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May 2004
Fatigued? Swedish experts point the way to high performance
PM
Leading steel powder producer Höganäs has built
up a unique knowledge bank on fatigue performance after embarking
on an unprecedented project to map the microstructure and
fatigue performance of its materials in the minutest detail...
There is a clear trend towards the increased use of PM steels
in highly stressed applications such as gears, where high
fatigue performance is required. Density and microstructure
are the key parameters for the fatigue performance of PM steels,
and in depth knowledge in these areas is an essential requirement
if PM components are to compete successfully with wrought
steel in high-performance applications.
A few years ago Höganäs AB embarked on an unprecedented
project to map the microstructure and fatigue performance
of its materials in the minutest detail. The result is an
unrivalled knowledge bank based on the work of the Fatigue
Research Team, a group of four that works exclusively on fatigue-related
research.
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A close-up view of microstructure reveals
a lot about a metal’s fatigue life.
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Sigurd Berg, Manager Product Development at Höganäs
says: "We are at the forefront of knowledge on fatigue
properties. We now have comprehensive knowledge on our materials'
fatigue performance in density areas up to 7.3 g/cm³."
"We can rank our materials from the starting point of
structure," he continues. "This means that from
the requirements of an application we can assist customers
in rapidly selecting the right material and process route
for a specific component."
"Our knowledge, particularly the fatigue performance
of our high-performance materials, is of great practical value
and is underpinning efforts to expand the PM market into the
huge potential market for highly loaded gears in applications
such as vehicle transmission and power train gears."
Anders Bergmark, a member of the team, says the project has
included a close study of nickel's ability to enhance fatigue
limits. "Traditionally, improved fatigue properties have
been achieved by adding a little nickel to the mixture in
order to raise the fatigue limit. However, powder mixes with
small additions of nickel cannot fully utilise the nickel's
performance, since the nickel particles are present as isolated
islands of austenite surrounded by a border of martensite.
It is not until a continuous network of martensite is formed
that the fatigue properties really improve."
"Nickel and copper additions of around 4 per cent and
2 per cent respectively efficiently create this continuous
network. But, at these high nickel levels, particle size segregation
can be a severe problem, disturbing the continuity of the
network. The remedy is to use diffusion-bonded grades, notably
Distaloy AE and Distaloy HP, which exhibit a microstructure
with a continuous network of martensite, and consequently
have good fatigue properties." Anders Bergmark presented
the findings of the Fatigue Research Team in a paper entitled
Microstructure Enhancement for Fatigue Improvement.
Another recent deposit in the knowledge bank is a paper by
another member of the Fatigue Research Team, Ola Bergman.
His presentation: Chromium-Alloyed PM Steels with Excellent
Fatigue Properties Obtained by Different Process Routes examines
the fatigue properties of the pre-alloyed water-atomised chromium
materials, Astaloy CrL and Astaloy CrM.
"We can show that these materials have the required
properties for applications where high fatigue performance
is essential," he says.
"The bending fatigue limits at a sintered density of
7.1 g/cm³ is around 260 MPa for Astaloy CrL + 0.8 per
cent graphite and around 290 MPa for Astaloy CrM + 0.45 per
cent graphite after conventional sintering at 1120ºC
in a nitrogen-based atmosphere with a low dew point.
"Even better results are achieved with high-temperature
sintering, which increases the fatigue limit of Astaloy CrL
to 310 MPa. A combination of conventional sintering and rapid
cooling improves the fatigue limit of Astaloy CrM to 380 MPa."
The explanation for this consider able rise in fatigue performance
lies in the microstructure shift from mainly bainitic to mainly
martensitic structure, which occurs as a higher cooling rate
is applied.
"What is significant here," emphasises Ola Bergman,"
is that you can optimise microstructure and heighten characteristics
with high-temperature sintering or rapid cooling in a cost-effective
single-stage process without secondary operations."
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Monica Carlsson in the Höganäs
fatigue laboratory.
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Words: Minett Media
Illustrations: Höganäs AB
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