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Forming
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Galvannealed sheet can be formed in much the same manner
and by the same variety of processes as galvanized sheet, since the
steel substrate is generally the same for both
materials.
Although the steel formability is similar for
both types of coating, there are differences in the |
steel forming
behavior of the galvannealed and galvanized coatings. The
zinc-alloy coating on galvannealed steel sheet is more brittle than
the zinc coating of galvanized sheet. When the sheet is bent or
formed severely, the zinc-alloy coating will exhibit some amount of
powdering on the compression side of the formation. The
amount of powdering that occurs during forming is generally not
detrimental to corrosion resistance after painting, but it can
cause "dirt" build-up in dies, presses, etc.
| Joining |
Soldering
The iron-zinc alloy coating on galvannealed sheet is substantially
more difficult to solder than galvanized sheet, although soldering
can be accomplished in commercial production operations. Often,
more reactive fluxes are required to make soldering easier.
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Welding
Galvannealed sheet can be readily welded by resistance
welding, and other more complex procedures including arc welding.
The equipment needed for welding galvannealed sheet is essentially
the same equipment that is required to weld galvanized sheet. As
with galvanized steel, precautions must be taken to avoid porosity
and cracking of the weld caused by penetration of zinc into the
weld pool while using arc and oxyfuel processes.
In electric
resistance welding, such as spot welding, the zinc-iron alloy
coating present on galvannealed sheet typically requires less
welding current than that needed for galvanized product. This is
especially true in that the coating is most often thinner than a
galvanized coating would be if used for the same application. At
lower currents, the coating reacts less with the electrodes; there
is less zinc pick-up, less redressing of electrodes, and thus, more
spot welds between redressing.
Similar benefits are
realized during seam welding of galvannealed sheet.
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Fastening
Galvannealed sheet can be joined by the same fasteners and
techniques as those applicable to galvanized sheet.
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Adhesive Bonding
Adhesive bonding techniques can be used for joining galvannealed
sheet either to itself or other materials. The same materials as
those used for the adhesive bonding of galvanized sheet are
needed. |
Sealants
Sealants are often used between overlapping areas of
steel sheet to form watertight joints and to achieve superior
environmental durability. Neutral-cure silicone rubber
sealants are recommended for use with galvannealed sheet. These
materials need no primer. They are flexible and noncorrosive as
well as resistant to heat, cold, water and ultraviolet rays.
Other types of sealants, like butyl rubber and styrene butadiene
rubber, may also be used successfully. For more information about
sealants for specific applications, consult a sealant
manufacturer.
Painting
U. S. Steel Galvannealed Sheet is an ideal material for
painted product applications. Whereas galvanized-coated steel
surfaces generally require that a paint pretreatment be applied
before painting to obtain good adhesion between the paint and the
galvanized surface, galvannealed sheet can be directly painted.
Improved performance will generally be attained if a
primer/washcoat is applied prior to painting, but it is not
absolutely necessary. Furthermore, the somewhat rougher finish of
the zinc-alloy surface provides better mechanical bonding to the
paint than the usually smoother surface finish of a galvanized
coating.
In service, since the galvannealed coating is less reactive than
zinc, there tends to be less blistering between the paint and
galvannealed coating. Also, there tends to be less undercutting
corrosion at sheared edges and other breaks in the paint. Thus, the
very nature of the corrosion behavior further enhances the adhesion
between the paint and the metallic coating.
Galvannealed sheet can be painted using many types of paints. In
most instances, the surface should be cleaned after fabrication to
remove oils and dirt so that good adhesion can be achieved during
and after painting. A clean surface also aids in maintaining good
adhesion during service in the environment. Although not necessary,
the application of an iron or zinc phosphate pretreatment provides
additional resistance to delamination of the paint during exposure
to the environment.
In all instances, discussions with paint suppliers are
recommended to achieve the best performance attainable for each
application.
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