SS · stainless
Stainless Steel 321
A stabilised austenitic - same machining character as 304 with a little more bite, and the titanium addition is there to stop sensitisation at high temperature.
Quoting this material
- Typical lead time
- 10–15 working days
- Condition
- Annealed
- 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.
A stabilised austenitic - same machining character as 304 with a little more bite, and the titanium addition is there to stop sensitisation at high temperature.
Properties as supplied
| Parameter | Value | Unit |
|---|---|---|
| Tensile strength | 515 | MPa |
| Yield strength (0.2%) | 205 | MPa |
| Elongation | 40 | % |
| Hardness | 217 | HB |
| Thermal conductivity | 16.1 | 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
Heat treatment
Not hardenable by heat treatment. The titanium stabilisation means it tolerates service at 425–815 °C without the chromium carbide precipitation that would corrode 304.
Achievable surface finish
Ra 1.6 µm
Ra 0.4 µm
Passivates normally; used where the part runs hot rather than where finish is cosmetic.
Tooling and cutting parameters
Cutting speed
- 100–180 m/min (carbide)
Feed
- 0.10–0.25 mm/rev turning · 0.08–0.15 mm/tooth milling
Tooling
- AlTiN coated carbide, sharp edge
Coolant
- Flood; keep the tool cutting rather than rubbing
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
- Exhaust and manifold components
- Heat exchanger and furnace hardware
- Aerospace engine-adjacent parts
- Chemical process at elevated temperature
- Expansion joints and bellows
DFM notes for this grade
- Specify it for temperature service, not machinability - at room temperature 304 is equally good and cheaper
- Stabilisation limits the service-temperature corrosion problem rather than the distortion problem: it still moves when welded
- Chip control is better than 304 in the annealed state; cold-drawn bar is noticeably stiffer to cut
Questions buyers ask
- Where is 321 the right answer rather than 304?
- Sustained service between roughly 425 °C and 815 °C, especially with welding. In that range plain 304 forms chromium carbides at the grain boundaries and the part corrodes from the inside; the titanium in 321 holds the carbon so it cannot.
- Does 321 machine differently from 304?
- Practically, no - the same feeds and speeds, the same work-hardening risk, the same need for flood coolant. It is slightly tougher on the tool because of the titanium additions, and we would quote a small premium for tool life rather than for cycle time.
Grades we would quote instead
304 / 304L
4/10Work-hardens faster than the tool can cut if the feed drops.
316L
3/10Harder to machine than 304 and it galls more, but it is the grade for chlorides, medical and marine.
Inconel 600
2/10The mildest of the common nickel alloys to machine - still awkward, but it work-hardens less aggressively than 625 and far less than 718.