Engineering steels 269due to the process of creep (stress relaxation). Therefore bolting materials must
have good creep strength in order to minimize relaxation and maintain steam
tightness. Repeated relaxation and retightening will lead eventually to creep frac-
ture and therefore the utilities limit both the operating period and the number of
retightening operations so as to avoid fracture during service.
In the UK, the majority of the bolting requirements are satisfied by a range
of Cr-Mo and Cr-Mo-V steels and the evolution of these steels has been
described by Everson et al. 4~ Details of the composition, heat treatment and
high-temperature properties of these steels are given in Table 3.20. The Cr-Mo
(Group 1) steel was introduced in the 1930s when the steam temperatures were
of the order of 450"C. By the late 1940s, steam temperatures had risen to about
480"C and higher strength bolts were required. This led to the introduction of
the Cr-Mo-V (Group 2) steel in which the higher strength is achieved by the
formation of a fine precipitate of V4C3 on tempering at 700"C. However, steam
temperatures continued to rise in the UK in pursuit of higher operating effi-
ciency and in 1955 reached a level of 565"C. This change necessitated a further
increase in strength and this led to the introduction of the 1% Cr 1% Mo 0.75% V
(Group 5) steel. However, this material proved to have poor rupture ductility due
to intergranular cracking after only short exposure at the operating temperature. A
major research programme was therefore undertaken on this problem and this led
to the development of the Cr-Mo-V-Ti-B (Group 6) steel. This composition
represented the addition of about 0.1% Ti and 0.005% B to Group 5 steel which
produced a significant improvement in rupture ductility. According to Everson,
Orr and Dulieu, the principal effect is due to boron, about 50% of which is incor-
porated in the V4C3 precipitates and the remainder is dissolved in the matrix.
This produces a stabilizing effect on the V4C3 near the grain boundary regions,
making the carbides more resistant to dissolution and so reducing the rate at
which denuded zones are formed. Titanium is added primarily as a nitrogen-fixing
agent and so preventing the formation of boron nitride, which is metallurgically
inactive. However, the formation of TiN leads to refinement of the austenite
grains which also contributes to improved rupture ductility. More recently, the
Cr-Mo-V-Ti-B steel has been used as boiler support rods, operating at temper-
atures up to 580"C.
Nimonic 80A (Ni-20.0% Cr, 2.4% Ti, 1.4% AI) is significantly stronger than
the low-alloy steels and allows the use of smaller bolts and more compact flanges
than are possible with ferritic bolting materials. It is also likely that Nimonic
80A would replace Cr-Mo-V (Group 6) steel as the principal turbine bolting
material if steam temperatures were to be raised above the present operating level
of 565"C.
Detailed studies have also shown that marked improvements in the rupture
characteristics of Cr-Mo-V bolting steels can be produced by restricting the
level of residual elements. Thus the rupture life of Cr-Mo-V steel can be related
to the 'R' factor (P + 2.43 As + 3.75 Sn + 0.13 Cu) and Cr-Mo-V bolting
steels are now made to a typical 'R' value of 0.07% compared to 0.2% in early
years. This has led to a substantial increase in rupture ductility. The 'R' value
can also be reduced to a level of 0.015 in VIM-VAR material to provide further
improvements in creep strength and ductility.