Is 303 stainless or 304 stainless better for machined parts?

What is the actual difference between 303 and 304?

The difference is one intentional addition: 303 carries sulfur on purpose, and 304 keeps sulfur as a low impurity. Because of that, the two grades are bought to different ASTM bar specifications, A582 for 303 and A276 for 304.

Both grades are austenitic chromium-nickel steels, often called 18-8. What sets 303 apart, per the British Stainless Steel Association, is a sulfur content of at least 0.15% (roughly 0.15 to 0.35%), enough to form a large population of manganese sulfide stringers along the rolling direction of the bar. Those inclusions are the whole point of the grade, and every trade-off discussed below comes from them.

The specification split follows the chemistry. The scope of ASTM A276/A276M covers hot- and cold-finished stainless bars and shapes in the commonly used grades, and explicitly sends the free-machining types to a separate document. That document is ASTM A582/A582M, which covers free-machining stainless bars “designed especially for optimum machinability”. So a drawing that says “304 per ASTM A276” and a drawing that says “303 per ASTM A582” are pointing at two different product standards, not two rows of the same table.

In Europe the equivalents sit in EN 10088-3 as 1.4305 (the 303 type) and 1.4301 (the 304 type). In Japan and in much of the Asian bar supply chain, the same pair appears as SUS303 and SUS304 under JIS G4303. The names line up closely, but the composition limits in each standard are not identical, which matters when you read a mill certificate later.

Where does 303 lose to 304?

303 gives up corrosion resistance, weldability, toughness and cold formability in exchange for machinability. For parts that will be welded, crimped, flared, rolled or exposed to chlorides, those losses usually outweigh the machining gain.

• Pitting and crevice corrosion. Sulfide inclusions that break the surface are preferred starting points for pits. The association’s list of free-machining drawbacks puts lower chloride pitting and crevice resistance first, followed by weaker stress corrosion cracking and corrosion fatigue resistance.
• Welding. Sulfur promotes hot cracking in the weld and heat-affected zone, so 303 is rated as having poor or limited weldability. If a machined fitting is later welded into an assembly, the drawing should call 304L instead.
• Forming. The stringers reduce ductility, so swaging, staking, crimping, roll threading and flaring are more likely to crack a 303 part. Cut threads are fine; rolled threads favor 304.
• Skin and water contact. The association notes that 303 is the only grade it lists as having failed nickel-release sweat tests to BS EN 1811, and that 303 is not approved in Europe for drinking-water contact or jewelry. Wearables, handles and potable-water fittings belong in 304 or 316.

None of this makes 303 a poor grade. Shafts, spacers, bushings, valve stems, manifold blocks, fasteners and instrument parts in dry or mildly humid service are exactly what it was designed for.

When should you skip both and specify 316?

Specify 316 (or 316L) when the part sees chlorides: seawater, splash zones, road salt, pool chemicals, many cleaning agents and some process fluids. The molybdenum in 316 raises pitting and crevice resistance above both 304 and 303.

The association’s guidance on seawater service is blunt: 304, and especially 303, should not be considered suitable for seawater, and even 316 has limits, and crevice risk climbs as seawater gets warmer. For welded 316 parts, 316L is the usual call.

There is a machinability price. 316 machines somewhat like 304, and there is no widely standardized free-machining 316 equivalent to 303. Bar producers do sell enhanced-machinability 304 and 316, made by controlling oxide inclusions rather than adding sulfur, which keeps corrosion resistance close to the standard grade. Those are proprietary products identified by trade name, so if a drawing allows them, it should still name the base specification that the bar must meet.

What should the RFQ say when sourcing stainless parts from Vietnam or elsewhere in Asia?

Name the grade and the specification you will accept, list any acceptable equivalents such as SUS303 to JIS G4303 or 1.4305 to EN 10088-3, and ask for mill certificates with each lot. Bar in the region is commonly supplied to JIS or EN standards, so deciding the equivalents up front avoids a query after the quote.

Equivalent does not mean identical. The composition and property limits in ASTM, EN and JIS documents overlap but are not the same, so if a property such as sulfur range, magnetic permeability or cold-drawn strength matters to the function, write the requirement explicitly instead of relying on the grade name. A mill certificate shows the heat chemistry; compare it with the specification you named, not with a generic “303” datasheet.

A practical RFQ line for stainless turned or milled parts covers five things: grade, specification (with acceptable equivalents), condition (annealed or cold finished), passivation specification and test, and any restriction such as “no free-machining grades”. The RFQ package guide lists the rest of what to send, and the tolerance guide covers the general tolerance note. Before sending stainless work anywhere, check that the supplier actually lists stainless among the materials it runs; our CNC turning page, for example, names the bar materials currently running. The quote page explains how to send a drawing package, and whether a material and finish fall within scope is confirmed at that first review.