Steels_ Metallurgy and Applications, Third Edition

266 Steels: Metallurgy and Applications

affect the rotor journals due to their smaller cross-sectional area. However, given
the temperature of operation, the creep and rupture behaviour of the HP rotor is
of major concern.
The IP rotor is similar in design but larger than the HP rotor and again there is
the need for high creep and rupture strength in the hotter regions. However, at the
exhaust end of the IP stage, the blades are longer and impose greater centrifugal
forces. This factor, coupled with the cooler operating conditions, therefore intro-
duces fracture toughness as an important design parameter. In addition, transient
operating phases, such as start-up and shut-down, can result in further thermal
stresses and reinforce the need for adequate fracture toughness at lower tempera-
tures. However, there tends to be an inverse relationship between rupture strength
and toughness and whereas the best creep and rupture strengths in 1% Cr-Mo-V
steel are obtained in an upper bainitic microstructure, such material has poor
toughness. On the other hand, the lower bainitic or martensitic structures that
promote good fracture toughness have poor creep strength. The final hardening
treatment for 1% Cr-Mo-V rotors therefore depends very much on whether
high-temperature strength or good toughness is considered to be the more impor-
tant design criterion. In the UK, rotors are oil quenched for improved fracture
toughness, whereas in the United States, air cooling is adopted in favour of creep
strength.
Both Reynolds et al. 36 and Viswanathan and Jaffe 37 have reviewed the factors
that might lead to an improved combination of creep strength and fracture tough-
ness. These include the use of vacuum carbon deoxidation, electroslag remelting
and modifications to the traditional 1% Cr-Mo-V composition to incorporate
higher levels of chromium (1.5%) and an addition of nickel (0.7%). It has also
been shown that improvements in the toughness of HP and IP rotors can be
obtained by reducing the level of impurity elements and suppressing temper
embrittlement. However, it is generally accepted that temper embritflement is
more pronounced in the 3.5% Ni-Cr-Mo-V LP rotor grade and this matter will
receive greater attention in the next section.

LP rotors

The LP rotor is the largest in the turbine assembly and operates at temperatures
between 270~ and ambient. The blades in this segment are also very long, e.g.
up to 1.12 m, and impose high stresses on the rotor shaft. Therefore the main
design criteria in LP rotors are a high proof strength and good fracture toughness
as opposed to the major requirement for high creep strength in the HP and IP
components. The 0.2% proof strength values for LP rotors are of the order of
750 N/mm 2 and the increasing demand for good fracture toughness is illustrated
by the data in Table 3.19 for FATI' values over the period from 1970 to 1985.
The material used for LP rotor forgings is 3.5% Ni-Cr-Mo-V steel to the
following composition:

0.25% C, 3.5% Ni, 1.5% Cr, 0.5% Mo, 0.1% V

In addition, the steel is made to low levels of silicon (0.1% max.) and manganese
(typically 0.2%), together with restricted levels of elements such as arsenic,