govt of techanology

1. Engineering
Heat Treatments
Treating of materials by
controlling cooling can produce
differences in material properties

2. Annealing
Engineering
• Makes a metal as soft as
possible
• Hypoeutectoid steels
(less than 0.83% carbon)
are heated above upper
critical temp., soaked and
cooled slowly.
• Hypereutecoid (above
0.83%) are heated above
lower critical temp.,
soaked and allowed to
cool slowly.

3. • Process Annealing.
Low carbon steels Engineering
may harden through
cold working. They
can be heated to
around 100 degrees
below lower critical
temp., soaked and
allowed to cool in air.
• Spheroidising. High
carbon steels may be
annealed just below
the lower critical
temp. to improve
machinability.

4. Engineering
• Normalising. Internal stresses caused by
rolling and rolling or forging are removed.
Steels are heated above upper critical
temp., soaked and cooled in air. The
cooling rate is faster than annealing giving
a smaller grain structure.
• Stress relieving. The component is
reheated and held at temperature for a
period of time and cooled slowly.

5. Hardening
Engineering
• Medium and High carbon steels (0.4 –
1.2%) can be heated until red hot and then
quenched in water producing a very hard
and brittle metal. At 723 degrees, the BCC
ferrite changes into Austenite with a FCC
structure.

6. Hardening 0.6% carbon steel
Engineering
• The metal is heated to over
780 degrees, which allows
the carbon to dissolve into
the FCC Austenite.
• Quenching the metal quickly
in water prevents the
structure from changing back
into BCC.
• A different structure, Body
Centre Tectragonal (BCT) is
formed. It is called
Martensite and is extremely
hard and brittle with a

7. Tempering
Engineering
• To remove some of the brittleness from
hardened steels, tempering is used. The
metal is heated to the range of 220-300
degrees and cooled.
• Tempering colours are an indicator of
temperature on polished metals. Colours
range from yellow to brown to violet and
blue.

8. Heat Treatments
Engineering
• A – Normalising
• B – Annealing or
Hardening
• C – Spheroidising or
Process Annealing
• D - Tempering

9. Quenching media
Engineering
• Brine (water and salt solution)
• Water
• Oil
• Air
• Turn off furnace

10. Case hardening
Engineering
• Low carbon steels cannot be hardened by
heating due to the small amounts of
carbon present.
• 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 as carburising.

11. Pack carburising
Engineering
• The component is packed
surrounded by a carbon-rich
compound and placed in the
furnace at 900 degrees.
• Over a period of time carbon
will diffuse into the surface of
the metal.
• The longer left in the furnace,
the greater the depth of hard
carbon skin. Grain refining is
necessary in order to prevent
cracking.

12. • Salt bath carburising. A molten salt bath (sodium
Engineering
cyanide, sodium carbonate and sodium chloride)
has the object immersed at 900 degrees for an
hour giving a thin carbon case when quenched.
• Gas carburising. The object is placed in a sealed
furnace with carbon monoxide allowing for fine
control of the process.
• Nitriding. Nitrides are formed on a metal surface in
a furnace with ammonia gas circulating at 500
degrees over a long period of time (100 hours). It is
used for finished components.

13. Induction hardening
Engineering
• Induced eddy
currents heat the
surface of the steel
very quickly and is
quickly followed by
jets of water to
quench the
component.
• A hard outer layer is
created with a soft
core. The slideways
on a lathe are
induction hardened.

14. Flame hardening
Engineering
• Gas flames raise the
temperature of the
outer surface above
the upper critical
temp. The core will
heat by conduction.
• Water jets quench the
component.

15. Age hardening
Engineering
• Hardening over a period of time
• Also known as precipitation hardening
• Occurs in duraluminium which is an
aluminium alloy that contains 4% copper.
This makes this alloy very useful as it is
light yet reasonably hard and strong, it is
used in the space industry.
• The metal is heated and soaked (solution
treatment) then cooled and left.

16. Pyrometry
Engineering
The measurement and control of
temperature in a furnace is called
pyrometry.

17. Seger cones
Engineering
• A traditional method
of gauging furnace
temperature.
• Cones with known
melting temperatures
are placed in the
furnace, temperature
is identified as cones
collapse.

18. Optical pyrometer
Engineering
• Also known as
‘disappearing
filament’.
• The light intensity of a
lamp, which can be
adjusted, is compared
to the light from a
furnace.
• Temperature is
measured when the
filament seems to
disappear in the glow

19. Thermo-electric pyrometer
Engineering
• A thermocouple uses
the principle that a small
current flows if two
dissimilar metals are
joined in a loop with
different temperatures at
the junctions.
• A galvanometer at the
cold junction detects a
change in current at the
hot junction in the
furnace

20. Case Hardening 1
Engineering

21. Case Hardening 2
Engineering

22. Case Hardening 3
Engineering

23. Case Hardening 4
Engineering

24. Chisel:
- cutting edge is hard and wear-resistant
- tang is tough and elastic
If the chisel would be hard throughout, it could break when theEngineering
hammer is
striked onto it!
Figure - Cut through a hardened chisel - 1 cutting edge (hard), 2 twig (tough)

25. Flame Hardening
Engineering

26. Engineering

27. Engineering