Steels_ Metallurgy and Applications, Third Edition

Engineering steels 271

Turbine blades

Apart from locations at the extreme ends of steam turbines, all the blades are
made in 12% Cr steels. Although the metallurgy of these steels will be discussed
in the next chapter, for the sake of continuity the use of 12% Cr steels in turbine
blades is better described at this stage.
In the HP turbine, the blades are short and operate under the maximum steam
temperature, i.e. 565"C. The creep strength of 12% Cr steels is not adequate to
operate at such temperatures and therefore the first few rows of blades at the
HP inlet are generally manufactured from Nimonic 80A. In the LP turbine, the
blades are long, and in large turbines the exhaust blades can exceed a length of
one metre. Such blades generate high centrifugal forces and again 12% Cr steels
are not strong enough to cope with the conditions imposed. In such situations,
the precipitation-strengthened FV520B steel may be employed, which has the
composition shown in Table 3.21.
However, in recent years, interest has grown in the use of titanium alloys,
typically Ti-6% AI, 4% V (Ti6AI4V), as a substitute for martensitic stainless
steels in the outlet stages of turbines. The reduced weight and high resistance
to corrosion fatigue of the titanium alloys allows the length of the blades to
be increased substantially, compared to what can be achieved in 12% Cr steels.
Thus the exhaust area of the LP turbine can be increased by about 50%, which
increases the power by the same amount.
For all stages between the inlet and outlet, the grades of 12% Cr steels shown
in Table 3.22 are employed. The tempering resistance and creep strength of these
materials is increased progressively with the addition of molybdenum, vanadium
and niobium to the 12% Cr base. As illustrated in the next chapter, nickel is added
to such steels in order to preserve an austenitic structure at high temperatures,
in the presence of ferrite-forming elements such as molybdenum, vanadium and
niobium.
Although the 12% Cr steels have adequate resistance to attack in moist air,
the formation of water droplets in the final stages of the turbine can result in
severe erosion problems. This is overcome by brazing strips of erosion-resistant
materials such as stellite (cobalt-based) to the leading edges of LP blades.

Table 3.21 Composition of FV 520B

C% Ni% Cr% Mo% Cu% Nb%

0.05 5.5 14 1.6 1.5 0.3

Table 3.22

Type C% Si% Mn% Ni% Cr% Mo% V% Nb%

12Cr-Mo 0.1 0.3 0.3 - 12.5 0.75 - -

12Cr-Mo-V 0.1 0.3 0.6 0.8 12 0.6 0.2 -

12Cr-Mo-V-Nb 0.13 0.5 1 0.8 11.2 0.6 0.2 0.4