303 vs 304 Stainless Machining: Which Grade to Order

By CHX Metal Engineering Team · Published 2026-11-08

For machined fittings, 303 buys faster cutting and longer tool life; 304 buys better corrosion resistance and weldability. On the 70 Tsugami Swiss-type lathes at CHX Metal both run to plus or minus 0.005 mm on critical diameters, with prototypes in 3-7 days. Choose 303 unless the part gets welded or sees chlorides.

Twins on the certificate, opposites on the floor

The confusing thing about 303 and 304 is not that they are different — it is how nearly identical they look on paper. Both are austenitic chromium-nickel grades with broadly the same chemistry, the same non-magnetic behaviour, the same familiar look after passivation. The real difference is a small sulfur addition in 303, put there for one purpose: making the steel cut faster. That single element splits the two grades into different jobs, different price behaviours and different failure modes, and drawings almost never say which one the designer actually needed.

Last quarter a buyer sent us a connector housing drawing marked 304. Before quoting we asked one question: will this part ever be welded? The answer was no — the callout had been inherited from an earlier revision that was welded. Switching it to 303 cut the unit price noticeably without touching a single functional requirement. Roughly half the time we ask that question, the answer opens the same door.

What the sulfur buys on the shop floor

Sulfur forms small inclusions that make chips break short instead of stringing. On a lathe that means higher feeds, longer tool life and cleaner threads. It matters most in exactly the features Swiss-type work lives on: deep cross-holes, fine threads, small bores with tight surface callouts. I have watched a bar of 304 occupy an operator for most of a morning on deep cross-drilling that a 303 equivalent would have cleared unattended overnight. There is no single honest percentage for the speed-up — it depends entirely on the features — and I am suspicious of anyone who quotes you one. What is reliably true is that on the same machine, the 303 version finishes sooner and with fewer tool changes.

Where 304 earns its slower cut

The sulfur inclusions that help cutting hurt corrosion resistance. 303 shows rust staining sooner than 304 in damp or salty conditions — we learned that the expensive way, when a batch of 303 shafts shipped to a coastal customer picked up surface stains in their warehouse before assembly. Nothing structural, but the customer saw rust, and reasonable people worry. 304 also welds properly; the sulfur in 303 causes hot cracking in the weld zone, so 303 belongs on parts that stay un-welded. For real chloride exposure, neither grade is the answer — that conversation moves to 316, and it is worth having before the order rather than after.

The short version, side by side

How the two grades run here

Both grades run as bar stock on our Tsugami Swiss-type lathes in Shajing, Bao'an, Shenzhen, where we have machined turned parts for 13 years. Critical diameters and bores are held to ±0.005 mm and verified on the CMM; unmarked dimensions default to ISO 2768-m. Minimum order is a single piece, priced honestly as what it is — mostly setup. Prototypes run 3-7 days before any production batch starts. Terms are EXW Shenzhen only - we are happy to recommend reliable freight forwarders. When a quote arrives with a substitution question in it, the answer is usually inside the quote, not after it.

Neither grade is the wrong answer; choosing by habit is. If your drawing says 304 and the part never meets a torch or salt spray, ask whether 303 saves money. If it says 303 and the parts are heading for a coastal install — well, that is a different conversation, and I would rather have it before the first humid summer than after.