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steel metallurgy steel is the widest
used metal but what constitutes a steel
how can we affect the properties and
what happens during the solidification
of steel these are a few key insights
that we will try to uncover in this
module steel is primarily iron with up
to 1% carbon plus other allying
additions in the majority of Steel's
this alloying addition generally totals
less than 5% but in some Steel's this
can be as great as 50% a steel
composition can be thought of as a
recipe different amounts of each
ingredient make up your final product in
steel these ingredients are known as
alloying additions each addition affects
the properties of the steel in a
different way
depending on the amount and type of
alloying additions added we can affect
the following properties in a different
way strength the ability to withstand
load in tension hardness the ability to
resist plastic defamation usually by a
penetration also described as resistance
to scratching or abrasion toughness the
ability to absorb energy ductility the
ability to deform without fracture
fatigue the weakening of metal caused by
repeatedly applied load for mobility the
ease in which metal can be molded into
the final product
machine ability the ease in which metal
can be processed into the final product
with cutting tools weld ability the ease
in which metal can be joined corrosion
the ability to withstand chemical
reaction through oxidation to affect the
steel in this manner a wide variety of
elements can be added these include
carbon silicon manganese phosphorus
sulfur chromium
molybdenum nickel aluminium niobium also
called columbium titanium vanadium
copper boron nitrogen tungsten cobalt
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carbon carbon is the most important
element for strength and hardness as the
level of carbon is increased the tensile
strength and hardness also increases
carbon is a cheap way of increasing the
strength and it is essential for the
formation of microstructures phosphorus
phosphorus is usually classed as an
impurity as it significantly reduces the
toughness and ductility of a steel but
it can be used as a solid solution
strengthening this is explained later in
this module sulphur sulphur is usually
classed as an impurity as it reduces the
skills ductility toughness and weld
ability sulphur can form with iron to
produce a low melting point in purity
called iron sulphide this can collect
along grain boundaries and cause the
steel to break up during hot working
this is called hot shortness sulphur is
sometimes added to steel to aid
machinability manganese manganese has a
great effect on harden ability and can
be found in most commercial steels
manganese can strengthen the steel
through solid solution strengthening
which is explained later in this module
it also combines with sulfur to prevent
hot shortness
chromium chromium increases the harden
ability of Steel it can join together
with carbon to form very stable carbide
which are excellent for wear and
abrasion resistance it is used in high
levels in stainless steels for corrosion
resistance it does this by creating a
protective oxide film on the surface of
the stainless field
molybdenum molybdenum also increases the
harden ability of steels when combined
with chromium and nickel
it has a strong multiplicative effect on
harden ability it is both in alloy and
stainless steels
nickel nickel as with chromium and
molybdenum increases the harden ability
of steel and can increase the toughness
of Steel's particularly at low
temperatures again it is present in
large amounts in stainless steels
silicon silicon is mainly used to remove
oxygen from steel in a process called
the oxidation the removal of oxygen in
Steel is important as oxygen can form
voids in steel known as blow holds and
porosity oxygen can also combine with
other elements to form brittle particles
known as oxides silicon can be used to
increase the fluidity when casting
Steel's Allen minium aluminium is
primarily used to D oxidize the steel it
can combine with nitrogen to form
nitrites which can restrict grain growth
niobium niobium also called columbium in
small amounts can increase yield
strength tensile strength and toughness
vanadium vanadium is used to increase
the harden ability and toughness of
steels through its ability to restrict
grain growth boron boron can
significantly increase the harden
ability of Steel's and can enhance the
effect of other alloying elements
nitrogen nitrogen is often added in
combination with other elements to form
nitrides these nitrides increase the
hardness and tensile strength but at the
expense of toughness and ductility
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when we add our elements they do not
always work in isolation
sometimes the elements work in
conjunction and can cause a
multiplication effect that would not be
expected from the sum of the individual
additions for example both chromium and
molybdenum may be added individually to
a steel in order to strengthen it but a
small amount of molybdenum used in
conjunction with chromium will result in
a much greater strengthening effect than
using one of the elements alone the
addition of carbon to iron is probably
the most important addition in cast
irons and steels this makes a diagram
called the iron carbon equilibrium
diagram very useful equilibrium means
that enough time has been allowed on
heating and cooling for any reactions to
fully complete many of the basic
features of this diagram influence the
behavior of the most complex steel
alloys the diagram is used to understand
what structures will be formed at what
temperatures and at what carbon contents
we can also see at what temperature
different compositions melt and we can
calculate how much liquid and solid will
be present at a given temperature and
can see when a steel will be fully solid
we can also calculate how much of each
structure or phase will be present at a
given temperature on the diagram there
are several points of interest a 1 the
temperature at which austenite turns to
pearlite the lowest temperature
austenite gamma-iron
does not exist this is 723 degrees
Celsius
a three the temperature when ferrite
alpha iron transforms to austenite
gamma-iron but pure iron this occurs at
nine hundred and ten degrees Celsius the
lowers in temperature along the line to
0.8 percent carbon then it increases in
temperature up to two percent carbon
liquidus temperature the temperature of
which steel of a given composition fully
turns to a liquid below 723 degrees
Celsius we can see fe3c which is iron
carbide this is called cementite which
is a ceramic compound of iron and carbon
steels with less than 0.8 percent carbon
consists of a structure of ferrite and
pearlite their light is a structure that
consists of iron carbide cementite and
ferrite alpha iron in parallel last
above 0.8 percent carbon cementite and
pearlite are primary constituents
if we take an example of a 0.3 percent
carbon steel the steel is molten until
we call to one thousand five hundred and
ten degrees Celsius at this point the
liquid iron starts to solidify into
Delta iron from one thousand five
hundred and ten to one thousand four
hundred and ninety five degrees Celsius
the amount of Delta iron increases while
the amount of liquid decreases at one
thousand four hundred and ninety five
degrees Celsius the body centered cubic
Delta iron transforms to face centered
cubic austenite as we continue to cool
to 1,450 four degrees Celsius the amount
of austenite increases and the liquid
decreases until we have a fully solid
austenite extractor as we decrease in
temperature further the structure
remains austenite until we hit eight
hundred and twenty degrees Celsius where
it starts to form body centered cubic
ferrite from 820 to 723 degrees Celsius
the amount of austenite decreases and
the amount of ferrite increases until
the remainder of the austenite will
transform but as austenite has a higher
solubility for carbon than ferrite the
ferrite that forms will not be able to
accommodate all the carbon that was
contained in the austenite and thus the
remaining austenite will firm a mixture
of ferrite alpha iron and iron carbide
cementite this structure is known as
pearlite here we can see some examples
of different carbon contents and the
structures produced low carbon structure
consists primarily of ferrite with small
grains of pearlite
medium carbon structure consists
primarily of pearlite with a small
percentage of ferrite high carbon at 0.8
percent carbon the structure consists
pearlite above 0.8 percent carbon the
structure consists of pearlite and
cement
in addition to the iron carbon
equilibrium diagram there are two other
diagram that are used extensively in
metallurgy these are the CCT continuous
cooling transformation diagram and the
TTT time temperature transformation
diagram we briefly outline these in this
module and we'll go into these in a
greater depth in a following module on
heat treatment where these are primarily
used while the iron carbon diagram
describes the structures of steel under
equilibrium conditions where enough time
has been allowed on heating and cooling
for any reactions to fully complete both
the continuous cooling transformation
and time temperature transformation
diagrams allow determination of
structures and various cooling rates
from slow to very fast
both these diagrams are helpful in
selecting the optimum steel and process
parameters to achieve a given set of
properties continuous cooling
transformation diagrams are generally
more appropriate for engineering
applications the diagrams show the
structures that are achievable
continuously cooling from the
austenitization temperature at a
constant rate these diagrams often show
the structure that can be achieved at
the center of different sized bars for
cooling in water oil and air some
diagrams also have the hardness that is
achieved from this structure time
temperature transformation diagram show
how long it would take for a structure
to be achieved by holding at a given
temperature this diagram allows you to
plot varying cooling rates and show the
structure that would be achieved
harden ability in mythology the harden
ability of a steel is a key parameter
and when we talk about harden ability in
Steel's we are often describing how deep
into the steel we can achieve hardening
if the steel is described as having a
low harden ability this will mean that
the steel will produce a shallower depth
of hardness when a steel has a high
harden ability it will be the same
hardness throughout the thickness of the
product highly hardenable steels are
more important in large components
harden ability is not to be mistaken for
hardness when describing the hardness we
are often looking at the microstructure
achieved during cooling for a given
steel it can be assumed that the quicker
the cooling rate the greater chance of
achieving a harder structure and if that
still has a high harden ability this
hard structure will be present deeper
into the thickness we can increase the
harden ability of a steel by adding
elements like manganese molybdenum
chromium nickel and boron when we add
these elements it increases the harden
ability and this will enable us to
achieve a harder structure deeper into
the thickness the ability of the
hardness to go deeper into the thickness
is because it is easier to achieve a
harder structure at a slower cooling
rate the structure of a steel can be
pure light with ferrite or cementite
which is a softer structure martensite
which is the harder structure and
bainite which is in between looking at
the continuous cooling transformation
diagram if the steel has a low harden
ability beright and pearlite
transformation will be shown in the
upper left hand side of the diagram
meaning it will be difficult or even
impossible to form martensite
if a steel has high heart and ability
transformation to martensite will be
shown at the bottom right-hand side of
the diagram meaning that a steel will
fully transform to martensite over a
large range of thicknesses strengthening
mechanisms in the introduction to
materials module we talked about
dislocations being present in metals
these dislocations reduce the strength
of the metal the principle of
strengthening mechanisms is to reduce
the ability of these dislocations to
move through the metal this can be
achieved by the reduction in grain size
cold working solid solution
strengthening and dispersion or
precipitation strengthening these
strengthening mechanisms can be applied
individually or in combination in metal
it is estimated that there are 10
million to 1 billion dislocations per
centimeter squared and each dislocation
has a strain field associated with it
with grain size the grains can interact
with the dislocations preventing further
movement of them if we reduce the grain
size we can increase the number of
grains interacting with the dislocations
preventing their movement and thus
strengthening the metal
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cold work introduces a large amount of
strain into the metal this strain
interacts with the dislocation strain
field impeding the movement of the
dislocations solid solution
strengthening is applied when we add
other chemical elements to a metal as
discussed in the introduction of
materials module the element added can
either fall between the atoms of the
bulk material or replace the atoms
within steel carbon atoms fall between
iron atoms and nickel atoms replace iron
atoms this will either be called
interstitial or substitutional solid
solution strengthening and will cause
distortion in the atomic structure this
distortion interacts with the
dislocations preventing the dislocation
movement and strengthening the steel
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dispersion or precipitation
strengthening is highly related to the
structure of the metal and takes place
when a phase is finally precipitated
through a softer matrix this hard
precipitate acts as a barrier to
dislocation movement the precipitate can
also produce a strain field that
interacts with the dislocation strain
field so in summary
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we have the ability to tailor the
properties of a steel by adding
different Aryan elements alloying
elements do not always work in isolation
sometimes they cause a multiplication
effect carbon to iron is probably the
most important addition to cast irons
and steel the iron carbon equilibrium
diagram continuous cooling
transformation diagram and time
temperature transformation diagram are
the most widely used diagrams in steel
metallurgy continuous cooling
transformation and time temperature
transformation diagram allow
determination of structures at a
variation of cooling rates harden
ability describes how deep into the
steel we can achieve hardening hardening
is not to be mistaken for hardness when
describing the hardness we are often
looking at the structure achieved from
cooling the structure of a steel can be
pearlite with ferrite or cementite which
is the softest structure martensite
which is the hardest structure and
bainite which is in between dislocations
reduce the strength of the metal the
principle of strengthening mechanisms is
to reduce the ability of these
dislocations to move through the steel
strengthening can be achieved by the
reduction in grain size cold working
solid solution strengthening and
dispersion or precipitation
strengthening