Wrought Nickel alloy Hastelloy X tube was subjected to Argon-Methane
mixture at 800oC and 1000oC to understand the carburization mechanism of alloy
utilized for fuel injection nozzles of micro-gas turbine combustors. Three
types of different internal carbides, (Cr,Mo)3C2, (Cr3Mo)7C3 and (Cr,Mo)23C6
were noticed in this order from the surface and the partial deformation to the external
surface of the sample tube seemed similar to the metal dusting. The internal
carburization mechanism on the inner and external components of tube were
followed. The carbon permeability in Hastelloy X was received and was nominally
lower than that of Nickel- 20%Chromium.
Hastelloy X is a key component for gas – turbine components like combustors
and fuel injection nozzles. It offers supreme oxidation resistance at the
elevated temperature oxidizing media by developing a security layer of chromium
oxide. Although this alloy is rapidly subjected to low oxygen potential, high
carbon containing media, specifically in combustion media with methane and
propane gases that are commonly used. The oxide layer is anticipated to become
unstable in these media and may damage to offer security.
Carburization analyses of iron and nickel based chromium alloys have
been widely shown. Iron-Chromium-Nickel alloys in C3H6/H2 conditions at 900 –
1100oC and the development of partial outer Cr23C6 and Cr27C3 was noticed. Normally
commercial alloys comprise of different alloying elements and various
concentration of iron and nickel. This difference in alloy chemistries makes it
very tough to state the corrosion nature of various alloys, for instance,
Hastelloy X in real service media.
Although Nickel based super alloy Hastelloy X is widely utilized for combustor components,
carburization analyses on alloy X are limited. Hastelloy X utilized solid
carbon for their carburization analyses. To understand the alloy’s attack utilized
for fuel injection nozzles in micro-gas turbines, carburization performance of
alloy X is tested in gas combination of argon and methane at 800oC and 1000oC.
Experiment setup
Carburization specimens with length of 20mm were taken from a wrought
Hastelloy X tube with internal and external diameters of 9.1 and 10.7mm. Plate
shape specimens with 1.5mm thick were utilized for few corrosion analyses to
recognize the products.
Argon travelled from the bottom of alumina tube, by the middle of the
tubular sample, then in the reaction quartz tube. The carburization test was
performed up to 800oC and 1000oC. The reaction tube was flushed with argon gas
many times before every corrosion analysis. The furnace temperature was
increased at a rate of 10oC per minute to the carburization temperature of 800
or 1000oC with a argon stream at a speed of 200Cm3/min. At the test
temperature, Ar gas was replaced by methane -10% mixture with argon at a speed
of 150 Cm3/min. Subsequent the test, the sample was furnace quenched in the
carburization gas stream with a flow speed of 50 cm3/ min.
At 800oC, no internal carburization was noticed for initial 25 hours of
the exposure, however it was noticed in few regions after 100 hours in the
internal side of the tube. At 1000oC, the internally carburized layers were
noticed to form after 60 minutes and depth of every layer increased with time. The
growth rate of the internal and external regions carburization was different
during smaller reaction periods.
The elements like iron, molybdenum and silicon may decrease the carbon
permeability in grade X as these elements decrease carbon diffusivity.
The external tube surface was affected noticeably and graphite
deposition was observed around the affected regions. Graphite can be
precipitated on the reaction surface, can develop and include nickel particles
and these corpuscles increase the reaction rate resulting into metal dusting.
Hastelloy X in argon-methane condition at 1000oC after a lengthy
exposure received metal dusting after exposure for 100 hours. Metal dusting was
one of the major causes of extreme corrosion of micro gas turbine parts created
from metal dusting on alloy X.
Outline
Scratches from the surface grinding can still be noticed after 25 hour
of carburization at 800oC and the surface was shielded with the needle like
reaction product after 100 hour. At 1000oC, the internal and external surfaces
were shielded by a fine grained reaction product after one hour of
carburization and fine grained product became coarser after prolong
carburization.
Carburization of Hastelloy X at 800oC – 1000oC in argon-10% methane
gas was conducted. The outcomes may be stated as:
Internal carburization was noticed at these temperatures in the given
gas mixture. Longer incubation periods were noticed at 800oC. Triple layered
carburization regions with M3C2, M7C3 and M23C6 were developed in this range
from the surface.
The development of the internal carburization regions developed on the
internal and external part of the tube followed parabolic mechanism. Carbon
permeability in alloy X was slightly
smaller than Ni-20Cr grade.
Metal dusting was noticed below graphite accumulation and may be one of
the reasons of extreme corrosion of micro gas turbine parts.
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