The grades, and when each earns its place
| Grade | EN number | Character | Choose it for |
|---|---|---|---|
| 303 | 1.4305 | Free-machining: sulphur added for chip control | High-volume turned parts where corrosion duty is mild; not for welding or marine |
| 304 | 1.4301 | The general-purpose austenitic workhorse | Brackets, housings, general fittings in normal atmospheres |
| 316L | 1.4404 | Molybdenum-bearing, low carbon: better chloride and chemical resistance, weldable | Marine, chemical, food, pharma and anything washed or dosed |
| Duplex (e.g. 2205) | 1.4462 | Roughly twice the strength with strong corrosion resistance | Oil and gas, offshore and structural duty in aggressive environments |
| 17-4PH | 1.4542 | Precipitation hardening: machinable then hardened | Shafts, valve internals and parts needing strength plus corrosion resistance |
Two buying notes. Specify "316L or 1.4404" rather than one national label, since UK stock moves under both names; allowing stated equivalents quotes faster and cheaper, as our RFQ guide explains. And resist paying for 316L out of habit: in a dry indoor atmosphere 304 does the same job for less, while in chloride splash it absolutely does not.
Why stainless machines differently
Austenitic stainless work hardens: the cut surface gets harder as the tool passes, so a hesitant cut hardens the very material the next pass must remove. Machining it well means sharp tools, positive feeds and rigid setups, no dwelling, no rubbing. It also runs hot, holds heat in the cut, and produces stringy chips that demand proper chip control.
None of this is a problem for a shop that does stainless daily. It is a problem for a shop that does not, and it shows up as chatter marks, burnished patches, blunt-tool surfaces and blown tolerances on thin walls. It is one of the quiet reasons the same drawing quotes differently across suppliers: the stainless-fluent shop plans the heat and the hardening in, the occasional one discovers them. When we place stainless work across our network, fluency with the grade is a selection criterion, not a hope.
Costs run higher than mild steel for honest reasons: the material itself costs more, cutting is slower, and tooling wears faster. The five cost inputs in our cost guide all move up a band. What should not change is the price logic: one number for the delivered, inspected part.
Corrosion resistance is a process, not just a grade
Stainless resists corrosion through a passive chromium-oxide layer that heals itself, provided the surface is clean. Machining, and especially contact with carbon-steel tooling, benches and swarf, can embed free iron that later blooms as rust spots on a "stainless" part.
Passivation, a controlled acid treatment, strips free iron and restores the passive layer, and hygienic or marine work often specifies it as standard. Segregated handling matters too: stainless machined and handled in a carbon-steel environment picks up contamination that no certificate mentions. Where the duty is serious, specify passivation and ask how cross-contamination is controlled. Both are normal parts of a finishing specification, and surface finish itself does real work here: smoother surfaces resist corrosion and clean better, which is why hygienic work carries Ra callouts.
The certificate is half the product
Stainless grades look identical on the bench. A 304 part sold as 316L works perfectly until the chlorides arrive, which is why serious buyers treat traceability as a requirement rather than an extra: an EN 10204 3.1 certificate tying the delivered part's heat number to actual mill test results for that batch of material.
That is how TrueNorth supplies stainless as standard: grade-fluent UK shops from the vetted network, machining, finishing and passivation coordinated as one order, dimensional inspection before dispatch, and the traceability pack with the 3.1 certificate in it. The sectors that lean on us for stainless, oil and gas, rail, food and process plant, are exactly the ones where the certificate matters as much as the part. Send the drawing with the environment the part lives in, and the DFM review will confirm the grade or suggest a better one within 48 hours.