Quick answer
Choose 304 unless the part will actually see chlorides. For most CNC machined parts — brackets, enclosures, fixtures, shafts, indoor equipment — 304 stainless costs less, machines faster, and holds up perfectly well. Move to 316 when the part will live near salt water, brine, de-icing salts, or aggressive chemicals: marine hardware, food processing equipment, chemical plant components, medical instruments, and anything mounted outdoors on the coast.
The gap in material price is typically 20–40%, and 316 is harder on cutting tools. That premium is only worth paying when the environment justifies it.
304 vs 316 at a glance
| Property | 304 | 316 |
|---|---|---|
| Alloy makeup | 18% Cr, 8% Ni | 16% Cr, 10% Ni, 2% Mo |
| Tensile strength (annealed) | ~515 MPa | ~515 MPa |
| Yield strength | ~205 MPa | ~205 MPa |
| Elongation | ~40% | ~40% |
| Hardness | ~70 HRB | ~79 HRB |
| Density | 8.0 g/cm³ | 8.0 g/cm³ |
| Pitting resistance (PREN) | ~19 | ~24 |
| Machinability rating | ~45% | ~40% |
| Material cost | Baseline | +20–40% |
| Magnetic? | No (annealed) | No (annealed) |
The mechanical numbers are almost identical — and that is the point. You are not choosing 316 for strength. You are choosing it for corrosion resistance.
1. The real difference: molybdenum
Both grades are austenitic stainless steels with a chromium oxide layer that self-heals in oxygen. What separates them is roughly 2% molybdenum in 316.
Molybdenum dramatically raises resistance to pitting and crevice corrosion in chloride environments. The standard way to express this is the Pitting Resistance Equivalent Number (PREN = %Cr + 3.3 × %Mo + 16 × %N). 304 sits around 19; 316 sits around 24. That ~25% jump is the entire reason 316 exists.
In practice: a 304 bracket on a machine indoors will outlast the machine. The same bracket bolted to a railing 200 metres from the sea will show rust staining within a year or two. Same bracket in 316 will not.
2. Machinability: 316 costs more to cut, too
Both grades are “gummy” compared with carbon steel — they work-harden instead of shearing cleanly, and they produce tough, stringy chips. 316 is the worse of the two:
- Lower machinability rating (~40% vs ~45% relative to free-cutting steel) — slower feeds, shorter tool life.
- Work-hardens faster. A tool that dwells or rubs instead of cutting will produce a hardened skin that wrecks the next pass.
- Built-up edge is more common, which degrades surface finish if parameters are not tuned.
What this means for your quote: a 316 part is not just more expensive in raw material, it usually carries a higher machining cost as well. On complex parts the total difference can exceed the raw material premium.
3. Corrosion in the real world
- Dry indoor air, general industrial environments: 304 is fine.
- Fresh water, food products, mild chemicals: 304 is fine.
- Salt spray, coastal outdoor, de-icing salt: use 316.
- Brine, chlorides, acids, bleach, chlorinated cleaners: use 316.
- Welded assemblies that will never be annealed: choose the low-carbon variants (304L / 316L) to avoid sensitisation in the heat-affected zone.
One caveat that surprises people: 316 is not “rust-proof”, it is “rust-resistant”. In hot, stagnant, high-chloride conditions even 316 pits. If your environment is that aggressive, surface treatment matters as much as grade — see our surface finishes page on passivation and electropolishing.
4. Welding and fabrication
Both weld readily by TIG and MIG. Two points worth specifying on the drawing:
- Ask for the L grade if you weld. 304L and 316L have lower carbon, which prevents chromium carbide precipitation at the weld line — the classic cause of rust appearing right next to a weld.
- Passivate after welding and machining. Machining embeds free iron particles from the cutting tool into the surface, and those particles rust. Passivation removes them and rebuilds the oxide layer. It is cheap and it is the single most commonly skipped step.
5. Cost: what you are actually paying for
Expect 316 to run 20–40% above 304 on raw material, with the exact spread moving with nickel and molybdenum markets. On top of that, budget for:
- Slower cycle times and more frequent tool changes
- Higher scrap risk on thin-walled parts (distortion from heat)
- Passivation, if not already in your specification
Our rule of thumb: if the part will never see chlorides, the premium buys you nothing. Spend it on tighter tolerances or a better finish instead.
Which grade for your application?
| Application | Recommended grade | Why |
|---|---|---|
| Machine frames, brackets, fixtures (indoor) | 304 | No chloride exposure, lower cost |
| Equipment enclosures, control panels | 304 | Finish and corrosion are more than adequate |
| Food & beverage machinery (no brine) | 304 | Standard food-grade material |
| Food processing with salt / brine / pickling | 316L | Chloride resistance + weldability |
| Marine hardware, boat fittings, coastal outdoor | 316 | Salt spray is the classic 304 failure mode |
| Chemical processing, pumps, valves | 316 / 316L | Acid and chloride resistance |
| Medical instruments and device housings | 316 / 316L | Body-fluid and sterilant resistance |
| Shafts, pins, fasteners near the sea | 316 | Crevice corrosion at threads kills 304 |
| Decorative / architectural (indoor) | 304 | Polishes beautifully, costs less |
Design and machining tips
- Keep the tool cutting. Never let it dwell — stainless work-hardens under a rubbing tool.
- Use sharp, positive-rake carbide inserts and replace them on schedule rather than on failure.
- Avoid very light finishing passes that ride over the work-hardened layer; leave 0.2–0.3 mm for a real finishing cut.
- Use plenty of coolant. Stainless conducts heat poorly — heat goes into the tool, not the chip.
- Specify 304L / 316L on any part that will be welded.
- Let parts reach room temperature before final inspection. Stainless expands and moves noticeably with heat; measuring a warm part gives you a false reading.
- Add passivation to the drawing. It is the difference between “stainless” and “stainless that stays stainless”.
FAQ
Is 316 worth the extra cost?
Only if chlorides are present. If your part lives indoors or in a normal industrial environment, 304 will perform identically for less money. If it goes on a boat, near the coast, into a food line with brine, or into a chemical process — yes, and it is the cheapest insurance you will ever buy.
Can I use 304 outdoors?
Inland and away from salt, yes. Within a few kilometres of the coast, or anywhere de-icing salt is used, 304 will develop surface rust staining. It is cosmetic at first and structural eventually.
Why did my stainless parts rust after machining?
Almost always one of two causes: free iron from the cutting tool embedded in the surface, or heat tint from welding. Both are removed by passivation. It is a surface contamination issue, not a material defect.
Why is my “non-magnetic” 304 part now magnetic?
Heavy cold work — including aggressive machining — can transform a small amount of austenite into martensite, which is magnetic. It is normal and does not harm corrosion resistance, but if magnetism matters to the application, say so on the drawing.
Should I specify 304L or 316L instead?
Yes, if the part will be welded. The L grades have lower carbon and resist sensitisation at the weld. Mechanically they are marginally softer, which is irrelevant for almost every machined part.
Get a quote for your stainless parts
Send us your drawing and tell us where the part will be used. If 304 will do the job we will say so. If the environment calls for 316, we will tell you before you find out the hard way. Every part ships with dimensional inspection records, and we provide inspection reports on request.
More on material choices: see our stainless and alloy steel CNC machining page, or send us your drawing for a quotation within 48 hours in the normal case.
