Invar Hardness and Machinability: Why the Rating Understates the Job

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

Invar machinability is rated at roughly 40 to 50 % of the B1112 free machining steel reference, the same band as 304 stainless, and its annealed hardness is only about 70 to 80 HRB. Both numbers make the alloy sound easy. Shops that cut it know the rating understates the job: Invar is gummy, work hardens when a tool rubs, throws long stringy chips and needs a stress relief between roughing and finishing. This guide puts the numbers next to familiar alloys, explains why the rating misses the real cost and shows what that means for cycle time and price.

Hardness: soft, and not the point

Annealed Invar 36 to ASTM F1684 is typically 70 to 80 HRB, about 130 to 160 HB. That is softer than 4140 in most conditions and about the same as annealed 304 stainless. Cold drawn bar can read higher, and the surface of a machined part reads higher still because the cutting has work hardened it.

Buyers sometimes see the hardness and assume a shop that cuts alloy steel will find Invar easy. Machinists know better. Hardness predicts tool wear on a material that behaves like steel. Invar does not behave like steel. It behaves like an austenitic stainless with even worse chip control, and the hardness of the bar tells you nothing about that. The glossary has the hardness scales and the terms used below.

Machinability rating next to familiar alloys

Machinability ratings compare cutting speed for a given tool life against B1112, a free machining steel set at 100 %. The values below are typical handbook figures for orientation. Ratings vary by source, condition and tooling. The aluminum figure in particular is often quoted on a different basis, so treat the table as a ranking rather than a measurement. The Wikipedia article on machinability explains how the ratings are built.

Material, typical condition	Typical rating, % of B1112	Typical annealed hardness	Character on the machine
12L14 leaded steel	about 160 to 170	about 80 to 90 HRB	Free cutting, short chips
6061-T6 aluminum	about 90, different basis	about 95 HB	Fast, easy chips, thermal growth during cutting
1018 carbon steel	about 70 to 78	about 70 HRB	Predictable, moderate chips
4140 annealed steel	about 65	about 90 to 95 HRB	Harder, but well behaved
Invar 36, annealed	roughly 40 to 50	about 70 to 80 HRB	Gummy, work hardens, stringy chips
304 stainless, annealed	about 45	about 70 to 85 HRB	Gummy, work hardens, stringy chips
Kovar, annealed	roughly 40 to 50	about 75 to 85 HRB	Same family as Invar
316 stainless, annealed	about 36	about 75 to 85 HRB	Tougher than 304
Ti-6Al-4V	about 20 to 22	about 36 HRC	Slow, heat at the edge, tool wear

Two things stand out. First, Invar sits with 304 and Kovar, not with titanium; on cutting speed alone it is an ordinary difficult alloy. Second, it sits far from 6061 and 12L14, which is why a buyer who is used to aluminum prototypes is surprised by the first Invar quote. The buyer’s guide to Invar machining sets that expectation up front.

Why the rating understates the difficulty

A machinability rating is measured in a single cut on a lathe. It captures how fast a tool can run before it wears. It does not capture four things that dominate an Invar job.

Gumminess and built up edge

Invar is ductile and smears rather than shears at the wrong speed. Material welds to the cutting edge, the built up edge breaks off and takes part of the tool with it, and the surface finish suffers. Shops counter this with sharp positive rake tools, coatings chosen for the alloy and speeds kept in the typical window of roughly 30 to 60 m/min for carbide milling and 15 to 25 m/min for HSS. The Invar 36 machining parameters guide lists typical starting points.

Work hardening

If the tool rubs instead of cutting, the surface hardens and the next pass has to cut through a harder skin, which dulls the tool, which rubs more. On Invar the cold worked layer also raises the local CTE and lets the part drift, so work hardening is a stability problem as well as a tool problem. Feed has to stay up on every pass, including the finishing pass, and tools are replaced before they dull. The mechanism is explained in Invar work hardening.

Chip control

Chips come off long, stringy and hot. On a lathe they wrap around the part and the chuck. On a mill they pack into pockets and recut. Chip breaking inserts, high pressure or well aimed flood coolant and programmed pecks or dwells all help, but every one of them costs cycle time that a machinability rating never sees. Invar is also magnetic, so fine chips cling to the part and the fixture and have to be cleaned off before inspection.

The sequence around the cutting

A precision Invar part is roughed, stress relieved, then finished with light cuts. That is a furnace run, a second setup and a second fixture. The part is then soaked at 20 °C and measured. None of that appears in a rating measured on one pass through a bar, and it is usually the largest single difference between an Invar quote and a stainless quote. Our guide to stress relief in the Invar machining sequence explains why the step cannot be skipped on stable parts.

What the rating means for cycle time

The table gives typical multipliers for the same part across materials, relative to 6061 aluminum, for a milled bracket with a few bores and a flat mounting face. These are illustrative shop floor rules of thumb, not measurements; the shop’s quote governs.

Material	Typical machine time, relative to 6061	Typical extra steps	Typical total time, relative
6061-T6	1.0	None	1.0
1018 steel	about 1.3 to 1.6	None	about 1.5
304 stainless	about 2.0 to 2.5	Deburring	about 2.2
Invar 36	about 2.0 to 3.0	Stress relief, second setup, deburring, 20 °C soak and inspection	about 3 to 4
Ti-6Al-4V	about 3.5 to 5	Careful cooling, tool changes	about 4 to 5

The point of the last column is that Invar’s total time lands closer to titanium than to stainless, even though its cutting rating matches stainless. The cutting is only part of it.

Worked example: the same bracket in three materials

Take a hypothetical mounting bracket, 120 mm by 80 mm by 25 mm, milled from plate with a pocket, six tapped holes and two bores toleranced at ±0.02 mm on position, quantity 20. A shop estimating machine time might arrive at something like this, treating the aluminum version as one hour of spindle time per part.

Item	6061-T6	304 stainless	Invar 36
Roughing and finishing, per part	1.0 h	2.2 h	2.0 h rough + 0.8 h finish
Tool changes and deburring, per part	0.1 h	0.3 h	0.4 h
Stress relief, per batch	none	none	1 furnace run, shared by 20 parts
Second setup and fixture, per part	none	none	0.3 h
Soak and inspection at 20 °C, per part	0.1 h	0.1 h	0.3 h
Typical total, per part	about 1.2 h	about 2.6 h	about 3.8 h plus furnace share

All figures are hypothetical and illustrative. The Invar bracket takes about three times the shop hours of the aluminum one and about half again more than the stainless one, before the furnace run and before the higher material cost, which is several times that of stainless per kilogram. Multiply the shop hours by the shop’s rate and add material and heat treatment: the quote gap that surprises buyers is fully explained. The Invar 36 machining cost guide walks through those lines in more detail.

Notice where the Invar time goes: not into a slower spindle, since the roughing pass is similar to stainless, but into the finishing pass, the second setup, the deburring and the inspection. A buyer who loosens tolerances that do not matter, allows a standard stress relief cycle and gives a flexible date reduces exactly those lines.

The free machining grade

There is a free machining version of Invar 36, with a small addition of selenium or a similar element that breaks chips into short pieces and reduces built up edge. Shops report tool life and cycle time improvements, sometimes quoted as 20 to 30 % on turned parts, though the figure depends heavily on the part and the tooling. It is common for fittings, spacers and small turned parts made in volume.

Before specifying it on a precision part, check three things with the shop and the mill:

• Dimensional stability. The free machining additions form inclusions that do not change the bulk CTE much, but the part still needs the same stress relief. Do not expect the grade to remove the heat treatment.
• Weldability and plating. Selenium bearing grades can be harder to weld and can behave differently in plating baths. If the part is welded or coated, say so.
• Availability. It is stocked in fewer sizes and forms, usually bar rather than plate, so a thick plate part may not have the option.

The full trade is covered in Invar free machining grade. For most milled optical and metrology parts the standard grade to ASTM F1684 remains the default, and the saving comes from the drawing rather than the alloy.

Reading a quote with the rating in mind

When a quote for an Invar part looks high next to stainless, the machinability rating is not the place to argue. Both alloys cut at similar speeds. Ask instead:

1. Is a stress relief included and where in the sequence?
2. How much of the time is finishing and inspection rather than roughing?
3. Which tolerances drive the light passes and the CMM time?
4. Would the free machining grade help on this geometry and what would it change?

Shops that cut Invar regularly answer those in a minute. Shops that are quoting from the rating alone cannot, and their number will be wrong in one direction or the other.

What to put on the RFQ

The rating tells a shop how fast to run the spindle; the RFQ has to tell it everything else. Include:

• Alloy and spec. Invar 36 to ASTM F1684, standard or free machining grade, with the condition.
• Heat treatment. Stress relief between roughing and finishing. Otherwise “shop to recommend” with the operating range.
• Critical features. The bores and faces that carry the tight tolerance, so the light passes and inspection time go where they matter.
• Surface and edge requirements. Burr limits and finish, since deburring is a real line on Invar.
• Quantity and timing. Furnace runs are batched, and the second order is cheaper.

When you send an RFQ with the drawing, we match it with up to three shops that cut Invar regularly and price it from experience rather than from a handbook rating. Each shop quotes you directly with the same sequence in front of it.

Frequently asked questions

Reviewed against

• ASTM F1684, Standard Specification for Iron-Nickel and Iron-Nickel-Cobalt Alloys for Low Thermal Expansion Applications
• Wikipedia, Machinability

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.

Keep reading

• Why Invar Work-Hardens and How Shops Control It During Machining · Materials
• Free-Machining Invar 36: What the Added Sulfur Changes · Materials
• Speeds, Feeds and Tool Life for Machining Invar 36 · Process
• Why Invar 36 Parts Are Costly and How Shops Price Them · Cost

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