POWER August 2010 - 44

ADVANCED MATERIALS
havior; all other 2% to 9% chromium steels
are subject to severe oxidation. Of the austenitic
and nickel-based alloys, those containing
cobalt exhibited the best oxidation
behavior. The results also indicate that all
austenitic and nickel-based alloys formed
a dense chromium oxide that results in low
oxidation rates.
Steamside oxidation testing has also
been performed at 1,475F for up to 1,000
hours on 20 different ferritic, austenitic,
and nickel-based alloys and coated materials.
Results indicate that, for steels,
oxidation susceptibility is independent of
chromium level once a threshold of about
10% is reached. As expected, all alloys experienced
greater oxidation at 1,475F than
at 1,200F.
Overall, the oxidation rates of VM12,
austenitics, and nickel-base alloys are very
low and should not be life-limiting, but additional
research is ongoing to better understand
oxide exfoliation in these alloys
that may limit their applicability.
Fireside Corrosion. These tests evaluated
the relative resistance of various advanced
alloys to fireside corrosion over
the full temperature range expected for an
A-USC plant. Three different coals were
evaluated, representing eastern, midwestern,
and western coals. Laboratory tests
were conducted, wherein specimens coated
with the appropriate deposit composition
were exposed to a gas mixture simulating
fireside corrosion. Tests simulated both
waterwall and superheater conditions.
Lab test results have shown that corrosion
behavior is primarily a function
of chromium level. Corrosion decreases
rapidly as the chromium level increases to
22% to 27% and then levels off. Nickelbase
alloys containing iron function better
than nickel-base alloys without iron.
Alloys containing more than 1% molybdenum
functioned poorly under all test
conditions.
Field tests are also being conducted. In
one type, steam loops formed by welding
together spool pieces of the various materials
were inserted in the superheater circuit
for exposure to actual boiler conditions
(Figure 3). In another type, air-cooled
probes were inserted inside the boiler and
evaluated after various exposures.
Welding. This task involved weld studies
of six different alloys in two product
forms (tubing or plate). Welding procedures
were developed for different combinations
of alloys and product forms.
Thin-section welding could be performed
successfully in all alloys. Welding trials on
thick-section nickel-based alloys showed
limitations for some processes. After 5+
years of research, 3-inch-thick welds are
now achievable in Inconel 740, a major
accomplishment for an age-hardenable alloy.
Work is ongoing for dissimilar metal
welds and alternate filler metals to alleviate
the potential for weld strength reduction
factors. (See " Improved Filler Metal
3. Testing steam loops. Fireside corrosion tests use a superheater test loop formed by
welding together different alloy tubes. The relative resistance of alternative alloys to different types
of coals can then be determined under actual operating conditions. Courtesy: Babcock & Wilcox
Enables Higher-Temperature Dissimilar
Metal Welds, " July 2009.)
Fabricability. Fabrication studies were
completed for six different alloys, showing
that typical boiler fabrication procedures
could be used for A-USC alloys. Three
types of procedures were studied:
■ Forming (press forming of headers and
piping, bending of tubing, and swaging of
tube ends).
■ Machining (weld grooves for header and
pipe longitudinal and circumferential
seams, socket weld grooves for tube-toheader
joints, and weld grooves for tube
circumferential seams).
■ Welding (submerged arc welding for
header and pipe longitudinal and circumferential
seams, gas tungsten arc welding
for tube-to-tube joints, shielded metal arc
and gas tungsten arch welding for tube-totube
socket joints).
To demonstrate the fabrication capabilities
achieved in the course of this project, a
mock-up of a header was fabricated (Figure
4). The mock-up illustrates capabilities with
respect to fabrication of CCA 617 alloy into
the header shape by bending of plate, girth
welding, seam welding, socket welding, machining,
swaging, hole drilling, and dissimilar
metal welds.
Coatings. Claddings, spray coatings, and
diffusion coatings capable of withstanding
corrosive conditions at 1,400F have been
4. Welding workout. A mockup steam
header was fabricated to demonstrate the capabilities
of CCA-617 alloy, including plate bending;
socket, girth, and seam welding; and dissimilar
metal welds. Courtesy: Alstom Power
Super 304 H
T 91
CCA 617
header (with
girth and
long seam
welds)
CCA 617
44
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POWER August 2010

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