FeCrAl alloys, trade name Kanthal®, are temperature resistant alloys with high
electrical resistance and excellent oxidation resistance and are used for resistance
heaters and components for high temperature applications (up to 1400 ° C). The
oxidation resistance is related to the formation of a thermodynamically stable layer of
aluminum oxide (alumina, Al2O3), which hinders the diffusion of oxygen into the interior
of the material and protects the material from further oxidation. However, the lifetime
of these elements is limited when the elements are subjected to cyclic thermal loads
at temperatures above 1000 °C. The problem is that the alumina oxide layer cracks
due to the relatively rapid temperature changes and the difference in the coefficient of
thermal expansion of the oxide layer and the base alloy. As long as there is a
sufficiently high concentration of aluminum in the solid solution α-Fe(CrAl) in the
underlying layer, the resulting crack will be quickly filled with a new layer of alumina.
Aluminum in solid solution is thus consumed, and when its concentration in the
subsurface falls below a critical value (⡈ 3 mas. %), the protective layer of alumina no
longer forms. This leads to the oxidation of iron and other alloying elements and the
gradual deterioration of the heating or structural element. Since we cannot arbitrarily
increase the aluminum content (max. 7.5 mas. %) in the production of these alloys,
since they can no longer be rolled into strips or drawn into wires due to the loss of
ductility, the main objective of our research was to determine whether it is possible to
increase the aluminum concentration in the subsurface layer of the alloy product
without the formation of an aluminide layer of intermetallic AlxFey phases on the
surface. This would increase the ability to renew the oxide layer and extend the life of
the product. The enrichment of the subsurface area with aluminum was carried out by
the pack aluminizing process.
The main objective of this master thesis was to increase the concentration of aluminum
in the subsurface layer of α-Fe(CrAl) solid solution in order to improve the ability to
form oxide layers during the lifetime of the element made of this alloy. For this purpose,
the surface of the studied FeCrAl alloy samples was treated with a process called pack
aluminization.
Pack aluminization took place in a tube furnace where the samples were buried in a
mixture of pure aluminum powder or FeAl master alloys, alumina powder and activator
(halide salts: AlCl3 or NH4Cl). Treated samples were analysed with a scanning electron
microscope and the surface texture, the depth of aluminization and the structure of the
surface layer were studied.
We find that the aluminum source and the ratio of activator to aluminum in the pack
play a key role in the rate of aluminum deposition on the workpiece surface, while the
deposition rate is the key factor that determines which phases form on the surface and
to what depth the aluminum diffuses after a given annealing time at a given
temperature. If we want to enrich only the subsurface area of the alloy Kanthal AF with
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aluminum without forming an aluminide layer on the surface, it is recommended to use
the master alloy FeAl as a source of aluminum in the pack.
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