Stainless steels 361of-40"C would need to provide a minimum Charpy V value at a test tempera-
ture of -50"C in the as-welded condition compared with a temperature of 0*C
in the post-weld heat-treated condition. The minimum Charpy V value depends
upon the strength of steel employed as indicated in Table 4.15.
Steels in fossil-fired power plants
Since the early 1970s, major events have taken place which have had a profound
effect on the choice of fuels for power generation. First, the oil crises of 1973
and 1979 curtailed the use of oil for steam-raising purposes but, currently, this
fuel accounts for about 8% of power generation. At a later stage, the fire at Three
Mile Island in the United States and the devastating melt-down at Chemobyl had
a very significant effect in limiting further commitment to nuclear power. More
recently, the privatization of the electricity industry in the UK has prompted the
dash for gas and in 1993, natural gas accounted for 9.2% of power generation.
As a result, the use of coal is declining but, currently, coal makes up 53% of
the power requirements. However, with proven reserves for more than 300 years,
coal-fired power stations are likely to feature prominently well into the twenty-
first century.
Boiler layout and operation
The selection of steels for the various parts of fossil-fired boilers will be appre-
ciated more readily by first providing an outline of boiler construction and the
steam-water circuit. A schematic cross-section through a boiler is shown in
Figure 4.38 The boiler has a large rectangular combustion chamber in which
high-temperature gases are generated by burning coal or oil, providing maximum
temperatures of up to 1600 and 2000"C respectively. The walls of the furnace
section are formed with the evaporator tubes which are arranged in closely
packed, vertical rows. Each tube has two longitudinal fins at 180" and the fins
on adjacent tubes are welded together to form an airtight wall, except for entry
ports for burners or exits for flue gas. 32 Although the temperatures of the furnace
gases are very high, the cooling effect of the steam/water mixture generally limits
the outer metal (fire-side) temperature of the evaporator tubing to about 450"C.
On the other hand, significantly higher metal temperatures can be generated in
some circumstances, e.g. when a flame impinges directly onto the surface of the
evaporator tubes.
The atmosphere in the combustion chamber of fossil-fired boilers can be
extremely aggressive, leading to severe attack and premature replacement of
the boiler tubes. This arises from the chlorine content of coals whereas sulphur
and vanadium are the damaging elements in fuel oils. Under normal oxidizing
conditions, a protective oxide scale is formed on the surface of the tubing which
limits the corrosion rate. However, reducing conditions promote complex reac-
tions between sulphur, chlorine, carbon and oxygen, leading to the formation
of non-protecting scales. Particularly damaging conditions are produced when
unburnt particles of coal impinge on the side-wall of the evaporator tubes, the