The document discusses the hardening process used to increase the hardness of metals. Hardening involves heating metals to their hardening temperature, holding at that temperature, and then rapidly cooling via quenching. This rapid cooling results in the formation of martensite, giving the metal a high hardness. The main purposes of hardening are to improve wear resistance, tensile strength, and yield strength. Factors like chemical composition, size and shape of the metal, and the quenching medium used affect how effective the hardening process is. Various hardening methods are discussed, like conventional quenching, quenching in stages, spray quenching, and quenching with self-tempering. Hardened metals find

1. SUBJECT:- MATERIAL SCIENCE & METALLURGY
TOPIC:- HARDENING PROCESS
INSTITUTE OF TECHNOLOGY &
MANAGEMENT
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2. WHAT IS HARD OR HARDNESS???
• The ability of a material to withstand
scratching, wear & penetration but harder
bodies is known as “HARDNESS”.
• HARDNESS is dependent on ductility, plasticity,
strain, strength & viscosity.
• HARDNESS is measured by determining the
resistance to penetration by different methods
such as “ROCKWELL HARDNESS TEST”.
• Some e.g. of hardness are Ceramics, concrete,
certain metals & super hard materials.http://alltypeim.blogspot.in/

3. HARDENING
• High hardness values can be obtained but the
process of “HARDENING”.
• Hardening consists of “heating to hardening
temperature, holding at that temperature”,
followed by “RAPID COOLING” such as
quenching in water, oil or salt bath.
• High hardness is obtained due to phase
transformation at considerably low
temperature in non equilibrium phase
namely “Martensite”.
• It is also called as “Hardening by quenching”.
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4. PURPOSES OF HARDENING
• Main purpose to develop “High Hardness”
• To improve wear resistance for tool steel
• To improve Tensile strength for structural
steel
• Improve Yield strength spring steel
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5. HARDENING TEMPERATURES
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6. FACTORS AFFECTING HARDENING
PROCESS
• The properties of hardened steel depends
upon the following factors :-
(a) Chemical composition
(b) Size & shape of steel part
(c) Hardening cycle
(d) Homogeneity & grain size of Austenite
(e) Quenching media
(f) Surface condition of steel part
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7. Chemical composition :- It determines
hardening temperature since it decides critical
temperatures. Alloying elements, their nature
& amount are very important. The Austenite
stabilizer results in high amount of RA. Carbide
former improves wear resistance of steel.
Size & shape of steel part :- Larger parts with
thickness heated slowly. Holding time must be
sufficient to ensure uniform temperature
throughout the specimen. This results in
production of high magnitude of internal
stresses due to volume change.http://alltypeim.blogspot.in/

8. Hardening cycle :- Hardening temperature
should be such that homogeneous austenite
with minimum grain size is produced. The
cooling rate must be just equal to CCR. Slow
heating rate are to be avoided in case where
the problem of oxidation & decarburization is
encountered.
Homogeneity & grain size of austenite :- The
properties of steel largely depends upon the
nature of austenite since martensite directly
formed from austenite. Coarse grains are not
desirable.
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9. Quenching media :- Quenching media must
provides the cooling rate higher than CCR. The
quenchant properties such as thermal
conductivity, latent heat of vapourization,
specific heat, its temperature affects cooling
rate.
Surface condition of steel part :- presence of
oil, greases, scales & other particles on steel
surface may leads to incomplete hardening.
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10. HARDENING OR QUENCHING
METHODS
Hardening can be done in many ways,
depending upon quenching procedures. The
different methods are :-
A. Conventional quenching
B. Quenching in stages in sequence
C. Spray quenching
D. Quenching with self tempering
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11. Conventional or direct quenching :-
Simplest & extensively used method.
The component is quenched from hardening
temperature in quenching bath & allowed too cool
up to temperature of quenching bath. Sever
internal stresses, distortion & cracking are
developed due to drastic cooling. Suitable for only
smaller sections & simple sections.
Quenching in stages :-
It consists of quenching the steel
parts from hardening temperature to a bath
maintain at a predetermined temperature higher
than Ms temperature. The is then quickly transfer
to a milder quenchan to cool down to room
temperature.
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12. Quenching with self-tempering :-
Here the part is quenched from hardening
temperature & is withdrawn from quenching bath
after some time without allowing it cools
completely in the quenching medium. i.e
considerable amount of heat will retained in the
central portion. Then component is cooled to room
temperature in a mild coolant like oil or air, first
cooling forms martensite up to certain extent and
second cooling undergo austenite to pearlite
transformation and tempering of martensite
formed in first stage. Process result in tough core
& hard surface. Additional tempering is not
required.
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13. Spray quenching :-
The cooling from hardening temperature
is done by spraying quenchant contiuously on
steel part. The heat extraction rate is higher
than direct quenching as quenchant is always
in contact with the steel surface. There is no
formation of vapour film over the steel surface.
Suitable for selective hardening.
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14. APPLICATIONS
• Construction materials – High strength reduces the
need for the material thickness which generally saves
weight& cost.
• Machine cutting tools (drill bits, taps, laths tools)
needs be much harder.
• Knife blades – a high hardness blade keeps a sharp
edge.
• Bearings – necessary to have a very hard surface that
will withstand continued stresses.
• Armor plating – High strength is extremely important
both for bullet proof plates & for heavy duty
containers for mining & construction.
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15. THANK YOU
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