Heat treatment: Normalizing, Annealing, Hardening, Tempering, Case hardening, Carburizing, Cyaniding, Nitriding, Surface hardening, Induction hardening, Flame hardening.

Heat treatment




Heat treatment is a heating and cooling
process of a metal or an alloy in the solid state with the purpose of changing
their properties. It can also be said as a process of heating and cooling of ferrous metals especially
various kinds of steels in which some special properties like softness,
hardness, tensile-strength, toughness etc, are induced in these metals for
achieving the special function objective. It consists of three main phases
namely (i) heating 
of the metal (
ii) soaking of
the metal and (
iii) cooling of the metal. The theory of heat
treatment is based on the fact that a change takes place in the internal
structure of metal   by heating and
cooling which induces desired properties in it. The rate of cooling is the
major controlling factor. Rapid
cooling the metal from above the critical range, results in hard structure.
Whereas very slow cooling produces the opposite affect i.e. soft structure. In any heat treatment operation, the rate of
heating and cooling is important. A hard material is difficult to shape by
cutting, forming, etc. During machining in machine shop, one requires
machineable properties in job piece hence the properties of the job piece may
requires heat treatment such as annealing for inducing softness and
machineability property in workpiece. Many types of furnaces are used for
heating heat treatment purposes. The classification of such heat treatment
furnaces is given as under.

Objectives of Heat Treatment




The major objectives of heat treatment are given as under

1. It relieves internal
stresses induced during hot or cold working.

2. It changes or refines grain
size.

3. It
increases resistance to heat and corrosion.

4. It improves mechanical properties such
as ductility, strength, hardness, toughness, etc.

5. It helps to improve machinability.

6. It increases wear resistance

7. It removes gases.

8. It improves electrical and magnetic properties.

9. It changes the chemical composition.

10. It
helps to improve
shock resistance.

11. It improves weldability.

The above objectives of heat treatment may be
served by one or more of the following heat treatment processes: 




Types of Heat treatment process

1. Normalizing

2. Anne
aling.

3. Hardening.

4. Tempering

5.  Case
hardening

(a) Carburizing

(b) Cyaniding

(c) Nitriding 

6. Surface hardening

(a)  Induction hardening,

(b)  Flame
hardening.




Normalizing,  Annealing, Hardening, Tempering, Case hardening, Carburizing, Cyaniding, Nitriding, Surface hardening, Induction hardening,  Flame hardening.

Heat treatment process





NORMALIZING

Normalizing is a defined as softening process
in which iron base alloys are heated 40 to 50°C above the upper-critical limit
for both hypo and hyper eutectoid steels and held there for a specified period
and followed by cooling in still air up to room temperature. Fig shows the heating temperature ranges for normalizing
process of both hypo and hyper carbon
steel.  Fig.
 shows the structure obtained
after normalizing of medium carbon
steel. 

Objectives of Normalizing

1. To soften metals

2. Refine grain structure

3. Improve machinability after forging
and rolling

4. improve grain size

5. Improve structure of weld

6. Prepare steel for sub heat treatment




ANNEALING

It is a softening process in which iron base
alloys are heated above the transformation range held there for proper time and
then cool slowly (at the of rate of 30 to 150°C per hour) below the
transformation range in the furnace itself.
Heating is carried
out 20°C above upper critical temperature point of steel in
case of hypo eutectoid steel and the same degree above
the  lower critical
temperature point in case of type eutectoid steel. Fig shows the heating
temperature ranges for annealing or softening process of both hypo and hyper
carbon
steel. Fig.  shows the
structure obtained after annealing of medium carbon steel. The structure of
steel on slow cooling changes into ferrite and pearlite for hypo eutectoid
steel, pearlite
for eutectoid steel
and pearlite and cementite for hyper eutectoid steel. The time for holding
the article
in furnace is ½ to 1 hour. As
ferrous metals are heated above the transformation
  range, austenite structure
will be attained
at this temperature.

For a particular type of structure
specific cooling rate is required to have good annealing properties for free
machining. As metal is slowly cooled after
heating and holding in and with the furnace and buried in non conducting media
such sand, lime or ashes, carbon steels
are cooled down at particular rate normally 150-200°C
per hour while alloy steel in which austenite is very stable and should be
cooled
much 
lower (30°C to 100°C per hour). Very  slow cooling  is required in annealing to enable austenite
to decompose at two degrees of super cooling so as to form a pearlite and
ferrite structure in hypo-eutectoid steel, a pearlite structure in eutectoid
steel and pearlite and cementite structure in hyper eutectoid steel. In
successfully annealed steel, the grains of
ferrite are large and regular while pearlite consists of cementite and ferrite.
Hypo-eutectoid hot worked steel may under go full annealing to obtain
coarse grain structure for free machining.
When steel is cold worked the hardness (Brinell hard) considerably increases
and ductility decreases
slightly. The
ductility of steel may be then restored
by so called recrystallisation or process annealing.




Objectives of Annealing

The purpose of annealing is to achieve the following

1. Soften the steel.

2. Relieve internal stresses

3. Reduce or eliminate structural in-homogeneity.

4. Refine grain size.

5. Improve machinability.

6. Increase or restore ductility
and toughness.

Types of Annealing 

(a)  Process annealing

(b)  Full
annealing

Process annealing

In process annealing, ductility
is increased with somewhat decrease in internal stresses. In this, metal is
heated to temperature some below or close to the lower critical temperature
generally it is heated 550°C to 650°C holding at this temperature and it is
slowly cooled. This causes completely recrystallisation in steel.

Full annealing

The main purpose of full
annealing of steel is to soften it and to refine its grain structure. In this,
the hypo-eutectoid steel is heated to a temperature approximately
20° to
30°C above the higher critical temperature and for hypereutectoid steel and
tool steel
        is heated to a
temperature 20 to 30°C above the lower critical temperature and this
temperature is maintained for a definite time and then slowly cooed very slow1y
in
the furnace itself.

 SPHEROIDIZATION

It is lowest temperature range of annealing
process in which iron base alloys are heated 20
 
to 40°C below the lower critical temperature, held therefore a
considerable period of time e.g. for 2.5 cm diameter piece the time recommended
is four-hours. It is then allowed to cool very slowly at room temperature in
the furnace itself. Fig 1 shows the heating temperature ranges for
spheroidizing process of carbon steel. Fig.2  shows the structure obtained
after annealing of carbon steel. During this process, the cementite of steel
which is in the combined form of carbon becomes globular or spheroidal leaving
ferrite in matrix, thus imparting softness to steel. After normalizing of
steels, the hardness of the order of 229
BHN and as such machining becomes difficult and hence to improve machining,
these are
spheroidised first and then
machined. This treatment is carried out on steels having 0.6 to 1.4% carbon.
The
objectives of spheroidising are given as under.

SPHEROIDIZATION It is lowest temperature range of annealing process in which iron base alloys are heated 20  to 40°C below the lower critical temperature, held therefore a considerable period of time e.g. for 2.5 cm diameter piece the time recommended is four-hours. It is then allowed to cool very slowly at room temperature in the furnace itself. Fig 8.7 shows the heating temperature ranges for spheroidizing process of carbon steel. Fig. 8.9 shows the structure obtained after annealing of carbon steel. During this process, the cementite of steel which is in the combined form of carbon becomes globular or spheroidal leaving ferrite in matrix, thus imparting softness to steel. After normalizing of steels, the hardness of the order of 229 BHN and as such machining becomes difficult and hence to improve machining, these are spheroidised first and then machined. This treatment is carried out on steels having 0.6 to 1.4% carbon. The objectives of spheroidising are given as under.
Fig 1: Structure of normalized medium carbon steel




Structure of annealed medium carbon steel
Fig 2: Structure of annealed medium carbon
steel 


Objectives of spheroidising

1. To reduce
tensile strength

2. To increase ductility

3. To ease machining

4. To impart structure for subsequent hardening process

 HARDENING

Hardening is a hardness inducing kind of heat
treatment process in which steel is heated to
 
a temperature above the critical point and held at that temperature for
a definite time
and then quenched
rapidly in
water, oil or molten salt
bath. It is some time said as rapid quenching also. Steel is hardened by
heating 20-30°C above the upper critical point for hypo eutectoid steel and
20-30°C above the lower critical point for hyper eutectoid steel and held
  at this temperature for some time and then
quenched in water or oil or molten salt bath.
   Fig shows the heating temperature ranges for hardening process of
both hypo and hyper carbon steel. Fig. 
 (a) shows the structure obtained on
water quenching on hardening of medium
carbon steel.
Fig.  (b)  shows the structure obtained
on oil quenching on hardening of medium carbon steel. Fig. (c) shows the structure obtained on water quenching on hardening of medium carbon
steel and followed
by tempering.

HARDENING Hardening is a hardness inducing kind of heat treatment process in which steel is heated to  a temperature above the critical point and held at that temperature for a definite time and then quenched rapidly in water, oil or molten salt bath. It is some time said as rapid quenching also. Steel is hardened by heating 20-30°C above the upper critical point for hypo eutectoid steel and 20-30°C above the lower critical point for hyper eutectoid steel and held  at this temperature for some time and then quenched in water or oil or molten salt bath.   Fig shows the heating temperature ranges for hardening process of both hypo and hyper carbon steel. Fig.  (a) shows the structure obtained on water quenching on hardening of medium carbon steel. Fig.  (b)  shows the structure obtained on oil quenching on hardening of medium carbon steel. Fig. (c) shows the structure obtained on water quenching on hardening of medium carbon steel and followed by tempering.
Structure of hardened carbon steel




Metal is heated up to austenite
formation and is followed by fast and continuous cooling of austenite to
temperature 205° to 315°C or even lower than that. Due to such rapid cooling,
austenitic structure changes to new structure known as martensite. It is
evident that
faster the rate of
cooling harder will be the metal due to formation of more martensitic
structure. Martensite
has a tetragonal crystal
structure. Hardness of martensite varies
from 500 to 1000
BHN depending upon the carbon content and fineness of the structure. Martensite
is a body centered phase produced by entrapping carbon on decomposition of
austenite when
cooled rapidly. It is the main constituent of
hardened steel. It is magnetic and is made of a
needle like fibrous mass. It has carbon content up to 2%. It is
extremely hard and brittle.
The  decomposition of austenite below 320°C starts
the formation of martensite. 


Sudden
cooling of tool steel provides thermal stresses due to uneven cooling. It
provides unequal specific volume of austenite and its decomposition product.
The structural transformations are progressing at different rates in outer
layers and central portion of the article. When martensitic transformation
takes place in the central portion of the article, due to tension stress
produces cracks. The harness depends upon essentially on the cooling rate. The
effect of cooling on austenite transformation is given in Fig.




Effects of coooling of austenite transformation
Effects of coooling of austenite transformation





TEMPERING

If high carbon steel is quenched for hardening
in a bath, it becomes extra hard, extra brittle and has unequal distribution
internal stresses and strain and hence unequal harness
and toughness in structure. These extra hardness, brittleness and
unwanted induced stress
and strain in hardened metal reduce the usability the metal. Therefore, these undesired needs must
be
reduced for by reheating
and cooling at constant bath temperature. In tempering, steel after
hardening,
is reheated to a temperature below the lower critical temperature and then followed by a desired rate of cooling.
Reheating the of hardened steel
is done above critical temperature when the structure is purely
of austenite and then quenching it in a molten salt path
having temperature in the range of
150-500°C. This is done to avoid transformation to ferrite
and pearlite and is held quenching
temperature for a time sufficient to give complete formation to an intermediate
structure referred to as bainite then cooled to room temperature. The
temperature should not be held less than 4 to 5 minutes for each millimeters of
the section. After tempering structure is changed into secondary structure like
martensite, troostite, sorbite and spheroidised. Fig. shows different
tempered states of martensite, troosite, sorbite and spherodite. Depending upon
the temperature of reheat, the tempering process is generally
classified in to three main categories. Which are discussed
as under.


Structures of tempered states of martensite, troosite, sorbite and spherodite
Structures of tempered
states of martensite, troosite, sorbite and spherodite



Low Temperature Tempering

Hardened steel parts requiring tempering are
heated up to 200°C and then quenched in oil. Tempering is used to retain hard
micro-structure of martensite which increases brittleness. Above fig represents
the microstructure of martensite. 


  Medium Temperature Tempering

 Hardened steel parts requiring tempering are
heated in the temperature range of 200-350°C. This
process gives troosite
structure. Troosite structure is another
constituent of steel obtained
by quenching tempering martensite. It is composed of the cementite phase in a
ferrite
matrix that cannot be resolved
by light microscope. It is less hard and brittle than martensite. It is also produced by cooling the metal slowly
until transformation begins
and then cooling
rapidly to prevent its completion. It has a dark appearance on etching.
It is weaker than martensite. Fig 
represents the microstructure of troosite.


High Temperature Tempring

Hardened steel parts requiring tempering are
heated in the temperature range of 350-550°C. This process gives sorbite
structure. Sorbite structure is produced by the, transformation of tempered
martensite. It is produced when steel is heated at a fairly rapid rate from the
temperature of the solid solution to normal room temperature. It has good
strength and is practically pearlite. Its properties are intermediate between
those of pearlite and troosite. Parts requiring tempering are heated in the
temperature range of 550-750°C. This process gives spheriodite structure. Fig represents the microstructure of sorbite. However there are other
special kinds of tempering also which are discussed as under.



Structures obtained tempering of hardened steel
Structures obtained tempering
of hardened steel




Aus-Tempering

It is a special type of tempering process in which and steel is heated
above the transformation range then suddenly quenched
in a molten salt bath at a temperature 200 to 450°C.
The piece is held at that temperature until the and
outside temperature are equalized. The part is then reheated and cooled at
moderate
rate. Aus-tempering
produces fine bainite structure in steel but

with minimum distortion and residual stresses. Fig.  shows
the process of aus-tempering
for medium C-steel.
Aus-tempering
is mainly used tempering for aircraft engine
parts.



Aus-Tempering It is a special type of tempering process in which and steel is heated above the transformation range then suddenly quenched in a molten salt bath at a temperature 200 to 450°C. The piece is held at that temperature until the and outside temperature are equalized. The part is then reheated and cooled at moderate rate. Aus-tempering produces fine bainite structure in steel but with minimum distortion and residual stresses. Fig.  shows the process of aus-tempering for medium C-steel. Aus-tempering is mainly used tempering for aircraft engine parts.
Aus tempering and mar tempering process


Advantages of Aus-Tempering

1. Quenching cracks are avoided.
2. Distortion and warping are avoided.
3. A more uniform
microstructure is obtained.
4. Mechanical properties of bainite are
superior to conventional hardening micro- structure.


Limitations of Aus-Tempering

1.      
The process is very costly.
2.      
The process is time consuming.


 Mar Tempering


It is a type of tempering process in which and its base alloys are heated above the transformation range then suddenly quenched
in a molten salt bath at a temperature 80 to 300°C. The piece is held at that
temperature until the and outside temperature are equalized. The part is then
reheated and cooled at moderate rate. Mar-tempering produces martensite in
steel but with minimum distortion and residual stresses. Fig. 8.16 shows the
mar tempering process
for medium
C-steel and its micro structures of given stages. Cold chisels are hardened at
the cutting edge and followed by tempering.
Because these processes increase the hardness of chisel
and increase the cutting ability.


CASE HARDENING

Some times special characteristic are required
in metal such as hard outer surface and soft, tough and more strength oriented
core or inner structure of metal. This can be obtained by casehardening
process.
It is the process of carburization i.e. saturating the surface layer of steel with carbon or some other substance
by which outer case of the object
is hardened where as the core
remains soft. It is applied to very low carbon steel. It is performed for
obtaining hard
and wear resistance
on surface of metal and higher mechanical properties with higher fatigue, strength
and toughness in the core. The following are the case hardening process.
(1)  Carburizing
(2)  Nitriding.
(3)  Cyaniding.
(4)  Induction hardening.
(5)  Flame
hardening

These processes are discussed as under.


Carburizing

Carburizing can be of three types

1. Pack
carburizing
2. Liquid carburizing and
3.  Gas carburizing
The above carburizing processes are discussed as under.



 Pack Carburizing

Metals to be carburized such as
low carbon steel is placed in cast iron or steel boxes containing a rich
material in carbon like charcoal, crushed bones, potassium Ferro-cyanide  or
charred
leather. Such boxes are made of heat resisting
steel
which are then closed and sealed with clay. Long parts to be carburized are kept vertical in -boxes. The boxes are heated to a temperature 900°C to 950°C
according to type of steel for absorbing carbon on the outer surface. The
carbon enters the on the metal to form a solid solution with iron and converts
the outer surface into high carbon steel. Consequently pack hardened steel
pieces have carbon content up to 0.85% in their outer
case. After this treatment, the carburized parts are cooled in boxes. Only plane carbon
steel is carburized in this process for hardening the outer skin and refining
the structure of the core to make it soft and tough. Small gears are case
hardened by this process for which they are enclosed in the cast iron or steel
box containing  a material rich in
carbon, such as small piece of charcoal and then heat to a temperature slightly
above the critical range. Depth of hardness from 0.8-1.6 mm is attained in three
to four hours. The gears are then allowed to cool slowly with-in the box and
then removed.
The second stage
consists of reheating the gears (so obtained) to about 900°C and then quenched
in oil so that its structure is refined, brittleness removed and the core
becomes soft
and tough. The metal is then reheated
to about 700°C
and quenched in water so that outer surface
of
gear, which had been rendered
soft during the preceding operation, is again hardened.


Liquid Carburizing

Liquid carburizing is carried
out in a container filled with a molten salt, such as
sodium cyanide. This bath is heated by electrical immersion
elements or by a gas burner and stirring is done to ensure uniform temperature.
This process gives a thin hardened layer up to 0.08 mm thickness. Parts which
are to be case-hardened are dipped into liquid bath solution containing
calcium
cyanide and polymerized hydro-cyanide acid or sodium or potassium cyanide along-with some salt. Bath
temperature is kept from 815°C to 900°C. The furnace is usually carbon steel
case pot
which may be by fired by
oil, gas or
electrically. If only
selected portions of the components are to be carburized, then the remaining
portions are covered by
copper plating.
There are some advantages of the liquid bath carburizing which are given as under.

Advantages of Liquid Carburizing

1.      
Greater depth of penetration possible in this process.

2.      
Selective carburizing is possible
if needed.

3.      
Uniform heating will occur in this process.

4.      
Little deformation or distortion of articles occur
in this process.

5.      
Ease
of carburizing for a wider range of products.

6.      
It
is time saving
process.

7.      
Parts
leave the bath with a clean and bright finish.

8.      
There
is no scale in this process as occur in pack hardening.



 Gas Carburising

In gas carburizing method, the parts to be gas carburized are surrounded by a hydrocarbon
gas in the furnace. The common carburizing gases are methane, ethane,
propane, butane and carbon monoxide
are used in this process. Carbon containing gas such as carbon monoxide (CO), methane
(CH
4), ethane (C2H6) or town gas is introduced in the furnace
where low 
carbon steel is placed. The furnace is either
gas fired or electrically heated. Average gas carburizing temperature usually
varies from 870° to 950°C. Thickness of case hardened portion up to 11 mm can
be easily obtained in 6 hours. The carburized parts can heat   treated after carburizing. Steel components
are quenched in oil after carburizing and then heated
again to form fine grain sized austenite and then quenched
in water to form martensite in surface layers. This gives
maximum toughness of the core and hardness of the surface      of
product.

Cyaniding

Cyanide
may also be used to case harden the steel. It is used to give a very thin but
hard outer case. Cyaniding
is a case hardening process
in which both C and N
2 in form of cyaniding salt are added to surface of low
and medium carbon steel. Sodium cyanide or potassium cyanide may be used as the
hardening medium. It is a process of superficial case hardening which combines
the absorption of carbon and nitrogen to obtain surface hardness. The components
to be case hardened are immersed in a bath having fused sodium cyanide salts
kept at 800-850°C. The component is
then quenched in bath or water. This method is very much effective
for increasing the fatigue limit of medium and small sized parts such as gears, spindle, shaft etc. Cyanide
hardening has some advantages and disadvantage over carburizing
and nitriding method. Cyaniding process
gives bright finishing
on the product. In it, distortion
can be easily avoided and fatigue limit can be increased. Decarburizing can be
reduced and time taken to complete
the process is less. But the main disadvantage of this process is that  it is costly and highly toxic process in
comparison to other process of case hardening. There are some common applications of cyaniding process
which are given as under.

Application of Cyaniding

Cyaniding is generally applied
to the low carbon steel parts of automobiles (sleeves, brake cam, speed box
gears, drive worm screws, oil pump gears etc), motor cycle parts (gears, shaft,
pins etc.) and agriculture machinery.

Nitriding

Nitriding is a special case hardening process
of saturating the surface of steel with nitrogen by holding it for prolonged
period generally in electric furnace at temperature from 480°C to 650°C in
atmosphere of Ammonia gas (NH3). The nitrogen from the ammonia gas enters into
on the surface of the steel and forms nitrides and  that impart extreme hardness to surface  of the metal. Nitriding is a case hardening
process in which nitrogen instead of carbon is added to the outer skin of the
steel. This process is used for those alloys which are susceptible to the
formation a chemical nitrides. The article to be nitride is placed in a
container (made  of high nickel chromium
steel). Container is having inlet and outlet tubes through which ammonia
gas is circulated. Ammonia gas is used as the nitrogen
producing material. The alloy
steel containing
Cr, Ni, Al, Mo, V and Nitre-alloy are widely used for
this
process. Plain carbon steels
are seldom nitirided. There are some common applications of this process which
are given as
under.

Application of Nitriding 

Many automobile, diesel engines
parts, pumps, shafts, gears, clutches, etc. are treated with
the nitriding process.
This process is used for the parts
which require high wear resistance
at elevated temperatures such as automobile and air plane valve’s and valve parts, piston pins, crankshafts, cylinder
liners etc. It is also used in ball and roller bearing
parts die casting dies, wire drawing dies etc.

Flame Hardening

It consists of moving an oxyacetylene flame,
over the part where hardening is required. Immediately
after this, the heated portion
is quenched by means of water spray or air passing
over it.
Temperature attained by the
surface is controlled and the rate of cooling is controlled by selecting a
suitable medium. Flame hardening is suitable for large sized articles where
only some portions of the surface requiring hardening and hence there is no
need to heat
the whole article in the
furnace. Metal is heated by means of oxy-acetylene flame for a sufficient time
unto hardening range and than quenched by spray of water on it. The hardened
depth can be easily controlled by adjusting and regulating the heating time,
temperature, flame
and water spray.
The main advantages of the process is that a portion of metal can be
hardened  by this process, leaving rest
surface unaffected by confining the flame at relevant part only where hardening
is required. This process is best suited to smal1 numbers of jobs which
requiring short heating time. This method is highly suitable for stationary
type of larger
and bulky jobs.

 Induction Hardening

Induction hardening is accomplished by placing
the part in a high frequency alternating magnetic field. It differs from
surface hardening in the way that hardness of surface is
not due
to the increase in carbon
content but due to rapid heating followed
by controlled quenching.
In this process,
a high frequency current is introduced in the metal surface and its temperature is raised up to hardening range. As this temperature is attained,
the current supply is cut
off instantaneously
water is sprayed on the surface. Heat is generated by the rapid reversals of
polarity. The primary current is carried
by a
water cooled copper tube and is
induced into
the surface layers of
the work piece. Thin walled sections require high frequencies and thicker
sections must require low frequencies for adequate penetration of the
electrical
energy. The heating
effect is due to induced eddy currents and hysteresis losses in the surface material.
Some portion of the metal part is heated above the hardening temperature and is
then quenched to obtain martensite on the metal surface. There are some
advantages of this process
which are given as under.

Advantages of  Induction Hardening

Induction hardening is comparatively
quicker. A minimum distortion or oxidation is encountered because of the short
cycle time. The operation is very fast and comparatively large parts can be
processed in a minimum time. There are some applications of this process which
are given as under.

Application of Induction Hardening

Induction hardening is widely
used for hardening surfaces of crankshafts, cam shafts, gear automobile
components, spline shafts, spindles, brake drums etc. It is also used for
producing hard surfaces on cam, axles, shafts and gears.


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