Stainless is the second most common material in our metal CNC queue. Most projects do not need to specify a particular grade, but for medical, food, marine, or pressure-rated parts the choice matters.
Headline properties
| Property | 304 | 316L | 17-4 PH (H1025) |
|---|---|---|---|
| Tensile (MPa) | 515 | 485 | 1170 |
| Yield (MPa) | 205 | 170 | 1070 |
| Hardness (HRC) | ~90 HRB | ~80 HRB | 33-38 |
| Magnetic | Slightly (cold-worked) | Non-magnetic | Magnetic |
| Pitting resistance (PRE) | ~18 | ~25 | ~16 |
| Weldability | Excellent | Excellent | Good (with PWHT) |
| Machinability | Tough | Tough | Better when annealed |
| Relative cost | 1.0x | 1.4x | 1.6x |
304: the default stainless
304 (also known as A2 or 1.4301) is the cheapest, most available austenitic stainless. It works beautifully in kitchens, indoor industrial environments, food contact, and most architectural applications.
316L: when chloride is in the picture
316L (A4 or 1.4404) adds 2-3 percent molybdenum, dramatically improving resistance to chloride pitting. It is the go-to for coastal, marine, pharmaceutical, and process plant equipment. The 'L' designation means low carbon — better for welded structures because it resists sensitization.
304 in marine atmosphere will pit and rust. 316L lasts decades. The price difference is small; the lifetime difference is large.
17-4 PH: when strength matters
17-4 PH is precipitation-hardenable and reaches twice the strength of 304/316. It is the stainless of choice for shafts, valve stems, fasteners, and wear-prone parts. Note that its corrosion resistance is closer to 410 than 316.
Decision matrix
| Need | Pick |
|---|---|
| Indoor general use | 304 |
| Food contact, dairy, beverage | 304 (electropolished) |
| Marine / coastal | 316L |
| Pharma, sterile, CIP/SIP | 316L (electropolished) |
| High-strength shaft, fastener | 17-4 PH |
| Magnetic compatibility critical | 316L (non-magnetic) |
Machining notes
All three are tough on tools. Run lower speeds, higher feeds, and rigid setups. Coolant matters more than on aluminum. 17-4 PH is best machined in the annealed (Condition A) state and heat treated to H1025 or H1075 after machining where shape allows.
Passivation and finish
Stainless parts machined and handled in a metals shop pick up free iron. Passivation per ASTM A967 (citric or nitric acid bath) restores the chromium oxide layer. Specify passivation explicitly on the drawing for any part exposed to corrosive service.
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