ASTM F1684 Explained: The Specification Behind Invar 36 Stock

By Invar Machining editorial team· Published October 13, 2026 · How we write these guides

Most buyers meet ASTM F1684 as a line on a material certificate: “Invar 36 per ASTM F1684”. It is easy to nod at that line and move on. It is worth understanding what the line promises, because it defines what the supplier is bound to deliver and, just as important, what it leaves for you to specify. This guide covers the standard from a buyer’s point of view and does not reproduce it. Always read the current edition from ASTM before you write a purchase order.

What ASTM F1684 is

ASTM F1684 is a specification published by ASTM International for iron-nickel and iron-nickel-cobalt alloys used in low thermal expansion applications. It belongs to the F series, which covers materials for electronic devices and similar uses. The best known alloy it covers is the 36 percent nickel grade called Invar 36. The standard also covers related low-expansion alloys, such as the cobalt-bearing Super Invar type.

A material specification is a contract document. It lists what the producer must meet and how it is checked. When a purchase order cites it, the supplier’s certificate can be held against it.

What the standard typically controls

A specification of this type covers a short list of items. The exact limits come from the current edition, and the table below shows the type of control rather than the numbers.

Control	What it means for the buyer
Chemical composition	Nickel within a narrow band, limits on carbon, manganese, silicon, phosphorus, sulfur and other elements
Thermal expansion	A required mean CTE over a stated temperature range, measured by a stated method
Product forms	Bar, rod, wire, plate, sheet or strip, each with its own manufacturing notes
Condition	The supply condition, typically annealed, and any permitted variations
Dimensions and tolerances	Standard tolerances on thickness, width and diameter for each form, or reference to another standard
Surface and workmanship	Freedom from harmful defects, surface condition
Testing and certification	Which tests are run, how often and what the certificate must show
Marking and packaging	How the material is identified and shipped

Table shows categories only. Read the current edition for the actual limits.

The two controls that matter most for precision work are chemistry and CTE. Nickel content decides the expansion curve, since a point of nickel moves the CTE by a noticeable amount near the Invar minimum. The standard keeps nickel in a band narrow enough that the CTE stays low and consistent from heat to heat.

What it does not control

The standard describes the stock, not your part. Several things that matter for stability sit outside it.

• Heat treatment of the finished part. Stress relief and stabilization after machining are set by your drawing or a process specification. They are covered in Invar 36 heat treatment and stabilization.
• Residual stress from machining. Cutting adds stress that no mill standard can predict.
• Long term drift. A part cut from compliant bar can still change size over months if it was not stabilized, as described in long term dimensional stability of Invar.
• Part geometry. A thin or slender part moves more than a thick block, regardless of the stock.
• Your specific CTE range. If you care about the CTE between 20 and 40 °C, the standard’s mean over a wider range may not tell you enough.

A buyer who treats “meets F1684” as “will stay stable” is leaving the more important half of the job unspecified.

How to cite it on a purchase order

A clear material line removes room for argument. Include these items:

1. Standard and edition, for example “ASTM F1684, edition year as in current catalogue”.
2. Alloy designation, using the name or UNS number the standard gives for the grade.
3. Product form and size, with dimensions and the tolerance class.
4. Condition, for example annealed, as required by the standard.
5. Certification, a mill test report showing chemistry and CTE with the heat number.
6. Traceability, so that each piece or bundle carries its heat number into the shop.
7. Any extra requirement, such as CTE tested over a narrower range or on a sample of your lot.

If the part will see a seal or a critical temperature range, extra testing is worth adding at the material stage. Testing a coupon at the mill or in an outside laboratory costs little compared with the machining time on a large part. How CTE is measured is explained in measuring the CTE of Invar with a dilatometer.

Reading the certificate

A mill test report shows what the mill measured on the heat, not what you will see in the part. Check that it covers the items listed in the table above.

Line on the certificate	What to look for
Heat number	Matches the markings on the bar or sheet
Chemistry	Each element listed, within the limits of the standard
Mean CTE	Range stated, method stated, value within the limit
Condition	Annealed or as ordered
Standard and edition	Matches the purchase order
Lab or mill identification	Producer name, date and a signature or stamp

A certificate with a CTE line but no range is as incomplete as a drawing without a tolerance. Ask the supplier to fix it, and ask for the test method. Invar material certification gives a longer list of what to check and what to do when something is missing.

Hypothetical worked example

A buyer needs a 250 mm Invar 36 reference bar. The purchase order says “Invar 36, ASTM F1684, annealed bar 40 mm diameter, MTR with chemistry and mean CTE 20 to 100 °C.” The numbers below are hypothetical.

• The certificate shows nickel at 36.1 percent, inside the band the standard allows, and the other elements well below their limits.
• It shows a mean CTE of 1.4 ×10⁻⁶ per K over 20 to 100 °C and cites the test method.
• The stock arrives with the heat number stamped on each bar.
• The shop machines the bar, stress relieves it after roughing and stabilizes it at the end. The drawing carries these steps, because the standard does not.

If the buyer had written only “Invar 36”, the shop could have supplied bar made to a different specification or to a mill standard with looser limits. That might still be fine, but it would be the shop’s call and not the buyer’s.

Product forms and what each means for machining

The form you order affects the part as much as the alloy. A standard like this one applies to several forms, but each one carries its own grain structure and stress state.

Form	Typical use	Machining note
Round bar	Shafts, pins, small housings, turned parts	Stress along the axis, can bow after rough turning
Flat bar and plate	Frames, bases, optical benches	Rolling stress, may warp when one face is removed
Sheet and strip	Shims, thin plates, etched parts	Thin and stressed, flatten after etching or cutting
Wire	Fasteners, springs, small components	Cold drawn, often needs an anneal before machining

A plate that is cut into a frame by removing most of its volume can twist as the stress balance shifts. Buying plate that is stress relieved or precision ground, and leaving enough stock for the shop to relieve again after roughing, is cheaper than fixing a warped part. The effect on the supply condition is described in Invar stock bar, plate and sheet. Where possible, order stock close to the finished size to save machining time and material, but not so close that no stock is left after the last stress relief.

Why the edition year matters

Standards are revised. A new edition can change a chemistry limit, add a product form, change the test method or reword the certification clause. A certificate that cites an older edition is not wrong, but it may not match what your quality system expects.

Three practical steps:

1. Write the edition year on every purchase order and drawing, not just the standard number.
2. If a customer drawing cites an edition, use that one even when a newer one exists, unless the customer agrees to change.
3. Keep a copy of the edition you cite in your files, since the standard itself is copyrighted and cannot be reproduced from memory.

A buyer who inherits a drawing with no edition should ask the engineer who wrote it. A wrong guess can mean rejecting good material or accepting material to an outdated limit.

Testing frequency and sample location

A mill certificate is usually based on a test of the heat or the lot, not of every bar. Chemistry comes from the ladle or a sample of the heat. CTE is measured on a sample taken from a representative piece. If your part is large or critical, the sample may not tell you about the exact bar that you machine.

Options for tighter control:

• Ask for a CTE test on a sample taken from your lot, at the mill or at an independent laboratory
• Ask for sample location to be stated, for example from the bar end opposite the heat marking
• Retain an uncut piece from the bar for later checks if the part fails in service
• Run a second CTE check on a coupon after your own heat treatment, which confirms the condition of the finished part, not only the stock

Each step adds cost and time, so match the effort to the risk. A general-purpose shim does not need a dilatometer test. A reference standard or an optical bench for a space instrument might.

Related standards and sources of confusion

Several documents sit near F1684 and are sometimes mixed up with it:

• ASTM F15 covers the iron-nickel-cobalt sealing alloy known as Kovar. See ASTM F15 for Kovar.
• ASTM F30 covers iron-nickel sealing alloys such as Alloy 42.
• Trade names such as Invar, Nilo 36 or FeNi36 are brand or national names for the same alloy. A table of equivalents is in Invar trade names and specifications.
• Company standards from mills and OEMs can be tighter or looser than ASTM, so a drawing that cites one should say where the buyer can obtain it.

When two specifications differ, the order of precedence should be written on the drawing, usually with the more specific requirement first.

Checklist before you place a material order

• ASTM F1684 named with the edition year
• Alloy, form, size and condition written out
• Certificate content listed, with CTE range and method
• Heat number traceability agreed
• Heat treatment of the finished part kept on the drawing, not assumed from the stock

Send the drawing and the material line through the RFQ form and we match you with shops that buy certified Invar stock and keep heat traceability through machining. For handbook values see the Invar reference data and the Invar 36 machining page. For sourcing routes see the Invar material sourcing guide.

See also: ASTM F1684 for Invar 36 and Super Invar, scope, UNS numbers and its 2024 withdrawal.

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 Mill Certifications: What a Certified Material Report Should Show · Choosing a Supplier
• Invar 36 by Other Names: Regional Trade Names and Specs · Materials
• Sourcing Invar 36, Kovar and Alloy 42: Mills, Distributors and What Your Shop Already Has · Choosing a Supplier

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