What is Heat Treatment?
• Heat treatment is a general term referring to a cycle of
heating and cooling which alters the internal structure of a
metal and thereby changes its properties
• Metal and alloys are heat treated for a number of
purposes however the primarily to:-
1. Increase their hardness and strength
2. To improved ductility
3. To soften them for subsequent operations (cutting etc)
4. Stress relieving
5. Eliminate the effects of cold work
HEAT TREATMENT OF STEEL
The mechanical properties of materials can be changed by
heat treatment. Let’s first examine how this applies to
In order to understand how carbon steels are heat treated
we need to re-examine the structure. Steels with carbon fall
between the extremes of pure iron and cast iron and are
classified as follows.
All metals form crystals when they cool down and change from liquid
into a solid. In carbon steels, the material that forms the crystals is
complex. Iron will chemically combine with carbon to form IRON
CARBIDE (Fe3C). This is also called CEMENTITE. It is white, very
hard and brittle. The more cementite the steel contains, the harder and
more brittle it becomes.
When it forms in steel, it forms a structure of 13% cementite and 87%
iron (ferrite) as shown. This structure is called PEARLITE. Mild steel
contains crystals of iron (ferrite) and pearlite as shown. As the %
carbon is increased, more pearlite is formed and at 0.9% carbon, the
entire structure is pearlite. 4
0.1 – 0.15
pressed steel body panels
Medium carbon steel
High carbon steels
0.15 – 0.3
0.5 – 0.7
0.7 – 1.4
2.3 – 2.4
steel rods and bars
springs, drills, chisels
• A solid solution of Carbon in face-centred
cubic iron (Allotropic), containing a maximum
0f 1.7 % carbon at 1130oC
• It is soft, ductile and non-magnetic and also
exist in the plain carbon steel above the
upper critical range.
• Ferrite is nearly pure iron. A solid solution of Carbon
in body-centred cubic iron, containing a maximum
of 0.04 % Carbon at 695oC.
• At room temperature, small amounts of manganese,
silicon and other elements may be dissolved in iron
as well as up to 0.007 % Carbon.
• Found only in Hypoeutectoid steel
• A hard brittle compound of iron and Carbon with
the formula Fe3C
• The hardest constituent of steel
• This may exist in the free state usually as a grain
boundary film, or as a constituent of the
• This is the eutectoid structure consisting of
alternate lamination of ferrite and
• It contains 0.83% Carbon and is formed by
the breakdown of the austenite solid
solution at 695oC
• The properties of pearlite are harder and
stronger than ferrite, but softer and more
ductile than cementite
If the carbon is increased further, more cementite is
formed and the structure becomes pearlite and
cementite as shown.
HEAT TREATMENT of CARBON STEELS
Steels containing carbon can have their properties
(hardness, strength, toughness etc) changed by heat
treatment. Basically if it is heated up to red hot and then
cooled very rapidly the steel becomes harder. Dead mild
steel is not much affected by this but a medium or high
carbon steel is.
Principle of heat treatment of steel
• Metals are never heated to the melting point in heat
• Therefore, all the reactions within the metal during the
heating and cooling cycle, take place while the metal is
in the solid state
• During ordinary heat treating operations, steel is seldom
heated above 983oC.
• In using the iron-iron carbide diagram, we need only to
concern ourselves with that part which is always solid
• The area where the Carbon content is 2% or less and
the temperature is below 1130oC
• Cooling rate is the most important part of heat treatment.
• Different cooling rates are now considered as they have a
significant effect on the properties of the metal.
• Austenite is transformed to course pearlite.
• Slightly more rapid cooling may produce fine pearlite in which
the layers of ferrite and cementite are thinner.
• Austenite transforms to a material called Bainite instead of
the usual pearlite.
• When etched, Bainite gives a dark appearance and shows a
circular or needle like form.
• By quenching in water, the transformation of
austenite is suppressed until about 318oC at which
point a new constituent called Martensite(quite brittle)
begins to form instead of the Bainite or pearlite of
slower cooling rate.
• As the temperature drops lower, the transformation
• This temperature vary with the alloy content of the
TIME TEMPERATURE TRANSFORMATION
• In order to obtain steels with the desired
properties, we must have some control over
the transformation process, and this is
indicated in the TTT diagram
• TTT diagram are used to predict the
metallurgical structure of a steel sample
which is quenched in the austenite region
and held to constant elevated temperature
• This is known as Isothermal transformation
TIME TEMPERATURE TRANSFORMATION DIAGRAM
This is used to produce sheets, bars and sections. If the
rollers are cylindrical, sheet metal is produced. The hot slab
is forced between rollers and gradually reduced in
thickness until a sheet of metal is obtained. The rollers may
be made to produce rectangular bars, and various shaped
beams such as I sections, U sections, angle sections and T
sections. Steel wire is also produced this way. The steel
starts as a round billet and passes along a line of rollers. At
each stage the reduction speeds up the wire into the next
roller. The wire comes of the last roller at very high speeds
and is deflected into a circular drum so that it coils up. This
product is then used for further drawing into rods or thin
wire to be used for things like springs, screws, fencing and
The process is similar to hot rolling but the metal is
cold. The result is that the crystals are elongated in
the direction of rolling and the surface is clean and
smooth. The surface is harder and the product is
stronger but less ductile. Cold working is more
difficult that hot working.
In this process the metal is forced into shape by
squeezing it between two halves of a die. The dies
may be shaped so that the metal is simply
stamped into the shape required (for example
producing coins). The dies may be a hammer and
anvil and the operator must manipulate the
position of the billet to produce the rough shape for
finishing (for example large gun barrels).
Cold working a metal by rolling, coining, cold forging or drawing leaves
the surface clean and bright and accurate dimensions can be
produced. If the metal is cold worked, the material within the crystal
becomes stressed (internal stresses) and the crystals are deformed.
For example cold drawing produces long crystals. In order to get rid of
these stresses and produce “normal” size crystals, the metal can be
heated up to a temperature where it will re-crystallise. That is, new
crystals will form and large ones will reduce in size.
If the metal is maintained at a substantially higher temperature for a
long period of time, the crystals will consume each other and fewer but
larger crystals are obtained. This is called “grain growth”.
Cold working of metals change the properties quite dramatically. For
example, cold rolling or drawing of carbon steels makes the stronger
and harder. This is a process called “work hardening”.
Most metals (but not all) can be shaped more easily
when hot. Hot rolling, forging, extrusion and drawing is
easier when done hot than doing it cold. The process
produces oxide skin and scale on the material and
producing an accurate dimension is not possible.
Hot working, especially rolling, allows the metal to re-
crystallise as it is it is produced. This means that
expensive heat treatment after may not be needed.
The material produced is tougher and more ductile.
LIQUID CASTING AND MOULDING
When the metal cools it contracts and the final product is
smaller than the mould. This must be taken into account in
The mould produces rapid cooling at the surface and
slower cooling in the core. This produces different grain
structure and the casting may be very hard on the outside.
Rapid cooling produces fine crystal grains. There are many
different ways of casting.
A heavy component such as an engine block would be cast
in a split mould with sand in it. The shape of the component
is made in the sand with a wooden blank. Risers allow the
gasses produced to escape and provide a head of metal to
take up the shrinkage. Without this, the casting would
contain holes and defects.
Sand casting is an expensive method and not ideally suited
for large quantity production. Typical metals
used are cast iron. Cast steel and aluminium alloy.
Die castings uses a metal mould. The molten metal
may be fed in by gravity as with sand casting or forced
in under pressure. If the shape is complex, the
pressure injection is the best to ensure all the cavities
are filled. Often several moulds are connected to one
feed point. The moulds are expensive to produce but
this is offset by the higher rate of production achieved.
The rapid cooling produces a good surface finish with
a pleasing appearance. Good size tolerance is
obtained. The best metals are ones with a high degree
of fluidity such as zinc. Copper, aluminium and
magnesium with their alloys are also common.
This is similar to die casting. Several moulds are
connected to one feed point and the whole
assembly is rotated so that the liquid metal is
forced into the moulds. This method is especially
useful for shapes such as rims or tubes. Gear
blanks are often produced this way.
Machining processes involve the removal of
material from a bar, casting, plate or billet to form
the finished shape. This involves turning, milling,
drilling, grinding and so on. Machining processes
are not covered in depth here. The advantage of
machining is that is produces high dimensional
tolerance and surface finish which cannot be
obtained by other methods. It involves material
wastage and high cost of tooling and setting.
• heat treatment that alters the
microstructure of a material causing
changes in properties such as strength,
hardness, and ductility
• It the process of heating solid metal to
high temperatures and cooling it slowly so
that its particles arrange into a defined
Stages in annealing
Heating to the desired temperature ,
Holding or soaking at that temperature,
Cooling or quenching ,usually to room
• In practice annealing concept is most widely
used in heat treatment of iron and steels.
Purpose of annealing
• It is used to achieve one or more of the
following purpose .
1. To relive or remove stresses
2. To include softness
3. To alter ductility , toughness, electrical,
4. To Refine grain size
5. To remove gases
6. To produce a definite microstructure .
Annealing process is employed in following
• Rolled stock
• Press work ….
3 Types of Annealing:
I. Process Annealing
II. Full annealing
i. Process Annealing
Carried out on cold-worked low carbon steel
sheet or wire in order to relieve internal stress
and to soften the metals.
• The steel is heated to 550 to 650oC below the
Increase in ductility reduce in TS & hardness
ii. Full Annealing
It carried out on hot-worked and cast steels in
order to obtain grain refinement with high
It also produces a softer steel with better
• For steels
– heating above critical point (30 - 50oC) then
- holding at this temperature for a time (thickness)
- followed by slow cooling usually in furnace.
iii. Spheroidizing Annealing
To remove coarse pearlite and making machining process
It forms spherodite structure of maximum soft and ductility
easy to machining and deforming.
• The process is limited to steels in excess of 0.5% carbon.
This steel can be softened by annealing at 650 – 750oC just
below the lower critical point, when the cementite of the
pearlite balls up or spheroidizes.
Defects uncontrolled temperature
Heated above the actual temperature or to long
maintained at annealing temperature: austenite grain
growth will occur and make the metal weak and brittle
If heated above the upper critical point to temperature,
Brittles films of oxide are formed which make the steel
unsuitable. For further use and must be remelted.
iii. Under annealing
The original pearlite will have change to several small
For hypoeutectoid steels
- heating above critical point (30 - 50oC)
- holding at this temperature for a time (thickness) &
- followed by cooling in still air.
• Produces maximum grain refinement and
consequently the steel slightly harder and stronger
than a fully annealed steel.
• However the properties will vary with section
Hardening is process in which Medium and High
carbon steels (0.4 – 1.2%) is heated to a
temperature above the critical point (until red
hot), held at this temperature and quenched
(rapidly cooled) in water, oil or molten salt baths.
• Hardening producing a very hard and brittle
metal. At 723 Deg C, the ferrite changes into
Tempering is a process of heat treating, which is used to
increase the toughness of iron-based alloys.
To remove some of the brittleness from hardened steels,
tempering is used. The metal is heated to the range of 220-
300 deg C and cool in the air.
• Tempering is usually performed after hardening, to reduce
some of the excess hardness, and is done by heating the
metal to some temperature below the critical temperature
for a certain period of time, then allowed to cool in still air.
• To harden by quenching, a metal (usually steel or cast
iron) must be heated into the austenitic crystal phase
and then quickly cooled.
• Quenching Media:
Brine (water and salt solution)
Turn off furnace
• Low carbon steels cannot be hardened by
heating due to the small amounts of carbon
present. So, Case hardening seeks to give a
hard outer skin over a softer core on the metal.
• The addition of carbon to the outer skin is known
• When temperature region 200 – 450oC the
martensite decomposes into ferrite and the
precipitation of the fine particles of carbide occurs
known. as troostite
• At higher temperatures 450 – 650oC the carbide
particles coalesce thus producing fewer and larges
particles which provide fewer obstacles to
dislocations resulting further increasing toughness
while decrease in strength and hardness and known
• Sorbite is ideal for components subject to dynamic
stresses such as crankshaft and connecting rod