Abstract
Four cast superalloys, Fe-base and (Fe,Ni)-base alloys, all containing 30%Cr and 0.4%C, were elaborated with addition of 3% and 6% of tantalum. Their oxidation behaviours were studied at 1000, 1100 and 1200°C during 50 hours. The oxidized surfaces of the samples were quantitatively characterized by measurements of thicknesses and surface fractions of internal oxides. The thermogravimetry files were treated according to the {m×(dm/dt) = Kp - Kv×m} equation, to obtain simultaneously the parabolic constant and the chromia volatilization constant. The internal tantalum oxides are more present in the Fe-base alloys and the carbide-free zones are less developed for the (Fe,Ni)-base alloys than for the others. The Fe-base alloys oxidize faster than the (Fe,Ni)-base and Ni-base alloys. The comparison with the corresponding Ta-free ternary alloys shows that the presence of Ta tends to accelerate the oxidation.
Highlights
The equiaxed superalloys obtained by classical foundry process are often reinforced by the presence of both interdendritic carbides appeared during solidification, and eventually by secondary carbides obtained by applying a specific heat treatment
The as-cast alloys present microstructures which are composed of a dendritic matrix and interdendritic carbides
Tantalum seems diffusing from the carbides towards the oxidation front to be oxidized in internal oxides, a phenomenon that promotes the development of a carbide-free zone
Summary
The equiaxed superalloys obtained by classical foundry process are often reinforced by the presence of both interdendritic carbides appeared during solidification, and eventually by secondary carbides obtained by applying a specific heat treatment. It is notably the case of cobalt-base alloys, but this principle of strengthening can be used for nickel and iron-base alloys. In addition to chromium, the role of which is to bring a good resistance against high temperature oxidation to the conventionally cast superalloys [1], tantalum, which is a very oxidable element, can play a role in the high temperature oxidation of (Fe and/or Ni) -base alloys. In this work the high temperature oxidation behaviours of two iron-base alloys and two ironnickel base alloys, containing 0.4%C and two amounts of tantalum 3% and 6%, are studied, with comparison with results obtained previously for ternary M30Cr0.4C alloys [2] and two nickel-base alloys with 0.4C and 3% to 6%Ta [3]
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