Stainless Steel Options: 303, 304, 316, and 17-4
Last updated September 26, 2026
Stainless steels are iron alloys with at least about 10.5 percent chromium. The chromium forms a self-repairing oxide layer on the surface, which is what stops the steel from rusting. Four grades cover most CNC work. The quote editor shows which grades are currently offered, with live pricing for your part; this guide explains what each grade is for.
303: the free-machining grade
303 has added sulfur, which makes it the easiest stainless to machine. It cuts fast, holds a good surface finish, and suits high-volume turned parts like fittings, shafts, and fasteners.
The tradeoff: slightly lower corrosion resistance than 304, and it cannot be hardened by heat treatment. If corrosion exposure is mild and you want the most economical machined stainless part, 303 is the usual pick.
304: the general-purpose grade
304 (often called 18/8, for roughly 18 percent chromium and 8 percent nickel) is the most common stainless in the world. It has good corrosion resistance, welds well, and is food safe.
It is gummier to machine than 303, so parts cost a bit more. Use 304 for enclosures, brackets, kitchen and lab equipment, and general hardware that sees moisture.
316: the corrosion-resistant grade
316 adds 2 to 3 percent molybdenum, which improves resistance to chlorides: saltwater, de-icing salts, and many chemicals. It is often called marine grade and is common in marine hardware, medical devices, and chemical processing equipment.
It machines much like 304 and costs more. If a part will live near salt water or aggressive cleaning agents, 316 is the safe choice.
17-4 PH: the high-strength grade
17-4 PH is a precipitation-hardening stainless that can be heat treated to very high strength, roughly 1,170 MPa yield in the common H900 condition, several times stronger than 304. It is magnetic and has moderate corrosion resistance, similar to 304.
Use it for high-stress shafts, gears, aerospace components, and anywhere you need steel-like strength plus corrosion resistance. Heat treatment adds cost and lead time compared to the austenitic grades above.
Quick comparison
| Grade | Yield strength | Corrosion resistance | Machinability | Typical uses |
|---|---|---|---|---|
| 303 | ~240 MPa | Good | Best of the four | Fittings, fasteners, shafts |
| 304 | ~215 MPa | Very good | Moderate | General hardware, enclosures |
| 316 | ~205 MPa | Excellent | Moderate | Marine, medical, chemical |
| 17-4 PH | Up to ~1,170 MPa | Good | Moderate | High-strength mechanical parts |
Yield values are typical annealed numbers; 17-4 PH is shown in the hardened H900 condition.
When stainless beats aluminum
- Thin or small features that must not bend or wear. Stainless is far harder and stiffer than aluminum.
- Wear surfaces and threads that see repeated use or sliding contact.
- Elevated temperatures where aluminum softens.
- Corrosive environments where you do not want to rely on a coating.
- Food, medical, and hygiene applications.
When aluminum wins
Aluminum is about one third the density of stainless and much faster to machine, so aluminum parts are usually lighter and cheaper. Unless you need stainless for one of the reasons above, 6061-T6 aluminum is the more economical choice.
Stainless parts on OpusFab can be ordered Raw (As-Machined), bead blasted, powder coated, or PVD coated. The Surface Finishes Overview explains what each finish does.