Hard Temper Stainless Steel Foil

A hard temper is not a grade, a coating or a finish. It is the work hardening left inside the metal by the last rolling pass, and it survives only until somebody brazes or cures the part. Hard Temper Stainless Steel Foil from Qihong leaves the mill in Ningbo, China between 0.008 mm and 0.30 mm with the hardness measured in HV and recorded per coil, so the number qualified on a sample is the number that arrives on the pallet.

The Last Pass

Reaching foil gauge takes a long series of reductions, and each one hardens the metal because the crystal lattice has no time to recover. Between passes the mill anneals that hardness away so the next reduction can bite without cracking the edge. What happens after the final pass decides what you receive: anneal once more and it is soft, stop there and it is hard.

1. Pass schedules are set per grade and gauge — thinner stock means more passes and a lighter bite each time, which is why a 0.008 mm coil can run through eighteen stands before it reaches size.
2. Intermediate annealing restores ductility between passes. Skip or shorten it and the edge cracks before the gauge is reached.
3. The final pass sets gauge and temper in the same operation, so thickness tolerance and hardness are won or lost together, not separately.
4. Slitting, leveling and rewinding only relieve stress. A coil can come out of them softer or less flat, never harder.
5. Every later heat treatment relaxes the work hardening for good — soldering, brazing, welding, powder-coat curing and lamination all anneal the material locally.

Everything worth knowing about Hard Temper Stainless Steel Foil follows from that one fact: the property is inherited from the last pass, so it has to be protected through slitting, packing, shipping and the customer's own assembly process.

Hardness vs Flatness

Hard and flat pull against each other. The residual stress that makes the metal spring back also makes a thin coil curl, wave at the edge and buckle in the middle. A mill can flatten almost any coil with enough tension and bending, but the same operation that flattens it also relaxes it.

1. Coil set comes from winding tension and core diameter. A small eye, a long sea voyage and a coil stored on its edge all make the curl worse.
2. Edge wave appears when the edges were rolled a shade thinner than the centre, which is common on narrow material taken from a wide master coil.
3. Slitting releases stress, so camber is created at the slitting line. It has to be measured on the finished narrow coil, not on the master coil before slitting.
4. Tension leveling improves flatness and costs a little hardness. How much is a process decision that should be agreed on the order, not discovered on arrival.
5. Flatness needs a number — millimetres of deviation per metre of length — because "good flatness" means something different to a mill, a slitter and a press operator.

Ask for flatness and hardness as two separate figures on the certificate, and ask which one was allowed to move. That single question separates Hard Temper Stainless Steel Foil that runs in a progressive die from material that looks perfect on the pallet and fights the feeder all afternoon.

Grade Ladder

The hardness ceiling belongs to the alloy. Cold rolling can only take a grade as far as its chemistry allows, which is why the same temper name means different things across the 200, 300 and 400 series.

1. Start with the strength the part actually needs at its working deflection, not with the hardest temper available.
2. Then check the environment: if the answer is chloride, wet or food contact, the grade changes before the temper does.
3. Then check the forming route: a part with one shallow bend and a part with a drawn cup cannot share a specification.

Matching all three is what makes Hard Temper Stainless Steel Foil a specification rather than a wish — and it is why the useful input at enquiry stage is a drawing with a bend radius on it, not a grade name on its own.

Specifications

The window below is what the lines run routinely. Combinations outside it are quoted individually rather than promised from a table.

Written this way, Hard Temper Stainless Steel Foil arrives against numbers that can be checked on receipt: gauge by micrometer, hardness by indentation, flatness on a plate, camber on the slit coil.

Bend Test First

A certificate that says full hard tells you what the mill intended. A bend test tells you what the strip will do in your tool, and the two are not always the same sentence.

1. Bend a sample 90° across the grain at the radius drawn on the part. Cracking on the outside fibre means the temper is too high for that radius, whatever the certificate says.
2. Measure hardness at a light load — HV 0.1 or HV 0.2 on thin gauge. A heavy indenter reads through the foil into the anvil and returns a number that belongs to the test rig, not the metal.
3. Check camber on the finished slit coil over a measured metre on a flat plate, because that is the condition the feeder sees.
4. Cut a metre, lay it free and measure the curl. That is the coil set your decoiler has to straighten before the first stroke.
5. Keep the pass schedule for repeat orders. A new schedule is a new material even at the same grade, gauge and temper name.
6. Qualify on a trial coil of 5 – 20 kg run on the production tool at production speed, then release the balance.

Inspection is not a formality on material this hard and this thin; it is the only way to know whether Hard Temper Stainless Steel Foil will survive the second station of your die. A sample coil and a bend report settle in a week what a bulk order settles in a claim.

Where It Fails

Hard temper foil fails in a small number of predictable ways, and all of them are cheaper to design out than to argue about afterwards.

1. Forming beyond a shallow bend — the elongation that would let the outside fibre stretch has already been used up by rolling. A tight radius cracks rather than forming.
2. Heat after forming — soldering, brazing, welding, powder-coat curing and lamination all anneal the material. Hardness does not come back, and a part that was qualified hard may arrive at the customer soft.
3. Sheared or punched edge followed by a bend — shearing leaves micro-cracks in the cut zone, and the bend opens them into a fracture.
4. Wet chloride under sustained load — high residual stress plus chloride is the classic stress-corrosion pair, and hard temper carries more residual stress than any other condition.
5. Large dead-flat panels — thin hard stock oil-cans between supports. If the part must be optically flat, specify flatness and accept that temper may have to give.
6. Deep drawn or three-dimensional parts — these belong in annealed stock, hardened afterwards if the application needs it.

None of these are defects in the material. They are the boundary of the condition, and stating the boundary is part of selling it honestly — which is why every enquiry for Hard Temper Stainless Steel Foil starts with the forming step and the service environment, not with the price per kilogram.

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