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steel is the most important material to
human society it has been the skeleton
of human industry for centuries and the
advent of the methods to mass produce it
from iron ore was the impetus to
transform human society from a mostly
agricultural lifestyle to urban
industrialization it forms the backbone
of our skyscrapers it paves the way for
our railways it shapes the engines that
powers our society and the very tools
that forge these objects are made from
steel we have spoken before about how
the refining process of iron determines
whether the resulting material will be
iron or steel and how the exact
percentage of carbon present in the
material has dramatic results on the
final material properties and how the
production methods through time has
taken steel from an expensive material
reserved for swords armor and tool
making to one that has permeated into
nearly every technology we use in our
lives but i failed to tell you why that
simple addition of carbon has such a
huge effect on the iron turning it from
a relatively weak material to one
capable of launching an industrial
revolution which is what we're going to
learn today
much of our knowledge in crafting steel
was passed down over the centuries from
blacksmith to blacksmith creating tools
for their communities so to learn more
about this amazing material and how
blacksmiths carefully tailor its
material properties i visited alex
steele's workshop to create my own knife
from scratch we started our forging
process with round stock 1055 steel with
0.55 percent carbon content and placed
it in the forge and gradually shaped it
with a power hammer to a rectangular bar
that could be more precisely shaped into
the shape of our blade using a three
pound hammer that my wii arms struggle
to swing after an hour once we roughed
out the shape we began to grind and
refine our blade eventually producing
our blade blank that would be later
grinded to its final shape but before
that could happen we needed to perform
some metallurgy wizardry through the
heat treatment process to drive home how
important this step is we tested four
samples of the same material at
different stages of the heat treatment
process this was our sample before the
heat treatment process which you will
quickly see was the weakest of the bunch
this is the normalized sample which was
ductile with a low yield stress it took
several hammer blows which it absorbed
through plastic deformation not ideal
characteristics for a sword or knife
next we tested our quench hardened
sample which is stupid and dangerous and
should not be tried at home this
fractured explosively and tore a hole
through alex reflector finally we tested
the tempered material which absorbed
every hammer blow with minimal plastic
deformation and only broke when we cut a
notch into the material to create a
stress concentration point this material
is tough capable of absorbing energy
without deforming permanently and hard
allowing it to resist damage to the
cutting edge it is the ideal material
for a blade if any of these terms
confuse you i created a video called
material properties 101 that you can
check out to get a better understanding
of material property terminology so how
can the same steel alloy change so
radically by simply applying heat well
this is the magic of the iron carbon
alloy we can carefully control how the
internal metallic crystal structure
forms with heating and cooling cycles
first let's see how adding carbon to the
iron affects the crystalline structure
with no carbon present pure iron forms a
crystal structure called body centered
cubic with an iron atom at each of the
eight corners and another in the center
each crystal structure has a direction
it most easily wants to deform called a
slip plane for body centered cubic the
slip plane occurs along this plane
metals with body centered cubic crystals
like iron and tungsten tend to be harder
and less malleable than metals with face
centered cubic crystals like aluminium
lead and gold when a metal is cooling
these crystals grow from individual
nucleation points and form grains where
each grain has the same orientation of
slip plane but neighboring grains may
not have the same slip plane let's think
about this two dimensionally when a
force is applied the grain wants to slip
in a particular direction and passes the
force onto the next grain in that
direction too but this grain slip plane
is at a different angle so that force
needs to be greater in order to cause
deformation it's like trying to push a
train down a railway track by pushing on
its side it's not going to go anywhere
easily so smaller and more numerous
grains results in a stronger material
pure iron tends to always have the same
crystal structure as it cools and its
crystal structure doesn't change
meaningfully with heat treatment this is
where alloying with carbon comes in but
to explore this let's look at our phase
diagram for carbon steel on this diagram
we have our carbon content percentage on
the x-axis and the temperature of the
metal on the y-axis this tells us the
crystalline structure of the metal at
various temperatures and carbon contents
on the left-hand side we have pure iron
which as we explained earlier forms only
one crystal structure called ferrite as
we move across the diagram to the right
hand side less and less of the crystal
structure forms ferrite and more forms
an iron carbide alloy commonly called
cementite now if we move up in
temperature we start to see these lines
that represent transitional temperatures
where the crystal structures of the
steel begin to transform into a new
crystal structure called austenite
moving further up again we see these
lines representing the transition of the
material to a liquid state austenite's
primary difference to ferrite is that it
forms that face centered cubic crystal
structure that we saw earlier while
ferrite is body centered cubic and while
this packing pattern is denser than body
centered cubic it does open up spaces in
the crystal structure that interstitial
carbon atoms which are smaller than iron
can snugly fit allowing austenite to
have a higher solubility to carbon over
ferrite using all this information let's
take our 1055 steel with 0.55 carbon
content and see how it transforms from
the start of our heat treatment cycle to
the end
the first step is called normalization
normalization's primary function is to
relieve internal stresses and strains
that form during the forging process and
return the material to its original
crystal structure before forging began
it's effectively a reset button for the
steel and creates a nice even grain size
and distribution increasing its strength
here we place the knife inside a steel
tube to prevent the metal from receiving
heat directly from the flame but instead
a more even radiate of heat from the
tube once it reaches this transition
temperature we let it soak to give the
crystal structure time to settle the
next step in normalization is to allow
the steel to slowly air cool what
happens now depends on the carbon
content if we take a 0.8 carbon steel no
hang on we're definitely going to need a
bigger graph for this
okay if we take a 0.8 carbon steel and
cool it through the transitional
temperature the austenite and the
interstitial carbon will slowly
transform to a mixture of ferrite and
cementite which takes this laminar
structure called perlite perlite
only forms at a 0.8 carbon solution now
if we take a 0.2 percent steel and
slowly cool it to its first transitional
temperature here where ferrite first
begins to form ferrite is pure iron so
as it forms the carbon percentage begins
to rise this will continue to happen
until the remaining austenite has a
carbon percentage of 0.8 percent and
will then form perlite from this point
on this forms a crystal structure
dominated by ferrite showing here as a
lighter color surrounded by the darker
perlite comparing these microstructures
to another two we can see the effect
carbon has on the microstructure here we
have pure iron with 100 ferrite showing
us the light color you can even see the
grain boundaries this is 0.5 carbon
steel similar to iris where a very small
amount of ferrite formed before reaching
the point of perlite formation and this
is 0.8 carbon steel where the entire
structure is perlite with our previous
example showing perlite under 500 times
magnification where you can readily see
that laminar structure how perlite
strength and steel is not well
understood it has little effect on the
steel stiffness but increasing the
pearlite content has a dramatic effect
on the material's yield point making it
much more capable of absorbing energy
without permanently deforming but we can
increase the material stiffness and
hardness with our next step if we heat
the metal back up to form austenite once
again but this time instead of letting
it cool slowly we rapidly cool it in oil
while casually burning your arm hair off
and barely flinching the carbon atoms
that spread out throughout the hot
austenite structure cannot diffuse out
of the crystal lattice to form cementite
and instead gets stuck in solution
creating a new crystal structure called
martensite this crystal structure has a
huge amount of internal stretching in
part because the carbon trapped within
the crystal structure causes the crystal
lattice to deform but also because
during the rapid cooling the surface
cooled much faster than the internal
material this causes internal tension in
the material and these internal strains
make it harder for additional
deformation to occur but this does not
make the material stronger it simply
means that it will not stretch and bend
before breaking and when it finally does
fracture all of this internal tension is
suddenly released in an explosive
expansion if you've watched smarter
every day's video on the saint rupert's
drop you'll have seen this principle
demonstrated in incredible slow motion
this material property is called
hardness and we want our cutting edge to
be hard to resist damage when cutting
but we do not want our entire blade to
be heard as it will not be able to
absorb a lot of energy it needs to be
able to dissipate some of that energy
through heat and deformation this is
where the final step of the process
comes in called tempering the tempering
raises the temperature enough to allow
the carbon trapped in solution to escape
we just use an oven set at 200 degrees
the carbon then coalesces to form
cementite once again but instead of
forming perlite like before it gathers
in globules surrounded by ferrite
tempering also relieves some of that
internal tension caused by the rapid
cooling this reduces the hardness but
increases toughness this produces a
material that has the perfect balance of
characteristics between the normalized
material and the hardened material it is
ductile enough to absorb hammer blows
without shattering but strong enough to
not permanently deform and with enough
hardness to ensure it doesn't gather
damage on the cutting edge this process
gave our steel the perfect material
properties and a lot of that is thanks
to the quality of the steel we started
off with learning these skills with alec
was a lot of fun and i highly recommend
you check out his channel his incredible
attitude to work really inspired me to
start learning some new hands-on skills
this year and i'm hoping to start that
off by learning how to program and
create robotics with an arduino this
course i found on skillshare is the
perfect jumping off point for anyone
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