Why Invar 36 Parts Are Costly and How Shops Price Them
An Invar 36 quote often surprises buyers who know what the same part costs in stainless. The gap is real, and most of it comes from five places: material, cutting time, heat treatment, inspection and paperwork. Once you see where the money goes, you can read the quote line by line, compare two shops on equal terms and change the drawing where it helps.
Where the money goes
Material
Invar 36 is about 36 % nickel, and nickel is expensive. Bar and plate cost several times more per kilo than 304 or 316 stainless. Fewer mills produce it, so stock in odd sizes is thin and thick plate can take weeks. The spec most drawings cite is ASTM F1684, and buying to that spec with a mill cert is normal and does not add cost.
Two details on the drawing move this line more than people expect:
- Stock size. If your part is 52 mm thick and the nearest plate is 60 mm, you pay for the 60 mm. A few millimetres of design freedom can drop to a standard size.
- Yield. A ring cut from solid plate throws most of the material into chips. A shop may suggest a tube or a ring blank for repeat orders.
How the per kilo price is built, and what stock forms exist, is covered in Invar cost per kg and Invar stock: bar, plate and sheet.
Cutting time
Invar is gummy and work hardens, so shops run it at roughly the speeds they use for 304 stainless (about 30 to 60 m/min with carbide) and change tools earlier. Chips are stringy and need clearing. A part that takes an hour in aluminum can take several in Invar. For small quantities, programming and setup still dominate the price, as with any CNC part. For larger quantities, cycle time takes over.
Heat treatment
A precision Invar part is usually machined in stages: rough, stress relieve, finish. That means:
- A furnace run, in house or at a heat treater, at a typical 315 to 345 °C for 1 to 2 hours with a slow cool
- Handling and packing the part in and out
- A second setup to finish, with fixturing that does not re-stress the part
If the drawing calls for a full stabilization cycle, add more furnace time and a longer lead time. This is the line buyers most often question, and it is the one that makes the part behave like Invar after it ships. The buyer’s guide to Invar machining explains why the sequence matters, and stress relief for Invar machined parts covers the cycles.
Tolerance and inspection
Tight features cost more in any material. With Invar there is an extra point: to trust a flatness or position number, the part has to be measured at 20 °C after it has soaked in the room. That means waiting time and CMM time on every critical part, not just a quick check at the machine.
The table gives a typical view of how the tolerance on a single feature changes what the shop has to do. Cost effects are relative and typical. The shop’s quote governs.
| Tolerance on a feature | Typical achievable on Invar 36 | What the shop adds | Typical effect on that feature’s cost |
|---|---|---|---|
| ±0.1 mm | Any stable part, one setup | Nothing beyond normal practice | Baseline |
| ±0.025 mm | Stable part after stress relief | Finishing pass, sample inspection | Small increase |
| ±0.01 mm | Well supported feature, finished after relief | Light finishing passes, CMM at 20 °C on every part | Noticeable increase, mostly inspection |
| ±0.005 mm | Small feature, extra passes, full sequence | Extra pass, soak time, per-part CMM, scrap allowance | Large increase, often two to three times the ±0.025 mm line |
Loosen what does not matter. A mirror mount needs the mounting face and bores tight. The outer profile can usually be ±0.1 mm. The full argument is in Invar machining tolerances.
Certs and records
A mill cert with chemistry comes with the material and costs nothing extra. These cost more:
- Measured CTE data for your specific heat lot, usually by dilatometer to ASTM E228
- Full dimensional reports on every part instead of first article only
- Heat treatment charts and traceability packs for aerospace or space programs
Ask for them when your program needs them. Say so on the RFQ, because a shop that learns about them after quoting will have to re-quote.
Worked example: a hypothetical quote for ten mirror mounts
Take a hypothetical batch of ten Invar 36 mirror mounts, each 80 by 60 by 25 mm, milled from plate, with a flat mounting face at 0.01 mm and three bores at ±0.01 mm. The drawing calls for stress relief between roughing and finishing, inspection at 20 °C and a mill cert. Every shop builds its quote differently, but the shares below are a typical shape for a batch like this. They are illustrative percentages, not a price.
| Quote line | What is in it | Typical share of the batch price |
|---|---|---|
| Programming and setup | Two programs, two fixtures, paid once | 18 % |
| Material | One plate cut to ten blanks plus allowance, with mill cert | 22 % |
| Roughing | Machine time, tooling wear | 14 % |
| Stress relief | One furnace load for all ten, handling | 8 % |
| Finishing | Second setup, light passes, deburr | 16 % |
| Inspection | First article plus three bores and one face on every part, at 20 °C | 15 % |
| Certs and packing | Report, cert copies, protective packing | 7 % |
Read it as a shape rather than a formula. Setup and machine time together are close to half. Material is about a fifth. The two lines that stainless does not carry, stress relief and 20 °C inspection, are near a quarter. On a stainless version of the same part, those two lines shrink or vanish, the material line drops to a fraction and roughing is faster. That is where the gap comes from. It is also why asking two shops to quote without stating the heat treatment gives you two numbers that cannot be compared. One shop may have left it out.
How quantity changes the price
Unit price on Invar falls fastest in the first two dozen pieces because the one-off lines, setup, programming, first article and the furnace load, spread over more parts. After that, material and cycle time set the floor. The index below is a typical shape for a small milled part like the mirror mount above. It is illustrative, not a quote.
| Quantity | What dominates the price | Typical unit price index (10 pieces = 100) |
|---|---|---|
| 1 | Programming, setup, a furnace load for one part | 350 to 500 |
| 5 | Setup still large, furnace shared | 140 to 170 |
| 10 | Balanced between setup, material and machine time | 100 |
| 25 | Setup spread thin, material and cycle time lead | 75 to 85 |
| 100 | Material, cycle time and batch heat treatment. Fixtures pay back | 60 to 70 |
| 500 | Material and cycle time. Stock and furnace loads planned ahead | 50 to 60 |
Heat treatment is charged per furnace load, not per part, at most shops. Ten parts in one load cost little more than one part, which is another reason a single prototype looks expensive per piece. If you are deciding between one prototype and a small batch, see prototype vs production Invar machining and minimum order quantity.
Lead time, deadlines and repeat orders
A short deadline costs more in any material. With Invar it can cost more than you expect, because the shop may need to buy material at a premium, push a furnace run into a busy week or run inspection overtime. If the deadline has room, say so. A shop that can batch your stress relief with other work can quote lower.
The second order of the same part is almost always cheaper per piece. The program and fixtures exist and the shop knows how the part moves after stress relief. If you expect to reorder, mention the likely yearly volume on the first RFQ. Some shops will hold material or quote a blanket order.
Drawing changes that lower the quote
- Tolerance only the features that affect function
- Use standard hole sizes and inside radii the shop has tools for
- Avoid thin walls and deep narrow pockets that need special fixturing
- Design around standard bar and plate sizes
- Say “shop to recommend” on heat treatment when you are not sure, instead of specifying the longest cycle
- Give a realistic yearly volume so the shop can plan material
A tolerance loosened from ±0.005 to ±0.02 mm on a non-critical bore, or a pocket made 1 mm shallower to fit a standard tool, can move a small batch quote noticeably. More of these are in design for machinability in Invar.
Questions to ask about a high quote
Before you push back on price, ask the shop five questions:
- Which line is largest? A shop that has done the work can tell you whether material, machine time or heat treatment drives the number.
- What stock did you price? If the plate is much thicker than the part, ask whether a smaller standard size or a different orientation would work.
- Does the quote include stress relief and inspection at 20 °C? If one quote includes them and another does not, the lower quote is not cheaper. It is a different part.
- Which features drove the inspection line? The shop may point at one tolerance that adds a CMM pass on every part and does nothing for function.
- What would you change on the drawing? Shops that cut Invar often suggest a thicker wall, a standard radius or a looser tolerance on a feature that does not matter.
Most of the time the answers point to one or two lines you can influence. A quote that is high because of material and heat treatment is probably right. A quote that is high because of setup and inspection can often be brought down on the drawing.
What to put on the RFQ
Quotes are only comparable when every shop quotes the same thing. Put these on the RFQ:
- Alloy and spec, for example Invar 36 to ASTM F1684
- Heat treatment: stress relief between rough and finish, full stabilization or “shop to recommend”
- The two or three critical features and their tolerances, with everything else at a general tolerance
- Inspection temperature, 20 °C after soak, if any tolerance is under about ±0.02 mm
- Certs and reports you need
- Quantity now and expected yearly volume
Send the drawing and those notes through the RFQ form. When we match your request, each shop sees the same details, so the quotes you receive line up and the one you pick is the one that priced the right part.
To see where the price lines sit between the RFQ and acceptance, read our guide to buying custom Invar parts.
Frequently asked questions
Why does an Invar part cost more than the same part in stainless?
The bar costs several times more, it cuts more slowly and a precision Invar part usually needs a stress relief between roughing and finishing plus inspection at 20 °C. Each of those adds a line that a stainless part does not carry, and together they can double or triple the price.
Can I skip stress relief to save money?
On parts that are not thermally critical and have loose tolerances, sometimes. On optical, metrology or tooling parts it is the step that keeps the part stable, so skipping it usually costs more later when the part moves out of tolerance in the customer's hands.
How can I get a lower Invar machining quote?
Tolerance only the features that matter, design the part to be cut from standard bar or plate sizes, avoid thin walls that need special fixturing, ask for measured CTE data only when you need it and give a realistic yearly volume so the shop can plan material and batch the furnace.
Why do two Invar quotes differ so much?
Usually because the shops are not quoting the same part. One may have included stress relief, 20 °C inspection and a full report, while the other priced a single setup and a mill cert. State the heat treatment and inspection requirements on the RFQ and the numbers will line up.
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.
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