched and stress relieved, operating range 0 to 60 °C, inspection at 20 °C. The RFQ asks whether the shop can supply a dilatometer report for the heat lot if the program later needs it. That single line of context lets a shop quote the right sequence.

What to put on the RFQ

A CTE requirement only means something if it comes with the interval, the condition and the inspection temperature. If the operating band runs above about 150 °C, first check the Invar 36 vs Kovar vs Super Invar comparison, because a matched expansion alloy may fit better. Then include:

• Alloy and spec. Invar 36 to ASTM F1684, with the UNS number from the Invar specifications reference if your program needs it.
• Condition. Annealed and quenched, stress relieved or stabilized. “Shop to recommend” with the operating range stated is also acceptable.
• Operating temperature range for the part, including the assembly temperature.
• Whether you need measured CTE data for the heat lot and over which interval, so the shop can arrange the dilatometer test up front.
• Inspection temperature, normally 20 °C, plus the soak time before measurement.

When you send an RFQ with the drawing, we match it with up to three shops that cut Invar and can run the stress relief and stabilization the part needs. Each shop quotes you directly, and each one sees the same operating range and condition, so the quotes and the parts line up.

Frequently asked questions

Reviewed against

• ASTM E228, Standard Test Method for Linear Thermal Expansion of Solid Materials With a Push-Rod Dilatometer
• 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
• Super Invar: The Lower CTE Option, Its Limits and How Shops Machine It · Materials
• Why Invar Work-Hardens and How Shops Control It During Machining · Materials
• Machining Invar for Cryogenic and Vacuum Applications · Materials

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