Stainless 303 & 304 Parts

Two grades, one decision

Both are 18-8 austenitic stainless: same family, same look, similar corrosion behavior in ordinary service. The difference is a small sulfur addition in 303 that makes chips break cleanly, which means faster cycles, better as-cut surface finishes, and lower part prices on machining-heavy work. The same sulfur is why 303 welds poorly and gives up a little corrosion resistance at the margins.

So the decision is mostly about what happens to the part after machining. Nothing welded, no food contact, indoor or mild outdoor duty: 303, and pocket the savings. Welded into an assembly, washed down, touching food or potable water, or living somewhere genuinely wet: 304.

What gets made from each

303 dominates turned work: fittings, shafts, pins, standoffs, and valve and instrument components. Anywhere a lathe-shaped part needs to shrug off rust without paying a stainless premium in machining time. 304 shows up as brackets, covers, enclosures, food-process and fluid-handling parts, and anything destined for a welded frame. Typical tolerances match the rest of the site: ±0.005 in on milled features, ±0.001 in on critical turned diameters.

Passivation and finishes

Machining can leave traces of free iron on a stainless surface, and that iron, not the stainless, is what rust-stains first. Passivation, a citric- or nitric-acid bath commonly specced to ASTM A967, strips it and lets the chromium-oxide layer do its job. It’s a cheap callout that’s standard practice for food, fluid, and outdoor parts. Beyond that: bead blast for a uniform matte, brushed or polished where looks matter.

The magnet test, and when to step up to 316

Fully annealed 303 and 304 are non-magnetic, but cold work (drawn bar, machined surfaces, bent flanges) makes them mildly magnetic. A stainless part that tugs a magnet slightly isn’t counterfeit; it’s physics. If a requirement genuinely demands non-magnetic, put it on the print.

And when the environment involves chlorides (salt spray, road salt, coastal air, aggressive washdown) step up to 316, which resists pitting where 304 eventually loses. It’s quotable like everything else on the materials page.