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Cédric Bellot
Abstract
Understanding and modelling of the precipitation in ultra-high strength martensitic steels
Cédric Bellot - 8 February 2012
In the lightening objective of aeronautic structures, two new
families of martensitic steels appeared since about twenty
years. The first family, hardened by the formation of M2C
carbides during the tempering, contains a high cobalt content
which is a very expensive element. The second family appeared
afterward with a double precipitation of carbides and
intermetallics, what allowed to strongly decrease the cobalt
content.
The characterization of the precipitation formed during the heat
treatment was carried out using complementary techniques such as
transmission electronic microscopy, X-rays diffraction, small
angle neutron scattering and atom probe. These techniques
allowed to have access to relevant parameters of microstructure,
which are crystallographic structures, distribution size,
chemistry and volume fraction of precipitates. Steels studied do
not present precipitation of iron carbides M3C type as it is the
case on other martensitic steels. Aluminium and intermetallics
precipitation allows to maintain a high dislocations density and
to reduce the cobalt content which have the same effect.
In order to systematize the studies on the understanding of
precipitation mechanisms, it is necessary to use modelling
tools, thus to know the sequence of precipitation. Kinetics
simulations were carried out using Matcalc software developed at
the University of Vienna (Austria). A new calculation method,
taking into account the chemical composition of critical nuclei,
was developed and allowed to simulate with a good accuracy the
evolution of the intermetallics precipitation during the
tempering. A new phase was also created in thermodynamic
database to take into account the carbon segregation on
dislocations.
Keywords: martensitic steels, heat treatment, precipitation, carbide, intermetallic, thermokynetic, thermodynamic, Matcalc
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