Environmental Engineering Reference
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cast or wrought ferrous alloys involved in furnaces, kilns, and ovens for a variety
of industrial and domestic purposes. These suffered oxidation and hot corrosion,
leading to heavy metal loss by scaling and at times catastrophic failure of compo-
nents. Subsequently, improved resistance to oxidation at moderate levels of
strength requirement was obtained by alloying cast irons with high silicon and
later by austenitic alloys containing 18-25 wt % Cr and up to 25 wt % Ni together
with small amounts of Si and Al. The high-temperature capability of iron-based
alloys increases with increasing chromium content. The strength of these alloys
also increases at higher temperatures with increasing chromium content. There-
fore, the two essential requirements for structural ferrous materials—strength and
corrosion resistance—are compatible, as illustrated in Fig. 6.41a and b, respec-
tively. However, the temperature capability of these alloys is limited to less than
1273 K where a combination of strength and oxidation rate invariably becomes
unacceptable.
Electrical heating elements for domestic equipments and industrial furnaces,
which require ductile wrought alloys, also undergo degradation. For the relatively
small cross-section of the heating elements, the importance of scale adhesion to
avoid generation of local hot spots has long been recognized. The incorporation
of reactive elements (e.g., Ce, Y, La, Zr, or Hf ) or their oxide dispersions to
minimize scale flaking was first used in such alloys (refer to Sec. 6.6) and has
more recently found applications in the coatings of high-strength superalloys.
The widespread use of steam power both for electricity generation and for rail
and marine transport led to the progressive increase in temperature in the search
for higher thermal efficiency, but the involved temperature remained generally
modest in comparison with those for the gas turbines. Failures of boiler and
superheater tubings under the influence of internal pressure led to the recognition
of creep as an important mechanism.
The gas turbine has been known in principle for many years. Such engines
are used in a wide variety of applications. The most demanding of these in terms
of material durability and reliability requirements under severe service conditions
are aircraft propulsion, marine propulsion, and electric power generation. But
successful operation of the engines was limited by the difficulty of matching the
efficiencies of the compressor and turbine to make the combination self-sus-
taining as well as by the limitation on the operating temperature imposed by
available materials of construction. Gradually, stationary gas turbines producing
shaft power were recognized to be thermally less efficient than steam turbines.
The outbreak of World War II stimulated further development in use of the con-
cept of turbine exhaust as a jet for aircraft propulsion. Soon the shortcomings
of the high-temperature materials for turbine blades were recognized, and some
researchers concentrated on air cooling of the blades using best available iron-
based alloys, whereas others tried the use of high-alloy austenitic steels and
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