Chapter 6 Mechanical Engineering Materials 22343

Program: Diploma(Mechanical)
Class: SYME
Course: Mechanical Engineering Materials(22343)
Unit 06: Heat Treatment Processes
Lecture 15: Annealing , Normalizing , Hardening , Tempering

1. Name of the Trainer :- Prof. S. B. Deshmukh
2. Years of Experience :- 8 Years
3. Domain Expertise :- Mechanical Engineering
www.sandipuniversity.edu.in
Presented By 02
https://www.sandipfoundation.org/
Department Of Mechanical Engineering,Sandip Polytechnic,Nashik

Unit 6 Heat Treatment Processes 03
Topic to be covered
6.1 Annealing: Purposes of annealing, Annealing temperature range, Types and
applications
6.2 Normalizing: Purposes of Normalizing, Temperature range, Broad applications of
Normalizing
6.3 Hardening: Purposes of hardening, Hardening temperature range ,application
6.4 Tempering: Purpose of tempering, Types of tempering and its applications
6.5 Case hardening methods like Carburizing, Nitriding, and Cyaniding.
6.6 Heat treatment Furnaces – Muffle , Box type

4
Heat treatment
Introduction :
Heat treatment is the controlled heating and cooling of metals, to alter their physical
and mechanical properties without changing the product shape.
Heat treatment is often associated with change in the nature, form, size, and
distribution of the micro-constituents.
Thus, it is a very enabling manufacturing process that can not only help other
manufacturing processes, but can also improve product performance by increasing
strength or other desirable characteristics.
The various heat treatment processes are :
Annealing (full, process and spheroidising annealing)
Normalising
Hardening (case hardening & surface hardening)
Tempering (austempering & martempering)

5
Heat treatment

6
Heat treatment
Definition of heat treatment process :
Heat treatment is a series of operations involving the heating and cooling of a metal or
alloy in the solid state, for the purpose of obtaining certain desirable
characteristics.
The rate of heating and cooling determines the crystalline structure of the
material.
In general, ferrous metals (metals with iron bases) and nonferrous metals, as well as
their alloys, respond to some form of heat treatment.
Almost all metals have a critical temperature at which the grain structure changes.

7
Heat treatment
The purpose of heat treatment :
To refine grain structure.
To improve machinability.
To relieve internal stresses.
To increase strength and wear resistance.
To increase hardness and toughness of
metal surface.
To improve mechanical properties
like tensile strength, ductility etc.
To increase corrosion resistance.
To improve magnetic and electrical properties.
To stabilize the microstructure, to minimize
growth.

8
Heat treatment
Principles of heat treatment :
The results that may be obtained by heat treatment depend, to a great extent, on the
structure of the metal and the manner in which the structure changes
when the metal is heated and cooled.
In this operation, the cooling rate plays an important role on which the structural
modification is mainly based.
A pure metal cannot be hardened by heat treatment because there is little change in
its structure when heated.
On the other hand, most alloys respond to heat treatment because their structures
change with heating and cooling.

9
Heat treatment - Annealing
Annealing :
Annealing involves the steady heating of a metal at a certain temperature above the
recrystallisation phase, followed by a gradual cooling (slow cooling) process.
Annealing process that heats the metal below the austenite phase to restore ductility
after cold working.
Types of annealing processes :The various types of annealing processes are
a. Full annealing
b. Process annealing
c. Spheroidising annealing
Objective / Purpose of the annealing :
1. Relieve internal stresses.
2. Improve machinability.
3. Reducing hardness.
4. Refines and remove structural in homogeneity

10
Objective / Purpose of the annealing :
5. Removing trapped gases during casting process.
6. Improve mechanical, physical and magnetic properties.
7. Produce the desired structure.
8. Prepare steel for further treatment.
a. Full annealing :
Full annealing is accomplished by heating a hypoeutectoid steel to a temperature
above the upper critical temperature.
In practice, the steel is heated to about 50-75 ºC above the upper critical
temperature.
It is then cooled in the furnace very slowly to room temperature.
The formation of austenite, destroys all structures that have existed before heating
and produce a coarse structure.
Slow cooling yields the original phases of ferrite and pearlite

11
a. Full annealing :
The hardness of the steel greatly reduced where as there is increase in ductility,
improves formability, machinablility,mechanical and magnetic properties.

12
b. Process annealing (Subcritical annealing) :
The process annealing comes under the
category of subcritical annealing.
In this process the steel is heated to a
temperature little below critical range and then
cooled slowly.
This process is usually carried out to remove
internal stress which is produced during cold
working.
Upon the extent of cold working, grain size,
composition and time held at heat.
This process is useful in the sheet and wire
drawing industries.

13
C. Spheroidise annealing :
This is a form of annealing in which cementite in the
granular form, is produced in the structure of steel.
This process causes the agglomeration of all
carbides in the steel in the form of small globules or
spheroids.
This process is usually applied to high carbon steels
which are difficult to machine.
The process consists of heating the steel slightly
above the lower critical point (730 to 770 ºC), holding
at this temperature and then cooling slowly to a
temperature below lower critical point.

14
C. Spheroidise annealing :
The rate of cooling in the
furnace is from 25 to 30 degree
per hour.
Another method is to use, a
high temperature isothermal
transformation of the austenite.

15

16
Heat treatment - Normalizing
Normalizing :
A heat treatment process that has the object of relieving internal
stresses, refining the grain size and improving the mechanical
properties.
The steel is heated to the 40 50 ºC above the recrystallisation
temperature according to analysis, held at the temperature for a
short duration and subsequently cooled in still air at room
temperature.
This heat treatment process is also called air quenching.
This process is used in operations like hot rolling and forgings.
Microstructure produced by normalizing, consists of ferrite and
pearlite for hypoeutectoid steel and pearlite and cementite for
hypereutectoid steel.
This structure consists of sorbitate and ferrite.

17
Heat treatment - Normalizing
The purpose of normalizing :
1. The purpose of normalizing is to remove
the internal stresses induced by heat
treating, welding, casting, forging, forming
or machining.
2. It can produce uniform microstructure.
3. To obtain desired microstructure.
4. To improve mechanical properties of
steel, that is strength and toughness.
5. To increase hardness and reduce ductility
gives a better surface finish.
6. The process is less expensive than
annealing.

18
Difference between Annealing and Normalizing
Point Annealing Normalising
Definition The steady heating of a metal at a
certain temperature above the
recrystallisation phase, followed by a
gradual cooling process.
The steel is heated to 40-50 OC above
recrystallisation temperature, according to
analysis, held at temperature for a short
duration and subsequently cooled in still air
at room temperature.
Cooling In furnace At room temperature
Microstruct-
-ure
Coarse pearlite with coarse grain size. Fine pearlite with fine grain size.
Hardening Process
Hardness
Reduced hardness, tensile strength &
toughness
Improved hardness, tensile strength &
toughness
Ductility More ductile Less ductile
Economy Costly and inconvenient process Economical & more convenient process.
Applications Steel, castings, rolled components
etc.
Large size castings, forgings.

19
Hardening
Hardening
Hardening involves heating a steel above the critical temperature and cooling
(quenching) rapidly in suitable fluid. e.g oil, water or molten salt bath (Brian solution).
When the steel is heated to its hardening temperature, it becomes austenitic. When
cooled quickly, the equilibration transformations, into pearlite and ferrite or pearlite and
cementite, do not have time to take place.
Hardening is applied to all tools and machine parts made from carbon steel and alloy
steels.
The procedure is carried out in three steps :
1. Heating the work to a temperature above critical point.
2. Holding the work at that point for a definite point.
3. Cooled rapidly (quenching) in a suitable medium (oil, water, salt bath etc.)

20
Hardening
Purpose of hardening :
1. To develop high hardness to resist wears.
2. To improve strength, elasticity, ductility and
toughness.
The degree of hardness produced in a steel
depends upon the following :
1. Adequate carbon percentage to produce
hardening.
2. Nature of quenching medium.
3. Size of the austenite grains.
4. Heating rate and time.
5. Size of workpiece.
6. Quenching rate.
7. Surface condition of workpiece.

21
Hardening
Hardening may cause some defects due to
uneven or prolonged heating or improper
cooling. Some of them are :
1. Uneven hardness due to uneven and
prolonged heating.
2. Insufficient hardness due to low hardening
temperature or too fast, uneven heating.
3. Change in dimension due to heating and
cooling.
4. Fracture during cooling due to uneven
heating or too fast cooling.
5. Oxidation and decarburization due to high
temperature.

22

23
Summary
In this lesson, We have learned
•Annealing: Purposes of annealing, Annealing temperature range, Types and
applications
•Normalizing: Purposes of Normalizing, Temperature range, Broad applications of
Normalizing
•Hardening: Purposes of hardening, Hardening temperature range ,application

24

Class: SYME
Lecture 16: Tempering-Purpose, Types ,applications
Case hardening methods

Topic to be covered
•Tempering: Purpose of tempering, Types of tempering and its applications
•Case hardening methods

28
Tempering
Tempering
Tempering is a heat treatment technique for
metals, alloys and glass.
In steels, tempering is done to "toughen" the metal by
transforming brittle martensite or bainite into a combination of
ferrite and cementite or sometimes tempered martensite.
The brittle martensite becomes tough and ductile after it is
tempered.
Tempering is used for previously quenched material which is
brittle and possesses hardening stresses.
Tempering releases the stresses and reduces the brittleness.
Tempering is accomplished by a controlled reheating of the work
piece to a temperature below its lower critical temperature.

29
Tempering
Purpose / Benefits of tempering :
Reduce internal stresses.
To stabilize structure
Improve ductility and toughness.
Reduce cracking.
Improve machinability.
Increase impact resistance.
Improve malleability.
Decrease hardness.

30
Tempering
Necessity of tempering :
The structure of steel obtain hardening is not suitable directly for engineering
applications due to following reasons :
1. Structure obtained after hardening is extremely hard brittle and highly stressed which
may cause failure by cracking and distortion.
2. Due to sudden cooling high internal stresses develop in components which may
results in cracking of part during service for this purpose tempering is followed by
hardening.

31
Tempering
Stages of tempering :
1.In stage one, heating a hardened steel up to a temperature of 200 ºC relieves the
hardening stresses only.
2.In stage two, it is further heated to above 300 ºC, causes any austenite that was
retained by the steel after quenching, to decompose into ferrite and cementite. Some
softening accompanies this transformation.
3.During third stage, tempering to about 400 ºC causes the epsilon carbide to transform
to cementite and ferrite. This portion of the tempering causes significant softening.
4.In the last and final stage that is 400 ºC to 700 ºC the structure become an aggregate
of ferrite with cementite in quite fine spheres, referred to as tempered martensite and
tempered bainite.

32
Tempering
Tempering is classified into three types :
a. Low temperature tempering.
b. Medium temperature tempering.
c. High temperature tempering.
a. Low temperature tempering:
This is done in the range of 150 ºC to 250 ºC purpose is to reduce the internal
stresses, increase ductility and toughness without any change in hardness.
Low temperature tempering is applied in the heat treatment of carbon and alloy steel
cutting tools.
b. Medium temperature tempering :
This method requires heating of work to 350-450 ºC.
It is used to reduce the hardness and strength of the metal and an increase in
elongation and ductility.

33
Tempering
b. Medium temperature tempering :
Martensite and any austenite decompose into ferrite which is precipitated as
extremely fine particles of cementite.
It is mainly used for parts subjected to impact load.
e.g. chisels, hammers, springs, spring plate etc
c. High temperature tempering :
It is done in the range of 500-650 ºC.
At this temperature, sorbite is formed in the steel and the internal stresses are almost
completely eliminated and provides favorable ratio of strength to toughness for
structural steels.
It is mainly used for parts subjected to high stresses and impacts.
e.g. gear, wheels, shafts, connecting rods, etc.

34
Martempering (Stepped Quenching)
Martempering (Stepped Quenching) :-
This is a hardening method that produces martensite.
It is also known as stepped quenching.
In this method steel is heated to the hardening temperature then it is quenched in the
medium having a temperature from 150-300 ºC.
The work is held until it reaches the temperature of medium and then it is
cooled further to room temperature in air or oil.
The holding time in quenching medium or bath, should be sufficient to get uniform
temperature to be reached through the cross section but not enough to
cause austenite decomposition.
Austenite is transformed into martensite, during the subsequent period of cooling to
room

35
Martempering (Stepped Quenching)
Advantages of martempering :
1. Less danger of quenching cracks
in the work.
2. Less volume charge occurs due
to the presence of a large amount
of retained austenite and
possibility of self tempering of the
martensite.
3. Less wrapping since the
transformations occur
simultaneously in all parts of the
articles.

36
Austempering (Isothermal quenching)
Austempering is not a hardening treatment.
It is performed in the same manner principally
as martempering but with a longer quenching
time in salt bath.
Another difference in this method is that, the
salt bath temperature for austempering is
above the martensite point, to ensure
sufficiently complete austenite decompositions
into bainite (circular troosite).
After which it is allowed to cool in room
temperature.

37
Advantages of Austempering
(Isothermal quenching)
1. Bainitic structures
produced are free from
cracks.
2. Softer than martensite.
3. Good impact resistance.
4. Greater ductility and
toughness.

38
Difference
Differentiate
between
austempering
and
martempering
Martempering Austempering
Martempering is a
hardening treatment
Austempering is not a
hardening treatment
It gives higher hardness It gives less hardness
Martempering gives
martensite product.
Austempering gives bainite
product.
Low ductility and toughness Grater ductility and toughness
More cracks on quenching Less cracks on quenching
Distortion is more Distortion is less
Tempering is needed Tempering is not needed
Cycle time is less Cycle time is high

39
Hardening
Surface Hardening
This is also one of the methods of heat treatments, in which the surface layers of a
metal are hardened to a certain depth while the core is maintained relatively
soft.
Low carbon steel are tough hence cannot be hardened.
High carbon steel can be hardened but are not so tough. However, both these
requirement may be met, by employing low carbon steel with suitable core properties
and then adding carbon, nitrogen or both, to the surface of the steel object in order to
provide a hardened case /layer to a definite depth.
This heat treatment process is known as 'case hardening'.

40
Hardening
Case hardening
Case hardening is also known as a process of chemical heat treatment, in which the
saturation of the surface of low carbon steel takes place with a certain element
(carbon), by diffusion of these element from the surrounding medium at a high
temperature.
The applications of surface hardening are crankshaft, camshaft, gears and bearing
surface etc.
The various surface and case hardening processes are :
1. Flame hardening
2. Induction hardening
3. Carburising
4. Nitriding
5. Cyaniding
6. Carbonitriding

41
Hardening
Purpose of case hardening :
1. To obtain a hard and wear resistance surface on machine parts with enrichment of
carbon up to 0.75 to 1.25 %.
2. To obtain tough core.
3. To obtain close tolerance in machine parts.
4. To obtain a higher fatigue and high mechanical properties in the core

42
Hardening
Differentiate
between case
hardening and
surface
hardening :
Surface hardening Case hardening
It is done by rapid heating and
cooling of surface (Quenching)
It is done by changing
chemical composition of the
surface
Low carbon steel cannot be
surface hardened
Low carbon steel can be case
hardened
No core obtained To obtain tough core.
Surface hardening is done after
normalising
For case hardening
normalising is not necessary
Surface hardening processes are:
Flame hardening, induction
Case hardening processes are:
Carburising, nitriding,

43
Differentiate
between case
hardening and
surface
hardening :

44
Case hardening processes
Flame hardening
Flame hardening is the process that is used to harden the surface of metal parts.
When you use an oxyacetylene flame, a thin layer at the surface of the part is rapidly
heated to its critical temperature and then immediately quenched by a combination of
a water spray and the cold base metal.
This process produces a thin, hardened surface, and at the same time, the internal
parts retain their original properties.
Whether the process is manual or mechanical, a close watch must be maintained,
since the torches heat the metal rapidly and the temperatures are usually determined
visually.
It is usually applied to medium to large size components such as large gears,
sprockets, slide ways of machine tools, bearing surfaces of shafts and axles,
etc. Steels most suited have carbon content within the range 0.40-0.55 %.

45
Advantages of flame hardening :
1. It is a fast process.
2. There is less distortion of surface.
3. Hardening is restricted to parts which are
affected by wear.
4. It is economical and useful method.
5. Large parts can be surface hardened
economically.

46
Disadvantages of flame hardening :
1. Danger of overheating.
2. Uncontrollable hardness produced at
different locations due to uncontrolled
temperature.
3. It is difficult to produced hardened zone
less than 1.5 mm in depth.

47
Induction hardening :
Definition :
A widely used process for the surface hardening of steel.
The components are heated by an alternating magnetic
field to a temperature within or above the transformation
range, followed by immediate quenching.
The core of the component remains unaffected by the treatment and its
physical properties are those of the bar from which it was machined.
Whilst the hardness of the case can be within the range 37/58 Rc.
Carbon and alloy steels with carbon content in the range 0.40 to 0.45 % are
most suitable for this process. It is used to harden the
It is used to harden the teeth of gears, shafts, crankshafts, camshafts,
camsand pulleys etc.

48
Advantages of Induction hardening :
1. Time required for this heat treatment operation is
less, thereby increasing labour productivity.
2. No scaling or surface oxidation.
3. Deformation due to heat treatment is
considerably reduced.
4. Permit automation of heat treatment processes.
5. Closer tolerances on dimension are maintained.
6. Induction hardened steel have high hardness,
high wear resistance, higher impact strength and
higher fatigue limits in comparison with ordinary
steels.
7. Can be applied to both internal and external
surfaces.

49
Disadvantages of Induction hardening : -
1. Cost of equipment is high.
2. Restricted to medium carbon and alloy steel only.
3. Not economical for mass production.
Applications of Induction hardening : -
1. Piston rod
2. Crankshaft
3. Camshaft
4. Spur gears
5. Automobile parts etc

50
Differentiate
between
flame
hardening
and
inducting
hardening :
Flame hardening Inducting hardening
Components are heated by
oxyacetelylene flame
Components are heated by an
induction coil
As the process is manual, case
depth controlling is not possible
Case depth can be controlled easily by
selecting proper frequency
Skilled operator is required
Unskilled operator can do this
operations
Economical for small components Economical for mass production
Used for external surfaces
External as well as internal surface can
be hardened
No power supply required High electric current required
Slow process Fast and quick process
Overheating can damage the
components
No overheating
Equipment is portable Equipment is not portable

51
Summary
•Tempering: Purpose of tempering, Types of tempering and its applications
•Case hardening methods

52

Class: SYME
Lecture 17: Case hardening methods,
Heat treatment Furnaces

Topic to be covered
•Case hardening methods like Carburizing, Nitriding, and Cyaniding.
•Heat treatment Furnaces – Muffle , Box type

56
Nitriding : -
Nitriding is a process of producing hard surface layer
on alloy steels only.
Nitriding consist essentially of heating the steel in an
atmosphere of ammonia gas at temperature of 500 to
650 ºC without further heat treatment.
The ammonia is dissolved and the nascent nitrogen is
combines with element in the steel to form nitrides.
These nitrides give extreme hardness to the surface.
A hard surface layer usually from 0.2 to 0.4 mm in
depth is produced in 50 hrs.
It is the last operation after shaping and heat
treatment.

57
Advantages of Nitriding : -
1. Very high surface hardness may be obtained.
2. Distortion and cracks are minimized as quenching is illuminated.
3. High wear resistance.
4. Nitride parts retain hardness up to 500 ºC
5. Economical for mass production.
6. Better process than carburizing.
7. No machining is required after nitriding.
Disadvantages of Nitriding
1. Long cycle time, up to 100 hrs for 0.038 mm depth.
2. Medium used is explosive.
3. Only special alloy steel (containing Al, Cr and V) can be treated.
4. Oxidation due to prolonged heating occurs.
5. Technical control required.

58

59
Carburizing : -
Carburizing is another surface hardening process caused by
changing the composition of surface layers, known as carburising.
The process is normally carried out on steel containing less than
0.2 % of carbon.
To have adequate hardness the component has a carbon content
of 1.0 % approx.
In this carburising process the carbon is added to the surface
layers upto 0.7 to 0.9 % by a carefully regulated depth which is
followed by quenching process, to convert the surface layer of
hard martensite.
This process is particularly used for gears, shafts and bearings
etc.

60
Methods of carburising :There are three methods of carburizing, namely
a. Pack carburising
b. Gas carburising
c. Liquid (Salt bath) carburising
a. Pack carburizing :
In peak carburising the component to be carburised, is heated to a temperature
between 8501060 ºC for up to five hours in a sealed metal box containing charcoals and
barium carbonate (BaBO3).
Oxygen present in the box reacts with the carbon to produce carbon monoxide (Co).
When carburising is complete, it is allowed to cool down.
The components are then cleaned and ready for subsequent heat treatment.
It is generally used for components where thick layer has to be hardened.

61
Pack carburizing :

62
Advantages of Pack carburizing :
1. Rapid heat transfer hence process is quick
2. Less distortion to parts
3. Verities of shape and sized components can be handled in single bath.
4. Parts can be directly quenched in bath after carburising.
Disadvantages of Pack carburizing :
1. Carburising salts are poisonous.
2. Needs proper arrangements for disposal of salt bath fumes.
3. Complex parts salt cleaning is difficult.
Applications of Pack carburizing :
1. Bearing recesses
2. Pinion gear camshaft

63
b. Gas carburizing :
In gas carburizing, the components are heated to a temperature between 850 to 1000
ºC in the carbon rich natural gas (methane, propane or butane gas) atmosphere.
Carbon diffuses into surface austenitic layer.
Gas carburising is used for the mass production of components upto 1 mm deep.
Advantages of Gas carburizing :
1. Case depth can be obtained accurately.
2. Fast process than peak carburising.
3. The process can be automated.
4. Less floor space is required than peak carburising.
5. Skilled labour is required.

64
Disadvantages of Gas carburizing :-
1. Higher skilled workers required as compare to pack carburising
Application of Gas carburizing :-
1. Used in mass production, batch or continuous
productions.

65
C. Liquid carburizing :
This process is mostly used for producing shallow case depths in thin sections.
The components are heated quickly in a bath containing a suitable sodium cyanide salt
and sodium carbonate.
The proportion of NaCN being maintained 20 % to 30 % by controlled
feed strong NaCN.
The normal case, depths for this process are about 0.25 mm with bath strengths of 20
% to 30 % NaCN.
High bath strengths 40 % to 50 % NaCN are required for case depths of 0.5 mm.
The case resulting from this process includes carbon and nitrogen.
The nitrogen does provide a hard surface but can also encourage retained undesirable
austenite in the surface layer.

66
C. Liquid carburizing :
This process normally works with bath temperatures of 800-950 ºC, for immersion
times from 2 to 7 hours, depending on the depth required.
For thicker case depths (up to 1.6 mm) activated salt baths are used.
These are based on cyanide and alkaline earth chlorides which act as the activators.
Advantages of Liquid carburizing :
1. Uniform heating.
2. Less distortion of work.
3. Possibility for direct quenching from the bath.
4. Heating is faster and uniform than peak carburising.
5. Time required for heat treatment is reduced.
6. Suitable for small and medium size parts.

67
Advantages of Liquid carburizing :
1. Use of poisonous salt so it requires careful
operations in the presence of fumes.
2. Risk of explosions.
Applications of Liquid carburizing :
1. Suitable for mass production of thin cases in
small and medium size components.

68
hardening processes

69
Heat Treatment Furnace : Muff, Box Type
Heat treatment furnace :
What is a furnace ?
A furnace is essentially a thermal enclosure and is
employed to process raw materials at high
temperatures both in solid state and liquid state.
Several industries like iron and steel making, non ferrous
metals production, glass making, manufacturing, ceramic processing,
calcinations in cement production etc. employ furnace.
Principle objectives of furnace
a) To utilize heat efficiently so that losses are minimum
b) To handle the different phases (solid, liquid or gaseous) moving at
different velocities for different times and temperatures such that
erosion and corrosion of the refractory are minimum.

70

71
Box type furnace :
This box type furnace and muffle type furnace is highly efficient and very easy to
operate.
It has a cubical construction and can be used upto 1100 ºC.
This furnace can heat treat processes requiring heating upto this temperature.
Widely used for hardening, annealing, tempering, age hardening of ferrous and non
ferrous metals and alloys, our products is used in the following applications such as :
1. Tempering
2. Annealing
3. Hardening
4. Stress relieving
5. Normalising
6. Heating

72
Key benefits of Box type furnace :
Temperatures up to 1200 Deg C
High efficiency
Low running cost
Easy to operate
Available in various sizes.

73
Muff or muffle furnace :
A muffle furnace is a type of furnace that is used to heat a material while keeping it
isolated from contaminants.
A muffle furnace is rectangular in shape and is loaded in the front.
They are very heavy insulated which helps to reduce the amount of heat that escapes.
This promotes long furnace run times at temperatures up to about 1800 degrees
Celsius.
Muffle furnaces first got their name back when high temperature electric heating was
not available. Back then, the heat was created by combustion.
The byproducts of combustion could negatively affect the material being heated.
It was thus deemed necessary to separate the combustion source and the material
being heated.

74
Muff or muffle furnace :
This "muffling" allowed for ideal heating
conditions that reduced the risk of
Contamination
Muffle furnaces are used today in a variety
of applications.
They are often used in lab settings to
conduct experiments since the possibility of
material contamination is low.
Muffle furnaces are also used in joining
applications such as brazing or soldering.
The high temperatures that a muffle
furnace can reach also make them a good
choice melting glass.

75
Heat Treatment Furnace : Muff

76
Summary
Case hardening methods like Carburizing, Nitriding, and Cyaniding.
Heat treatment Furnaces – Muffle , Box type

77

Chapter 6 Mechanical Engineering Materials 22343

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