Stainless 303 & 304 Parts

Two grades, one decision

Both are austenitic stainless built on roughly 17-20% chromium and 8-11% nickel; 304 is the grade known as 18/8 (sources 1-3). The difference is a sulfur addition in 303, 0.15% minimum, which makes it easier to machine (sources 3 and 4): the British Stainless Steel Association (BSSA) puts its machinability about 60% better than 304 (source 3). The same sulfur is why 303 has poor weldability and significantly lower pitting resistance than 304 (source 3); BSSA notes that sulfide inclusions at the surface of 303 are preferential pitting sites (source 5).

So the decision is mostly about what happens to the part after machining. Nothing welded, no food contact, indoor or mild outdoor duty: 303. Welded into an assembly, washed down, touching food, or living somewhere genuinely wet: 304, and 304L when it is welded, because carbon held to 0.030% maximum avoids the grain-boundary corrosion that weld temperatures can cause (source 6).

What gets made from each

303 is the usual grade for turned work: fittings, shafts, pins, standoffs, and valve and instrument components. BSSA lists fittings, threaded fasteners, shafts and valve bodies among its uses, where only basic corrosion resistance is required (source 3). 304 shows up as brackets, covers, enclosures, food-process and fluid-handling parts, and anything destined for a welded frame; it is used in food handling and processing equipment, and 304L is preferred for corrosive service where welding is required (source 1). Milled features are quoted from the print, and critical turned diameters to ±0.001 in. Long, slender turned parts suit Swiss-type machining, where the stock feeds through a guide bushing and the tool cuts near the point of support (source 7); Swiss screw machining covers small 303 parts in volume.

Passivation and finishes

Passivation removes iron from the surface of a stainless part and restores the passive oxide layer that keeps it from rusting (source 8). It is specified as a nitric acid, citric acid or electrochemical treatment, and ASTM A967 is a common specification for it (sources 8 and 9). If a part will see moisture or washdown, put passivation on the print. A finish called out on the print is quoted with the part; the surface finishes guide explains what bead blast, brushed and polished looks do.

The magnet test, and when to step up to 316

Austenitic stainless is usually non-magnetic, but work hardening can make it slightly magnetic, and so can bending or cutting (source 2). A slight pull on a magnet is not a sign of the wrong grade. If a requirement genuinely demands non-magnetic, put it on the print.

When the environment involves chlorides (salt spray, road salt, coastal air, aggressive washdown), step up to 316. Its 2-3% molybdenum gives more resistance than 304 to localized attack by chlorides (source 10), and BSSA says 304, and more especially 303, should not be considered for seawater service (source 5). BSSA also calls 316’s own resistance in seawater limited (source 5). It’s quotable like everything else on the materials page.

Sources

All accessed 4 October 2026. Where a figure here differs from your print, the print governs.

1. Wikipedia, "SAE 304 stainless steel": en.wikipedia.org (304 at 18.0-20.0% chromium and 8.0-11.0% nickel; also known as 18/8; used in food handling and processing equipment; 304L preferred for corrosive service where welding is required, such as tanks and pipes).

2. Wikipedia, "Stainless steel": en.wikipedia.org (the 18/8 name and AISI type 304; austenitic grades usually non-magnetic, slightly magnetic through work hardening; magnetism when austenitic steel is bent or cut; a self-repairing chromium oxide surface film).

3. British Stainless Steel Association, "Stainless Steels to Behold: 303": bssa.org.uk/stainless-steels-to-behold-303-%E2%9C%A8/ (approximately 17% chromium, 8% nickel, 0.15% minimum sulfur; machinability index 60% better than 304; pitting resistance significantly lower than 304; poor weldability; used where only basic corrosion resistance is required, including shafts and threaded fasteners).

4. Wikipedia, "Machinability": en.wikipedia.org (stainless steels machine poorly against carbon steel because they are tougher, gummier and work harden quickly; 303 and 416 are easier to machine because of added sulfur and phosphorus).

5. British Stainless Steel Association, "Selection of 316, 304 and 303 types of stainless steels for seawater applications": bssa.org.uk/bssa_articles/selection-of-316-304-and-303-types-of-stainless-steels-for-seawater-applications/ (304, and more especially 303, not to be considered for seawater service; sulfide inclusions at the surface of 303 are preferential pitting sites; 316’s corrosion resistance in contact with seawater is limited).

6. British Stainless Steel Association, "Comparison of 304 or 316 and 304L or 316L type compositions and effect on corrosion resistance": bssa.org.uk/bssa_articles/2-comparison-of-304-or-316-and-304l-or-316l-type-compositions-and-effect-on-corrosion-resistance/ (carbon 0.08% maximum for 304 and 0.030% for 304L; below 0.030% the intercrystalline corrosion after exposure to weld temperatures does not occur).

7. Peter Zelinski, "Understanding Swiss-Type Machining", Modern Machine Shop, 26 June 2013, updated 14 July 2022: mmsonline.com/articles/understanding-swiss-type-machining (the stock feeds through a guide bushing, so the tool cuts near the point of support; effective for long and slender turned parts).

8. Wikipedia, "Passivation (chemistry)": en.wikipedia.org (passivation removes exogenous iron and creates or restores a passive oxide layer; ASTM A967 and AMS 2700 the most prevalent standards; nitric acid commonly used, citric acid growing in use).

9. ASTM International, "A967/A967M-25: Standard Specification for Chemical Passivation Treatments for Stainless Steel Parts": store.astm.org (immersion treatment in nitric acid or citric acid solutions, and electrochemical treatment).

10. Wikipedia, "SAE 316L stainless steel": en.wikipedia.org (molybdenum 2-3%; greater corrosion resistance than 304 with respect to localized attack by chlorides).