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Sourcing Invar 36, Kovar and Alloy 42: Mills, Distributors and What Your Shop Already Has

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

A drawing calling for Invar 36, Kovar or Alloy 42 is only half the order. Somewhere between the drawing and the finished part, a bar, a plate or a coil of strip has to come from a mill, through a distributor or a service center, and land at a machine shop that can cut it. Buyers who have never ordered a controlled expansion alloy before often assume it works like ordering 304 stainless from a local supplier, and are surprised by the lead time, the minimum order quantity, or the fact that the shop asks to source it instead. This guide covers how the supply chain for these three alloys actually works, what a mill certificate does and does not tell you, and why letting the shop source the material is usually the faster path to a finished part.

Who actually makes this material

Invar 36, Super Invar, Kovar and Alloy 42 are specialty iron-nickel alloys, not commodity steel. A handful of mills worldwide melt them to ASTM F1684 (Invar 36 and Super Invar), ASTM F15 (Kovar) or ASTM F30 (Alloy 42), and the volume produced each year is small compared to stainless or tool steel. That has two consequences for a buyer. First, there is no local steel yard with these alloys on the shelf in every size. Second, the further your required size or form sits from a mill’s standard run, the longer the wait and the higher the minimum order quantity.

From the mill, material moves one of two ways. Large distributors and metal service centers buy mill runs and break them into smaller lot sizes, cut to length, and hold limited stock in the most common bar and plate sizes. Smaller specialty distributors focus on aerospace and optical alloys and carry deeper but narrower stock, often including forms a general service center does not bother with. Most machine shops that regularly cut Invar buy from two or three of these distributors rather than direct from a mill, because mill minimum orders are usually well beyond what a single job needs.

What a mill certificate actually tells you

Every legitimate lot of Invar 36, Kovar or Alloy 42 ships with a mill certificate, sometimes called a certificate of conformance or a material test report. Read closely, it usually confirms chemistry against the ASTM spec and the heat or lot number the material came from. It rarely includes a measured coefficient of thermal expansion unless it was ordered as a separate, chargeable test, because CTE testing by dilatometer takes time the mill does not spend by default.

That distinction matters because CTE is the entire reason most buyers choose these alloys. The typical values on the Invar specs reference, such as 1.2 to 1.6 ppm/°C for Invar 36, come from handbook data across many heat lots, not a promise for your specific bar. If a design genuinely depends on a tight CTE number, say so on the purchase order and expect the distributor or the shop to quote the dilatometer test as a line item. Our material certification guide covers what to ask for and when it is worth the cost.

Typical stock forms and lead time by alloy

Alloy Common stock forms Typical lead time for common sizes Typical lead time for less common sizes
Invar 36 Round bar, flat bar, plate, sheet 1 to 3 weeks from a distributor’s stock 8 to 16 weeks direct from a mill run
Super Invar Round bar, flat bar, limited plate 4 to 8 weeks, fewer distributors stock it 12 to 20 weeks or more, mill minimums apply
Kovar Round bar, sheet, strip, tube 2 to 4 weeks from a distributor’s stock 8 to 14 weeks for uncommon tube or thick plate
Alloy 42 Strip, sheet, some round bar 1 to 3 weeks for strip, longer for bar 8 to 12 weeks for bar sizes outside the common range

These are typical figures for planning a schedule, not a quote. A distributor’s actual stock changes week to week, and a heavy plate or an odd diameter can push any of these alloys into mill lead time even when the alloy itself is common. Our stock lead time planning guide walks through how to build material lead time into an RFQ so it does not become a surprise after the quote is accepted.

Minimum order quantities catch buyers off guard

A distributor selling commodity steel will often cut a single piece to length with no minimum. Specialty alloy distributors frequently will not, because breaking a mill lot into small pieces still costs them the same handling regardless of order size. A buyer who needs one bracket in Invar 36 can find themselves quoted a minimum order of a full bar length, or a minimum dollar value per order, well beyond what the part itself requires.

This is one of the clearest cases where a machining shop’s existing relationships pay off. A shop that regularly buys Invar 36 or Kovar already holds drop stock from previous jobs, or can add a small quantity to a standing order with its usual distributor, avoiding the minimum a new account would face. Our minimum order quantity guide covers how this plays out on both the material side and the finished-part side of a quote.

A hypothetical example

A buyer needs twelve Invar 36 brackets, each machined from a 40 mm round bar about 60 mm long. Total material required is under half a meter of bar. Calling a specialty distributor directly, the buyer is quoted a minimum order of a 3 m bar length at a price close to what the full job would have cost through a shop, plus a two to three week wait for the cut piece to ship before machining even starts.

The same buyer sends the drawing as an RFQ instead. The matched shop already stocks 40 mm Invar 36 bar from a prior job, or can add the length to an existing distributor order at its standing account pricing. Machining starts within days of the drawing being accepted rather than weeks after a separate material purchase, and the buyer pays one line item instead of two. This is the ordinary case, not the exception, for small and mid-volume orders in these alloys.

What to put on the RFQ so a shop can quote the material correctly

  • The alloy and its spec, for example “Invar 36 per ASTM F1684” rather than just “Invar,” since trade names vary between mills.
  • Whether you are supplying material yourself or want the shop to source it. If you are supplying it, say when it will arrive and include the mill certificate.
  • Any measured CTE, decarburization or other test requirement beyond a standard chemistry certificate, since that changes both cost and lead time.
  • Your real need date, so the shop can tell you upfront whether it has drop stock or is starting from a fresh material order.

The full list of what belongs on a drawing and RFQ, beyond material sourcing, is in our RFQ drawing checklist. If you already have a drawing ready, send it through our RFQ form and we match it with up to three shops that machine the alloy you specified and can source or confirm the material as part of the quote.

Most buyers who ask about sourcing Invar, Kovar or Alloy 42 material directly are trying to save time or money before they have priced the alternative. In practice, a shop that already buys the alloy on a standing basis is usually faster and no more expensive than sourcing it yourself, and it removes one more handoff where a mismatched certificate or a missed lead time can delay the part. Material sourcing is worth understanding, mainly so you know what a fair quote looks like when your shop handles it for you.

Frequently asked questions

Can I buy Invar 36 bar stock myself and hand it to a shop?

Yes, some buyers do, usually when a spec requires a specific mill or when material is already on hand from a prior project. Most shops will machine customer-supplied material, but ask first: some charge a handling fee, and the shop will still want the mill certificate to plan the stress relief and heat treatment correctly.

Why is Super Invar harder to source than Invar 36?

Fewer mills melt the iron-nickel-cobalt chemistry, and the volume ordered each year is much smaller than for Invar 36. Expect longer lead times, higher minimum order quantities, and fewer stock sizes on the shelf at a distributor.

Does a distributor's stock CTE match what I need?

Chemistry is controlled to the spec, but CTE varies a little by heat lot and by the heat treatment the mill or the shop applies afterward. A distributor's certificate usually gives you chemistry only. If your design depends on a specific CTE number, ask for measured expansion data on that heat lot, and expect to pay for the test.

Is it cheaper to source Invar myself or let the shop do it?

For most one-off and low-volume orders, letting the shop source it is cheaper and faster. Shops buy stock in shop-lot quantities at better pricing than a single buyer typically gets, and they already know which distributors carry which alloy and form.

Do you sell Invar, Kovar or Alloy 42 material?

No. We are a matching service, not a material distributor. We connect your RFQ with shops that machine these alloys, and those shops handle material sourcing as part of the job unless you tell them you are supplying it.

Reviewed against

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.

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