Engineering steels 267Table 3.19 FATI' Values in LP rotors
Period FA77,oc a
Average " iange1970-1975 +25 0 to + 50
1976-1980 + 16 - 18 to + 40
1981-1985 + 10 -4 to + 20aFracture appearance transition temperature.antimony, tin and phosphorus in order to minimize the effects of temper
embrittlement. The evolution of the current composition will be described briefly
to reflect the various changes that have taken place to accommodate the demands
for increased turbine size and improved fracture toughness.
Boyle et al. 39 have given a very detailed account of the development work
that was undertaken in the United States in the 1950s following failures in a
number of turbine and generator rotors. These failures were attributed to inade-
quate fracture toughness and prompted an industry-wide development to improve
the performance of the 2.7% Ni 0.5% Mo 0.1% V steel that was used for large
turbine rotors at that time. It became evident that temper embrittlement was a
significant factor in contributing to low toughness and the manganese content
of the steel was reduced from 0.7% to 0.4%. In order to compensate for the
loss in hardenability and to improve toughness, the nickel content was raised
from 2.7% to 3.5%. Boyle et al. state that nickel contents as high as 5% were
investigated but that the benefits derived from additions greater than 3.5% were
insufficient to compensate for the difficulties introduced by the depression of
the Acl. Presumably these difficulties related to the fact that the Acl can be
depressed to a temperature close to or below the nominal tempering temperature
with the potential risk of reaustenitization.
Although changes to the manganese and nickel contents realized significant
improvements in toughness, there was also the need to develop a composition for
larger diameter rotors which required higher tensile and yield strengths, together
with good toughness and ductility. This led to the addition of 1.5% Cr to the
modified Ni-Mo-V to provide the current 3.5% Ni-Cr-Mo-V grade. Since
the 1950s, a considerable amount of work has been carried out in the UK to
improve the fracture toughness of LP and generator rotors, particularly in relation
to the suppression of temper embrittlement. In addition, the original FATI' data
have been augmented with K ic fracture toughness values and a good correlation
between the two parameters has been derived via the expression:
KIcMNm_3/2 _ 6600
60 - B
where B = (test temperature- FATT value)~
Following homogenization treatments at temperatures around 11500C, rotors
are heated slowly to a solution treatment temperature of 840~ and then lowered
vertically into an array of high-pressure water sprays. Modem LP rotors have a
diameter in excess of 1700 mm and the above treatment results in the formation