Stainless Steel Grades for CNC Machining: 303 vs 304 vs 316
Use 303 for general machined parts (best machinability), 304 for food and general corrosion resistance, 316 for marine and chemical environments, and 17-4 PH when you need both corrosion resistance and high strength (aerospace, valves).
Stainless steel grade choice affects machinability, corrosion resistance, and final cost by 2–4×. Picking the wrong grade for a marine or food application can cause part failure.
Quick Selection Guide
- **303** — easy machining, indoor parts, fasteners, fittings - **304** — food contact, kitchen, general outdoor (not marine) - **316** — marine, chemical, medical implants, brewery - **17-4 PH** — high strength + corrosion (valves, aerospace fittings)
Stainless Steel Grades Comparison
| Grade | Machinability | Corrosion | Strength | Cost |
|---|---|---|---|---|
| 303 | Excellent | Good | Medium | 1.0× |
| 304 | Fair | Very good | Medium | 1.1× |
| 316 | Fair | Excellent | Medium | 1.4× |
| 316L (low carbon) | Fair | Excellent (weldable) | Medium | 1.5× |
| 17-4 PH (H900) | Difficult | Very good | Very high (1300 MPa) | 2.2× |
Frequently asked questions
- Can 304 be used in seawater?
- Not recommended. 304 will pit in chloride-rich environments. Use 316 or duplex stainless (2205) for marine applications.
- Why is 303 easier to machine than 304?
- 303 contains added sulfur (0.15–0.35%) which acts as a built-in chip breaker, allowing higher feed rates and longer tool life. 304 work-hardens aggressively and is harder to machine.
- Is 17-4 PH magnetic?
- Yes — 17-4 PH is a martensitic stainless and is magnetic. 303, 304, and 316 are austenitic and essentially non-magnetic.