Invar specs reference: ASTM, UNS, composition and typical properties
The four low-expansion alloys shops quote under the Invar name, on one page. Use it to write the material line on a drawing, to check a mill cert or to decode a trade name a supplier used. All property values are typical handbook figures. The mill certificate for your heat lot governs.
Alloy pages: Invar 36 machining, Super Invar machining, Kovar machining, Alloy 42 machining.
Specifications and names
What to write on the drawing and what a supplier might call the same material.
| Alloy | System | UNS | ASTM | Other references | Trade names |
|---|---|---|---|---|---|
| Invar 36 | Fe-36Ni | K93600 (std), K93601 (free machining), K93603 | ASTM F1684 | AMS 2454 (heat treatment), EN 1.3912, JIS equivalent grades | Invar 36, NILO 36, Pernifer 36, Nilvar |
| Super Invar | Fe-32Ni-5Co | K93500 (typical designation) | ASTM F1684 (Fe-Ni-Co grade) | Supplier data sheets | Super Invar, Super Invar 32-5 |
| Kovar | Fe-29Ni-17Co | K94610 | ASTM F15 | EN 1.3981, DIN 17745, MIL-I-23011 class 1 (historic) | Kovar, NILO K, Pernifer 29/18, Dilver P, Rodar |
| Alloy 42 | Fe-42Ni | K94100 | ASTM F30 | EN 1.3917, MIL-I-23011 class 5 (historic) | Alloy 42, NILO 42, Pernifer 42, Glass Sealing 42 |
Trade names belong to their owners and are listed so a buyer can recognise them. Historic MIL specs are shown because they still appear on old drawings. Full story in Invar 36 by other names.
Composition
Typical limits in weight percent. The governing spec sets the exact range.
| Alloy | Ni | Co | Mn max | Si max | C max | Fe |
|---|---|---|---|---|---|---|
| Invar 36 | 35 to 37 | 0.5 max | 0.6 max | 0.35 max | 0.10 max | balance |
| Super Invar | 31 to 33 | 4 to 6 | 0.5 max | 0.3 max | 0.05 max | balance |
| Kovar | 29 | 17 | 0.5 max | 0.2 max | 0.04 max | balance |
| Alloy 42 | 41 to 43 | 0.5 max | 0.8 max | 0.3 max | 0.05 max | balance |
Typical properties
Annealed condition near room temperature. Machinability relative to B1112 free cutting steel.
| Alloy | CTE, ppm/°C | Over | Density, g/cm3 | Curie, °C | Hardness, HB | Machinability |
|---|---|---|---|---|---|---|
| Invar 36 | 1.2 to 1.6 | 20 to 100 °C | 8.05 | ~230 | 130 to 160 | 40 to 50 % |
| Super Invar | 0.3 to 0.6 | 20 to 90 °C | 8.1 | ~230 | 130 to 160 | 40 to 50 % |
| Kovar | 5.0 to 5.5 | 30 to 200 °C | 8.36 | ~435 | 150 to 170 | 45 to 55 % |
| Alloy 42 | 4.0 to 6.0 | 30 to 300 °C | 8.12 | ~380 | 140 to 170 | 45 to 55 % |
Typical mean CTE by temperature band
ppm/°C. The number to use is the mean over your operating range, not the room temperature figure. Read Invar 36 CTE vs temperature for how to apply it.
| Range | Invar 36 | Super Invar | Kovar | Alloy 42 |
|---|---|---|---|---|
| −200 to −100 °C | 1.5 to 2.0 | not recommended | 4.0 to 4.5 | 3.5 to 4.5 |
| −100 to 0 °C | 1.2 to 1.5 | 0.6 to 1.0 | 4.5 to 5.0 | 4.0 to 4.5 |
| 0 to 100 °C | 1.2 to 1.6 | 0.3 to 0.6 | 5.0 to 5.3 | 4.5 to 5.0 |
| 100 to 200 °C | 2.5 to 5 | 1.5 to 6 | 5.3 to 5.5 | 5.0 to 5.5 |
| 200 to 300 °C | 6 to 11 | 8 to 12 | 5.5 to 6.0 | 5.5 to 6.5 |
Super Invar can transform below about −60 °C and lose its low expansion permanently. Invar 36 and Kovar rise sharply above their Curie temperatures. Values are typical, from handbook curves, and vary with heat treatment and cold work.
Typical heat treatment cycles
What the cycles are for and what a typical version looks like. The drawing, the spec or your process document governs. Details in the heat treatment guide.
| Alloy | Step | Typical cycle | Purpose |
|---|---|---|---|
| Invar 36 | Stress relief after roughing | 315 to 345 °C, 1 to 2 h, slow cool | Removes machining stress so the part does not move after finishing |
| Invar 36 | Full stabilization (AMS 2454 style) | Anneal 830 °C, water quench, 315 °C 1 h, 95 °C 24 to 48 h | Sets lowest CTE and long-term stability for optics and metrology parts |
| Super Invar | Stabilization | Anneal 830 °C, water quench, 315 °C 1 h, 95 °C 24 to 48 h | Required to reach and keep the 0.3 to 0.6 ppm/°C band |
| Kovar | Anneal and decarburize before sealing | 900 to 1100 °C in wet hydrogen, controlled cool | Clean, low-carbon, stress-free surface that glass wets |
| Alloy 42 | Anneal | 850 to 1000 °C in protective atmosphere | Softens for forming, relieves stress before sealing or plating |
Test methods you may see on a cert
- ASTM E228Linear thermal expansion by push-rod dilatometer. The usual way measured CTE for a heat lot is reported.
- ASTM E831Linear thermal expansion by thermomechanical analysis. Small samples, common in labs.
- ISO 1Sets 20 °C as the reference temperature for dimensional measurement. The reason precision Invar parts are soaked and inspected at 20 °C.
- ASME Y14.5Geometric dimensioning and tolerancing. Datums, flatness and position callouts on the drawing follow it.
- Mill test reportChemistry per heat lot, sometimes hardness and grain size. CTE only if it was ordered.
Questions about specs
Which spec covers Invar 36 bar and plate?
ASTM F1684 is the usual specification in North America for iron-nickel and iron-nickel-cobalt low expansion alloys in bar, plate, sheet and strip. UNS K93600 is the standard grade and K93601 the free machining grade. European drawings often cite 1.3912.
What is the difference between ASTM F15 and ASTM F30?
ASTM F15 covers the iron-nickel-cobalt sealing alloy known as Kovar, UNS K94610. ASTM F30 covers iron-nickel sealing alloys such as Alloy 42, UNS K94100. Both are sealing alloy specs with chemistry, expansion and test requirements.
Are these CTE values guaranteed?
No. They are typical handbook values for planning. The expansion of a real part depends on the heat lot, the heat treatment and the cold work it has seen. If a design depends on a number, ask for measured CTE data for the heat lot per ASTM E228 on the RFQ.
Which alloy should I specify?
Invar 36 for the lowest expansion over a wide range including cryogenic use. Super Invar when Invar 36 is still too much and the part lives between about 0 and 90 °C. Kovar or Alloy 42 when the part must seal to glass or ceramic, matched to the glass expansion.