303 vs 304 Stainless Steel for Machined Parts

303 and 304 are the same stainless steel with one deliberate difference: 303 has 0.15–0.35% sulfur added to the 304 base alloy. That tiny addition transforms how the material behaves on a machine — sulfur breaks the chips short and lubricates the cut, turning a gummy, work-hardening steel into a free-machining one. But the same sulfur that makes 303 easy to cut slightly degrades its corrosion resistance and makes it hard to weld. Everything else about the two grades — the 18% chromium, 8% nickel, the look, the density — is essentially identical. The whole 303 vs 304 decision is a trade of machinability for corrosion and weldability.

303 is the free-machining stainless: the sulfur forms manganese-sulfide inclusions that break chips short, so cuts are crisp, tools last, and cycle times drop. It is among the easiest stainless steels to machine, period. 304 is the opposite — gummy and work-hardening. It smears rather than shears, welds chips to the tool edge, and hardens the surface the moment a tool rubs instead of cuts, so it demands slower speeds, sharper tools, and constant positive feed. On a part full of threads, small bores, and turned features, 303 can machine significantly faster and cheaper than 304 — which is exactly why it exists. Our stainless steel machining page covers the process details for both.

Mechanically the two are near twins. 304 runs about 205–215 MPa yield and 505–515 MPa tensile; 303 lands around 190–275 MPa yield and 500–690 MPa tensile, with more spread because published values vary by form and supplier. The strength difference is not the decision driver. On price they are effectively even — 303 is sometimes quoted a hair cheaper (around 0.95× vs 304’s baseline), but the real savings from 303 show up in machining time and tooling, not the material line. So pick 303 for cut cost, not because the stock is cheaper.