κ-Carbides are a special class of
carbide structures. They are most known for appearing in steels containing
manganese and
aluminium where they have the
chemical formula .
Properties
Structure
κ-Carbides crystallise in the
perovskite structure type with the
space group Pm3m (Nr. 221).
This structure was, inter alia, elucidated with XRD-measurements on
steel alloys containing κ-carbide precipitates but also on single crystals of
manganese-κ-carbides with a molecular formula of Mn
3.1Al
0.9C and a lattice parameter of
a=3.87Å.
In
steel where diverse arrangements of the atoms are possible, a considerable effect of the short range ordering, e.g. of
iron and
manganese on the microscopic properties of the
alloy, has been observed.
This is especially important for the role as
hydrogen-traps in
.
Composition
A first glance at the composition of a steel alloy is achieved by analysing its surface with EDX-technique.
Depending on the content of the alloying Chemical element of the steel, different types of κ-carbides can form. They occur in both ferritic (α-Fe) and Austenite (γ-Fe) steels.
Magnetism
SQUID measurements on
Crystallite Mn
3.1Al
0.9C revealed a soft
behaviour of this κ-carbide with a Curie temperature of 295±13 K, a
remanence magnetic moment of 3.22
μB and a
Coercivity of 1.9 mT.
DFT-simulations confirmed these findings and indicated that other κ-carbides behave similarly.
Occurrence
κ-carbides are typically found as
in high-performance steels.
A common example is the TRIPLEX
steel with the generic composition Fe
xMn
yAl
zC containing 18-28 %
manganese, 9-12 %
aluminium and 0.7-1.2 %
carbon (in mass %).
It is a high-strength, low-
density steel consisting of
austenite γ–
solid solution,
Nano- size κ-carbides and α– ferrite.
Other similar
are known for their high
ductility.
κ-carbides are usually formed from areas enriched in carbon through
spinodal decomposition and are key determinants of the properties of these steels.
The low
density is e.g. obtained after a hot rolling post-process.
Upon cooling, different domains of
austenite and ferrite are formed and κ-carbides form at the boundaries of these domains.
Continuing the cooling process leads to a phase transition of
austenite to ferrite and the κ-carbides are released as a result of an
eutectoid transformation in form of a
precipitate.
The κ-carbides can have an additional strengthening effect on steels
See also
External links
-
κ Carbide in Steels (Phase Transformations & Complex Properties Research Group, University of Cambridge)
