Process
Stress relief annealing for clad sheet: 540 °C and what it fixes
Roll bonding leaves stress in the sheet. A controlled anneal removes it without wrecking flatness or the bond. Here is the regime, why each number exists, and which fields belong in your specification.
At a glance
Specification at a glance
| Parameter | Regime | Why it matters |
|---|---|---|
| Annealing temperature | 540 ± 25 °C | High enough to relax rolled-in stress, low enough to stay clear of aluminium melting and stainless sensitizing |
| Holding time | Under 3 hours at temperature | Long dwell adds nothing after stress relaxation and starts to grow intermetallic layers at the bond line |
| Heating rate | 80 to 200 °C/h | Slow enough to avoid thermal gradients between steel and aluminium |
| Cooling rate | 80 to 200 °C/h | Fast cooling re-introduces the gradient the anneal just removed |
| Atmosphere | Bright or air, agreed per order | Decides whether the surface needs pickling afterwards |
| Flatness after anneal | Agreed limit | The real acceptance test for the process, not the temperature chart |
Why clad is annealed at all
Roll bonding works by forcing three metals to reduce together until the interfaces are metallurgically joined. The steel skins and the aluminium core do not reduce by the same amount, and they do not spring back by the same amount either. What comes off the mill therefore carries residual stress that is locked into the sheet, distributed differently through each layer.
Left in place, that stress shows up later in ways that look like a material defect but are really a process omission: sheet that bows after it is cut, discs that go out of flat after blanking, earring in deep drawing, and occasionally a shear crack at the cut edge where the layers separate under load. Annealing removes it before the material reaches your press.
Where the numbers come from
The plant works to 540 ± 25 °C, holding for under three hours, with heating and cooling held to 80 to 200 °C/h. Each figure is a compromise rather than a preference.
The temperature has to be high enough to relax the rolled-in stress in both the stainless skins and the aluminium core. It also has to stay well below the point where the aluminium core would soften enough to squeeze out under its own layer pressure, and below the range where 304 begins to sensitize at the grain boundaries. 540 °C sits in that window with margin at both ends, which is why the tolerance is ± 25 °C rather than a single setpoint.
The hold is short for the same reason. Once the sheet has reached temperature and the stress has relaxed, extra time buys nothing and costs something: at the bond line, longer dwell at temperature grows the intermetallic layer between aluminium and stainless, and a thick intermetallic layer is what makes a bond brittle. Cooling is controlled for a simpler reason: if one layer cools faster than the other, the gradient you just removed comes straight back.
What belongs in the specification
Buyers rarely specify the anneal, and then argue about flatness at destination. Six fields prevent that. State the temperature band and the hold time (or accept the plant's standard band in writing). State the atmosphere if the surface must be bright, because air annealing leaves oxide that needs removing. State flatness as a measurable limit on the form you are buying, not on the coil: flatness of a 2.5 mm sheet and flatness of a 240 mm disc are different problems. State hardness or temper if your drawing operation is sensitive to it, since annealing changes it. State whether the anneal applies to the whole order or only to the structures that get deep drawn. And state what happens if the flatness limit is missed: re-anneal, re-level, or reject.
How to check it without a laboratory
Two field checks catch most problems. Cut a disc or a square from the middle and from the edge of a sheet, lay them on a surface plate and measure the gap under a straight edge after 24 hours. Stress that was not removed shows up as a change between the two, not as an absolute number. Then shear a strip across the bond and look at the fracture: a bond that fails at the interface with a clean, bright surface is a process problem, while a bond that tears through one of the layers is doing its job.
Questions
Common questions
Is annealing always necessary?
Not for every structure. Thin three-ply sheet that will be cut into discs and pressed once often runs without a separate anneal. It becomes necessary when the sheet is thick relative to the disc diameter, when the part is deep drawn, or when flatness after cutting is a customer requirement.
Why 540 °C specifically?
It relaxes rolled-in stress in both the stainless skins and the aluminium core while staying clear of aluminium softening and stainless sensitizing. The ±25 °C band is the window that survives both constraints.
Does annealing affect the bond strength?
A correct anneal does not reduce it. An over-long hold at temperature grows the intermetallic layer at the bond line, and that is what makes a bond brittle, which is why the hold is capped.
Can you anneal after cutting discs?
It is possible but unusual: the flatness benefit is largely lost once the sheet is cut, and handling cost rises. The anneal is normally applied to sheet or coil before blanking.
What flatness limit should I ask for?
Set it on the form you buy and the operation that follows. A limit quoted in millimetres per metre on sheet is not the same as a limit measured across a disc, and the two should not be interchanged in a purchase specification.