Stainless steel stamping: design and finish considerations

A stainless part rarely fails because the press was too small. It fails at the second draw, where 304 that was annealed at 40% elongation has already work-hardened past the point where the wall can stretch, and a crack opens where the punch radius meets the cup wall. Stainless steel stamping is a design problem before it is a press problem: grade, temper, draw ratio and die surface decide the outcome, and the finish you specify decides whether the part still looks like stainless after handling.

This guide is for design engineers specifying a stamped stainless component. It gives the grade table, the tonnage and clearance numbers, the springback allowance, and the finishing sequence that keeps a 2B or No.4 surface intact through blanking, forming, welding and packing.

Which stainless grade should the part be stamped from?

Start with the service environment, then the forming severity. Austenitic grades, 304 and 316L, form well and weld well. Ferritic 430 is cheaper and weaker in ductility. Martensitic and precipitation-hardening grades are rarely a stamping choice.

Two numbers in that table drive most decisions. Elongation tells you how far the material can stretch before it tears, and the work-hardening exponent tells you how quickly that stretch capacity is used up. 304 starts near 0.45 and 430 near 0.22, which is why a shape that draws cleanly in 304 can split in 430 at the same depth.

Temper matters as much as grade. Annealed strip is the forming grade. Quarter-hard and half-hard strip are chosen deliberately for spring parts, and they will not tolerate a draw. If the drawing says "301" without a temper, the tool design is undefined.

How much tonnage does a stainless blank need?

Blanking force is the blank perimeter multiplied by thickness and shear strength. For 304, shear strength is roughly 480 MPa at a 600 MPa tensile, so a blank with a 900 mm perimeter in 1.5 mm material needs about 648 kN, or 66 tonne-force, before any stripping or penetration allowance.

Add 20–30% for stripping and press inefficiency and the job wants an 80–85 T press. The same blank in 1.5 mm CRCA, at a shear strength near 300 MPa, needs about 405 kN. That 1.6× ratio is the number to carry into the quotation: stainless does not merely cost more per kilogram, it consumes press capacity faster. The press tonnage calculator runs the same calculation with your perimeter and thickness.

Drawing adds a second force. A 120 mm diameter cup in 1.5 mm 304 needs roughly 340 kN of draw force before blank-holder load, calculated as π × diameter × thickness × tensile strength. Blank-holder pressure for stainless typically sits at 1.5–2.5 MPa, higher than for mild steel because the material resists flowing inward.

What does a second draw do to the material?

The first draw spends the ductility. For 304 at 1.5 mm, a blank-to-cup diameter ratio of about 1.8 is the practical limit in one draw; the second draw should stay near 1.3 and the third near 1.2. Past those ratios the wall thins faster than the punch descends, and the crack starts at the nose radius where the strain is highest.

Wall thinning is the number to watch. A 20% reduction in wall thickness at the radius is a design warning, not a process detail, and it usually appears first on the side of the cup opposite the blank-holder gap. Measure it on the first article and on the last part of each coil lot.

When the geometry genuinely needs more than the ratios allow, an interstage anneal restores ductility. Budget for it honestly: the anneal adds a heat treatment and a scale or oxide removal step, and the part may need to be re-lubricated and re-located before the next draw. A three-draw part with two anneals is a different cost structure from a single-draw part, which is why the draw sequence should be agreed before the tool is designed. The deep drawing rules set out the geometry that keeps a part inside one draw.

What die clearance and radii should the tool use?

Clearance for stainless runs at 8–12% of thickness per side, against 5–8% for mild steel. At 1.5 mm that is 0.12–0.18 mm per side. Too little clearance raises punch load and burr height; too much produces a rolled edge and a poor cut face that shows through a brushed finish.

The punch and die radii follow the thickness. A minimum inner bend radius of 1t is normal for 304, so 1.5 mm at 1.5 mm thickness, and 1.5t for 430. On a drawn cup, the punch nose radius should be at least 4t and the die entry radius 6–10t, or the material thins at the nose before the wall has drawn.

Die inserts in D2 or an equivalent cold-work steel at 58–62 HRC, with TiN or AlCrN on the draw radii, are the practical baseline. Progressive tools for stainless also need a stripper that holds the strip flat between stations, because a lifted strip changes the punch entry angle and starts a burr.

How much springback should the design allow?

Expect 3–6° of springback on a 90° bend in 1.5 mm 304, roughly twice the mild steel figure at the same radius. The cause is the yield strength relative to elastic modulus: stainless carries more elastic strain before it yields, and releases more when the punch withdraws.

Three controls work. Overbend the tool by the measured angle rather than the nominal one. Bottom the punch into the material at the end of the stroke where the geometry allows. Or add a restrike station, which costs a station but holds the angle across coil-to-coil variation. For 301 in a harder temper, springback can exceed 10°, and a restrike is usually the only stable answer.

Springback also moves hole-to-bend distances. A flange that measures correctly on the tool can sit 0.3 mm out after release, which matters when the part locates on that flange in an assembly. The springback guide covers the compensation method.

Which finish should the drawing call for?

The mill finish is part of the specification, not a default. 2B is the standard cold-rolled, annealed and pickled finish with a mild sheen. BA is bright annealed, smoother and more reflective. No.4 is a brushed grain, and No.8 is a mirror polish. Each behaves differently under a tool.

A brushed No.4 surface has a direction. If the grain runs across the bend, the surface stretches and the line breaks visibly at the radius. State the grain direction relative to the bend line, and state which face is cosmetic, because the tool cannot protect both.

What damages the finish before the customer opens the box?

Four things, and none of them is the material. Free iron from a carbon steel brush, a grinding wheel used on mild steel, or dust in the shop settles on the surface and rusts within days. Heat tint from welding leaves a chromium-depleted oxide that must be removed before passivation. Adhesive residue from peel film traps moisture. And bare handling at packing puts fingerprints into a BA surface that no amount of cleaning will fully remove.

The sequence that works: keep stainless tooling and brushes separate, remove heat tint mechanically or with a pickling paste, passivate to ASTM A967 so the chromium oxide layer reforms, rinse and dry, then apply peel film to cosmetic faces and handle with clean gloves. ASTM A967 covers chemical passivation treatments; it does not remove heat tint on its own, which is why the order of operations matters.

Laser-cut edges deserve a note. Cutting stainless with nitrogen gives a clean, oxide-free edge that needs little finishing. Cutting with oxygen leaves a dark oxide that must be removed before welding or coating, and that removal cost belongs in the route.

How should a stainless assembly be welded and finished?

Low heat input, a matching filler and a cleaning step are the three controls. 308L filler suits 304, and 316L filler suits 316L; a mismatched filler leaves a weld with less corrosion resistance than the parent metal. Keep the interpass temperature low on thin sheet, because 304 is sensitised by prolonged time in the 425–870 °C band, and use 304L where the weld will see corrosive service.

Heat tint colour is a useful shop-floor guide. A straw or light gold tint is borderline and still needs removal on a cosmetic part. Blue, purple and grey tint indicate a thicker chromium-depleted oxide and must come off before passivation, by pickling paste, an electrochemical brush or mechanical removal with stainless-only abrasives.

Then wash, dry and passivate to ASTM A967. On a component that will be visible, agree the acceptable weld appearance on a sample before production. Weld dressing on a No.4 surface leaves a patch that is hard to blend, and the blending cost is real on a 0.8 mm panel.

What should the inspection plan record?

Inspection for a stamped stainless part should record more than the outside profile. Measure hole position, flange height and draw depth against the drawing. Check wall thinning at the draw radius, because a wall that thins from 1.5 mm to 1.1 mm is a crack waiting for the next coil lot. Record hardness rise on a formed part, since work hardening is the mechanism that limits a second draw.

For cosmetic parts, keep a signed appearance sample under the same lighting the customer uses. For coated parts, record dry film thickness and cross-hatch adhesion. For welded assemblies, record the passivation batch. The stamping defect guide maps each symptom to its physical cause.

What does Ganesh Multitech run?

Ganesh Multitech presses stainless, CRCA, mild steel, aluminium, brass and copper on mechanical presses from 20 T to 300 T, with 250 T punching and a tool room that designs, builds and maintains press tools, progressive dies, welding fixtures and checking fixtures in-house. Tooling is quoted separately from the piece price, so the capital and the conversion cost stay visible.

Press work has run at Ganesh Multitech since 1992, across five units in Mumbai (Vasai) and Nagpur, with ISO 9001:2015 and ISO 14001:2015 certification behind the inspection records. Where a tool is not justified, the same part is cut on the 12 kW fibre laser, 40 ft × 16 ft bed, formed on a 200 T 5-axis brake, 4,450 mm bed, to 10 mm, welded on three CLOOS robotic cells or 30-plus manual stations, and finished through seven-tank pretreatment and powder coating, with DFT and cross-hatch adhesion checks. Formed tolerance is ±0.5 mm typical, tighter on critical features by agreement, and first article inspection and PPAP documentation are available. Customers include Mahindra, Minda, Gunnebo, Godrej & Boyce and Indian Railways.

When stamping stainless is the wrong route

Below roughly 300 pieces a year, a tool is a cost with no payback; laser cut and form the part and revisit when the volume repeats. Above 4–6 mm thickness, tonnage and tool wear climb faster than the piece price falls, and cutting plus forming is the better route. A mirror-polished deep draw is usually a polishing problem rather than a stamping problem, and the handling cost can exceed the press saving.

We do not machine mating faces, cast, forge or run presses above 300 T. If the design needs a machined pad or a 400 T draw, the route changes and the quotation should say so rather than absorb it.

What the press drawing has to settle

• Grade and temper, with the standard reference, for example 304 annealed to ASTM A240.
• Mill finish, 2B, BA, No.4 or No.8, and the grain direction relative to the bend line.
• Cosmetic faces marked, and the surfaces that must not be touched by the tool.
• Critical dimensions with the tolerance class from ISO 2768-1 plus any tighter callouts.
• Weld notes and whether passivation to ASTM A967 is required after welding.
• Annual volume, release pattern and the expected part life, so the tool decision can be priced.

Send the drawing and the volume to request a quote. The tool review checks draw ratio, tonnage and clearance before the price is written, which is cheaper than discovering the second draw after the tool is cut. More design guidance sits in the stamping and press tools hub and in the deep drawing design rules.