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how do we tailor the properties of steel
for a desired application
how can we make a metal strong or tough
how can we optimize properties and what
information can help in defining how we
heat treat metals
in this video we will try to answer
these questions by looking at how heat
treatment is critical in achieving the
optimum properties and how we can either
soften or harden a metal through
producing different microstructures
we will review the different heat
treatments the microstructures formed
and how we predict what these might be
along with the addition of chemical
elements
heat treatment is probably the most
important process in controlling the
properties of metal
it involves heating solid metal to a
defined temperature
followed by suitable cooling rate
in order to achieve the desired material
properties
heat treatments are primarily conducted
to either soften or harden the steel
depending on its final application and
manufacturing process
this softening treatment is often
referred to as a conditioning process
and will lower strengthened hardness
while increasing toughness and ductility
conditioning includes two heat
treatments called annealing and
normalizing
the hardening process does the opposite
it increases strength and hardness while
lowering toughness and ductility
this includes two processes called
quenching and tempering and aid
hardening
the specific heat treatment used in
manufacturing will depend on the metal
chemistry the size of the part and the
required properties
the three main structures that are
achievable through heat treatment are
perlite binite and martensite
for the majority of heat treatments we
cool from the austenitic temperature
region
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the austenitic region is where most of
the alloying additions go back into
solution and this is shown here in the
iron carbon equilibrium diagram
this diagram shows the structures formed
under slow cooling in iron
as a function of temperature and carbon
content
as we cool through the transformation
region
we can create different structures by
altering the cooling rate
if we want to achieve pearlite the
softest structure we need to cool slowly
enough so that this is formed
to achieve martensite the hardest
structure
we need to cool quickly enough to ensure
that perlite or binite is not formed
the speed of these cooling rates depends
on a given steel composition
and there are various diagrams
that help to predict the different
structures formed
these will be discussed later in this
module
quenching tempering is probably the
widest use of the heat treatments to
harden steel and consist of heating the
material to approximately 15 degrees
celsius or 60 degrees fahrenheit to the
austenitic range
for steels this is usually somewhere
between 700 degrees to 1000 degrees
celsius
or
1290 degrees
to 1830 degrees fahrenheit
it is then held at this temperature
until the material is fully transformed
to astonite
this whole time takes into account the
dimensions of the component
the material is then removed from the
furnace and submerged
usually in an agitated liquid in a
process called quenching
this causes changes in the micro
structure
as the component cools rapidly down to
around
200 degrees celsius or 390 degrees
fahrenheit
this quenching causes none or very
little carbon to precipitate as iron
carbide
it produces a feather-like structure
called matenzite
which distorts the internal structure
this additional stress makes it
difficult for the facts on the atomic
scale called dislocations to move around
in response to the applied load causing
the material to become stronger
the formation of martensite is only
achieved if the material exceeds a
critical cooling rate
if not the micro structure formed will
be either binaite pearlite or ferrite or
a combination of these
this is the reason why components of
different sizes processed the same way
may have vastly different properties and
also why the surface properties may
differ from the center which would cool
slower than the surface
the quenching operation will increase
the hardness and strength of the steel
but will drastically decrease the
ductility and toughness
the aim of tempering is to soften the
steel back achieving the desired
combination of properties
tempering will considerably improve
toughness and ductility while still
maintaining a high strength level
tampering consists of heating the steel
to a temperature below the aesthetic
range
usually somewhere between 500 degrees to
650 degrees celsius or 900 degrees to
1200 degrees fahrenheit
and holding at this temperature for a
specific time period
during this process the hard brittle
martensite associate into ferrite and
iron carbide
producing a structure called tempered
martensite which has lots of fine
particles dispersed through the ferrite
this will give the optimum combination
of strength hardness toughness and
ductility
ratios of these properties can be
refined through the time and temperature
of the temporary
age hardening sometimes called
precipitation hardening is used to
increase the strength and hardness of
some stainless steels and nickel based
alloys it follows similar steps to
quench and tempering and such that the
material is heated to a temperature to
dissolve all the atoms into solution
it is then quenched at a cooling rate
fast enough for the atoms to remain
frozen in solution
the final step differs from tempering as
it is designed to harden the steel as
opposed to soften it back
it is called aging and involves the
controlled reheating to around
450 degrees
to 650 degrees celsius or
840 degrees to 1200 degrees fahrenheit
to coast some atoms to precipitate these
create strain in the structure and again
this makes it difficult for the
dislocation defect
move in response to the applied load
making the material stronger
conditioning treatments can be used to
obtain the final properties of a steel
or it can be used as an intimidate step
in order to ensure that the material
will not crack following rolling casting
forming or forging
it can also be used to get the material
into a state to aid in machining
in both normalizing and annealing the
material is heated to the austenite
range and then slow cool this is done in
still air for normalizing and in a
furnace for a nailing
a normalized structure can also be
tempered to alter the properties
slightly any gases that need removing
for example hydrogen will be done as
part of this process
both treatments leave the material
homogeneous soft and ductile but because
normalizing is cooled in air the
strength and hardness of this structure
will be greater than the annealed one
some nickel alloys and stainless steels
are used in the annealed condition to
aid in corrosion resistance
due to the slow cooling rates present in
normalizing and annealing a perlite
structure is likely to form per light
consists of alternate last of iron
carbide
also known as cementite and a pure iron
structure called ferrite
during cooling the austenite which has a
higher solubility for carbon
starts to turn to ferrite
but because this ferrite cannot
accommodate the higher concentration of
carbon the structure develops in
alternate labs of cementite and ferrite
this perlite is often present with
ferrite and depending on the carbon
content the perlite and ferrite will be
present in differing ratios
if the cooling rate is too fast for
perlite but not quick enough to produce
martensite a structure called binite
will form
finite is of medium hardness and lays at
a medium cooling rate
in between perlite and martensite
initially ferrite is produced but
because the cooling rate is quicker than
perlite the structure does not form in
lasso ferrite and iron carbide
but instead the iron carbide starts to
precipitate from the remaining
carbon-rich asteroid forming a structure
that consists of ferrite and particles
of iron carbide
depending on the cooling rate these iron
carbide particles can be more elongated
this is called upper binate and is the
slower cooled of the two structures
the other lower binate forms at faster
cooling rates and contains ferrite with
finer disc-like particles of iron
carbide
an annealing process used in cold
rolling or forming is a sub-critical or
process annealing
this is used to reduce the internal
stresses after manufacture
or to enable further processing
in this process the steel is not heated
all the way up to the austenitic range
but is heated just below the ferrite to
the austenite transformation temperature
usually around 500 degrees to 650
degrees celsius over 930 degrees to 1200
degrees fahrenheit the material is then
held for the desired amount of time to
cause softening through structural
changes primarily grain
recrystallization and grain growth
generally the higher the alloy content
the easier it is to form mattenzite and
this ability to form mountainside is
known as hardenability
meaning a steel with low harden ability
requires a faster cooling rate to attain
the same hardness at a given location
than a steel with high hard enability
knowing what structures will form in a
steel can be aided by two diagrams
called the continuous cooling
transformation and time temperature
transformation diagrams
both these diagrams show what structures
will be achieved under different heat
treatment conditions
they will also show the effect that
chemical elements will have on the
structures formed
for example elements like carbon
manganese
nickel molybdenum chromium and vanadium
make the steel more hardenable
making it easier to form martensite at
slower cooling rates
time temperature transformation diagrams
are used to understand what
microstructures will form as a function
of temperature and hold time
this means that heat treaters can use
this diagram to understand what
structures might be formed if a
component is called from austenite and
held at a given temperature for a given
time
this is possible because if we hold
austenite at a temperature where it is
unstable
eventually austenite will form into a
different microstructure in a process
called isothermal transformation
the diagrams assist in conducting two
treatments that help to minimize
distortion residual stresses and risk of
cracking
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these are called mu tempering in alz
tempering and they are essentially the
same process
they involve cooling at a sufficiently
quick rate
to intermediate temperature to form the
desired structure
holding to enable the temperature at the
center to equalize with the surface to
reduce internal stresses and then cool
in the material to room temperature
for mud tempering
the material is held in the martensite
region
and in else tempering the component is
held in the binite region
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a salt or oil bath is often used to hold
a sufficiently high intermediate
temperature
time temperature transformation diagrams
provide a good starting point to aid in
heat treatment but because they are
interested in the isothermal
transformation of astonite they are only
of limited use
to accurately predict the heat treatment
response and thus microstructure of
different steels of different sizes
quenched in different liquids we need to
use a different chart called a
continuous cooling transformation
diagram
this has greater practical use for heat
treaters
continuous cooling transformation
diagrams present data in two ways
the first is to plot microstructures as
a function of temperature and cooling
rate
from this we can superimpose the cooling
rate of our component either at the
surface
center or any distance in between and we
can predict which microstructure will be
formed
the second is to represent the cooling
rate at the center of different diameter
bars for cooling in air oil and water
this type of cct diagram is a convenient
way to look up the bar diameter we are
quenching
and the cooling medium used and again
predict the structure at the center of
the bar
both of these ways are often presented
on the same diagram along with the
hardness and this will enable the heat
treater to start to predict the
properties of the steel
there are more specialized heat
treatments that we have not covered here
that use special processes and
environments to control specific
material properties
these processes include quenching with
polymers and brines using surface
hardening by just heating and quenching
the outer few millimeters
continuous furnaces can be used to
automate the process and special
environments like in nitriding can also
be used
so to summarize
heat treatment is critical in achieving
the optimum properties and is used for
either softening or conditioning a metal
like a normalizing and annealing
treatment
or it can be used to harden the metal
like in quenching tempering
or age hardening
these treatments will produce three main
microstructures either perlite binite or
matinzite
the specific heat treatment used in
manufacturing will depend on the metal
chemistry the size of the pot and the
required properties
two diagrams can aid heat treaters
in predicting the structure and
properties in metals
these are called the continuous cooling
transformation
and time temperature transformation
diagrams
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you