Stainless steel stampings: work hardening and tool life

Stainless steel stampings cost more than the material price difference suggests, and work hardening is the reason. Annealed 304 enters the press at about 210 MPa yield; after 20% cold reduction the same metal is at roughly 480 MPa, and after 40% it is near 860 MPa. The second stroke on a part is therefore cutting metal that the first stroke already hardened.

That single behaviour explains three costs that surprise buyers. The draw force rises through a multi-stage part, so the press and the tool are sized on a peak rather than an average. The die wears faster because the sheet is harder at the cutting edge. And the part may need an anneal between draws, which adds a furnace step, a scale removal step and a straightening step that no stamping quotation shows by default.

The grade table, the tonnage arithmetic and the tool-life numbers below are the ones that decide those costs. They are worth working through before the drawing is released, because a change of grade after the tool is cut is a new tool.

Why work hardening decides the second draw

Austenitic stainless has a high strain hardening exponent, around 0.45 for 304. Flow stress rises steeply with true strain, which is the natural log of the original thickness divided by the current thickness. A 20% reduction in thickness is a true strain of 0.22; a 40% reduction is 0.51. On the 304 curve those strains correspond to the yield values above.

The practical effect is that a single draw at the limit leaves very little ductility for the next stage. Elongation in a tensile test falls from about 45% annealed to roughly 20% after 20% cold work, and the forming limit curve moves down with it. That is why multi-stage stainless draws are planned around a total reduction of about 60% before an anneal, and why a three-stage 304 part often becomes a two-stage part plus an anneal plus a final form.

What the anneal costs

A solution anneal for 304 sits at roughly 1,040–1,120 °C followed by a fast cool, and it does three things to the production plan. It resets the mechanical properties, it leaves an oxide scale that has to be removed before the next draw or before passivation, and it distorts a part that was flat.

The distortion is the part nobody quotes. A drawn cup that comes out of the furnace slightly oval has to be straightened or re-struck, and that is a second handling step on a part that already has three. It is also why the anneal decision belongs at the DFM stage: a design that keeps the total reduction under the limit avoids the step entirely.

Planning a draw around the hardening limit

Take a 100 mm diameter cup, 45 mm deep, in 1.5 mm 304. The blank is the square root of 10,000 + 18,000, or 167.3 mm, which gives a draw ratio of 1.67 against the 304 limit of about 1.72. The part fits one draw, with almost no margin.

The force calculation shows why that margin matters. Using the annealed tensile strength of 515 MPa, the draw force is π × 100 × 1.5 × 515 × (1.67 − 0.7) = 235 kN. A blank holder at 30% adds 70 kN, and a 30% margin brings the requirement to 397 kN, or 40 tonne-force. The part lands on a 63 T press, not a 40 T, because the force is calculated at the end of the stroke where the material has already hardened.

Push the depth to 70 mm and the blank grows to 194.9 mm, a ratio of 1.95 that is past the limit. The part now needs two draws: 194.9 mm to 130 mm at a ratio of 1.50, then 130 mm to 100 mm at 1.30. The second stage is cutting metal that the first stage hardened to roughly 480 MPa yield, so its force cannot be read from the annealed data sheet. That is exactly where a press sized on stage one runs out of margin.

What tooling do stainless steel stampings need?

Stainless is harder to shear than CRCA, and the numbers are small enough to calculate. Shear strength is about 310 MPa against 240 MPa, so a 10 mm hole in 1.5 mm sheet needs 14.6 kN in 304 against 11.3 kN in CRCA, a factor of 1.29.

Three tool decisions follow from that factor:

• Clearance. Stainless runs 8–10% of thickness per side, where CRCA runs 5–8%. At 1.5 mm that is 0.12–0.15 mm per side against 0.075–0.12 mm. Tighter clearance on stainless raises the punch load, burnishes the hole wall and accelerates wear without improving the hole.
• Die material. D2 at 58–60 HRC is the default. Above roughly 500,000 strokes a year, a powder metallurgy insert or a TiCN-coated punch holds the edge longer, and the coating also reduces the tendency to gall.
• Springback. A 90° bend at R = t springs back 4–6° in 304 against 1.5–2.5° in CRCA, so the tool has to overbend by that amount. A tool cut to 90° produces a part at 84–86° and a fit problem at assembly.

Galling is the failure that scraps parts

Tool wear is gradual and shows up as a rising burr. Galling is sudden: a patch of stainless welds itself to the die radius, tears the surface it touches, and every part after it carries a score mark on the same face.

Three controls work together. The die radius is polished to a mirror finish and kept that way, because a 0.4 µm surface does not pick up metal the way a ground surface does. The lubricant carries extreme-pressure additives and is applied to both faces, not just the top. And the stroke rate drops 20–30% against the CRCA rate, because the heat generated at the die radius is what breaks the lubricant film in the first place.

Speed, lubricant and the anneal residue

A stainless job runs at 20–30% fewer strokes per minute than the same tool would run on CRCA. A 30 stroke/min press becomes 22–24, which is a real capacity difference on a part that was quoted against a steel cycle time.

Stainless also conducts heat poorly, at about 16 W/m·K against roughly 50 W/m·K for steel, so heat builds in the die instead of flowing into the bolster. A die that stays cool on CRCA will run warm on stainless, and once the die radius is hot enough to thin the lubricant film, galling starts. A contact check on the die block after the first hour of a run is a cheap way to catch it.

Lubricant choice then splits along the process route. Chlorinated extreme-pressure lubricants give the best galling protection, but the residue has to be removed before an anneal, because it burns on and stains the surface, and again before passivation. Where an anneal follows the draw, a chlorine-free extreme-pressure lubricant and a wash step are the safer combination, even though the tool needs a little more attention.

Stainless scrap also has to be segregated by grade. Mixed 304 and 430 scrap is worth less than either sorted grade, and a magnetic check separates the ferritic grades from the austenitic ones in seconds. On a high-volume part the scrap credit is a line in the piece price, not a rounding error.

Grade selection and what each grade does to the tool

The table is the working set for stamping. Mechanical values are minimums for annealed sheet; the last column carries the tooling consequence.

Grade selection is usually driven by corrosion rather than by forming. 304 covers most indoor and general outdoor work, 316L is specified where chlorides are present, and 430 is a cost reduction for dry indoor parts that do not need nickel. The forming consequence is that 430 needs a larger bend radius and has less elongation, so a design that was developed on 304 cannot always be switched to 430 without checking the radii.

Contamination, passivation and the visible face

Stainless that has been formed is not automatically corrosion resistant. Shearing smears the cut edge and can drag iron from the tool or the press bed into the surface, and those particles rust in service while the surrounding metal stays clean. The result is a part that looks defective and a claim that is hard to argue.

Four controls keep it clean:

• Separate the handling. No carbon steel brushes, no grinding dust from the CRCA line, and no steel slings or chains on the stainless parts.
• Cover the bolster and the die faces that the sheet slides over, so embedded iron from previous jobs does not transfer.
• Passivate after forming, using a citric or nitric treatment that removes free iron and restores the chromium oxide layer. Where there is heat tint from welding or annealing, pickling comes first.
• Protect the visible face with a film during handling and packing, because a brushed or 2B finish shows every mark and a scratch cannot be polished out on a formed part.

Intergranular corrosion is the other risk, and it appears when a sensitised grade is held in a temperature range that lets chromium carbides form at the grain boundaries. Low-carbon 304L and stabilised grades are the answer where a weld or an anneal is unavoidable, and ISO 3651-2 is the test method behind the acceptance decision.

What we press stainless on

Stainless runs on mechanical presses from 20 T to 300 T, with 250 T punching capacity for the piercing stations that share a tool with forming. The in-house tool room designs the die, chooses the insert material and builds the checking fixture, so a stainless tool that needs a coating or a changed clearance is dealt with by the people who built it rather than by a supplier who has never seen the part.

Tooling is quoted separately from the piece price, and that matters more on stainless than on CRCA. A die with twice the wear rate and a third of the regrind interval carries a maintenance cost that a CRCA tool does not, and the buyer should see it before committing to a volume. The press tonnage calculator gives the forming and piercing loads, and the stamping capability page sets out the press range and the material set.

Where the part is a flat blank with holes and a few forms, the stainless design and finish guide covers the feature-level rules, and the progressive die maintenance guide covers the regrind schedule that a stainless tool needs.

What to put on a stainless drawing

1. Grade and condition, written as 304, 304L, 316L, 430 or 409 with the annealed or cold-rolled temper, because the temper changes both the load and the springback.
2. Surface finish, such as 2B, brushed or No. 4, and which face is visible.
3. Bend radii and the bend direction relative to the rolling direction.
4. Draw or form stages, if the part needs more than one stage, with the total reduction and the anneal condition.
5. Passivation and pickling, including whether the finish is required before or after assembly.
6. Burr limit and burr side, because a burr on a visible edge is a reject on stainless.
7. Protective film and packing, since a scratched stainless part cannot be repaired the way a painted part can.

When stainless stamping is the wrong route

Stainless is the wrong route when the part is a flat panel or a simple bracket at low volume. Laser cutting and press-brake forming avoid the tool entirely, and the laser cut edge on stainless is clean enough for most visible work after deburring.

It is the wrong route when the part needs a bright annealed finish after forming, because that is a mill or a finishing line, not a press operation. It is also the wrong route when the grade is 301 in a spring temper and the design expects deep forming; the material is chosen for its strength after forming, and a deep draw fights that choice.

And it is the wrong route when the tool life arithmetic does not close. A part at 5,000 pieces a year on a tool that needs a regrind every 200,000 strokes is a tool that never pays back, and the honest answer is the laser route with a formed or welded detail.

Send the drawing with the grade, thickness, annual quantity and finish through the RFQ route, and the quotation will separate the die, the piece price and the passivation step. The rest of the press and tooling library sits under the stamping and press hub, including the stamping defect catalogue that covers the galling and scoring failures described here.