1. SURFACE HEAT TREATMENT
PROCESSES
Prepared By
Dr.S.S.Saravanakumar, M.E., Ph.D.,
Associate Professor,
Department of Mechanical engineering,
Kamaraj college of engineering and technology,
Virudhunagar.

2. Case hardening
• 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.

3. Advantages of case hardening :
Less distortion compared to through hardening steel
Fatigue properties of a part can be controlled and frequently
improved
Relatively inexpensive steel can be given wear-resisting properties
which would be normally attained through the use of more highly
alloyed and more expensive steels
Hardening of the surface of steels which cannot be normally capable
of being hardened to a high degree by altering the surface
composition
Combination of case and core properties can be attained that are not
possible with conventional hardening treatment
.

4. • Scaling and decarburisation are minimized during surface
• hardening, offering advantages in producing machined parts
• Can be applied to very large parts, which due to very large mass
• or because of danger of cracking would be impractical to harden,
• by conventional heating and quenching Selected area can be
• hardened on any sized place that are difficult with conventional
• heating and quenching
• High surface hardness, improve resistance to wear and galling,
• improve fatigue life, improve corrosion resistance (stainless steel is
• an exception)

5. Carburising Methods
•Pack carburising
•Salt bath Technology or Liquid
Carburising
•Gas Carburising
•Vacuum or Low pressure carburising

6. CO2 + C ---> 2 CO
Reaction of Cementite to Carbon Monoxide:
2 CO + 3 Fe --->Fe3C + CO2

7. Pack carburising
• 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.

8. Pack carburising
•This process is the simplest and earliest
carburising process.
•The process involves placing the components to
be treated in metal containers with the caburising
mixture, based on powdered charcoal and 10%
barium carbonate, packed around the
components.
•The containers are then heated to a constant
temperature (850 oC to 950 oC)for a time period to
ensure an even temperature throughout and
sufficient to enable the carbon to diffuse into the
surface of the components to sufficient depth.
:

9. • Salt bath carburising. A molten salt bath (sodium 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.

10. EFFECT OF CARBURIZING GEAR TOOTH

11. Limitations of Pack Carburising
• It is difficult to control case depths of less than 0,6mm
• The normal case depths produced are 0.25mm to 6mm
• Poor control of surface carbon content and inability to produce
close tolerances of case depth are disadvantages and is
seldom used now a days.

12. Liquid Carburising
• This process is mostly used for producing shallow case depths in
thin sections.
• The components are heated in a bath containing a suitable mix of
sodium cyanide salts and sodium carbonate.
• The normal case depths for this process are about 0,25 to 0.5mm
with bath strengths of 20% to 30% NaCN. High bath strengths 40%
to 50% NaCN are required for case depths of 0,5mm and above.

13. Liquid Carburising
• The case resulting from this process includes carbon
and nitrogen.Which makes this process ideal for low
carbon sheet metal pressed and machined
components.
• This process normally works with bath temperatures
of 800oC to 930oC for immersion times from 2 to 7
hours depending on the depth required.
• However the disposal problems in eliminating the
solid waste & wash water have made this process
not environmental friendly and is becoming obsolete.

14. Gas Carburising
• Gas carburising is very popular and widely used for case depths ranging
from 0.2 mm to 3 mm.
• Possible to achieve narrow bands of case depth requirements.
• Good Repeatability of end results possible
• Different Furnaces and atmospheres are used to suit the end
requirements of the product and cost impact.

15. Carbonitriding
• Carbo nitriding is the variation of Carburising where
nitrogen is additionally introduced into the surface
along with carbon.
• This achieved by passing Ammonia along with regular
gases up to 5% of the Total volume of gases used for
carburising
• Ammonia breaks and gives necessary source of
Nitrogen
• Carbonitriding is done at temperatures of 830-890 oC
and is used for case depths ranging from 0.1 mm to
0.7 mm.

16. CARBO NITRIDING
GAS NITRIDING
The nitrogen source - Ammonia (NH3).
At the nitriding temperature the
ammonia dissociates into Nitrogen and
Hydrogen. 2NH3 ---> 2N + 3H2

17. PRINCIPLE OF INDUCTION HARDENING
GEAR TEETH

18. Induction hardening
• 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.

19. Induction Hardening

20. Flame hardening
• 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.

21. Age hardening
• 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.

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

23. Optical pyrometer
• 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
from the furnace.

24. Thermo-electric pyrometer
• 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

25. Vacuum hardening
Why vacuum hardening?
Development of vacuum concept for heat treatment

26. Process details
1. evacuation of the furnace is started and vacuum level of better than 1X 10-3
mbar is achieved.
2. heating of the furnace starts when the vacuum level of 1.33X10-1 mbar is
reached..
3. Heating cycle
Room Temperature
|10ºC/min
650ºC -120 mts soaking at furnace temperature
|10ºC/min
850ºC-120mts
|5ºC/min
1000ºC-60mts
|5ºC/min
1100ºC-60mts
|
1170ºC-30mts
|5ºC/min
1250ºC-30min
• Note:- partial pressure of 4X10-1 mbar is maintained above 800ºC
4. Cooling cycle

27. Temperature v/s time diagram for vacuum hardening process

28. Benefits
1. Optimum hardening
2. Distortion and crack free hardening of the workload.
3. Absence of oxidation, decarburization or carburization on
the surface of work piece.
4. Reduced or no post-hardening and finished costs.
5. Prevent surface reaction such as oxidation or decarburizing
on work pieces thus retaining a clean surface intact.
6. Remove surface contaminants such as oxide films and
residual traces of lubricants resulting from other operations.

29. Disadvantages
1. Cost for hardening increases.
2. The components are to be thoroughly cleaned before hardening
in vacuum furnace

30. Limitations and problems
1. Volatilization and dissociation in vacuum furnace
2. Deformation or distortion

31. Vacuum furnaces
 TYPES OF VACUUM FURNACE
Vacuum furnaces can be grouped into one of the three basic
designs
I. Top loading furnaces.
II. Bottom loading furnaces.
III. Horizontal loading furnaces.

32. Vertical loading vacuum furnace

33. Horizontal loading vacuum furnace

34. Main parts of vacuum furnace
FURNACE VESSEL
HEATING ELEMENT
INSULATION
PUMPING SYSTEM

35. Vacuum measuring and control
 Hot filament ionization gauge
 Pirani gauge
 Cooling system:
The following media with increasing intensity oheat
transfer are used for the cooling of components.
1. Vacuum.
2. Stagnant gas (Ar, N2).
3. Agitated recirculating gas (Ar, N2).
4. Agitated recirculating gas at pressure (Ar,
N2, He)

36. Industrial Usage Of Vacuum Furnaces
 HARDENING
 BRAZING
 PLASMA NITRIDING
 PLASMA CARBURIZING
 LOW PRESSURE CARBURIZING

37. Plasma Nitriding
1. Introduction
Plasma nitriding, known also as ion nitriding is a form of case hardening
process.
It is an extension of conventional nitriding process, utilizing plasma discharge
physic to diffuse nitrogen into the surface of a ferrous alloy.
Plasma nitriding can be further branched out into plasma nitrocarburising.
In this process, carbon together with nitrogen was introduced into the metal
surface.
The harden case, which is the nitriding layer is commonly known as
‘diffused case’ or ‘diffusion zone’.

38. Plasma nitriding is achieved using a D.C glow discharge technology, whereby
the nitrogen gas inside the furnace is converted into nitrogen ions and
absorbed by the metal.
Molecular nitrogen is first broken into atomic nitrogen through direct plasma
dissociation.
N2 + e- → N + N + e-
Atomic nitrogen is then further converted into nitrogen ion through plasma
ionization N + e- → N+ + 2e-
The nitrogen ion, N+, will then diffuse into the metal surface as finely
dispersed nitrides, imparting high hardness to the surface.
Thus, case hardening is achieved.
Plasma Nitriding

39. In Plasma nitriding process, the job part and the cathode inside the furnace will be
emitting a purple glow.
This is because voltages had dropped sharply at these regions.
This provided a large amount of discharged energy, which causes the cathode and
job part to glow.
Plasma Nitriding Process

40. Advantages for utilizing plasma nitriding
Ability to automate the system which gives good reproducibility of results
Shorter cycle time , No environmental hazard ,improve control of case depth
Ability to select the compound layer type to suit the required usage
Good friction, wear, and fatigue properties
High hardness of the treated surface
Flexibility to nitride stainless steels, titanium alloys
Possibility to lower nitriding temperature and to limit distortion

41. Typical plasma nitriding process

42. Carburising Methods
•Pack carburising
•Salt bath Technology or Liquid
Carburising
•Gas Carburising
•Vacuum or Low pressure carburising

43. Pack carburising
•This process is the simplest and earliest
carburising process.
•The process involves placing the components to
be treated in metal containers with the caburising
mixture, based on powdered charcoal and 10%
barium carbonate, packed around the
components.
•The containers are then heated to a constant
temperature (850 oC to 950 oC)for a time period to
ensure an even temperature throughout and
sufficient to enable the carbon to diffuse into the
surface of the components to sufficient depth.
:

44. Limitations of Pack Carburising
• It is difficult to control case depths of less than 0,6mm
• The normal case depths produced are 0.25mm to 6mm
• Poor control of surface carbon content and inability to produce
close tolerances of case depth are disadvantages and is
seldom used now a days.

45. Liquid Carburising
• This process is mostly used for producing shallow case depths in
thin sections.
• The components are heated in a bath containing a suitable mix of
sodium cyanide salts and sodium carbonate.
• The normal case depths for this process are about 0,25 to 0.5mm
with bath strengths of 20% to 30% NaCN. High bath strengths 40%
to 50% NaCN are required for case depths of 0,5mm and above.

46. Liquid Carburising
• The case resulting from this process includes carbon
and nitrogen.Which makes this process ideal for low
carbon sheet metal pressed and machined
components.
• This process normally works with bath temperatures
of 800oC to 930oC for immersion times from 2 to 7
hours depending on the depth required.
• However the disposal problems in eliminating the
solid waste & wash water have made this process
not environmental friendly and is becoming obsolete.

47. Gas Carburising
• Gas carburising is very popular and widely used for case depths ranging
from 0.2 mm to 3 mm.
• Possible to achieve narrow bands of case depth requirements.
• Good Repeatability of end results possible
• Different Furnaces and atmospheres are used to suit the end
requirements of the product and cost impact.

48. THANK YOU