UNIT IV HEAT TREATMENT

1. HARDENABILITY

2. HARDENABILITY
• Hardenability of steel is defines as that
property which determines the depth and
distribution of hardness induced by
quenching by austenite condition.
• METHOD OF DETERMINING
HARDENABILITY :
– Jominy end quench test

3. JOMINY END QUENCH TEST
• The Jominy end quench test is used to
measure the hardenability of a steel.
• This describes the ability of the steel to be
hardened in depth by quenching.
• steel to partially or completely transform
from austenite to some fraction of
martensite at a given depth below the surface

4. • High hardenability allows slower quenches to be
used (e.g. oil quench), which reduces the distortion
and residual stress.
• The test sample is a cylinder with a length of 102
mm (4 inches) and a diameter of 25.4 mm (1 inch).
Jominy test specimen

5. • This is usually at a temperature of 800 to
900°C.
• The test sample is quickly transferred to the
test machine,
• where it is held vertically and sprayed with
a controlled flow of water onto one end of
the sample.
• This cools the specimen from one end,

7. • The hardness is measured at intervals from the
quenched end. The interval is typically 1.5 mm
for alloy steels and 0.75 mm for carbon steels.
• High hardness occurs where high volume
fractions of martensite
• Lower hardness indicates transformation to
bainite or ferrite/pearlite microstructures

8. CASE HARDENING
(OR)
SURFACE HARDENING
OF STEELS

9. DRAWBACK IN NORMAL
HARDENING
• During normal hardening inner surface become
high brittle material and it cannot withstand shock
loads.
• So we need a special heat treatment process for
hardening the outer surface called case hardening

10. PURPOSE OF CASE HARDENING
• to make wear resistant outer surface
• to obtain an inner surface which can with stand shock
load
• to improve the corrosion resistance
• to improve the thermal resistance
• to improve the life of the material, this made from low
cost material.

11. METHODS OF CASE
HARDENING
1. DIFFUSION METHODS
– Carburising
– Cyaniding
– Carbo Nitriding
– Nitriding
2. THERMAL METHODS
– Induction Hardening
– Flame Hardening

12. CARBURISING
• It’s a process of adding carbon on the
outer surface of the low carbon
material.
METHODS
1. PACK CARBURISING
2. LIQUID CARBURISING
3. GAS CARBURISING

13. PACK CARBURISING

15. Pack carburizing
• .

16. PACK CARBURISING
• In this the work piece is kept in a box.
• The box containing carburising mixture which consist of
charcoal, coke and barium carbonate.
• The box is closed and sealed as air tight with fire clay.
• Now the box is placed in a furnace and heated upto 900 to 950
degree celcius.
• Depend on the hardness level required the workpiece placed
inside the furnace for sufficient time in the process.
• During this CO (Carbon monoxide ) released and it
react with iron on the work piece surface and forms a
hard iron carbide
• 2CO + 3 Fe -------- Fe3C + CO2
• Then the work piece are cooled along with the Box.

17. Dis-advantages of Carburising
• Not suitable for mass production.
• It will take more time for packing , un-packing, heating
and cooling.
• Its difficult to control the depth of case to be hardened.

18. LIQUID CARBURISING
Here the work pieces are immersed in a liquid salt bath
kept at a temperature of about 900`c.
The salt bath contains up to 20% sodium cyanide, sodium
carbonate and barium chloride are also added
The work pieces are immersed in the salt bath from 5 minutes
to 1 hour according to the required depth of hardened
surface.
In this process the work pieces absorb large amount of
carbon and a small amount of nitrogen
carbon monoxide is released due to the reaction of sodium
cyanide with oxygen. this reacts with iron on the work piece
surface and forms iron carbide.
2NaCN+2O2Na2 CO3 +2N+CO
2CO+FeFe3C+CO2

19. .
Advantages:
1.It is suitable for mass production.
2.Hardening can be done up to 7mm depth
3.Uniform case depth can be obtained
4.It is a fast process
5.No Oxidation takes place
6. It is suitable for work pieces of various shapes and sizes.
Disadvantages:
1.It is costly process
2.Cyanide used in this various shapes.
3.Explosion may occurs if the work pieces not properly dried before
immersing in sodium cyanide solution
4.It is necessary to wash the work pieces thoroughly after carburizing.

20. GAS CARBURISING
• Here the work piece is kept in an air tight furnace and heated to a
temp of 925 0
C .
• Gases containing high carbon such as methane or butane is
mixed with air and passed inside the furnace.
• The work piece kept in the same temperature for 3 – 4 hrs.
according to the req. depth of case to be hardened.
• CO (carbon monoxide) released from the process and it react
with the iron on the work piece surface and forms IRON CARBIDE

21. Gas carburizing
• .

22. APPLICATION –
GAS CARBURIZING
• used to surface hardening of small and medium
sized components such as gear wheels and
automobile parts

23. .
ADVANTAGES
• Suitable for mass production
• We can easily control the case depth
• Its fast and economic process.
• No distortion occurs.

24. NITRIDING

25. • NITRIDING – is a process of producing
nitrogen atoms in the surface of the steels
• Nitrogen is absorbing by the surface of the
steel then harden the surface.
• ATMOSPHERIC NITROGEN is not
suitable for the process

26. NITRIDING

27. CYANIDING
• Its a process of hardening the surface of
low carbon and alloy steels by adding
carbon and nitrogen.

28. CYANIDING
• .

29. • The work piece immersed in a sodium
cyanide solution kept at a temperature
8000
C - 9000
C.
• 30 Minutes – 3 hours immersed according to the
required depth case to be hardened.
• Then its taken out and quenched in water or oil
• Carbon and nitrogen released from sodium
cyanide at the high temperature.
• These react with iron on the work piece surface
and form hard carbide nitride case.
• A hardened case depth of 0.1 – 0.5 mm can be
obtained by cyaniding.
• Its possible to obtain hardness up to 65 Rockwell
hardness.

30. Applications of cyaniding
• Suitable for making hardness to less depth
in small components
• This process done for the components
used in automobiles such as small gears,
bolts , nuts, screws etc.

31. ADVANTAGES
• Fast process
• Good surface finish obtained
• Uniform case depth formed
• Less distortion occurs
• Fatigue strength increased.

32. CARBONITRIDING
• Here carbon and nitrogen are added on
the work piece surface by passing a
gas containing carbon and nitrogen.

33. process
• Work piece kept in a furnace and
heated to a temperature 8000
C - 9000
C.
• Natural gas containing ammonia and hydro
carbon is passed inside the furnace
• Carbon and nitrogen released from this gas
at high temp. and it react with iron on the work
piece surface and form iron carbide case.
• The w/p is kept inside the furnace for 20 min .
For obtaining req. depth of case to be hardened.
• 0.5mm case hardness obtained…..(65 Rc)

35. Application
• Small gears
• Bearings
• Valve seats
• Air craft parts
• Guide ways are hardened

36. Advantages
• Wear and corrosion resistance increased.
• Good fatigue resistance obtained.
• Complicated shape components also hardened.
Dis-advantages
• More time consuming process
• Costly process
• Not suitable for plain carbon steels

37. FLAME HARDENING
• It’s a process of hardening the surface
of the work piece by heating using oxy
– acetylene flame.
• Steels containing 0.3– 0.6 % carbon can
be hardened by this method

38. FLAME HARDENING
• .

40. process
• The work piece is heated rapidly above critical
temperature, then its quenched by water
immediately.
• A specially designed torch used….which has nozzle for
producing flames and orifices for spraying water.
• Torch is moved over the surface to be hardened and
heated.
• The water is sprayed through the orifice and the surface
is cooled immediately.
• The depth of hardened case depends upon the speed at
which the torch is moved.
• A hardened case depth of 3mm – 6mm can be obtained
by this method.

41. Application
• Machine guide ways
• Gears
• Shafts
• Mill rolls
• Piston pins
• Cams are hardened by this process

42. ADVANTAGES
• Simple and economical process
• Easy to carry the entire set up to usage area
• Suitable for hardening large size machine parts
• Distortion and scale formation is less
DISADVANTAGS
• Overheating may cause cracks

43. INDUCTION HARDENING
• By electrical induction heat is produced
and this heat is used for surface hardening
process.

44. INDUCTION HARDENING
• .

45. process
• Work piece is kept between two induction coils made
up of copper tubes.
• Orifices are provided for spraying of water for cooling
the work piece.
• When high frequency alternating current is passed
through the coils, magnetic field will set up around the
coils.
• This magnetic field induces high frequency eddy
current on the work piece surface.
• High heat is generated on the work piece surface due
to the resistance to flow of current from the work piece.

46. .
• Depend on current frequency the hardness depth
will varied.
• Can be hardened up to 3mm depth ----- 60 Rc

47. Application
• Suitable for medium carbon steels.
• Piston rod
• Cam shaft
• Cylinder liners hardened by this
process
• Bearings
• gears

48. Advantages
• Fast process
• Suitable for mass production
• Possible to obtain case with high quality
• Depth hardening can be easily controlled
• No distortion will be occurred
• No scale formation
• Internal surface of holes can also be hardened
Disadvantages
• Costly process
• Low carbon steels cannot be hardened
• Difficult to harden complicated shapes
• High maintenance cost
• Not suitable for less number of components