"394" is a British Standard designation (BS 394S17) for a copper-bearing variant of the familiar 18-8 austenitic family, better known internationally as AISI 304Cu, 302HQ, EN 1.4567 (X3CrNiCu18-9-4), or JIS XM-7. It starts from essentially the same chromium-nickel base as standard 304 but adds 3.0-4.0% copper, an addition that slows work hardening and dramatically improves ductility during severe cold deformation. That combination makes 394/304Cu the grade of choice for cold-heading and thread-rolling applications — stainless fasteners, screws, and rivets — where the blank has to survive aggressive upsetting and forming without cracking.

Beyond fasteners, 394 is also used in heat exchangers, coolers, tanks, and pumps for paper and food-processing plants, ship components, and electronics, wherever the combination of 304-type corrosion resistance and superior cold-formability is useful. Unlike plain 304, the copper-bearing 1.4567 variant remains resistant to intergranular corrosion in the as-delivered condition and after welding, and it offers somewhat better resistance to reducing acids such as dilute sulfuric acid. Like the rest of the austenitic family, it is not hardenable by heat treatment; its useful properties come from the copper addition working together with the standard 18-8 matrix, not from a hardening heat treatment.

The copper addition that makes 394/304Cu so good for cold heading also changes its machining behavior slightly compared with plain 304: copper tends to promote somewhat better chip breaking and reduces the material's tendency to smear across the tool face, so this grade generally machines a little more predictably than standard 18-8 stainless at similar hardness. That said, it is still a fully austenitic, non-heat-treatable grade, and it still work-hardens under a rubbing or dull tool the way any 300-series stainless does — the copper addition softens that tendency, it doesn't eliminate it.

Sharp, positive-rake tooling remains the right approach: it shears the material cleanly instead of burnishing it, which limits the work-hardened layer the next pass has to cut through. Feed rates should stay high enough to get under any hardened skin from a prior pass, and because parts made from this grade are frequently headed or cold-formed downstream, dimensional consistency and surface finish on the machined features matter as much as raw metal-removal rate.

Chip control benefits from the copper addition but still calls for a stainless-appropriate chipbreaker geometry rather than a general carbon-steel insert. Adequate coolant helps offset the alloy's low thermal conductivity and keeps the copper-bearing matrix from smearing onto the cutting edge.