Machining Alloy 42 (ASTM F30): Thin Parts, Flatness and Alloy Choice
Alloy 42 machining follows the same rules as Invar 36 and Kovar: sharp positive rake tools, real chip loads, flood coolant and a stress relief when flatness matters. The alloy, 42 % nickel with the balance iron and covered by ASTM F30 as UNS K94100, expands a typical 4.0 to 6.0 ppm/°C, which is why it appears in lead frames, thermostat bimetals and ceramic packages. Most of the parts a shop sees in Alloy 42 are thin, so the hard problem is holding them flat, not cutting them. This guide covers where the alloy is used, how thin parts are fixtured, when stamping beats machining and when a buyer should specify Alloy 42 rather than Kovar or Invar 36.
Where Alloy 42 is used
Alloy 42 is a controlled expansion alloy, not a lowest expansion alloy. Its CTE sits close to that of alumina ceramic and of the softer sealing glasses, and it is cheaper and more widely stocked as strip than Kovar. That combination puts it in a handful of applications, summarized on the Alloy 42 machining page:
- Lead frames for plastic and ceramic semiconductor packages, stamped or etched from strip 0.1 to 0.25 mm thick. Alloy 42 frames expand closer to the silicon die and the molding compound than copper frames, which helps in packages that see thermal cycling.
- Thermostat bimetals, where Alloy 42 or a similar low expansion grade forms the passive side of the strip and a high expansion alloy forms the active side.
- Ceramic package components, such as pins, seal rings and lids that braze or seal to alumina.
- Glass to metal seals with soft glass, at lower sealing temperatures than the borosilicate seals that Kovar is made for.
- Carriers, spacers and fixtures around those parts, where the matched expansion keeps the assembly aligned through test and processing temperatures.
Composition and expansion of the whole nickel iron family are covered in the Wikipedia entry on Invar, which explains why expansion falls to a minimum at about 36 % nickel and rises again toward 42 % and beyond. Alloy 42 sits on that rising slope, which is what gives it a CTE that matches ceramics rather than approaching zero.
Alloy 42 next to Invar 36 and Kovar
The three alloys are often confused because they come from the same family, cut the same way and cost more than steel for the same reason: nickel. The table lists typical values near room temperature; the mill certificate and the shop’s quote govern.
| Property | Invar 36 | Alloy 42 | Kovar |
|---|---|---|---|
| Specification | ASTM F1684 | ASTM F30 | ASTM F15 |
| UNS | K93600, K93601, K93603 | K94100 | K94610 |
| Nominal composition | 36 % Ni, bal Fe | 42 % Ni, bal Fe | 29 % Ni, 17 % Co, bal Fe |
| Typical mean CTE, ppm/°C | 1.2 to 1.6, 20 to 100 °C | 4.0 to 6.0, 30 to 300 °C | 5.0 to 5.5, 30 to 200 °C |
| Density, g/cm3 | 8.05 | 8.12 | 8.36 |
| Annealed hardness, HRB | about 70 to 80 | about 70 to 85 | about 75 to 85 |
| Typical use | Lowest expansion near 20 °C | Matched expansion, lead frames, soft glass | Matched expansion, hard glass and alumina |
The Invar 36 vs Kovar vs Super Invar comparison covers the first and third columns in depth; the reference page of Invar specifications lists the standards for all three.
How Alloy 42 behaves on the machine
Machinists treat Alloy 42 exactly as they treat Invar 36. It is soft in the annealed condition, roughly 70 to 85 HRB typical, it is gummy, it work hardens when a tool rubs instead of cutting and it makes long stringy chips that need breaking or clearing. Thermal conductivity is low, so heat stays at the edge. The recipe from Invar 36 machining carries over unchanged:
- Sharp carbide or HSS with positive rake, replaced before the edge dulls
- Feed kept up so the tool takes a real chip on every pass
- Typical carbide milling speeds of roughly 30 to 60 m/min, HSS 15 to 25 m/min
- Flood coolant for heat and chip evacuation
- Climb milling, no dwelling in corners
A shop that has cut Invar 36 or Kovar recently will not be surprised by Alloy 42. What surprises shops is the geometry. Invar 36 parts are often chunky optical mounts and benches. Alloy 42 parts are usually thin plates, rings and frames with a lot of surface and little stiffness, which is where the fixturing problem starts. The metallurgy of the cold worked layer is covered in Invar work hardening, and it applies to Alloy 42 too.
Fixturing thin Alloy 42 parts for flatness
A thin nickel iron plate distorts three ways during machining: it springs when clamping force is released, it bows as roughing stress on one face pulls against the other and it moves again when the cold worked layer relaxes over the following days. Shops that hold flatness on thin parts use a combination of the following, described in more detail in Invar fixturing and workholding and Invar thin wall machining:
- Low force holding. Vacuum chucks, wax or adhesive mounting and magnetic chucks with a soft fixture plate. Alloy 42 is magnetic, which helps, but a magnetic chuck can also pull a thin plate flat against the chuck and let it spring back when released, so shops use it with care.
- Alternating passes. Skim one face, flip, skim the other, flip again, taking equal light cuts so the stress on the two faces stays balanced.
- Stress relief between roughing and finishing. Typically 315 to 345 °C for 1 to 2 h with a slow cool, with the part laid flat on a clean plate in the furnace so it does not sag or pick up new stress.
- Finishing with light cuts. A final 0.05 to 0.1 mm per face with a sharp tool leaves very little new cold work.
- Inspection after a soak at 20 °C, on a surface plate or CMM, not on the machine.
Typical results on a stable, stress relieved thin part are flatness of 0.02 to 0.05 mm over 100 mm, with 0.01 mm possible on smaller parts with lapping or extra passes. Thickness tolerance of ±0.01 to ±0.025 mm is realistic. Tighter numbers are possible but add furnace runs and inspection time, so ask the shop before the drawing is released.
Stamping, etching or machining
Alloy 42 is unusual among the alloys on this site because most of its parts are never machined at all. The table sets out where each process fits. Values are typical and the shop or the stamper’s quote governs.
| Factor | Stamping | Chemical etching | CNC machining |
|---|---|---|---|
| Typical thickness | 0.1 to 1.0 mm strip | 0.05 to 0.5 mm sheet | 0.5 mm and up, thinner with care |
| Tooling cost | High, a progressive die | Low, a photo tool | None beyond fixtures |
| Break even quantity | Thousands and up | Hundreds to thousands | One to a few hundred |
| Edge quality | Burr and rollover on one side | Slight undercut, no burr | Machined edge, deburred |
| Flatness | Good on thin strip, coil set possible | Good, no mechanical stress | Depends on fixturing and stress relief |
| Typical fit | Lead frames, clips, bimetal blanks | Fine lead frames, screens, prototypes | Carriers, fixtures, lids, thick parts, short runs |
A buyer who sends a 0.15 mm lead frame drawing to a machine shop will get a polite decline or a very high quote. A buyer who sends a 3 mm carrier plate with a pocket, tapped holes and a flatness callout has a machining job. The middle ground, a 0.5 to 1 mm plate with a few features in quantities under a hundred, is where the choice is real and where a shop’s answer depends on its fixturing.
Worked example: a burn in carrier at 20 to 125 °C
Take a hypothetical carrier plate, 80 mm long and 2 mm thick, that holds ceramic packaged devices through a burn in cycle from 20 to 125 °C. The designer wants the carrier to move with the alumina packages rather than against them, and is choosing between Alloy 42, Kovar, Invar 36 and aluminum.
Growth of an 80 mm length over +105 K = 80 mm × CTE × 105.
| Carrier material | Typical CTE, ppm/°C | Growth over 80 mm, mm | Mismatch to alumina at about 7.0, mm |
|---|---|---|---|
| Alumina package (reference) | 7.0 | 0.059 | 0.000 |
| Alloy 42 | 5.0 | 0.042 | 0.017 |
| Kovar | 5.3 | 0.045 | 0.014 |
| Invar 36 | 1.4 | 0.012 | 0.047 |
| 6061 aluminum | 23.5 | 0.197 | 0.138 |
Values are typical and the calculation is illustrative. Alloy 42 and Kovar both stay within about 0.02 mm of the ceramic over the full cycle, which a spring contact or a clearance fit absorbs. Invar 36, despite being the most stable alloy in the family, mismatches the ceramic by nearly three times as much because the goal here is matching, not minimum expansion. Between Alloy 42 and Kovar the decision comes down to cost and availability, because no glass seal or braze is involved. Alloy 42 wins on both.
If the same carrier instead had to hold an optical reference at 20 ± 2 °C, the calculation would flip and Invar 36 would be the obvious choice.
When to specify Alloy 42, Kovar or Invar 36
- Specify Alloy 42 when the part must expand with a ceramic, a silicon package or a soft glass, when strip is the starting form, when quantity is high enough that material cost matters or when a stamping die will eventually replace the machined prototype.
- Specify Kovar when the part seals to borosilicate glass or brazes to alumina, when it sees several hundred degrees in processing or when the sealing house specifies it. See Kovar glass to metal seal parts.
- Specify Invar 36 when the requirement is the lowest possible expansion near room temperature, such as optical mounts, metrology and tooling, with no matched expansion partner in the assembly.
Whichever alloy you pick, the drawing should state the specification, the condition and the operating range. “Alloy 42” alone can mean annealed strip, cold rolled strip or bar with different flatness and hardness, and the shop will quote the easiest interpretation.
What to put on the RFQ
An Alloy 42 quote is only accurate when the shop knows the form, the thickness and the flatness you need. Include:
- Alloy and spec. Alloy 42 to ASTM F30, UNS K94100, with the supply form (strip, sheet, plate or bar) and the temper if it matters.
- Thickness and flatness. Both, with the inspection method and the inspection temperature, normally 20 °C.
- Stress relief. Between roughing and finishing. Otherwise “shop to recommend” with the operating range.
- Quantity now and expected volume. So the shop can tell you honestly whether machining, etching or stamping fits.
- The mating part. Ceramic, glass or silicon, with its CTE if you have it, so the shop understands why the alloy was chosen.
- Edge and burr requirements. Thin parts burr; say what is acceptable.
When you send an RFQ with the drawing, we match it with up to three shops that cut the nickel iron alloys and can fixture thin parts, run the stress relief and inspect flatness at 20 °C. Each shop quotes you directly against the same drawing and the same flatness callout.
Frequently asked questions
What is Alloy 42 and what is its CTE?
Alloy 42 is an iron alloy with about 42 % nickel, covered by ASTM F30 as UNS K94100. Its mean coefficient of thermal expansion is typically 4.0 to 6.0 ppm/°C from room temperature to about 300 °C, which places it between Invar 36 at 1.2 to 1.6 and Kovar at 5.0 to 5.5. The mill certificate governs the value for a given heat lot.
Is Alloy 42 easier to machine than Invar 36?
It machines in the same band. Alloy 42 is soft and gummy, work hardens if the tool rubs and produces long chips, just like Invar 36 and Kovar. Sharp positive rake tools, steady feeds and flood coolant are the recipe. Most machined Alloy 42 parts are thin, so fixturing and flatness are the harder problem, not the cutting itself.
Should lead frames be machined or stamped?
Stamped or chemically etched from strip, almost always. Machining a 0.15 mm thick frame with hundreds of leads is slow and the leads distort. Machining suits the parts around the frame: carriers, test fixtures, spacers, lids and short runs of thicker parts where a stamping die is not justified.
When should I specify Alloy 42 instead of Kovar?
When the part needs a matched expansion around 5 ppm/°C but does not seal to borosilicate glass or braze to alumina at high temperature. Alloy 42 costs less, is stocked in more strip sizes and seals to softer glasses. Kovar is the safer choice for hard glass seals and ceramic brazing.
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.
Need a quote for this part?
Send the drawing. We match you with up to 3 shops that machine Invar and reply within 24 business hours.
Get matched with an Invar shop