00:00:00
Hi, I’m Dr. Billy Wu and here we’ll be talking
about the different forms of steel and their
00:00:06
microstructures which dictate their properties.
This video follows on from an earlier talk
00:00:11
about equilibrium phase diagrams and the lever
rule, so if you’re not sure about these concepts,
00:00:17
please do check that out which is linked
above and below for your viewing pleasure.
00:00:23
So, first all of lets explore
why this is important.
00:00:27
In short, steels, which are
alloys of iron and carbon,
00:00:31
are one of the most widely used
engineering materials in modern day life.
00:00:36
This comes in a range of different
varieties depending on the composition.
00:00:40
At low concentrations of carbon
we have low carbon steels
00:00:44
which typically are quite ductile and are used
in everyday applications including paperclips.
00:00:51
As we increase the carbon content we get medium
carbon steels, where the hardness increases
00:00:56
and we can start to use them in
applications such as cutting tools.
00:01:01
This increasing carbon content can
continue to further increase the hardness
00:01:06
and ware resistance in high carbon steels which
are used in applications such as railway lines.
00:01:12
The hardness continues to increase
as carbon content goes up,
00:01:16
however the material starts becoming
increasingly brittle and prone to fracture.
00:01:21
In the case of cast irons with very high amounts
of carbon, this hardness is ideal, and a secondary
00:01:27
benefit of the high carbon content is that the
specific heat capacity increases, meaning that the
00:01:33
material can hold heat for longer, which is ideal
for applications such as kitchen pots and pans.
00:01:41
However, in order to select a suitable material
for an application, it is useful to understand
00:01:46
what causes these changes to the materials
mechanical properties and how we can control this.
00:01:54
In a previous video, we discussed what phases are
and how we can use equilibrium phase diagrams to
00:02:00
understand the impact of composition and
temperature on material microstructure.
00:02:06
Now, to remind ourselves, a
phase is a region of a material
00:02:09
with uniform physical and chemical properties.
00:02:13
In the copper-silver phase diagram example,
00:02:16
this is a system which exhibits limited solid
solubility which means that the solute atoms
00:02:22
are not completely soluble in the solvent atoms
for any composition of solute and solvent atoms.
00:02:30
If we quickly look at an example where
we have a low silver composition.
00:02:34
At high temperatures, we are in
the single phase liquid region.
00:02:41
Then as we gradually cool the material,
we pass through a liquidus line,
00:02:45
where a solid alpha phase
starts to precipitate out.
00:02:51
As we continue to cool the material,
we pass through a solidus line,
00:02:55
where all the remaining liquid
solidifies into the alpha phase.
00:03:02
As we continue to cool the material,
we pass through a solvus line
00:03:06
where a 2nd phase we call beta
starts to precipitate out.
00:03:11
Therefore, we can see that these phase diagrams
are very useful for helping us understand
00:03:17
what sort of equilibrium microstructures we would
get at different temperatures and compositions.
00:03:24
Now if we refocus our attention to ferrous alloys,
these are a subset of the broader metal alloys
00:03:30
category. You can see the importance of ferrous
materials as we often brake down metal alloys into
00:03:36
ferrous and non-ferrous materials. When we refer
to ferrous materials, we mean alloy compositions
00:03:43
which contain iron, and in the case of steel,
one of the main alloying components is carbon.
00:03:50
We can then further subdivide ferrous
materials into steels and cast irons,
00:03:55
where the main difference is the amount
of carbon we have in the material.
00:04:00
With steels, we can further subdivide this
into low alloy and high alloy materials.
00:04:07
In the case of high alloy steels which
include materials such as tool steel
00:04:12
and stainless steel, there is a relatively
high proportion of elements such as Chromium
00:04:17
and nickel which give rise to properties such as
high abrasion resistance and corrosion resistance.
00:04:24
In this video though, we will
focus on the low alloy steels,
00:04:28
which we further divide into low,
medium and high carbon variants.
00:04:35
Now, in terms of steels we broadly classify this
as iron-carbon alloys which have a carbon content
00:04:42
between 0.04 and 1.7 weight percentage.
00:04:48
With low carbon steels, or mild steel,
00:04:50
this normally has between
0.04-0.3 weight percentage carbon.
00:04:56
These are typically cheap, easy to machine and
weld though have relatively low strength. These
00:05:04
are often found in applications such as car body
panels or as re-bar reinforcements in concrete.
00:05:11
Then as we increase the carbon content to between
0.3 to 0.7 weight percentage carbon we get medium
00:05:19
carbon steel. This has a higher hardness than low
carbon steel but tends to be a bit more expensive,
00:05:25
with applications including
gears and cutting tools.
00:05:30
If we continue adding carbon, to between 0.7-1.7
weight percentage carbon we get high carbon steel,
00:05:38
which is very hard, which makes it abrasion
resistant but also more difficult to weld.
00:05:43
Applications here can include railway tracks.
00:05:47
Now beyond these compositions, as we
increase the carbon content to beyond 1.7
00:05:52
we form cast irons which are even harder but
become increasingly brittle. You’ll often find
00:05:58
applications such as kitchen pots and pans,
and workshop machinery using cast irons.
00:06:05
And for completeness, lets remind ourselves
that when we are talking about low alloy steels,
00:06:10
or carbon steels, we mean iron-alloys
where carbon is the main alloying element.
00:06:17
In alloy steels, such as stainless steel there
00:06:20
are large amounts of other alloying
materials such as chromium or nickel.
00:06:27
So now that we know about the broad categories
of iron-carbon alloys, lets take a deeper look
00:06:32
into the different phases that form.
Remember a phase is a region of material
00:06:37
which has uniform chemical
and physical properties.
00:06:41
Now for practical purposes,
lets assume that pure iron
00:06:45
is anything which has a carbon content
below 0.03 weight percentage carbon.
00:06:51
Here, 3 distinct phases form
depending on temperature
00:06:56
Below 910°C, we have alpha ferrite which is a
soft and magnetic form of iron which is used in
00:07:03
electric motors. This has a body centred cubic,
or BCC, structure which you can see on the right.
00:07:11
As we increase the temperature, we form something
called austenite or gamma phase iron. This exists
00:07:17
between 910 and 1391 °C and is non-magnetic
with a face centred cubic, or FCC, structure.
00:07:27
Finally, at even higher temperature, we form a
delta-iron phase which has a BCC structure again.
00:07:37
In most applications, this delta
phase isn’t commonly encountered
00:07:41
so we will focus our discussion on
the ferrite and austenite phases.
00:07:47
Now if we start to alloy the iron with carbon
we can control the mechanical properties
00:07:52
of the alloy. In this part of the discussion
00:07:56
we’ll continue to use the phase descriptors
of alpha and gamma but this time to describe
00:08:01
interstitial solid solutions of carbon in iron.
Interstitial basically means the smaller carbon
00:08:09
sits in the gaps of the larger iron
atoms, rather than substituting them out.
00:08:15
We retain the names of the ferrite and austenite
from the pure iron case due to the fact that the
00:08:20
crystal structures are retained, i.e.
BCC for ferrite and FCC for austenite.
00:08:28
Now in addition to ferrite and austenite, a 3rd
phase can also form which is called cementite.
00:08:34
This has a chemical composition of Fe3C with a
fixed carbon content of 6.7 weight percentage.
00:08:42
Cementite as a material is very hard and brittle.
00:08:47
Therefore, the mechanical
properties of steel largely
00:08:51
depend on the amount of cementite in the alloy.
00:08:54
Now to understand which phase forms at
a specific temperature and composition,
00:09:00
lets look at our iron carbon phase diagram.
00:09:04
At relatively high temperatures, and moderate
carbon contents, we have a pure liquid state
00:09:09
and at low carbon contents and also high
temperatures we get this delta BCC phase.
00:09:18
Then as we cool the material
at moderate carbon contents
00:09:21
we pass through a liquidus line to form a
2 phase liquid and solid austenite region.
00:09:28
If we continue to cool the material, this
can either form a pure austenite phase
00:09:32
at lower carbon contents or a 2-phase austenite
and cementite phase at higher carbon content.
00:09:40
At even lower carbon contents,
alpha ferrite can start to form
00:09:44
from the austenite, and at extremely low
carbon compositions a pure alpha phase forms.
00:09:52
Then as we cool down even more, we form a
2 phase ferrite and cementite phase region,
00:09:59
where the amount of cementite increase as the
carbon content increases upto 6.7 weight percent.
00:10:07
Notable points on this phase
diagram include the eutectic point,
00:10:11
which corresponds to the composition and
temperature of the lowest melting point.
00:10:16
In the case of the iron-carbon phase diagram
this is at 4.3 weight percentage carbon.
00:10:23
And the other major point to note is the
Eutectoid point, which is the point in a
00:10:28
phase diagram indicating a solid is in equilibrium
with 2 other solid phases. For iron and carbon,
00:10:37
this occurs at 0.76 weight percent carbon which
is an important number, we will revisit later.
00:10:45
Now, for most engineering applications
compositions between 0.04 and 1.7 weight
00:10:52
percentage carbon are the most relevant
so we will focus our attention there.
00:10:58
Now if we look at the eutectoid composition
which is 0.76 weight percentage carbon,
00:11:04
we can start to visualise what
sort of equilibrium microstructure
00:11:07
we will get. Here we’ve zoomed into our
iron-carbon phase diagram for clarity.
00:11:14
At temperatures above 727°C we will form a single
austenite phase which is soft and non-magnetic.
00:11:23
Then as we cool the eutectoid
steel, we start to form 2 phases,
00:11:28
cementite which is hard and brittle.
And ferrite which is soft and ductile.
00:11:35
Now at the eutectoid composition
as we cool the material,
00:11:39
these 2 phases will form the eutectoid
structure called pearlite, which consists
00:11:44
of alternating layers, or lamella, of cementite
and ferrite. This pearlite structure though,
00:11:50
isn’t a single phase, but rather it’s a 2 phase
arrangement of soft-ferrite and hard-cementite.
00:11:58
Now that was the equilibrium microstructure at the
eutectoid composition of 0.76 weigh percent carbon
00:12:06
but we saw earlier that adding in more
carbon generally results in a harder material
00:12:11
so lets have a look at why that is.
00:12:15
Here we have 2 cases of interest which
centre around the eutectoid composition.
00:12:20
If the carbon composition is less than 0.76
then we call this a hypo-eutectoid composition.
00:12:29
Now if we draw a vertical line at a
composition below the eutectoid composition,
00:12:34
we can see at high temperatures
we have a single austentite phase
00:12:41
Then as we cool the material, a ferrite
phase starts to form from the austenite.
00:12:48
As this continues to cool, these small
islands of ferrite will continue to grow
00:12:53
and likely form interconnected regions.
00:12:58
Finally, as we cool below 727 degrees, the
remaining austenite is converted to pearlite,
00:13:06
which again, is made up of alternating layers
of soft ferrite and hard cementite. Now,
00:13:12
something to note here is that the pearlite
grains are held together with soft ferrite,
00:13:17
which we term, pro-eutectoid ferrite, or
alpha p. We’ll revisit this in a moment.
00:13:26
For the case where we have a carbon
composition greater than 0.76,
00:13:31
we call this a hyper eutectoid composition
00:13:34
and can again draw a vertical line here to help
us visualise the equilibrium microstructure.
00:13:41
Again, at high temperatures, we
have a single austentite phase.
00:13:48
And as we decrease the temperature, we enter
into a 2phase region, but this time instead of
00:13:54
ferrite and austinite, cementite and austinite
is the more thermodynamically favourable state.
00:14:03
Finally as we cool even more, the
remaining austenite is converted into
00:14:07
pearlite. Now the main difference between
the hypo and hyper-eutectoid compositions
00:14:14
is that the pearlite grains are held
together with a much hard cementite phase,
00:14:18
which we call pro-eutectoid-cementite or Fe3C
p. Therefore, this is one of the main reasons
00:14:26
for the increase in hardness as carbon content
is increased. Pearlite grains are held together
00:14:33
with more and more hard cementite as
opposed to the softer ferrite phase.
00:14:42
So, to summarise.
00:14:45
Steel, which is an alloy or iron and carbon,
00:14:48
is one of the most commonly used
engineering materials in modern society.
00:14:54
Here the carbon content is
an extremely important factor
00:14:57
in determining the materials microstructure
and therefore mechanical properties
00:15:02
with common types including low, medium and
high carbon steels as well as cast irons.
00:15:09
The 3 main phases of interest are ferrite,
austenite and cementite, where ferrite and
00:15:14
austenite are often characterised as softer
materials and cementite as a hard material.
00:15:21
At the eutectoid composition of 0.76
weight percent carbon, pearlite forms
00:15:28
which is a 2 phase material made of alternating
layers of soft ferrite and hard cementite.
00:15:35
When decreasing the carbon content to below
0.76 percent, we form a hypo eutectoid steel
00:15:43
where the pearlite grains are held
together with soft pro-eutectoid ferrite.
00:15:49
Above 0.76 percent carbon, we form a
hyper eutectoid composition where the
00:15:55
pearlite grains are held together
with hard pro-eutectoid cementite.
00:16:09
So, hopefully this short video has helped you
to see how important the carbon content is, when
00:16:14
determining the mechanical properties of a steel
and how phase diagrams can help us to understand
00:16:20
the resulting equilibrium microstructure. Do check
out the other video on phase diagrams and the
00:16:25
lever rule which goes into more detail about the
background of phase diagrams, if this is unclear.
