Bending stainless steel without surface damage

Grade 304 sheet does not crack at the bend as often as people expect. It marks. A grey smear along the die line, a faint orange-peel texture on the outside radius, a bright line where the punch touched, a rust bloom three weeks later where a particle of carbon steel embedded in the surface — these are the complaints, and they are surface failures rather than structural ones. Bending stainless steel without surface damage is therefore a different discipline from bending it without cracking.

Three mechanisms account for almost all of it. The outside face is stretched beyond what a 2B finish tolerates. The tool picks up stainless and welds to it, because austenitic grades work-harden fast and conduct heat badly. And the film, the handling or the shared tooling introduces contamination that only shows up in service. Each has a numeric fix, and the fixes are not interchangeable — polishing a die will not remove orange peel, and increasing the radius will not stop galling.

The bending and welded assemblies hub gathers the sister material on forming limits, tolerances and fixture welding, for the parts that go to a brake and then to a fixture.

Why 304 marks where CRCA does not

Austenitic stainless has a higher strain-hardening exponent than mild steel, a lower thermal conductivity and a stable passive oxide layer that regenerates as soon as it is rubbed away. Those three properties combine into the behaviour you see at the die.

The higher tensile strength raises the contact pressure between tool and sheet for the same bend. The low conductivity keeps frictional heat at the interface instead of letting it spread into the sheet, so the surface stays hot where the pressure is highest. The passive layer, regenerating continuously, prevents the surface from oxidising into a low-friction film the way mild steel does. The result is adhesive wear: microscopic cold welds form between the sheet and the tool, then tear, transferring stainless onto the tool and tool material onto the sheet.

Once a smear exists on the die shoulder it acts like a file. Every subsequent part is scored in the same place, which is why a single bad part becomes a batch problem.

The numbers that keep the face acceptable

Radius, strain and the outside face

The outside of a bend stretches by a predictable amount:

ε = t / (2R + t)

A 2 mm sheet on a 2 mm inside radius, so R = t, gives 2 / (2 × 2 + 2) = 33.3%. Doubling the radius to 4 mm gives 2 / (2 × 4 + 2) = 20%. Tripling it gives 14.3%.

Where the 20% line sits depends on grain size. A coarse-grained austenitic sheet roughens visibly at lower strain than a fine-grained one, because each grain deforms as a unit and the surface steps at the grain boundaries. That is orange peel, and no amount of tool polishing removes it — the sheet is telling you the strain is too high for its microstructure. The practical response is more radius, a finer-grain sheet, or a different finish that tolerates the texture. Hardness and r-value testing to ISO 10113 tells you which sheets in a consignment are the coarse ones before they reach the brake.

For a cabin panel, a door skin or a lift car surface, the two-thickness radius is the default. For a bracket, a stiffener or a part that will be painted, one thickness is fine and cheaper.

Galling, and the four controls for bending stainless steel without surface damage

Galling is contact-pressure-driven, so every control works by reducing pressure, reducing sliding, or separating the surfaces.

Reduce pressure with a wider die. An 8 × t V-die spreads the load over a longer contact arc than a 6 × t die, and the die-shoulder pressure falls accordingly. Going to 10 × t on a cosmetic part is often the single most effective change, at the cost of a longer minimum flange.

Separate the surfaces. A 0.5 mm PET film on the die-side face is the cheapest intervention and the one most plants already own. Keep it off the punch side: a compressible layer between punch and sheet changes the effective radius and therefore the angle, and the film deforms unevenly at the tip.

Polish or coat the tool. A polished radius punch and a die with a hardened, low-friction insert raise the threshold at which pick-up starts. Dedicated stainless tooling is worth it on a repeat part, because cleaning a shared die between every run costs more than a second set of wear parts.

Do not share tooling with carbon steel. This is contamination control rather than galling control, and it matters for a different reason: a carbon steel particle pressed into a stainless surface becomes a corrosion site. It rusts, the rust stains the surrounding stainless, and the customer sees a defect that no amount of cleaning removes. Where a shared tool is unavoidable, clean the working faces and keep separate wear parts.

Deburring matters here too. A laser-cut edge with a heavy dross bead puts a hard, rough feature against the die shoulder on every stroke. The edge quality article covers what a 12 kW fibre laser leaves on stainless and where the burr has to go before forming.

Springback: the number that moves your flange

Austenitic stainless springs back more than mild steel because its yield strength is higher and its elastic modulus is lower — around 193 GPa for 304 against 210 GPa for CRCA, so the same stress produces more elastic recovery.

A 2–3° springback on a 90° bend does not sound like much until it is converted into a linear distance. A flange 100 mm long, springing back 2°, moves its edge by 100 × tan(2°) = 3.49 mm. On a 300 mm flange the same 2° moves the edge 10.5 mm. That is the real cost: the angle tolerance is fine and the part still does not fit, because the tip of the flange is where the mating hole lives.

Two things follow for the drawing and the programme. First, springback is a property of the grade and the temper, so the bend table has to be per grade, not per thickness. Second, the Delem control on the 5-axis brake stores the correction with the part programme, which is what makes a repeat order land in the same place as the first.

The minimum bend radius reference lists the starting radii by material, and the bend allowance calculator takes the K-factor and radius you are actually using rather than a default.

Tonnage: what a 200 T brake actually has at 304

Air-bending force scales with tensile strength, thickness squared, and length, and inversely with die opening:

F (kN/m) = 1.42 × σb (MPa) × t² × 1000 / (1000 × V)

For 3 mm 304 at 600 MPa in a 24 mm V-die: 1.42 × 600 × 9 × 1000 / 24,000 = 319.5 kN per metre, or 32.6 tonnes-force per metre. The same 3 mm CRCA at 450 MPa gives 239.6 kN/m, or 24.4 t/m — the stainless penalty is exactly the strength ratio, one third.

Across the full 4,450 mm bed, 319.5 kN/m × 4.45 m = 1422 kN, or 145 t. That fits inside a 200 T machine with reasonable margin. At 4 mm the same calculation gives 426 kN/m, and the full bed needs 193 t — inside the rating but too close to it for repeat work, because a machine run at 97% of rated tonnage deflects, and deflection is what takes the angle out of the middle of the part.

At 10 mm in an 80 mm V-die the force is 1065 kN/m, so 200 T buys about 1.84 m of bend. Ten millimetre stainless is a short-part job on this class of machine, whatever the specification sheet says about maximum thickness.

The film question

Protective film is a handling aid that becomes a process problem if it is not planned.

Film left on a sheet that goes through a fibre laser burns at the cut edge, leaves a hard adhesive residue along the cut, and that residue survives degreasing often enough to cause coating defects later. Film applied after cutting, on the die-side face only, gives most of the scratch protection with none of the adhesive risk. Film on the punch side should be removed before forming.

Where the part will be visible and unpainted — a brushed stainless cabin panel, a handrail, a kick plate — the handling discipline matters more than the film. Clean gloves, a covered bench, no stacking face to face, and a protective interleaf between parts.

How Ganesh Multitech runs stainless bending

Stainless work is formed on the 200 T, 5-axis CNC press brake with a 4,450 mm bed and Delem control, up to 10 mm, with the bend table held per grade so a repeat order does not inherit a CRCA correction. Material arrives from a 12 kW fibre laser with a 40 ft × 16 ft bed, which cuts a 3,000 mm panel in one piece and gives the brake a straight, square edge to locate against — a rough or tapered cut edge is one of the commoner causes of a bend that reads correctly at one end and marks at the other.

The bending capability page carries the machine list and the materials. Stainless cabin and trim work for lift makers follows the elevator parts route, where the visible face gets the handling discipline described above. Ganesh has manufactured sheet metal since 1992 across five units in Mumbai (Vasai) and Nagpur under ISO 9001:2015 and ISO 14001:2015, and first article inspection and PPAP are available.

Where the part is welded after forming, heat tint and sensitisation enter the picture. Welding 304 in the 450–850 °C range can precipitate chromium carbides at the grain boundaries, and the sulphuric acid/copper sulphate test in ISO 3651-2 is the method for checking resistance to intergranular corrosion in the affected zone. Low-carbon 304L removes most of the risk and costs little.

What to put on the drawing

• Grade and finish: 304 or 316, 2B, brushed, No. 4, and which face is visible.
• Inside radius, stated explicitly, with the cosmetic faces called out.
• Grain or brush direction relative to the bend line.
• Die-side face, if a film or a protective pad is expected.
• Bend sequence for parts where a formed flange would block the punch.
• Whether the part is welded after forming, and the post-weld finish or passivation required.
• Flat pattern or bend allowance basis per grade, not per thickness.
• Handling and packing instructions for unpainted visible surfaces.

The stainless stamping guide covers the tooling side of the same material, and laser cutting stainless for industrial parts covers the cut edge that the brake inherits.

When this is the wrong route

A press brake is the wrong process when the section is constant along a long length and the volume is high — a roll-formed profile produces a cleaner stainless surface at a lower piece price, because the material slides over forming rolls rather than being dragged over a die shoulder. It is wrong when the radius required is below one thickness and the grade cannot take the strain, because the answer is a different grade or a different process, not a tighter tool. It is also wrong when the part is a deep drawn housing, where the forming is a press and tool question rather than a bend question.

Ganesh does not roll-form profiles, and does not pickle or passivate a weld to restore a decorative finish that was damaged before welding. Where heat tint has to be removed, it is removed before coating or before dispatch, and the acceptance standard is agreed at quotation.

Send the drawing with the visible face marked and the finish named, or send it for quotation with the quantity and the assembly it fits. The review will come back with the radius, the die opening, the force per metre and the film call, so the first part and the four-hundredth part look the same.