Superposition of TTT (full colour) and CHT (faint
color) transformation curves.Coloured lines
connect equal fractions of transformation.
A graphic illustration of injecting external
lubricants to prosthetic joints as an alternative
approach to improve wear-resistance of implant
materials.
Modified heat treated 310 steel exposed to
KCl deposit in an oxidising gas at 600ºC for
168 hours: Preferential attack of sigma phase
precipitates.
Study of austenite transformation during continuous heating
In conventional heat treatment of steel, the material
is exposed to high temperature to obtain
a soft phase, austenite, which is transformed
into various products with strongly varying mechanical
properties during subsequent cooling
to room temperature. The final properties of the
material are determined by the cooling conditions,
so that the rate of austenite transformation
versus temperature and time is of major
technological interest.
In response to this interest, researchers developed
Continuous-Cooling-Transformation, CCT,
and Time-Temperature-Transformation, TTT, diagrams,
where the various stages of austenite
transformation are reported versus temperature
and time during continuous, and interrupted,
cooling, respectively. Data allows to study the
kinetics of transformation, yielding information
on the transformation mechanisms, which is of
interest to develop predictive tools. Significant
results have been obtained. Nevertheless,
none of the current transformation theory can
reconcile all experimental data obtained during
the formation of martensite that, among all the
transformation products, is the one yielding the
highest strength and hardness.
In the present project, various steels were
cooled to -196°C and the transformation of
austenite into martensite was systematically
investigated for the first time during heating.
Similarly, to the more established investigation
approaches, data can be presented versus
temperature and time, in this case in the form
of a Continuous-Heating-Transformation, CHT,
curves (see figure). The investigation revealed
that two types of martensites exist, named S
and U martensites, respectively. S martensite is
suppressible by fast cooling and cannot form at
an observable rate at temperatures approaching
absolute zero; its growth can be time
dependent.
U martensite is insuppressible and
can form at 4 K; its growth is instantaneous.
Currently, it is under investigation what kind of
information could be obtained by applying continuous
heating conditions during the formation
of products other than martensite.
Extending the lifetime of orthopaedic implants by lubricants
A new project, “ArthroLube”, funded from the
Villum Foundation (a new scheme, the Villum
Experiment) kicked off on October 1, 2017, as
led by Associate Professor Seunghwan Lee. As
artificial joint implants are functioning under
persistent loading and shear stress, generation
of wear debris is an unavoidable problem, and it
often triggers a cascade of immune responses.
Insufficient wear-resistance of implant materials
is known to be responsible for nearly half of
implant failures. For this reason, extensive research
efforts have been put forth to minimize
wear particles, typically by developing more
wear-resistant materials or implant designs.
In the project “ArthroLube”, it is attempted to
solve this problem by administering fluidic lubricants
to the prosthetic joints as an alternative
way to reduce friction and wear. This approach
has not been possible until recently, mainly due
to the lack of lubricants compatible with the human
body. Based on recent studies showing effective
lubrication by various macromolecules in
aqueous environment, the project will explore
the feasibility of extending this approach to
prosthetic joints. The focus of the research activities
will be placed on analyzing wear debris
from the implants as well as biocompatibility
tests on cells and animal models. Ultimately,
the research in “ArthroLube” is expected to
form a basis for further drug development to
enhance the lifetime of artificial joint implants.
High temperature corrosion in thermal power plants firing biomass
In accordance with the political energy strategy
in Denmark, there is a drive to reduce emissions
from fossil fuels. Therefore, an increasing
number of fossil fuel power plants have been
replaced by or converted to biomass power
plants. Biomass is more carbon dioxide neutral
and results in a net reduction of carbon dioxide
emissions, however, its use poses material challenges
for the high temperature components in
the boiler. Combustion of biomass results in a
flue gas containing highly aggressive species,
which can cause severe high temperature
corrosion. Especially the release of KCl which
condenses as a salt on heat exchangers has
been shown to result in faster corrosion rates
compared to fossil fuel plants.
The project “Biomass Corrosion Management”
aims to improve understanding and lifetime prediction
models for biomass power plants. This
project is financed by ForskEL and KME and is
in collaboration with DONG Energy. It was previously
observed that a high Cr content in a steel
above 20% resulted in higher corrosion rates
due to internal corrosion attack at grain boundaries.
However, depending on time and temperature,
the microstructure of a steel evolves with
nucleation of precipitates especially at grain
boundaries and could be a cause of internal attack.
This project focuses on how KCl corrosion
is influenced by changes in microstructure due
to ageing. Thermodynamic modelling, laboratory
exposures, and advanced materials characterisation
techniques are utilised. The corrosion
response of as received (solution treated)
steels are compared with steels heat treated
in the laboratory or aged steels received from
power plants. Where precipitates are present
such as the sigma phase (light grey) shown in
the figure, preferential attack of precipitates
compared to the matrix is observed. Thus, it is
not only the alloy composition that influences
corrosion but also the evolved microstructure
during heat treatment or ageing.
Contact:
Matteo Villa, e-mail: matv@mek.dtu.dk
Contact:
Seunghwan Lee, e-mail: sele@mek.dtu.dk
Contact:
Melanie Montgomery, e-mail: mmon@mek.dtu.dk
36 materials and surface engineering