Bending without cracks
Restored ductility lets the element follow the radii on the drawing – the tube bends predictably and holds its geometry.
Bright annealing
Rolling hardens the sheath. Annealing gives the ductility back.
Rolling compacts the MgO and cold-works the sheath: the tube leaves the mill harder, under residual stress and less ductile. Annealing under a protective atmosphere reduces hardness and residual stress and restores ductility – without oxidising the surface.
It sits between rolling and forming: only an annealed tube can be bent to tight radii without cracking. The heat also drives residual moisture out of the MgO.
Annealing furnace
What changes
Softer and stress-relieved – not stronger.
Annealing does not raise the strength of the tube; it makes it formable. The cold-worked structure recrystallises: elongation rises, hardness and residual stress fall.
| Property | After rolling | Annealed in air | Bright-annealed |
|---|---|---|---|
| Hardness and residual stress | High – cold-worked | Reduced | Reduced |
| Ductility for bending | Limited – tight radii risk cracks | Restored | Restored |
| Surface after the furnace | As rolled | Dark oxide scale, chromium-depleted beneath | Bright and oxide-free |
| Follow-up treatment | Anneal before tight bends | Pickling or mechanical cleaning | None required |
| Corrosion resistance at delivery | – | Restored only after pickling | That of the sheath grade |
| Brazing of fittings | – | Surface must be cleaned first | Clean surface, ready to braze |
The furnace
Heated and cooled under protective gas.
Hot stainless steel in air forms a dark oxide scale with a chromium-depleted layer beneath it. In a dry hydrogen-containing atmosphere (low dew point), at 1040 – 1100 °C for stainless sheaths, the conditions are reducing, so no oxide forms – and the tube cools under the same atmosphere before it leaves the furnace.
Twice in our plant: the welded sheath tube is annealed in our tube mill after welding and sizing, and the filled element is bright-annealed again after rolling. Our division Balçık Brazing also offers bright annealing for third-party components.
What it means for your element
Predictable bending, a clean surface, the full corrosion resistance of the grade.
Clean, brazing-ready surface
No scale to flake off, no pickling residue: a uniform surface for visible elements and for brazing flanges and nipples.
Corrosion resistance of the grade
Without scale and chromium depletion, the sheath delivers the corrosion resistance of its grade – no pickling needed.
How we verify
- Hardness
- measured on annealed samples to confirm the softening
- Surface
- bright, uniform, free of scale and discolouration
- Forming
- first pieces bent to the drawing radius and inspected for cracks
- Electrical
- insulation resistance and dielectric strength on every element
- Corrosion
- salt spray to ISO 9227 on samples, where specified
Limits: annealing does not change the alloy, so the sheath grade is still chosen for the medium first. Elements that run at red heat in air, such as grill elements, darken in service.
Field evidence
Twelve months by the sea.
We exposed conventionally annealed and bright-annealed tubes side by side for 12 months in a marine atmosphere – high temperature, salt and humidity. Afterwards each tube was split in half and its surface examined under an optical microscope.
Bright-annealed
Still bright and shiny
Annealed in hydrogen at 1040 – 1100 °C and cooled rapidly under the same gas: no oxide layer to start corrosion from.
Conventionally annealed
Inferior corrosion performance
Annealed in air, the sheath carries a dark oxide scale with chromium-depleted steel beneath – the weak point in a salty, humid atmosphere.
On our lines
Annealed on our own lines in Kahramankazan.
Related
Process technologies that work together.
Tight radii or visible elements in your design?
Send the drawing. We check bend radii, sheath grade and surface requirements and confirm the forming route before we quote.