Stainless steel stampings: work hardening and tool life

Why stainless behaves differently in a press

Stainless steel is chosen for corrosion resistance, appearance or heat resistance. Those properties come from alloying, mainly chromium, and they change the way the metal behaves under a punch. Austenitic grades such as 301, 304 and 316 work harden at a much higher rate than mild steel, which means the metal gets stronger and less ductile as it is formed.

The practical consequences are three. First, a bend that forms in one hit in mild steel may need two or three stages, or an interstage anneal, in a hard stainless temper. Second, the sheared edge is harder than the parent strip, which raises the load on the next station and on the punch that trims it. Third, the material conducts heat poorly compared with carbon steel, so the heat generated by shearing stays at the punch edge and accelerates wear.

None of this makes stainless a bad stamping material. It makes it a material that rewards a die designed for it. A die that was sized for mild steel will produce parts, but it will need sharpening more often, will show more burr and will be less predictable over a long run.

The grades a press shop actually sees

Grade choice is usually driven by the environment, then by strength and formability. The table summarises the families and the stamping note that goes with each.

Work hardening and the forming sequence

In an austenitic grade the first few per cent of strain raise the yield strength sharply. A 301 strip in a spring temper may already be at a high strength before it reaches the press. Bending it further raises the strength but reduces the remaining elongation, so the next bend has less margin. A part with three bends in the same area is therefore much harder to make than the same part with the bends spread out.

The usual solutions are to form in stages with a stress relief anneal between them, to soften the starting temper, or to redesign the part so that the severe bends are separated. Annealing adds cost and can distort thin parts, so it is worth checking whether the design really needs the hardness that caused the problem.

Work hardening also affects the sheared edge. The edge is harder than the strip, which is good for wear resistance in service and bad for the next trimming station. In a progressive die, the punch that trims a previously sheared edge sees a harder, more abrasive surface than the punch that cuts the parent strip.

The tooling penalty

Stainless is not especially hard in the annealed state, but it is sticky. It galls against tool steel, especially on draw radii and on the flank of a punch. Once a small pick-up forms, it grows, tears the part surface and increases friction. The heat from that friction cannot escape quickly because the material conducts it poorly, so the edge softens and wears faster.

The countermeasures are well known but often skipped on a quote. Use a premium tool steel or carbide for high volume, polish the working surfaces, coat the punches and dies with a hard low friction layer, open the clearance slightly compared with mild steel, and use a lubricant with extreme pressure additives. Keep the stripper in contact and the strip flat, because any lift encourages galling.

The result is a die that costs more to build and more to maintain than a mild steel die, but that holds tolerance for longer. The worst outcome is a cheap die that produces good parts for the first ten thousand strokes and then needs constant sharpening.

• Premium tool steel or carbide for cutting edges at high volume
• Polished flanks and draw radii to reduce pick-up
• Hard, low friction coating on punches and die inserts
• Clearance set for stainless, not copied from a mild steel die
• Extreme pressure lubricant, applied consistently

Springback and bend radius

Stainless has a higher yield strength than mild steel and a similar elastic modulus, so it springs back more after the punch is withdrawn. A 90 degree bend may need to be formed to 92 or 94 degrees to end up at 90, and the correction is not the same for every batch.

The practical approach is to trial the actual material and the actual bend sequence, then build the correction into the tool. Do not rely on a handbook value for a different grade. If the part has several bends, correct them in order, because each bend changes the residual stress in the next.

Tight bend radii are also a cracking risk, particularly in 301 and in 430. For austenitic grades a minimum inside radius of about one thickness in the annealed condition is a sensible starting point, and more for a hard temper. For 430 the limit is often set by the limited elongation rather than by springback.

Passivation, surface and magnetism

Stamping leaves free iron on the surface from the tooling, and free iron rusts. Passivation removes it and restores the chromium oxide layer that gives stainless its corrosion resistance. It is a chemical cleaning step, usually based on nitric or citric acid, and it is not a coating. A passivated part looks like a clean stainless part, which is exactly the point.

The starting surface matters too. A 2B finish is the common mill finish, a BA finish is brighter, and a brushed finish is directional. Stamping can mark or stretch the surface, so a cosmetic part may need a protective film during forming and handling.

Magnetism is a common surprise. Annealed 304 is essentially non-magnetic, but cold work makes it slightly magnetic, and a deeply formed or sheared part can attract a magnet. Ferritic 430 is magnetic all the time. If the application is sensitive to permeability, state the limit and test the formed part, not the flat strip.

What to send for a stainless quote

Send the grade, the temper, the thickness and tolerance, the finish, the bend radii, the burr requirement and the annual volume. Add the service environment so the supplier can flag a grade that will not survive it. If the part needs passivation, say so and name the standard.

Ask how the supplier will manage galling and heat at the die, what tool steel and coating will be used, and what the sharpening interval is expected to be. Ask for a trial report that includes the burr, the bend angles and the springback correction.

If the volume is low, a soft die and a secondary passivation step may be the right answer. If the volume is high, the extra cost of carbide or a coated premium tool steel is usually repaid in fewer stoppages. The crossover depends on the part and the press, and it is worth asking for both options in the quote.