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July 2009 Tube Products International
61
corrosion resistance. The best of the currently used
austenitic solid solution alloys raises the steam
temperature to 675°C (1,250°F), but steam pressure
is limited to circa 30MPa (4.35ksi). Coal ash corrosion
is also questionable for many of these alloys. Alloy
development became necessary to meet the European
superheater property requirements of both a 100,000
hours creep strength at 750°C (1,380°F)/100MPa
(14.5ksi) and a coal ash corrosion resistant target of less
than 2mm (0.079") metal loss in 200,000 hours.
These material property targets were made even more
demanding by the US DOE project. Only an alloy
with aerospace strength properties coupled with the
corrosion resistance of the best of the high-nickel
solid solution alloys would meet these new stringent
requirements. However, success would raise plant
efficiency to 50+ (LHV basis). It is estimated that this
development would result in a 30% reduction of CO
2
emissions.
Advanced nickel-based high strength alloys tradition-
ally have nickel and chromium as their foundation, with
elements such as molybdenum, tungsten and cobalt
added to confer additional solid solution strengthening.
These alloys, such as Inconel alloy 617 and VDM’s
CCA617 (compositions are given in
Table 1
) are
relatively easy to weld and generally lack a requirement
for post fabrication heat treatment. However, for the
current requirements of an advanced ultra-supercritical
superheater tubing alloy, these alloys lack the creep
strength and corrosion resistance for long life.
The lack of adequate corrosion resistance is possibly
overcome by the application of a suitable weld overlay
material such as Inconel FM 52 or FM 72. However,
alloy 617 and CCA 617 are deemed suitable for AD700
header and steam transfer piping where only steam
oxidation corrosion resistance is required. Headers are
thick-walled extruded pipes, located, outside the boiler,
to collect and homogenise the steam from the boiler
tubes and send it through insulated transfer piping to
the turbine (
Figure 2
).
Where high temperature strength is critical, additions
of aluminium, titanium and niobium, resulting in
precipitation hardening, must be added. Nimonic
®
alloy 263 is a typical example. However, where coal
ash corrosion is an additional requirement, increased
levels of chromium are mandated over alloy 263 and
most other nickel-base superalloys. The contribution
of chromium to coal-ash corrosion resistance has been
amply demonstrated in the literature.
3
Inconel alloy 740
With the technical challenge of the AD 700 project
requirements in mind, Special Metals Corporation was
asked to develop a nickel-base alloy for the very
hottest sections of the boiler, namely the superheater
and reheater. Using Nimonic alloy 263 as the starting
point, variations of the gamma prime hardener elements
were explored to assure that the strength target of
750°C/100MPa (1,380°F/14.5ksi) and 100,000 hours
Alloy
C Ni
Cr
Mo Co Al
Ti
Nb Mn Fe
Si
617
0.08 53
22
9.7
12
1.1 0.4
-
0.08 1.2 0.05
CCA 617
0.06 55
22
8.8 11.6 1.2 0.4
-
-
0.9 0.2
263
0.05 51
20
5.9
20
0.4 2.2
-
0.35 0.3 0.06
740
0.03 Bal
25.0 0.5 20.0 0.9 1.8 2.0 0.3 0.7 0.5
FM52
0.04 62
29
-
-
0.8 0.5 1.7
-
9.0
-
FM72
0.05 56
43
-
-
-
0.6
-
0.1 0.2 0.1
Table 1
▼
▼
: Nominal composition of the candidate advanced ultra-supercritical boiler alloys
Figure 2
▲
▲
: Depiction of a typical header under fabrication at
Mitsubishi Heavy Industries