What to Know Before You Send a Stainless Part Out to Quote

September 21, 2026

Stainless is the material where the cheapest grade to cut is often the one you cannot use. That single fact explains most of the surprise in stainless quotes. This guide covers what changes the price, what belongs on the drawing and where buyers usually lose time.

Stainless is not one material

The word covers four families that behave differently in a machine.

Austenitic grades, the 300 series, are the ones most buyers mean. They are not magnetic in the annealed state, they weld well and they work harden hard. 304 and 316L live here. So does 303, which is 304 with sulphur added to make it cut.

Martensitic grades, 410, 416, 420 and 440C, can be hardened by heat treatment. They hold an edge, which is why knives and shafts use them, and they resist corrosion least of the group.

Precipitation hardening grades, 17-4 PH and 15-5 PH, are machined soft in condition A and then aged to between 30 and 44 HRC. You get martensitic strength with corrosion resistance closer to 304.

Duplex grades, 2205 and 2507, are roughly half austenitic and half ferritic. Around twice the yield strength of 316L and much better in chloride, at the cost of being slow and hard on tooling.

Most parts land in the first group. The rest of this guide stays there, with a note where 17-4 PH behaves differently.

Work hardening is the whole story

Austenitic stainless hardens where it is deformed. A tool that rubs instead of cutting leaves a hardened skin, and the next pass has to cut through it. That is why the two usual reactions to a bad surface finish, slowing the feed and taking a lighter cut, make the problem worse.

Three consequences show up in your quote:

• Cycle times are longer than for carbon steel or aluminium at the same geometry. 316L is rated around 36 against free-machining carbon steel B1112 at 100. Aluminium 6061 sits above 300.
• Tool life is shorter, and inserts are a real line in the cost. A shop that runs stainless every week knows which grade and coating survive it. A shop that does not will either price the risk in or discover it on your job.
• Heat stays in the part. Stainless conducts heat poorly, so the heat that would normally leave with the chip goes into the workpiece and the tool. Deep pockets, thin walls and long threads get harder as the cut goes on, and the part grows while it is being measured.

That last point matters for tolerances. A tolerance of ±0.02 mm on a stainless part is not the same request as ±0.02 mm on an aluminium part. It usually means a separate finishing pass, a settling period before inspection and more time on the CMM.

What each grade costs you to cut

Machinability is rated against free-machining carbon steel B1112 at 100. PREN, the pitting resistance equivalent number, is calculated from nominal composition as chromium plus 3.3 times molybdenum plus 16 times nitrogen. Both are published typical values for the alloy, not a promise about your geometry. There is a fuller explanation of PREN on Wikipedia.

Grade	Machinability	PREN	Usually specified for
416	85	13	Dry indoor shafts and fasteners
303	78	18	Dry indoor parts with threads and cross holes
17-4 PH	50	16	Parts needing 30 to 44 HRC with some corrosion resistance
304 / 304L	45	19	General purpose, weldable, indoor and mild outdoor
316 / 316L	36	24	Chloride, cleaning chemicals, food contact, body contact
2205 duplex	30	35	Sea water, high strength, chemical process

Two numbers in that table carry the whole argument. 303 cuts more than twice as fast as 316L. 316L survives in places 303 will not. There is no grade that does both, which is why the 303, 304 and 316L comparison is worth reading before you fix the grade on a drawing.

17-4 PH deserves its own note. Machining happens in condition A, then the part is aged. H900 gives the highest hardness and the lowest corrosion resistance, H1150 the reverse. Ageing moves the size slightly, so tight tolerances mean either machining after ageing, which is slower, or a shop that knows the growth for that condition.

What belongs on the drawing

Most rounds of email between a buyer and a shop are about information that could have been on the print. The list below is what a stainless shop needs before it can quote without guessing.

1. Grade and condition. Not just 17-4 PH but 17-4 PH H1075. Not just 316 but 316L if the part is welded.
2. The environment, in words. Washed down daily with caustic. Immersed in sea water. Autoclaved. This is the single most useful sentence you can add, because it lets a shop offer a cheaper alloy that still survives.
3. Tolerances that matter, and only those. A blanket ±0.01 mm on every dimension costs real money and is almost never needed.
4. Surface finish, with a number. Ra 1.6 micrometre and Ra 0.4 micrometre are different processes, not different effort levels.
5. Passivation or electropolishing, with the standard. Usually ASTM A967 for passivation and ASTM B912 for electropolishing. See passivation against electropolishing for which one your part needs.
6. Documents you need back. Mill certificate with heat number, passivation certificate, first article report, CMM report. Each of these is office hours, so a shop that learns about them after quoting will either absorb the cost once or requote.
7. Quantity and whether it repeats. Stainless setups take longer to prove out. A repeating order justifies fixturing that a one off does not.

Where buyers lose money

Specifying 316L everywhere. It is the safe answer and it is expensive. If the part lives in a dry cabinet, 304 does the same job for less.

Specifying 303 for a wet part. Its chromium content looks close to 304, so it reads as safe. The sulphide stringers that make it machine well are exactly where pitting starts.

Leaving the finish vague. A shop that reads no finish callout quotes as machined. When the part arrives and the surface is not what you pictured, the fix costs more than the spec would have.

Ordering one at a time. The setup is the expensive part of a stainless job. Two of the same part rarely costs twice one.

Asking a general shop to learn on the job. A shop that cuts mild steel all week can machine 316L. It will just take longer, wear more tooling and price the uncertainty in. The parts that need a stainless specialist are covered in the self check on the home page.

The short version

Pick the grade from the environment, not from habit. Put the finish and the passivation standard on the drawing with their standard numbers. Say which certificates you need back. Then send the drawing to shops that run these grades regularly rather than to whoever is cheapest on a list.

Frequently asked questions

Figures in this guide are published specification limits or published typical values for the alloy, not measurements of any part. Machinability is rated against free-machining carbon steel B1112 at 100 and PREN is chromium plus 3.3 times molybdenum plus 16 times nitrogen. The conventions are set out in thereference section and the editorial standards.

Keep reading

• When to Machine 17-4 PH and When to Age It
• Duplex 2205 and the Parts That Actually Need It