SS · 10 grades
Stainless Steels
Corrosion resistance with a machining cost attached - the grade decides how much.
Stainless is specified for what it does not do: rust, react, contaminate. What it does do is work-harden, which is why 304 on a light finishing pass is the classic way to scrap a part. 303 is the free-machining stainless and the one to choose if the drawing allows it; 316L is the one for chlorides and medical; 17-4 PH is where strength after heat treatment matters.
What this family does well, and where it bites
- 303 machines like a mild steel; 304 and 316 do not - expect 1.5–2× the cycle time
- Work hardening is the main risk: never dwell, never take a light spring pass
- Passivation and electropolishing give a clean, verifiable surface
- Ferritic and duplex grades need different tooling and different feeds entirely
Stainless Steels grades we machine
Each grade page carries the properties as supplied, what heat treatment does or does not do to it, the achievable surface finish, starting cutting parameters and the DFM notes that matter for that specific grade.
303
6/10228 HB · 585 MPa
The free-machining austenitic, and the reason the rest of the family looks so hard by comparison.
304 / 304L
4/10201 HB · 505 MPa
Work-hardens faster than the tool can cut if the feed drops.
316L
3/10217 HB · 485 MPa
Harder to machine than 304 and it galls more, but it is the grade for chlorides, medical and marine.
17-4 PH
4/1032 HRC (annealed) · 44 HRC (H900) · 1,310 MPa (H900)
In the annealed condition it machines like a tough 304; in H900 it is hard enough that finishing has to be done by grinding or hard turning.
416
6/10262 HB (annealed) · to 40 HRC hardened · 585 MPa (annealed)
The martensitic free-machining grade - the best-machining stainless on this list and the one that hardens after machining, unlike the austenitics.
430
5/10155 HB · 450 MPa
Ferritic - no nickel, magnetic, and it cuts cleanly without the work-hardening fight of the 300 series.
2205 Duplex
2/10290 HB · 620 MPa
Roughly twice the yield of 316L and about twice as unpleasant to machine.
15-5 PH
4/1030 HRC (annealed) · 40 HRC (H1025) · 1,170 MPa (H1025)
The same family as 17-4 PH with better toughness and fewer large-section problems.
321
4/10217 HB · 515 MPa
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.
420
4/10235 HB (annealed) · to 50 HRC hardened · 1,450 MPa (hardened) · 655 MPa (annealed)
Martensitic and hardenable to about 50 HRC, which is the point of it.
Questions buyers ask
- Why is 303 not the default if it machines so much better?
- Because it is not weldable and its corrosion resistance is a step below 304. If the part gets welded or spends its life in a chloride environment, 303 is the wrong answer however cheap it is to machine.
- What does passivation actually do?
- It removes free iron left on the surface by machining and lets the chromium oxide layer form properly. On a machined 316L part it is the difference between a surface that resists pitting and one that starts pitting at the first chloride exposure.