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Mech 473 Lectures
Professor Rodney Herring
Group 3 Steels: Eutectoid Composition Steels
Steels with carbon contents just below the eutectoid to the
eutectoid composition (0.6 – 0.8 C) are used when high
strength and toughness is required throughout the thickness
of a part.
Examples include:
Heavy steel forgings such as marine crank shafts.
Small forgings for highly stressed parts such as connecting rods
for internal combustion engines and rails for locomotive
transport
The massive components cannot be heat treated so their strength
must come from a fine pearlite structure which is developed
by finishing the hot working of the steel at a low enough
temperature to permit recrystallization of the austenite, while
preventing austenite grain growth.
Steel Forgings
These are made from 1060 Steel, which contains 0.55 – 0.65 %C
and 0.8 %Mn.
It is essential that the composition be kept on the ferrite side of the
eutectoid because the presence of any proeutectoid cementite
would make forging very difficult and make the product
brittle on cooling to room temperature.
The forging process has two stages:
The first stage is “upset forging* ”, which breaks up the cast
structure of the billet and gives a fine austenite grain size.
The second stage shapes the component, eg., crankshaft, and
is also performed at temperatures above the eutectoid.
How would upset forging create finer grain size?
It increases the dislocation density, which act as nucleation sites,
especially at grain boundaries to enhance recrystallization during
austenizing, which enhances the formation of small grains.
Steel Forgings
For smaller components such as connecting rods for the Internal
Combustion engine, it may also have a final cold forming
forging treatment such as stamping.
The smaller components are often heat treated by quenching to
form 100% martensite followed by a tempering at 200 oC.
Tempered martensite formed in
AISI 1060 forging quenched
from 925 oC and tempered at
200 oC.
Pearlite-ferrite structure of hot
rolled 0.75 %C rail steel. The
ferrite in the grain boundaries is
due to the depression of the
eutectoid temperature caused by
rapid cooling. The microstructure
should be 100% pearlite.
Mechanical Properties of Eutectoid Composition Steels
We will look at some of the details of these alloys.
Steel Rails
Steel rails are also made from ASTM A1 steel. This was the first
alloy specified by ASTM.
This is similar to AISI 1080 and contains 0.78-0.80 %C, 0.70-1.0
%Mn and 0.1-0.50 %Si.
A microalloyed version, A1+ has with superior mechanical
properties contains 1.25 %Mn and 0.25 %Cr.
14 %Mn is added to rails intended to be used for switching, which
creates pockets of retained austenite. Under the heavy load of
a train, the retained austenite at the surface transforms to
martensite to give an extremely hard wearing surface layer,
which is progressively replenished by successive transits.
This is a good example of “stress-induced transformation” of g to
martensite.
Steel Rails
The cast structure of the steel is broken down by hot cross rolling
after which the billet is straight rolled to a size just greater
than the finished size.
The semi-finished rail is then moved over to finishing rolls, which
are machined with grooves that match the profile of the
finished rail.
During this final rolling stage the temperature falls to just above
the eutectoid and after rolling is completed, the rail is cooled
relatively rapidly in flowing air to give a fine pearlite
structure.
Rails for “Bullet Trains” have this process down to a fine art.
The End