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Choosing a stainless grade: 303, 304, 316 and 17-4 PH

July 21, 2026

Stainless steel is the second most common material in international machining requests and the one that surprises buyers most on price. The grade you choose changes machining time, tool cost and sometimes the entire supplier list.

The grades worth knowing

304, the general purpose austenitic. Good corrosion resistance, non magnetic in the annealed state, weldable, widely stocked. The default for brackets, enclosures, food contact parts and general hardware.

316, the chloride resistant one. Adds molybdenum, which resists pitting in salt and chemical environments. Slightly tougher to machine than 304. Specify it where the environment demands it, not as a general upgrade.

303, the free machining grade. Sulphur added for chip breaking, which makes it dramatically faster to turn. Lower corrosion resistance, poor weldability. Ideal for shafts, fittings and turned components at volume in benign environments.

416, the free machining martensitic. Can be hardened, magnetic, machines well. Used for shafts and valve components where some hardness is needed.

17-4 PH, the precipitation hardening grade. High strength, good corrosion resistance, hardenable to a range of conditions such as H900 or H1150. Common in pumps, valves and aerospace hardware. Machining in the annealed condition then ageing is the normal route, and the part moves slightly during ageing.

Grade	Relative machinability	Corrosion	Typical use
303	High	Moderate	Turned parts, fittings
304	Moderate	Good	General parts, enclosures
316	Moderate, lower than 304	Excellent in chlorides	Marine, chemical, medical
416	High	Moderate	Shafts, hardened components
17-4 PH	Moderate	Good	High strength components

What stainless does to your quote

Three effects, all in the same direction:

1. Cutting speed. Where aluminium runs at high surface speed, stainless runs at a fraction of it. The same pocket takes several times longer.
2. Tool wear. Carbide tooling wears steadily in stainless and the shop replaces inserts more often. That cost is in your hour rate.
3. Work hardening. If a tool dwells or rubs rather than cutting, the surface hardens and the next pass fights it. This is why interrupted cuts and thin walls are harder in stainless than in aluminium.

The practical consequence for design: features that are mildly expensive in aluminium become genuinely expensive in stainless. Deep pockets, small corner radii and thin walls should all be reviewed once you switch material.

Finishing and passivation

Machined stainless is not automatically corrosion resistant. Cutting embeds free iron in the surface, which rusts and makes the part look like it was never stainless. Passivation removes it and lets the chromium oxide layer reform.

Specify it where corrosion matters, name a standard such as ASTM A967 and say whether nitric or citric is acceptable. For medical and food applications this is not optional.

Electropolishing goes further, smoothing the surface and improving both appearance and cleanability. It removes a small amount of material, so tight tolerances need to account for it.

Magnetism, which catches people out

Austenitic grades such as 304 and 316 are non magnetic when annealed and become slightly magnetic after cold work or heavy machining. If your part must be non magnetic for a sensor or an instrument, say so in the request, give the acceptance criterion and expect a solution annealing step.

Availability in Vietnam

304 and 316 in plate, bar and tube are stocked locally and priced competitively. 303 and 416 are available with a little notice. 17-4 PH and specialty grades are usually imported to order, and a certified heat with a mill certificate adds lead time.

If your part needs full traceability, say it in the first message. It changes both the supplier list and the schedule.

What to put in your request

Material: stainless 316, local stock acceptable, no certified heat required. Passivate per ASTM A967 citric. Part is used outdoors in a coastal installation.

The last sentence is the useful one. A shop that knows why you chose 316 can tell you if 304 would do, or if the design needs something else entirely.

Design changes that pay for themselves in stainless

The same geometry costs several times more in stainless than in aluminium, so features that were harmless become expensive. Four changes usually recover most of the difference:

1. Open the internal corner radii. A larger cutter in stainless is worth far more than in aluminium, because the machine is limited by tool load rather than by spindle speed.
2. Reduce pocket depth or widen the pocket. Long tools chatter in stainless, and chatter work hardens the surface, which makes the next pass worse.
3. Cut the number of tapped holes, or use a coarser thread. Tapping stainless is where taps break, and a broken tap in a nearly finished part is a scrapped part.
4. Question every tight tolerance. Stainless moves as internal stress relaxes, so holding ±0.01 mm needs a rough pass, a rest and a finish pass. That is a real cost, not caution.

Asking the right question in the request

If the environment drives your material choice, describe the environment rather than only naming the grade:

Part sits in an outdoor enclosure 500 m from the sea, washed down with fresh water monthly. Currently specified 316. Advise if 304 with passivation is sufficient.

A shop that answers that question well has engineers, not just machines. A shop that just prices the 316 has told you something too.

See also the guides on material substitution and on what drives the price of a machined part. When your drawing is ready, send it here.

Questions we get on this

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