304 vs 316L
When should a machined part be 316L instead of 304 stainless?
Short answer
Choose 316L when the part meets chlorides, marine air, or a medical or pharmaceutical surface, because the molybdenum in 316L is what resists pitting and 304 cannot substitute for it. Otherwise 304 is the better machining grade: machinability 4 against 3, and 100–180 m/min against 80–150.
Side by side
| Property | 304 / 304L | 316L |
|---|---|---|
| Tensile strength | 505 MPa | 485 MPa |
| Yield strength (0.2%) | 215 MPa | 170 MPa |
| Elongation | 40 % | 40 % |
| Hardness | 201 HB | 217 HB |
| Density | 8.00 g/cm³ | 8.00 g/cm³ |
| Machinability (shop scale) | 4/10 | 3/10 |
| Cutting speed | 100–180 m/min (carbide) | 80–150 m/min (carbide) |
| Feed | 0.10–0.25 mm/rev turning · 0.08–0.15 mm/tooth milling | 0.10–0.25 mm/rev turning · 0.06–0.15 mm/tooth milling |
| Tooling | AlTiN or TiAlN coated carbide, sharp positive geometry, strong corner radius | AlTiN coated, sharp, strong edge; expect 2–3× the insert consumption of 304 |
| Coolant | Flood, high pressure ideally, aimed at the cutting edge | High-pressure flood; through-tool coolant wherever the geometry allows |
| Surface finish | Ra 1.6 µm typical, Ra 0.4 µm best | Ra 1.6 µm typical, Ra 0.4 µm best |
| Lead time | 7–12 working days | 8–14 working days |
| Condition as supplied | Annealed (or cold drawn bar with higher tensile) | Annealed, low carbon for welded assemblies |
Machinability is our shop ranking, not a published index. Cutting data is a starting range: the holder, the depth of cut and how the part is held move it.
Choose 304 / 304L when
The environment is not chloride-bearing
304 resists atmospheric and fresh-water corrosion. Paying for molybdenum in a dry indoor part buys nothing.
Cost and cycle time decide the quote
304 cuts at 100–180 m/min and 316L at 80–150, so the same part takes longer in 316L before material cost is counted.
The part is a shaft, pin, spacer or general stainless component
At 505 MPa tensile and 201 HB, 304 covers the ordinary load case without the galling that 316L brings.
Choose 316L when
The part sees chlorides, seawater or de-icing salt
Molybdenum is the alloying difference that resists pitting; 304 pits where 316L does not.
The part is a welded assembly that must stay corrosion-resistant
316L is the low-carbon grade, which keeps carbide precipitation out of the weld zone.
The surface will be electropolished for medical or pharmaceutical use
Electropolishing 316L leaves a cleanable surface that resists both corrosion and bacterial retention.
The parts are sterilised or cleaned with chloride-bearing agents
Repeated chloride exposure is the case that separates the two grades in service rather than on the drawing.
The short version
Buyers rarely regret 316L on a chloride part and often regret 304. On everything else, 304 machines faster and costs less, and choosing 316L for a dry indoor bracket is money spent on a property the part never uses.
What bites in 304 / 304L
- ·Never take a light finishing pass to “clean up” a dimension - that is exactly the cut that work-hardens the surface and ruins the finish
- ·On thin walls, the material work-hardens as it deflects, so support it and take fewer, deeper cuts
- ·Threading and tapping are the slowest operations on a 304 part; a form tap instead of a cut tap changes the cycle time and the tool life
What bites in 316L
- ·The molybdenum addition is what resists chlorides - do not accept a 304 substitution on a chloride job however similar the datasheet looks
- ·It galls: two 316L surfaces under load will seize unless one is a different material or the surfaces are hardened and lubricated differently
- ·Low carbon means lower yield: a 316L part is slightly less stiff in a press fit than a 304 part, which matters on thin-wall designs