304 stainless sheet metal: grade, thickness, formability

A 2.0 mm 304 panel and a 2.0 mm 304L panel look identical on the bench and fail differently three weeks after welding. The difference is carbon: 0.08% maximum in 304, 0.03% maximum in 304L. That single line of chemistry decides whether the heat-affected zone keeps its corrosion resistance, and it is the decision most often left to whoever quotes the job.

304 stainless sheet metal is the default austenitic grade for formed and welded parts: 18–20% chromium, 8–10.5% nickel, no molybdenum, and a passive film that rebuilds itself in air. It cuts, bends and welds predictably, and it avoids the molybdenum premium of 316L. Four properties drive the rest of the specification — 205 MPa minimum yield, 515 MPa minimum tensile, 40% elongation, and a strain-hardening exponent near 0.45.

This page covers the grade callout, the thickness a shop can cut and form, the forming numbers that fix the flat pattern, and what welding does to the joint. Corrosion and coating sit on 304 stainless corrosion and finish compatibility; the materials and grades hub carries the equivalent pages for CRCA, aluminium, brass and copper.

Which 304 grade belongs on the drawing

The 304 family is three grades with the same chromium and nickel and different carbon. 304 at 0.08% maximum is the general-purpose sheet. 304L at 0.03% maximum is the low-carbon variant that stays corrosion resistant after welding without a post-weld anneal. 304H runs 0.04–0.10% carbon and exists for strength at 500 °C and above, which is boiler work rather than sheet metal.

For a welded enclosure, a canopy frame or a lift car panel, the answer is 304L. Chromium carbides precipitate at the grain boundaries when austenitic stainless sits between 450 °C and 850 °C, and a weld pool surrounded by cold sheet passes through that band on every pass. The chromium that leaves solution to form the carbides is no longer available to rebuild the passive film, so the grain boundaries beside the weld corrode first. Low carbon keeps the carbides from forming at all.

The cost of that decision is strength. 304L is certified at 170 MPa minimum yield where 304 certifies 205 MPa, a 17% difference that matters on a bracket designed to the limit and not at all on a panel designed for stiffness. Read the certificate before approving the substitution in either direction: a mill that ships 304L against a 304 callout has met the chemistry but not the mechanical minimum, and a mill that ships 304 against a 304L callout has met neither the chemistry nor the intent.

What the certificate guarantees, and what it does not

ASTM A240 sets the composition and mechanical minimums for flat-rolled 304; EN 10088-2 sets the delivery conditions in Europe, including the surface finish and the tolerance class. A certificate to EN 10204 3.1 ties the heat number to the test results. None of them tells you the grain size, the flatness of the sheet as delivered, or how the material will bend on your tooling.

PREN is calculated as chromium plus 3.3 times molybdenum plus 16 times nitrogen. 304 has no molybdenum, so it sits at about 18 against roughly 24 for 316L, and that gap is the whole reason 316L exists.

Thickness range and where the force runs out

Cold-rolled 304 sheet is stocked from 0.4 mm to about 6.0 mm; above that the same grade is supplied as hot-rolled plate. Ganesh cuts 0.8–20 mm on the 12 kW fibre laser with a 40 ft × 16 ft bed and forms to 10 mm on the 200 T brake, so a 304 part from 0.8 mm to 10 mm can be cut, formed, welded and coated in one route.

Weight is the first constraint that bites, and it is pure arithmetic. At 8.0 kg per m² per mm, a 1.5 mm sheet weighs 12.0 kg per m², so a 1,250 × 2,500 mm sheet is 3.125 m² and 37.5 kg. That is above a comfortable one-person lift on a wet floor, and it is the point where a vacuum lifter pays for itself. In CRCA the same sheet is 36.8 kg, so stainless does not add weight — it adds hardness and slipperiness, which is a different handling problem.

Force is the second constraint. Air-bending force per metre is 1.42 times tensile strength times thickness squared divided by the die opening, with stress in MPa and dimensions in millimetres. At 3 mm in a 24 mm V-die and 600 MPa: 1.42 × 600 × 9 ÷ 24 = 319.5 kN per metre, or 32.6 tonnes-force per metre. Across the full 4,450 mm bed that is 1,422 kN, or 145 t of a 200 T machine.

At 6 mm in a 48 mm V-die the same sum gives 639 kN per metre, or 65 t per metre, so 200 T buys 3.07 m of bend and no more. At 10 mm in an 80 mm V-die it is 1,065 kN per metre, or 108.6 t per metre, which limits the part to 1.84 m. The thickness in the specification and the length in the drawing have to be read together; a 10 mm channel 3 m long is not a press brake part on this class of machine.

Formability: the radius is a strain limit

The outside of a bend stretches by a predictable amount. For an inside radius R and thickness t, the surface strain is t divided by (2R + t). At 2 mm on a 2 mm radius that is 33%; on a 4 mm radius it is 20%; on a 6 mm radius, 14%. The 33% bend still forms, but the surface roughens where individual grains step out of the plane, and on a 2B finish that texture is visible.

That is why the rule splits by what the surface has to do. Structural brackets and stiffeners bend at one thickness. Visible panels, door skins, handrails and lift car surfaces bend at two. The minimum bend radius reference lists the starting radii by material, and the stainless bending guide covers the tool pick-up and galling that come with the same grade.

Grain direction is the variable that never appears on a drawing. Bending across the rolling direction distributes strain over more grains and tolerates a tighter radius; bending parallel to it concentrates strain along the grain boundaries. On a part with one tight bend, add a note that the bend line must run across the grain and let nesting place the part, because a 2,500 × 1,250 mm sheet offers one orientation and a rotated part may not fit the nest.

For deep drawing, 304 is a better choice than 316L. A strain-hardening exponent of 0.45 spreads strain away from the punch nose and delays necking, which supports a limiting draw ratio of about 1.8–2.0 in a single draw. Anything deeper needs a second draw, an anneal, or a different process.

Springback on austenitic stainless runs 2–3° on a 90° air bend, against 1–2° for CRCA at the same radius-to-thickness ratio, because the yield strength is higher and the modulus is lower at 193 GPa. Converted to a linear error, 3° on a 100 mm flange moves the tip by 100 × tan 3° = 5.24 mm. The angle tolerance can pass inspection while the flange tip misses the mating hole, so the brake programme carries the correction per grade.

The bend allowance needs the same treatment. At 2 mm with a 2 mm inside radius and K = 0.45, the allowance for a 90° bend is 1.5708 × (2 + 0.9) = 4.56 mm, and the bend deduction is 2 × 4 − 4.56 = 3.45 mm. Flanges of 40 mm and 30 mm therefore develop to 66.55 mm. The same part in CRCA at K = 0.33 gives 4.18 mm allowance and 3.82 mm deduction, a 0.38 mm difference per bend that multiplies through every fold. The K-factor derivation is on the bend allowance page.

Welding 304 without giving back the corrosion resistance

The filler for 304 and 304L is ER308L, which matches the parent chemistry and deposits 5–10 ferrite number of delta ferrite in the austenitic weld metal. That ferrite is deliberate: a fully austenitic weld solidifies with a continuous liquid film at the grain boundaries and cracks under restraint. For 304 welded to carbon steel, ER309L provides the dilution buffer.

Heat input for 0.8–3.0 mm sheet sits around 0.3–0.8 kJ per millimetre, with interpass temperature held under 150 °C. Above that range the heat-affected zone widens, distortion grows, and on 304 rather than 304L the sensitisation band is reached over a larger area.

Distortion is worse than on mild steel for a simple reason: the coefficient of thermal expansion is 17.3 µm/m·°C against 11.7 for CRCA, so a weld that shrinks 1 mm in mild steel pulls about 1.5 mm in stainless. A 2,000 mm frame welded without a fixture will bow. The sequence and fixture logic are the same as for any frame, but the allowance is larger, and it is set on the welded assemblies capability page rather than at the bench.

Heat tint is the last step that is routinely skipped. The straw and blue oxides beside a weld are chromium-depleted and they must come off before the part is passivated, coated or put into service. A wire brush spreads iron and does not remove the oxide; pickling paste or an acid bath does. Where the part will be powder coated, tint under the coating is an adhesion failure waiting for a damp season.

How Ganesh runs 304

304 and 304L parts are cut on the 12 kW fibre laser at 40 ft × 16 ft with nitrogen assist, which leaves a bright, oxide-free edge that welds without a pickling step for thin gauges. Forming runs on the 200 T, 5-axis CNC brake with a 4,450 mm bed and Delem control to 10 mm, with the bend table stored per grade. Frames and enclosures go through three CLOOS robotic MIG cells or the 30-plus manual MIG, spot and SPM stations, on fixtures built in our own tool room.

Finishing follows the same route as carbon steel, with one addition: stainless is blasted with non-ferrous media before coating, because an iron phosphate conversion coating has almost no free iron to react with. Seven-tank pretreatment, the 16 ft × 16 ft blast chamber and the 18 ft × 14 ft powder booth are on the paint shop capability page. Typical sheet metal tolerance is ±0.5 mm, tighter on critical features by agreement, and first article inspection and PPAP are available.

The plant has manufactured sheet metal since 1992 across five units in Mumbai (Vasai) and Nagpur, with more than 200 people, under ISO 9001:2015 and ISO 14001:2015. Stainless work for lift makers and railway suppliers runs through the elevator parts and railway parts routes.

What to put on the drawing

1. Grade by designation and standard — "304L to ASTM A240", not "stainless steel".
2. Thickness with a tolerance or a reference to the mill tolerance class, never a bare "2 mm".
3. Inside radius for every bend, with the visible faces identified.
4. Grain direction requirement on parts with a tight bend or a brushed finish.
5. Whether the part is welded, and the filler designation if the joint is structural.
6. Finish: bare 2B, brushed, passivated, powder coated or wet painted, and which faces.
7. Flat pattern basis, or an instruction that the supplier develops it.
8. Certificate requirement — 3.1 inspection certificate with the heat number, where traceability is contractual.

When 304 is the wrong grade

304 fails predictably in three environments. Above roughly 200 mg/L chloride at ambient temperature it pits, and in a coastal splash zone it tea-stains within months. Above 60 °C with chloride present and residual stress from welding or bending, chloride stress-corrosion cracking becomes the governing risk and no coating prevents it. And in a hot, concentrated chloride stream — a pickling tank, a pool plant room — even 316L is a compromise.

It is also the wrong grade on cost where the environment is dry and indoor. A CRCA enclosure with a competent powder coat costs less per part and performs for the life of the machine, which is why the CRCA route still wins most control cabinets. The full comparison, with the failure modes and the alternatives, is on when 304 stainless is the wrong material.

Send the revision with the grade, thickness, radii and weld callouts, and the quotation will state the flat pattern basis, the die opening and the filler it assumes. Request a quote with the drawing attached.