Why 304 is the default austenitic stainless
304 stainless is an austenitic chromium-nickel steel with 18% Cr and 8% Ni — the famous “18-8” designation. Developed in the 1920s and standardized as the most widely produced stainless grade in the world, 304 strikes a balance between corrosion resistance, formability, weldability, and cost that fits roughly 70–80% of stainless applications. It is the steel used for kitchen sinks, brewery fermenters, elevator panels, food processing equipment, architectural hardware, structural brackets, and general industrial enclosures. If a part needs “stainless steel” without a specific corrosive environment driving the choice, the default answer is 304.
Our typical 304 customer is building food-service or commercial kitchen equipment (panels, brackets, prep tables, nozzles, valve bodies), brewery and dairy hardware (fittings, clamps, sanitary connectors that don't see CIP chemicals), architectural and decorative components (handrails, finish trim, fasteners, signage frames), or general industrial parts (enclosures, mechanical assemblies, structural plates, instrument housings). Orders range from 5-piece prototypes to 5,000-piece production runs, with the occasional high-volume Swiss-turn pin or fitting run into the tens of thousands.
304 vs 304L — and when to pay for the difference
304L is low-carbon 304 (maximum 0.03% C versus 0.08% for standard 304). The difference matters when the part will be welded after machining. During welding, carbon in standard 304 can combine with chromium at grain boundaries adjacent to the heat-affected zone, forming chromium carbide precipitates and locally depleting the boundary of corrosion-resistant chromium. This is sensitization, and it opens the door to intergranular corrosion in service. 304L prevents the problem by having so little carbon that carbides can't form.
Practical guidance: if the part will be welded (TIG, MIG, or laser, by you or downstream), specify 304L. If it will see only machining and assembly, standard 304 is mechanically equivalent and costs about 8% less. For elevated temperature service above 525 °C (rare in our customer mix, mostly power-generation and chemical), 304H is the high-carbon variant required by the spec — higher carbon improves creep strength at temperature. We stock 304 and 304L in common bar, plate, and sheet sizes; 304H runs on order with longer lead time.
Machining parameters — easier and cheaper than 316
The absence of molybdenum makes 304 substantially easier to machine than 316. We run 304 at roughly 20–25% higher cutting speeds and see correspondingly longer tool life. Our roughing parameters: 80–100 m/min surface speed, 0.12–0.18 mm feed per tooth, 1.5–3 mm axial depth of cut, flood coolant. Finishing runs 110–140 m/min with sharp-edge uncoated or AlCrN-coated carbide. We climb-cut exclusively on 304 (any stainless work-hardens instantly if the tool dwells), and keep coolant flowing — although 304 is more forgiving of brief coolant drop-out than 316.
Swiss-turn 304 features (sanitary fittings, brewery nozzles, architectural pins, threaded fasteners) run at 90–110 m/min with consistent feed. In production we often achieve ±0.008 mm diameter tolerance on Swiss-turned 304L — tight enough for press-fit and bearing-pocket assemblies. For deep pockets and thin-wall enclosures, we route to vibration-damped fixturing and trochoidal toolpaths to keep walls within ±0.05 mm flatness over 200 mm spans.
Finishes — from bead blast to mirror polish to electropolish
304 takes a wider range of finishes than most stainless grades. Common options:
- Bead blast: uniform matte finish, Ra ~1.6–3.2 µm. Standard for industrial enclosures and brackets. Adds 3–5% to part cost.
- #4 brushed (architectural): directional grain finish, Ra ~0.8–1.2 µm. Standard for kitchen panels, elevator interiors, range hoods, counter trim. Produced with 120–180 grit belt. Adds 8–15% depending on surface area.
- Mirror polish (#8): highly reflective finish, Ra ~0.05–0.1 µm. Decorative trim, signage, high-end architectural. Produced with sequential 240/320/400/600 grit then buffing wheel. Adds 25–40%.
- Passivation per ASTM A967 (Citric 2 or Nitric 2): removes free iron, restores passive layer. Standard for food-contact and corrosion-sensitive parts. Adds 4–8%.
- Electropolish: anodic electrochemical polishing, reduces roughness by ~50% and removes embedded iron. Standard for biopharmaceutical and semiconductor. Adds 12–25%.
Specify the finish on the drawing or call it out in the RFQ — combining 304 with the right finish is often what makes the part feel premium without paying for 316 material that would deliver the same visual result.
Where 304 doesn't belong — and why we'll tell you
Despite being the default austenitic, 304 has real limits. The biggest one is chloride pitting and crevice corrosion. Any 304 surface in contact with chloride solutions — seawater, swimming pool water, brine, chlorinated CIP sanitizers, road salt spray, body fluids — will eventually develop pits and crevice attack, even though the bulk material looks fine. The pitting is invisible until it perforates. For marine hardware, food-CIP equipment with chlorinated sanitizers, medical devices, and chemical processing with chloride exposure, you need 316 stainless instead — the 2–3% molybdenum addition dramatically improves chloride resistance.
Other 304 limits worth knowing: not suitable for sustained temperatures above 425 °C without specifying 304H; not suitable for sulfuric or hydrochloric acid (any concentration) — use 904L, AL-6XN, or 254 SMO for those services; not as galling-resistant as Nitronic 60 for sliding contact applications. When we see a quote where 304 is the wrong call, we'll say so on the response — switching specs early saves everyone time.
Certification documentation
Every 304 lot ships with: ASTM A276 or A479 mill certificate linking heat number to your PO, chemistry report (Cr, Ni, Mn, C, Si, P, S, N — the elements that matter for both spec compliance and the corrosion behavior we're selling), mechanical property report, passivation certificate per ASTM A967 if applicable, electropolish process record if applicable, first-article inspection report, and Certificate of Conformance. For food-contact we add 3-A Sanitary Standards compliance documentation and FDA 21 CFR 177 declaration. For welded architectural assemblies we add weld procedure qualification per ASME IX or AWS D1.6 on request. Specify the certification scope during the RFQ so pricing and lead time reflect it correctly — running food-grade documentation on a non-food part wastes time and money on both sides.
What to send for a 304 quote
Required: STEP file, PDF drawing with tolerances and surface-finish callouts (Ra values matter on 304 too — they drive electropolish vs mechanical polish vs as-machined cost), material callout (304, 304L, or 304H), target quantity, target delivery. Helpful: application context (architectural, food-contact, brewery, general industrial — affects fixture protocol and inspection), passivation / finish specification, required certification documentation scope, any AVL constraints, and any non-standard requirements like non-magnetic behavior or weld-prep callouts. See the quality process page for our full inspection scope. Quotes come back within 24 hours with per-piece pricing across 3–5 quantity tiers, lead time, and a DFM note flagging anything that's costing more than it should.