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Stainless Solutions

IMOA’s »Stainless Solutions« e-newsletter covers a different stainless steel issue each month, with tips on design and specification, and links to technical resources.

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Specifying Flat Panels

There are many reasons why an installed panel may not appear flat. Achieving flatness starts with appropriate specification and fabrication. The internationally recognized materials standards (i.e. ASTM, EN and others) include flatness requirements. They can vary with the specification, thickness and product form.

For example, if you are specifying wide cold-rolled coil for panels using ASTM, A480/A480M determines the dimensional tolerance requirements and has two flatness options – standard and “Stretcher-Leveled Standard of Flatness”. Table 1 shows the flatness requirement differences.  For architectural and aesthetic applications, the project specification and the fabricator’s order should require, “ASTM A480/A480M stretcher-leveled standard of flatness”. If this wording is not included, then standard flatness can be provided. Some stainless steel producers obtain stretcher-leveled quality of flatness by tension leveling coils, which is acceptable.

A Not specified to stretcher-leveled standard of flatness, and not including hard tempers of 2xx and 3xx Series, dead-soft sheets, and deep-drawing sheets.
B Maximum deviation from a horizontal flat surface.
C Not including hard tempers of 2xx and 3xx Series, dead-soft sheets, and deep-drawing sheets.

Left:
The fine No. 4 polish is consistent but reflections create a banded appearance.
Photo C. Houska, TMR Consulting

Right:
One Canada Square at Canary Wharf combines coined Cambric wall and column panels with reflective trim.
Photo Outokumpu

 

Coined finishes, such as Linen and Cambric, are particularly popular for building exteriors because their surface texture diffuses light. With coining, a coil or sheet is passed between two rolls where one is flat and the other is patterned. The shallow repetitive pattern appears flatter than more reflective finishes. Learn more about Finishes on IMOA’s website.

If the material is not flat before a polished finish is applied, then the polishing belt contact with the surface may not be uniform, producing an uneven finish with scratch length and depth variations. Even if the panel is subsequently flattened, finish variation can make the panel appear wavy.


Residual Stresses
All metal coils or panels can have residual stresses, which vary across their widths and thicknesses. Often the outer edges and surfaces are under tension while the center is under compression so material must be removed evenly, otherwise flatness problems can be created  during finishing and fabrication. While some fabricators have presses to flatten panels as the final processing step, this may not be 100% effective.

When possible the design should use a standard width and the full panel cross section. If a wider panel is purchased and excess metal is slit from one side, residual stress differences (i.e. removing tension from one edge) can cause panel movement. If the panel is too wide, it is better to slit an equal amount of metal from both sides.  Finishing only one side of a light gauge panel can cause panels to curve. Both sides should be finished, but the back of the panel can simply be ground to equalize the stresses and retain a flat panel.  Any metal finishing or fabrication operation should consider residual stress balance.

One common method for avoiding edge wrinkling and buckling during brake forming is to cut a notch or groove along the back of the bend before forming.  That eliminates the excess metal at the bend, which can wrinkle, and concentrate the stress along the bend.

For more information, see IMOA’s guidance on fabrication of Austenitic and Duplex stainless steel.


Looking for Clues
As you try to determine the cause of flatness problems, consider these questions:

  • Place a straight edge across the surface (vertically, horizontally and at 45 degree angles).  Is there physical variation in flatness and what pattern does it follow?
  • Review the specification, project document, metal purchaser’s requirements, and the mill certifications. Were the correct specifications used and was the material certified to them?
  • Is the finish consistent in appearance?  Use a magnifying glass if necessary.
  • Was there a manufacturing operation that might have changed the residual stress balance?
  • If there are bends, are they consistent in appearance?
  • How do the purchased and design widths compare?
  • Was bending or some other operation that might have changed the stress balance concentrated on one side or area?