Why is it necessary to reduce the internal stresses in sheet metal?
How do flatness defects occur?
What is the solution in case of flatness defects?
Sheet metal plates and sheet metal parts often contain flatness defects. These flatness defects, such as internal stresses in the sheet, are generally undesirable because they make further processing more difficult and negatively affect the properties, quality, and aesthetic appearance of products made from the sheet.
Roller leveling (leveling between rollers) is a widely used method by which flatness defects and internal stresses can be minimized through repeated bending deformation.
Origin of flatness defects
The stresses present in sheets and sheet coils can already originate in rolling mills during the production of coils and plates. Although rolling mills perform a certain degree of stress relief, practical experience shows that this is sometimes not sufficient.
If the material contains internal stresses, these may increase during further processing, or in insufficiently stress-relieved material, the stresses may be released, leading to deformation and waviness.
Internal stresses can arise:
- during the production of sheet coils in rolling mills
- during the cutting of sheet coils
- during laser cutting of sheets
- during punching (stamping)
- during bending
- during deep drawing
- during welding
The internal stresses present in sheets and sheet parts are released during further processing, which can result in failure to meet the required flatness tolerances of semi-finished and finished products. If the sheets are leveled before further processing, the internal stresses are reduced and do not manifest during machining.
For example, if sheet parts are bent without prior stress relief, the bending angle may vary between parts cut from different sheets. Similarly, welded parts without prior stress relief are more likely to deform due to heat.
It is also advisable to minimize internal stresses before punching or laser cutting. This way, when producing parts with many small contours, deformation during processing is minimized or avoided.
When the flatness of a sheet or sheet part is inadequate, external forces are applied to bring the material into a state of plastic deformation, after which it is leveled flat. During plastic deformation, the material flows, and the internal stresses are reduced to a minimum.
