Steels_ Metallurgy and Applications, Third Edition

128 Steels: Metallurgy and Applications

the enamel and the firing conditions. Special grades of steel specifically developed
for enamelling are, therefore, often used.
The main potential problem that may affect an enamelled component is called
fish scaling, whereby small slivers of enamel split off from the surface to leave a
fish-scale appearance. This may occur soon after enamelling or many weeks later.
Fish scaling arises because the solubility of atomic hydrogen in steel is as much
as 1000 times greater at the firing temperature than at room temperature. During
cooling the solubility limit is exceeded with the result that gaseous molecular
hydrogen is formed at high pressure. If this gaseous hydrogen forms at the enamel
steel interface and if there is not sufficient enamel adherence and toughness, the
pressure developed is able to crack the enamel and remove a fragment to form a
fish scale.
The tendency to form fish scales may be minimized by keeping the dew point
of the furnace atmosphere low since water vapour is the main source of atomic
hydrogen by a reaction with iron to form iron oxide. It is also beneficial to avoid
unnecessarily high firing temperatures since such temperatures lead to even higher
solute hydrogen contents. The most important method of avoiding fish scaling,
however, is to use a steel which contains sites that can accommodate high-
pressure gaseous hydrogen within the steel and which, therefore, prevent the
build-up of gaseous hydrogen at the steel enamel interface.
The sites that are used may be physical or chemical. Physical sites include
voids, cracks, vacancies and dislocations. Chemical sites include precipitates such
as TiN, TiC, AIN, Fe3C or BN and inclusions such as A1203, MnS and MnO.
Certain steels are hot rolled with a high coiling temperature to produce coarse
carbides that crack on cold reduction to provide physical sites. Titanium-bearing
IF steels are commonly used since they provide chemical sites as well as good
formability.
The enamelling process provides a hard, durable finish in a wide range of
attractive colours, which is resistant to heat, abrasion and chemical attack. Appli-
cations for vitreous enamelled steels include domestic cookers, baths, hot-water
tanks and cookware, architectural facing panels, flue pipes, silos, heat exchanger
panels and tanks for bulk storage.

References

1. Hewitt, B.J., Review of Annealing Technology, IISI, Technical Exchange
   Session, p. 51 (1996).


2. Imose, M., Trans. Iron Steel Inst. Japan, 25, 911 (1985).


3. Lubensky, P.J., Wigman, S.L. and Johnson, D.J., Microalloying '95, Conf.
   Proc., Pittsburgh, The Iron and Steel Society, p. 225 (1995).


4. Meyer, P. and Frommann, K., ABM Int. Congress, Sao Paulo, p. 249 (1994).


5. The Making, Shaping and Treating of Steel (Ninth Edition), Ed.
   Harold E. McGannon, United States Steel (1971).


6. Swartz, J.C. Trans. Met. Soc. AIME, 239, 68 (1967).


7. Hall, E.O. Proc. Phys. Soc., Series B 64, 747 (1951).


8. Petch, N.J.J. Iron and Steel Inst. 174, 25 (1953).