Steels_ Metallurgy and Applications, Third Edition

268 Steels: Metallurgy and Applications

of a rim structure which is essentially martensitic and a core structure which is
essentially bainitic. In order to avoid quench cracking, the large forgings are not
quenched directly to room temperature but are held for a period at a temperature
of 200"C in order to reduce internal stresses. The material is then tempered in the
range 560-640"C, the lower bound avoiding the formation of isothermal temper
embrittlement and the upper bound avoiding the risk of partial reaustenitization.
In turn, the rate of cooling adopted from the tempering temperature represents a
compromise between the fast cooling that inhibits temper embrittlement and the
slow cooling that minimizes internal stress. The compromise of fan cooling is
usually employed.
At one time, the requirement for very low levels of silicon and manganese
would have posed major problems in deoxidation but these have been overcome
by the introduction of vacuum carbon deoxidation (VCD). VCD treatment is
also thought to be beneficial in improving the solidification characteristics and
reducing the segregation effects in large forging ingots.

Turbine generator end rings

The copper windings in an electrical generator are not strong enough to resist the
rotation stress and end rings are fitted to retain the windings. These components
are very highly stressed and have 0.2% proof strength values greater than
1200 N/mm 2. A further requirement is that the material should be non-magnetic
in order to minimize electrical losses from the generator. Up until the late 1970s,
a cold-worked steel containing 18% Mn 5% Cr was used world-wide for this
application but the main disadvantage of this material was its susceptibility
to stress corrosion cracking (SCC). This could arise in moist conditions and
the threat of catastrophic disintegration during service necessitated frequent
inspection.
Because of these problems, attention was turned to the development of a more
SCC-resistant material, leading to the introduction of a steel of the following
composition:

0.08%C, 0.3%Si, 18.0%Mn, 18.0%Cr, 0.60%N

This material is substantially solid solution strengthened by nitrogen and,
additionally, this element also serves to preserve the austenitic (and therefore non-
magnetic) structure in the presence of a high chromium content. The 18% Mn
18% Cr steel also offers the facility for attaining higher 0.2% proof strength
values than 18% Mn 5% Cr steel, i.e. values up to 1400 N/mm 2. 18% Mn
18% Cr steel now represents the first-choice material for end tings and after
10 years trouble-free service has virtually eliminated the need for in-service
inspection.

Turbine bolts

Large bolts or studs are used to maintain steam tightness in the flanges of high-

temperature turbines. These are tightened to a tension that imposes an elastic

strain of about 0.15% but at the high operating temperatures this tension reduces