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The production of Vitrenamel Sheet requires attention to
processing details by both the steel manufacturer and the customer
who applies the vitreous enamel coating. The steel manufacturer has
to meet the requirements with respect to carbon content and its
distribution, the annealing practices and other in-house
steelmaking practices that are known to influence the
enamelability. The customer has to apply good practices with
respect to cleaning the steel before applying the frit, the
application of the frit, and the high temperature firing process
used to convert the loose powder frit into a hard vitreous
coating.
Outgassing and fishscale are the major difficulties associated
with enameling. The types of outgassing that are influenced by the
steel are those caused by carbides and hydrogen. Generally,
carbides are easily controlled and do not cause many problems.
However, if the carbides are close to the steel surface, the carbon
can combine with oxygen to form carbon monoxide or carbon dioxide
gases. These gases can then cause a rough enamel surface.
Hydrogen-related outgassing is a more common problem. This
problem is related to steel's increasing solubility for
hydrogen when heated to high temperatures. During the frit-firing
process, hydrogen may be introduced into the steel from moisture in
the atmosphere or in the enamel frit. As the part cools, the
hydrogen solubility in the steel decreases and is released from the
steel surface. If the frit is still fluid, the escaping hydrogen
gas will pass through the frit without causing any problems.
However, if the enamel coating has become fairly viscous, the
exiting gas can cause a rough surface on the finished vitreous
coating.
Fishscale is also related to hydrogen that has entered the steel
during the firing process. If the hydrogen does not escape while
the frit is still fluid, but is trapped between the steel and
hardening frit, eventually the internal pressure can reach such
high values that the hard enamel coating is fractured. The surface
roughness is known as "fishscale".
To assure good performance of the finished product, other issues
need to be attended to during application of the vitreous coating.
These include:
1. Warpage - Product distortion that occurs during
firing. Usually, it relates to the residual stresses in the formed
part.
2. Sag - Dimensional instability experienced by the
steel part during firing at high temperature. The purchased steel
can impact this behavior.
3. Strength after Fire - During the firing process,
the steel may be prone to changes that adversely affect the
strength of the finished part. Strength-after-fire- can be improved
by assuring that the correct steel grade is selected to match the
requirements during the firing cycle.
Enameling steels can be joined using a variety of joining
processes. These include resistance welding, arc welding, laser
welding, and mechanical fastening. The effects of each of these
joining practices needs to be addressed carefully by the end user
for each end application. Also, the specific type of frit used
during the manufacturing process can impact the performance in the
weld area; thus, evaluation of the performance of welded areas
should be investigated whenever a new type of frit is used
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