304 vs 316 Stainless Steel for Machined Parts

The one alloying element that decides it

304 and 316 are both austenitic stainless steels, both non-hardenable by heat treatment, and both built on the same chromium-oxide passive film that makes stainless steel stainless. If you put the two beside each other on a bench, you could not tell them apart.

The difference is molybdenum. 316 carries 2–3% of it; 304 carries essentially none. That is the whole story, and every practical consequence flows from it: what the grade survives, what it costs, and how it machines.

The chemistry, roughly:

	304	316
Chromium	18–20%	16–18%
Nickel	8–10.5%	10–14%
Molybdenum	—	2–3%
Carbon (standard / L grade)	≤0.08% / ≤0.03%	≤0.08% / ≤0.03%
Hardenable by heat treatment	No	No
Typical yield strength (annealed)	~215 MPa	~205 MPa
Relative material cost	Baseline	Typically 30–60% higher
Chloride pitting resistance	Moderate	Good
Machinability	Moderate — work-hardens	Slightly tougher

Notice how close the mechanical properties are. Yield, tensile and elongation are effectively the same. This is not a strength decision, and treating 316 as “the stronger stainless” is the most common misunderstanding in this comparison. Strength on a stainless drawing comes from 17-4PH or from cold work, not from moving between these two.

What molybdenum actually does

Stainless steel protects itself with a passive film of chromium oxide a few atoms thick, which re-forms instantly when scratched — provided there is oxygen and provided the film is not being attacked faster than it can rebuild. Chloride ions attack it. They break the film down at local points, the exposed metal corrodes, and the result is a pit that grows under a crust of corrosion product. On a machined part this looks like rust spotting, usually in a thread root, under a washer face, or in a pocket that holds liquid.

Molybdenum makes the passive film markedly more resistant to that specific attack. It does not make the alloy stronger and it does not make it more abrasion-resistant. It buys resistance to chloride pitting and crevice corrosion — and nothing else.

That narrow benefit is worth a great deal in the right place and nothing at all in the wrong one. A 316 bracket sitting in a dry indoor machine enclosure has paid a 40% material premium for a property it will never exercise.

Where 304 is the right answer

304 is the workhorse austenitic grade and covers a very wide range of duties honestly:

• Fresh water and potable water systems
• Food contact and beverage equipment, where the surface is cleaned and passivated
• Indoor machinery, enclosures, brackets, covers and frames
• Most industrial and rural atmospheres, including mildly polluted air
• Many mild chemicals and process fluids, where the concentration and temperature are known
• General fasteners, spacers, shafts and fittings in benign environments
• Anywhere the part is painted, powder coated or otherwise isolated from the environment

For these duties there is no performance argument for 316, and the cost difference is real. On a production run the saving is permanent and per-part.

Where 304 is the wrong answer

The failures are almost always chloride-driven, and they cluster in recognisable places:

• Marine and coastal. Salt spray carries chloride to surfaces that never get wet directly. Coastal hardware in 304 pits in months to a couple of years.
• De-icing salt. Road and airport hardware sees concentrated chloride seasonally, and it stays on the part in crevices.
• Bleach, chlorine and chloride-based sanitisers. Common in food, beverage and pool equipment, and frequently more aggressive than the duty was described as.
• Crevices. Under a washer, in a thread root, between two mating faces, under a gasket, in a weld undercut. Chloride concentrates where liquid sits and oxygen cannot reach, and this is where pitting starts even in moderate chloride.
• Stagnant or pooling liquid. A drain line that holds fluid, a pocket that traps water, a tank that is not emptied.
• Elevated temperature. Chloride attack accelerates with temperature. A duty that is fine at ambient can pit at 60 °C.

If any of those describe the part, 316 — usually 316L — is the specification, not the upgrade. See the marine parts page for how the same logic plays out on deck hardware and through-hull fittings.

One related note that is not really about grade choice: machining leaves free iron smeared into the surface from the tooling, and that iron corrodes first even on a perfectly correct alloy. Passivation removes it and restores the natural oxide film, which is why a machined stainless part is normally passivated after cutting rather than shipped as-machined. Our passivation page covers what it does and does not fix — it will not rescue a 304 part in a chloride duty.

Six specification mistakes on stainless drawings

These come up often enough to be worth listing, because each one costs either money or a failed part.

1. Writing “stainless steel” without a grade. Stainless is a family, not a material. The shop will pick something — probably 304 — and the shop’s choice may not match your environment. Name the grade.

2. Specifying 316 with no chloride in the duty. This is the most common way to spend money for nothing. If the part lives indoors, in fresh water, in food service or in a benign atmosphere, 304 does the job at a lower cost, and the saved premium can go into tolerance or finish where it buys something.

3. Specifying 304 for a coastal or marine part. The mirror image of the same mistake, and the more expensive one, because it fails in service. Coastal air carries chloride to parts that never get wet.

4. Forgetting the L grade on a welded part. A 304 part with a welded joint and no post-weld treatment will corrode beside the weld. The fix costs nothing at the drawing stage and cannot be applied after the fact.

5. Reading a hardness callout onto a 304 or 316 part. Neither grade hardens. A drawing that asks for 304 at HRC 30 is asking for something the alloy cannot deliver, and the quote will either come back with a question or the part will be supplied soft and fail later. If hardness is the requirement, the grade changes — usually to 17-4PH, 410 or 416.

6. Assuming a machined part is non-magnetic because the grade is austenitic. Cold work makes it slightly magnetic. If the specification matters — instrument hardware, sensor bodies, some electrical work — it belongs on the drawing as a requirement, and 316L is the better starting point than 304.

How the two grades are quoted

Both are quoted by the same variables: size and product form, quantity, tolerance, finish and the environment you describe. Three of those interact in ways worth knowing before you send the drawing.

Form drives the price more than the grade does on small quantities. 304 bar in a common diameter is the most available stainless product there is. 316 in an unusual diameter, or a heavy plate, may have to come from a mill order and add days. A change to a stocked size is sometimes a larger saving than a change of grade.

Quantity amortises the setup. Stainless work-hardens, so the machining sequence is planned rather than improvised, and the setup is a real part of the cost on a small batch. Orders start at 50 pieces per style and material, and samples from a single piece — enough to prove the part before a production run commits.

Environment changes the recommendation, not just the grade. “Marine duty, drains freely, not immersed” and “marine duty, permanently immersed, warm” are different problems. The first is 316 and a sensible design. The second is outside what these two grades should be quoted into, and we would rather say so at the quotation stage than supply a part that pits in the first season.

Material is bought against your drawing for your job rather than pulled from a shelf. If a grade, size or temper became difficult to source, you would hear about it before anything was cut — we do not downgrade a material quietly. Details of the mill traceability that ships with each batch are on the quality documents page.

Machining: both work-harden, 316 slightly more

Neither grade is free-cutting, and the reason is the same for both: austenitic stainless work-hardens under the cutter. A pass that is too light rubs the surface instead of cutting it, and the rubbed surface comes back harder than the material underneath. The next pass then cuts hardened metal, wears the tool faster, and hardens the surface further. The fix is a decisive depth of cut, consistent feed and a sharp tool — never a gentle finishing pass to “sneak up” on a dimension.

Stainless also conducts heat poorly, so the heat generated at the cutting edge stays there rather than spreading into the part. Controlled speeds and coolant protect tool life on both grades.

316 is the tougher of the two. It is slightly gummier, produces a stringier chip, and shows up as shorter tool life and a longer cycle than the same part in 304. The difference is not dramatic, but on a high-volume turned part it is enough to show in the unit price. Our stainless steel machined parts page covers how each grade behaves on the floor.

If machining cost is the driver and the environment is benign, the answer is usually not 316 — it is 303, the free-machining variant of 304. Sulphur added to the melt breaks the chip cleanly, so 303 turns and threads far faster and is the practical default for high-volume stainless fasteners. The trade is explicit: 303 gives up corrosion resistance and welds poorly, so it does not belong in chlorides or in a welded joint. Most of the stainless fasteners we produce run on CNC turning or automatic screw machines, where that machining advantage compounds across the batch.

Both 304 and 316 hold ±0.01 mm on precision features with ISO 2768-m as the general block. Grade changes cost, not tolerance — see tolerances we hold.

The L grades, and why welding decides them

When standard 304 or 316 is welded, chromium carbides can form at the grain boundaries in the heat-affected zone. Carbide formation pulls chromium out of the surrounding metal, and chromium is what makes the passive film. The result is a narrow band beside the weld that corrodes earlier than the rest of the part — sensitisation.

The low-carbon grades exist to prevent exactly that. 304L and 316L hold carbon below roughly 0.03%, low enough that the carbides do not form. That is why the L grade is specified whenever a part is welded and then returned to service without a post-weld solution treatment.

The practical rule:

• Machined only, no welding — 304 or 316 is fine.
• Welded, then in service — 304L or 316L.
• Welded and then annealed — the standard grade is acceptable, because the anneal dissolves the carbides.

There is a small strength trade: the L grades have marginally lower minimum tensile values than their standard counterparts. For most machined parts this is irrelevant, and it is a poor reason to accept a sensitised weld.

Temperature and magnetism

Temperature. Both grades hold useful strength at elevated temperature, and 316 is slightly better at the top of the range because of its higher nickel and molybdenum content. Neither is a high-temperature alloy in the sense that a nickel superalloy is — if the duty runs hot and loaded, that is a conversation to have with the drawing rather than a reason to pick 316 over 304.

Magnetism. In the annealed condition both are essentially non-magnetic. Cold working changes that: turning, forming and thread rolling transform some austenite to martensite, and the part becomes slightly magnetic. A heavily cold-worked 304 fastener can be noticeably magnetic, while 316 usually stays closer to non-magnetic because its higher nickel stabilises the austenite better.

If a specification demands non-magnetic, put it on the drawing. It is not a property you can assume from the alloy name on a part that has been machined.

Cost, and when the premium is wasted

316 typically costs 30–60% more per kilo than 304, depending on product form and the market at the time, and it machines a little slower on top. Over a production run that premium is permanent and per-part.

The honest test before specifying 316 is a single question: will this part see chlorides? If the answer is a clear yes — marine, coastal, de-icing salt, bleach, chloride process fluid, or a crevice that holds liquid — pay for the molybdenum. If the answer is no, or “probably not”, the money buys nothing and the design should address the real risk instead.

There is a second move that is often better than upgrading the alloy: design out the crevice. A part that drains, that avoids tight gasket faces, that has no pocket to trap liquid, and that can be cleaned will outperform the same geometry in a higher grade. A 304 part with good drainage beats a 316 part that holds salt water in a thread root.

When neither grade is right

Two requirements sit outside this comparison entirely, and both are common.

Hardness. Neither 304 nor 316 can be hardened by heat treatment, so neither is a wear part. Where the drawing needs hardness, the answer is 17-4PH (precipitation-hardening, high strength with useful corrosion resistance), or 410 and 416 (martensitic, hardenable, magnetic, with moderate corrosion resistance). 416 is the free-machining of the two and is the usual turned-part choice. All three are on our confirmed material list.

Load plus corrosion. Where a part has to carry significant load and survive a corrosive environment, 17-4PH is usually the right answer rather than a heavier section in 316. It approaches alloy-steel strength while keeping far better corrosion behaviour than carbon steel.

Strong acids and high-chloride process duty. Some duties sit beyond the grades in this comparison. Rather than quote something that will not hold, we will tell you the duty is outside our confirmed range and explain what the requirement actually calls for.

The callout that belongs on the drawing

Instead of	Write
Stainless steel	304, or 316L — name the grade
316	316L if the part is welded; 316 if it is not
Stainless, then passivate	304, passivate after machining, mask the threads
Non-magnetic stainless	316L, state the magnetic requirement explicitly
Stainless, hard	17-4PH (or 416 for a turned wear part) — not 304 or 316

Add the environment. “Marine duty, salt spray, part drains” tells us more than any grade name, because it lets us say whether 316 is sufficient or whether the duty needs something further up. Send the drawing, the grade and the service environment, and the quote comes back within 24 hours with DFM feedback. Orders start at 50 pieces per style and material, samples are available from a single piece, and a material certificate (MTR) tracing the grade to its mill heat ships with every batch — see what ships with your order.