Invar is made by alloying iron with 36% nickel, then carefully controlling impurities, cooling rates, and heat treatment to achieve its signature property: near-zero thermal expansion. The process starts with melting high-purity raw materials in a vacuum induction furnace and ends with a multi-step stabilization treatment that locks in a coefficient of thermal expansion as low as 0.5 × 10⁻⁶/°C, roughly 20 times lower than ordinary steel.
Why Invar Barely Expands
Before diving into production, it helps to understand what makes Invar special. Most metals expand steadily as they warm up. Invar does too, but its magnetic structure fights back. As the alloy heats, its atoms naturally want to spread apart. At the same time, the magnetic interactions between neighboring iron and nickel atoms produce a counteracting force called volume magnetostriction, which effectively pulls the lattice inward. These two effects nearly cancel each other out between about -100°C and 100°C, giving the alloy its famously low expansion rate. The name “Invar” comes from “invariable.”
This balance only works at one specific composition window: 35 to 37% nickel, with iron making up the rest. Shift the nickel content by even a few percent and the magnetic compensation breaks down, causing expansion to climb sharply.
Composition and Purity Requirements
Under the ASTM F1684 standard, Invar 36 (UNS K93600) must contain 35.0 to 37.0% nickel with iron as the balance. Small amounts of other elements are permitted but tightly capped:
- Carbon: 0.1% maximum. Carbon is the most damaging impurity. Even small amounts cause dimensional instability over time, because carbon trapped in the crystal structure can precipitate as carbides during aging, triggering unwanted volume changes. Charles Édouard Guillaume, who discovered Invar, identified carbon as the primary culprit behind dimensional drift.
- Manganese: 0.6% maximum. A minimum somewhere between 0.44 and 0.76% is needed to suppress an expansion anomaly that occurs around 700°C during processing.
- Silicon: 0.35% maximum
- Phosphorus and sulfur: 0.025% maximum each
- Chromium, copper, molybdenum: 0.5% maximum each
Keeping impurities this low requires starting with high-purity nickel and electrolytic iron. Scrap-based melting is possible but demands careful sorting and analysis.
Melting and Casting
Invar is almost always melted in a vacuum induction furnace. Vacuum melting serves two purposes: it prevents oxidation of the melt, and it pulls dissolved gases (oxygen, nitrogen, hydrogen) out of the liquid metal, both of which would compromise dimensional stability. A typical laboratory or small-production setup uses a vacuum induction melting system operating at around 10⁻² mbar with a partial argon gas pressure of 100 mbar to shield the melt.
The charge of iron and nickel is loaded into a crucible, heated by a high-frequency induction coil until fully liquid, and held at temperature long enough for the composition to homogenize. For production-scale ingots, the melt may be homogenized at approximately 1200°C before casting.
Why Cooling Rate Matters
How quickly the molten alloy solidifies has a direct effect on the quality of the final product. Faster cooling (higher undercooling) produces a finer grain structure: the microstructure transitions from large, tree-like dendrites to small columnar grains and eventually to fine equiaxed grains as the cooling rate increases. More grains mean higher hardness, and faster solidification also reduces the size of shrinkage cavities at the center of the casting, producing denser, more uniform material. In short, controlled rapid cooling yields a better starting ingot with tighter microstructure and fewer internal voids.
Hot Working
After casting, the ingot is typically hot-rolled at around 900°C to break down the as-cast grain structure and shape the material into plate, bar, or strip. Hot working at this temperature keeps the alloy in its fully austenitic (face-centered cubic) phase, where it’s soft enough to deform without cracking. This step also helps close any residual porosity from casting and begins to develop a more uniform grain size throughout the cross-section.
Cold Rolling for Thin Strip
For applications requiring thin sheet or strip, Invar is cold-rolled at room temperature after hot working. Cold rolling dramatically increases strength but reduces ductility. At 90% cold rolling reduction, Invar 36 can reach a tensile strength of 820 MPa and a hardness of 283 HV, compared to roughly 490 MPa (71 ksi) in the annealed condition. The tradeoff is that the material becomes brittle and its thermal expansion behavior is disturbed by the internal stresses.
To recover usable ductility while keeping high strength, manufacturers follow cold rolling with a brief annealing step. One approach combines 50% or 90% cold rolling reduction with pulsed current annealing for just 40 seconds. This produces thin strips (0.1 to 0.5 mm thick) with tensile strengths of 600 to 650 MPa, yield strengths of 520 to 575 MPa, and fracture elongation around 5.8 to 5.9%, a useful balance of strength and formability.
Three-Step Heat Treatment for Minimum Expansion
Heat treatment is where Invar’s low expansion is truly dialed in. The as-cast or as-rolled alloy has a coefficient of thermal expansion around 2.1 × 10⁻⁶/°C in the 20 to 150°C range. That’s already low compared to steel (around 12 × 10⁻⁶/°C), but a three-step heat treatment can push it below 0.6 × 10⁻⁶/°C.
The optimized sequence works as follows:
- Step 1, solution treatment: Heat to 850°C, hold for one hour, then water quench. This dissolves any carbides or precipitates back into the nickel-iron matrix and freezes the alloy in a clean, homogeneous state.
- Step 2, tempering: Reheat to 350°C, hold for one hour, then cool. This relieves internal stresses from quenching without allowing significant precipitation.
- Step 3, low-temperature aging: Hold at 100°C for 24 hours. This long, gentle soak stabilizes the magnetic domain structure and locks in the lowest possible expansion coefficient.
After this full treatment, the coefficient of thermal expansion stays between 0.5 and 0.6 × 10⁻⁶/°C from room temperature up to 150°C. Above 200°C, the magnetic compensation effect weakens and expansion rises noticeably, reaching about 1.6 × 10⁻⁶/°C by 250°C. This is why Invar is specified for precision applications that operate near ambient temperatures: scientific instruments, laser mounts, satellite structures, shadow masks for displays, and precision molds.
Mechanical Properties of Finished Invar
In the annealed condition, Invar 36 is relatively soft and highly ductile. At room temperature (68°F / 20°C), it has an ultimate tensile strength of about 71 ksi (490 MPa), a yield strength of 35 ksi (241 MPa), and 42% elongation. As temperature rises, strength drops steadily: by 1112°F (600°C), tensile strength falls to just 30 ksi and elongation climbs to 68%.
This softness makes annealed Invar easy to machine, though it tends to work-harden and can be gummy under the cutting tool. Sharp carbide or coated tooling with positive rake angles and steady feed rates helps. Cold-worked Invar machines more cleanly but dulls tools faster due to its higher hardness.
Dimensional Stability Over Time
Even after proper heat treatment, Invar can exhibit slow dimensional changes over months or years if impurity levels are too high. Carbon is the main offender. At elevated temperatures (above roughly 315°C), carbon-rich Invar undergoes large contractions as carbides precipitate out of the matrix. At lower temperatures, a reversible carbon-dependent reaction around 700°C can cause expansion during processing unless sufficient manganese is present to suppress it.
For applications demanding the highest long-term stability, such as length standards or satellite structures, manufacturers use extra-low-carbon grades (sometimes called “Super Invar” when cobalt is added) and perform extended stabilization cycles. The goal is to ensure that every atom is in its equilibrium position before the part enters service, so there’s nothing left to shift later.