Steels_ Metallurgy and Applications, Third Edition

Engineering steels 265

Table 3.18

Component Weight (tons)

....... ............

660 MW Fossil unit 1200 MW Nuclear unit

HP rotor 15 90

IP rotor 30 -

LP rotor 50 210

Generator rotor 90 190

and Gemmil134 for the weights of finished forgings. These authors also point out
that the ingot weights for these components are usually a factor of two or three
times that of the finished forgings and often involve the combined production of
several steelmaking and secondary refining units in order to achieve the required
ingot weights.
Attention will be focused on the rotors for turbines and generators but reference
will also be made to other components such as casings, bolts and blades. A very
detailed text on the materials used in both steam and nuclear power plant has
been prepared by Wyatt. 35

Turbine casings

The casings for the HP and IP stages of turbines are made from large castings,
and in the UK the 0.5% Cr-Mo-V grade is favoured:

0.15% C, 0.5% Cr, 0.5% Mo, 0.25% V

The casings are generally cast in two halves which are bolted together longitudi-
nally along a heavy flange. In fossil-fired plant, steam inlet temperatures of 540
or 565"C are employed and therefore the casings are subjected to high internal
pressure within the creep range. In addition, the two shifting operation of being
on load during the day and shut down overnight imposes significant thermal
fatigue stresses on the casing and other components in the HP and IP stages.
Repair welding in 0.5% Cr-Mo-V casings can give rise to stress relief
cracking which is associated with low creep ductility in the coarse-grained heat-
affected zone of weldments. This problem is generally overcome by adopting
titanium as opposed to aluminium deoxidation and this practice has proved to be
beneficial to both weldability and rupture ductility.

liP and IP rotors

The following 1% Cr-Mo-V composition is used world-wide for the rotor shafts

of both the HP and IP stages of turbines:

0.25% C, 1.0% Cr, 1.0% Mo, 0.3% V

As indicated earlier, the HP rotor is the smallest in the turbine and operates at

temperatures up to the main steam temperature of 540-565~ During operation,

the rotors are subjected to centrifugal forces and also to torques which mainly