2012 Annual Report - Jesus College - University of Cambridge

A Taste of Cold Steel
Lucy Fielding, Jesus Graduate Student
According to archaeologists, the Iron Age began in 1300 BC and lasted for around two
millennia. Today, steels (alloys of iron and carbon) comprise 95% of global metal
consumption and this trend shows no sign of declining. Glancing at the media, however,
one would be forgiven for assuming that steel is now a has-been. We are bombarded with
stories of novel materials: carbon nanotubes, metallic glasses, graphene, carbon fibre,
nickel superalloys . . . all of which are “stronger than steel”. “Now we can construct space
elevators!” claim the articles. “Let’s build a climbing frame to the moon! We’ll use this
stuff to make everything!”
The observant among us, however, will note that most cars, trains and buildings still
don’t feature superalloys, metallic glass or magic nanotubes. Neither are they invisible;
nor do they fly; nor do they do any of the other things that journalists tend to ‘predict’.
Instead, steels somehow remain the best – and cheapest – materials for the job. Also, they
are stronger than steel. This is because ‘steel’ is a vague construct used by sensationalists,
with an unspecified strength guaranteed to be less than that of a novel material.
Metallurgists rarely refer to ‘steel’, just as the Inuit have fifty words for snow, not one of
which is ‘snow’.
Steels, on the other hand, are alloys with a colourful variety of properties: strength,
toughness, hardness, ductility and wear resistance. They are everywhere: railways,
pipelines, bearings, jet engines, armour, nuclear reactors . . . and more. Many of these
technologies are cutting-edge in their design, and require highperformance steels to
match. The benefits of steels lie first and foremost in their versatility. Iron atoms can ‘stack’
themselves in various distinct configurations (just like a greengrocer stacking oranges –
there are different ways to do it). Each configuration is known as a ‘phase’. As well as the
two main iron phases (austenite and ferrite) there is a carbide phase – a compound of iron
and carbon (Fe3C) – called cementite. These phases often exist in combinations and have
different properties. For example: cementite is strong and brittle, whereas ferrite is softer
and tougher. If we combine them into a structure called pearlite (consisting of alternating
layers of ferrite and cementite), we produce a steel that is reasonably strong and tough.
Transmission electron microscope
image of superbainite structure.
α = ferrite and γ = austenite.
The inset image is a carbon
nanotube to scale
METALLURGY I Jesus College Annual Report 2012 29