SS · stainless
Stainless Steel 303
The free-machining austenitic, and the reason the rest of the family looks so hard by comparison. Published index around 78% on the B1112 = 100% scale, against roughly 45% for 304.
Quoting this material
- Typical lead time
- 7–12 working days
- Condition
- Annealed, or cold drawn for bar stock with a higher tensile
- Achievable finish
- Ra 1.6 µm typical · Ra 0.4 µm best
Machined in-house to your drawing: see the precision CNC machining service and the machine and inspection capability it runs on.
Machinability
Our own 1–10 shop ranking, not a published index: 10 is free-cutting brass C360, 1 is Inconel 718. It ranks how a material behaves on the floor - cycle time, chip control and tool life together. Where a published index exists it is quoted in the note.
The free-machining austenitic, and the reason the rest of the family looks so hard by comparison. Published index around 78% on the B1112 = 100% scale, against roughly 45% for 304.
Properties as supplied
| Parameter | Value | Unit |
|---|---|---|
| Tensile strength | 585 | MPa |
| Yield strength (0.2%) | 230 | MPa |
| Elongation | 40 | % |
| Hardness | 228 | HB |
| Thermal conductivity | 16.2 | W/m·K |
| Modulus of elasticity | 193 | GPa |
Hardness, tensile and thermal figures are typical published values for the condition stated. Actual mill certificates govern; send the specification with the drawing and we confirm against the certificate before the material is cut.
Condition
Annealed, or cold drawn for bar stock with a higher tensile
Heat treatment
Austenitic stainless is not hardenable by heat treatment - only by cold work. Annealing softens it if the part has been cold worked, but that is a stress-relief operation, not a strengthening one.
Achievable surface finish
Ra 1.6 µm
Ra 0.4 µm
Takes a good finish and passivates cleanly, though the sulphur that makes it free-cutting also gives slightly lower corrosion resistance.
Tooling and cutting parameters
Cutting speed
- 120–200 m/min (carbide)
Feed
- 0.10–0.25 mm/rev turning · 0.08–0.15 mm/tooth milling
Tooling
- TiAlN or AlTiN coated carbide, sharp edge, avoid a honed geometry
Coolant
- Flood, high pressure if available - never let the tool rub
Starting points for carbide on a rigid setup, not a guarantee. The right numbers depend on the machine, the toolholder and the fixturing - ask and we will say what we would actually run.
Typical applications
- Screws, nuts and fasteners
- Instrument fittings and adapters
- Valve and pump internals
- Shafts running in stainless housings
- Marine hardware not exposed to chlorides
DFM notes for this grade
- Not weldable in structural service, and the sulphur lowers corrosion resistance - if the part gets welded or sees chlorides, use 304 or 316L
- The gain is real: a 303 part often costs 30–40% less than the same part in 304 because the cycle time drops and the tooling lasts
- Cold-drawn bar machines better than annealed bar at the same nominal grade - specify the temper if the part is turned from bar
Questions buyers ask
- Is the corrosion difference between 303 and 304 significant in practice?
- For a dry indoor part, no. For anything that sees chlorides, condensation cycles or a washdown, yes - the sulphur inclusions are where pitting starts. We would push back on 303 for a food or medical part rather than quote it quietly.
- Why does 303 still gall if it machines so well?
- Machining and galling are different failure modes. Free-machining additives help the chip form; they do nothing about two stainless surfaces sliding under load, which is a galling problem. For that you change the pair - a stainless shaft against a bronze bush, not against another stainless part.
Grades we would quote instead
304 / 304L
4/10Work-hardens faster than the tool can cut if the feed drops.
416
6/10The martensitic free-machining grade - the best-machining stainless on this list and the one that hardens after machining, unlike the austenitics.
316L
3/10Harder to machine than 304 and it galls more, but it is the grade for chlorides, medical and marine.