The document provides an overview of various surface heat treatment processes for metals. It discusses techniques like surface hardening, case hardening, and nitriding that involve diffusing elements like carbon or nitrogen into the surface of the metal to create a hard case while leaving the core soft. Induction hardening, carburizing, and nitriding are described as common methods for surface hardening. The document also covers other surface hardening techniques like flame hardening, vapor deposition methods, and plasma nitriding.

1. Surface Heat treatment of
materials
By
P.SURESHKUMAR
ASST PROF
Dept. of Automobile Engineering
JCTCET
Coimbatore-105

2. Surface hardening is the process of hardening the surface of a
metal object while allowing the metal deeper underneath to
remain soft, thus forming a thin layer of harder metal (called the
"case") at the surface.
For example components like turbine shaft, gears, spindle,
automotive components and axle need to have a hard surface but a
soft core.
Heat treatment
Heat treatment is the process of heating and cooling metals to
change their microstructure and to bring out the physical and
mechanical characteristics that make metals more desirable.
The most common reasons metals undergo heat treatment are to
improve their strength, hardness, toughness, ductility and
corrosion resistance.

3. HEAT TREATMENT
BULK SURFACE
ANNEALING
Full Annealing
Recrystallization Annealing
Stress Relief Annealing
Spheroidization Annealing
AUSTEMPERING
THERMAL THERMO-
CHEMICAL
Flame
Induction
LASER
Electron Beam
Carburizing
Nitriding
Carbo-nitriding
NORMALIZING HARDENING
&
TEMPERING
MARTEMPERING
An overview of important heat treatments

5. A1
A3
Acm

T
Wt% C
0.8 %
723C
910C
Spheroidization
Recrystallization Annealing
Stress Relief Annealing
Full Annealing
 Ranges of temperature where Annealing, Normalizing and Spheroidization treatment are
carried out for hypo- and hyper-eutectoid steels.
 Normalizing Heat Treatment process is heating a steel above the critical temperature,
holding for a period of time long enough for transformation to occur, and air cooling.

8. Through hardening of the sample
Schematic showing variation
in cooling rate from surface
to interior leading to
different microstructures
 The surface of is affected by the quenching medium and experiences the best possible
cooling rate. The interior of the sample is cooled by conduction through the (hot) sample and
hence experiences a lower cooling rate. This implies that different parts of the same sample
follow different cooling curves on a CCT diagram and give rise to different microstructures.
 This gives to a varying hardness from centre to circumference. Critical diameter (dc) is that
diameter, which can be through hardened (i.e. we obtain 50% Martensite and 50% pearlite at
the centre of the sample).

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

10. Severity of quench values of some typical quenching conditions
Process Variable H
Air No agitation 0.02
Oil quench No agitation 0.2
" Slight agitation 0.35
" Good agitation 0.5
" Vigorous agitation 0.7
Water quench No agitation 1.0
" Vigorous agitation 1.5
Brine quench
(saturated Salt water)
No agitation 2.0
" Vigorous agitation 5.0
Ideal quench 
Note that apart from the nature of the
quenching medium, the vigorousness of the
shake determines the severity of the quench.
When a hot solid is put into a liquid
medium, gas bubbles form on the surface of
the solid (interface with medium). As gas
has a poor conductivity the quenching rate is
reduced. Providing agitation (shaking the
solid in the liquid) helps in bringing the
liquid medium in direct contact with the
solid; thus improving the heat transfer (and
the cooling rate). The H value/index
compares the relative ability of various
media (gases and liquids) to cool a hot solid.
Ideal quench is a conceptual idea with a heat
transfer factor of  ( H = ).
1
[ ]
f
H m
K


Severity of Quench as indicated by the heat transfer equivalent H
f → heat transfer factor
K → Thermal conductivity
 Before we proceed further we note that we have a variety of quenching media at our
disposal, with varying degrees of cooling effect. The severity of quench is indicated by the
‘H’ factor (defined below), with an ideal quench having a H-value of .
Increasing
severity
of
quench

11. Surface hardening: why & how?
Components like gear, shaft or spindle need a hard / wear resistant surface but a soft / tough
core.
Section size of such components is often too large to be uniformly hardened even on severe
quenching.
More over the time lag between the transformations at the surface and the core results in an
unfavorable tensile residual stress at the surface.
Recall the general thumb rule that the region that transforms later is likely to have compressive
residual stress.
The surface is likely to transform first in steel having the same composition all through its
section.
Therefore surface would have residual tensile stress. Depending on its magnitude it may lead to
cracking or distortion.
The presence of residual tensile stress is also harmful as it would reduce fatigue life of critical
components like turbine shaft or landing gear of an aircraft.
The purpose of surface hardening is to develop a hard surface with compressive residual stress,
to improve its wear resistance, to increase its fatigue life and to avoid susceptibility to
distortion and cracking.

12. There are several other ways the strength or the hardness of the
surface can be increased without adversely affecting the toughness
of the core.
Some of the most common techniques are as follows:
1. Induction hardening
2. Case carburizing + case hardening
3. Nitriding
4. Shot peening
5. Hard facing, coating or surface alloying
Formation of Martensite is primarily responsible for the
development of very high strength in steel.
However you need to cool a component made of steel very fast to
get martensite

15. Layer additions Substrate treatment
Hard-facing
Fusion hard-facing
Thermal spray
Coatings
Electrochemical plating
Chemical vapor deposition (electroless plating)
Thin films (physical vapor deposition, Sputtering,
ion plating)
Ion mixing
Diffusion methods
Carburizing
Nitriding
Carbonitriding
Nitrocarburizing
Boriding
Titanium-carbon diffusion
Toyota diffusion process
Selective hardening methods
Flame hardening
Induction hardening
Laser hardening
Electron beam hardening
Ion implantation
Selective carburizing and nitriding
Engineering methods for surface hardening

16. 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.

20. Induction hardening is very effective for surface hardening of plain carbon steel
having 0.35‐0.70%C.
The salient features of induction hardening are as follows:
• Heat the surface to a temperature above A3 (austenitic region)
• Core does not get heated : the structure remains unaltered
• Surface converts to martensite on quenching.
• Fast heating & short hold time: needs higher austenization temperature
• Martensite forms in fine inhomogeneous grains of austenite
• Applicable to carbon steels (0.35 – 0.7C)
• Little distortion & good surface finish

21. Once the process is complete the microstructure of the surface gets transformed into
martensite while that at its core remains unaltered.

23. Carburizing
Carburizing, also referred to as Case Hardening, is a heat treatment process that produces a
surface which is resistant to wear while maintaining toughness and strength of the core.
This treatment is applied to low carbon steel parts after machining as well as high alloy
steel bearings, gears and other components.
Carburizing increases strength and wear resistance by diffusing carbon into the surface of
the steel creating a case while retaining a substantially lesser hardness in the core. This
treatment is applied to low carbon steels after machining.
Most carburizing is done by heating components in either a pit furnace, or sealed
atmosphere furnace and introducing carburizing gases at temperature.
Gas carburizing allows for accurate control of both the process temperature and carburizing
atmosphere (carbon potential).
Carburizing is a time/temperature process; the carburizing atmosphere is introduced into
the furnace for the required time to ensure the correct depth of case.
The carbon potential of the gas can be lowered to permit diffusion, avoiding excess carbon
in the surface layer.

24. Carburizing cannot be done in ferrite phase as it has very low solid solubility for carbon at
room temperature. It is done in the Austenite region above 727°C in carbon rich
atmosphere.
Types of carburizing
i. Pack carburizing
ii. Gas carburizing
iii. Liquid carburizing
For iron or steel with low carbon content, which has poor to no hardenability of its own,
the case hardening process involves infusing additional carbon into the case.
Case hardening is usually done after the part has been formed into its final shape, but can
also be done to increase the hardening element content of bars to be used.
Because hardened metal is usually more brittle than softer metal, through-hardening (that
is, hardening the metal uniformly throughout the piece) is not always a suitable choice for
applications where the metal part is subject to certain kinds of stress.
In such applications, case hardening can provide a part that will not fracture (because of
the soft core that can absorb stresses without cracking) but also provides adequate wear
resistance on the surface.

25. Pack carburizing
• 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.
A major limitation of pack carburizing is poor control over temperature & carburization
depth.
On completion of the process the steel parts are cooled slowly. Direct quenching is not
possible as the job is surrounded by carburizing mixture packed in a sealed box having high
thermal mass.
This can be overcome by using gaseous or liquid carburizing medium.

26. • Salt bath carburizing. 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 carburizing. The object is placed in a sealed furnace with carbon monoxide allowing for fine control of the
process.
• Gas carburization is done by keeping the samples at the carburizing temperature for a specified time in a
furnace having a mixture of carburizing and neutral gas. CH4 and CO are the most commonly used
carburizing gas.
• It is usually mixed with de‐carburizing (H2 and CO2) and neutral gases (N2).
• This helps maintain a close control over carbon potential. It should be enough to maintain %C at in the range
1.0‐1.2% at the surface.
• In the presence of Fe the carburizing gases decompose to produce nascent carbon that diffuses into steel.
CH4 = C (Fe) + 2H2
2CO = C (Fe) + CO2
• It provides excellent control over the furnace temperature and atmosphere (carbon potential).
• Samples after carburization can be directly quenched.

27. Liquid carburization
It is done by keeping the job in a salt bath consisting of 8% NaCN + 82 BaCl2 + 10 NaCl.
It allows precise temperature control and rapid heat transfer. Carburization takes place due
to the formation of nascent carbon.
The chemical reactions that occur in the presence of Fe are as follows:
BaCl2 + NaCN = Ba(CN)2 +NaCl
Ba(CN)2 = C (Fe) + BaCN2
What is the chemical CN?
A cyanide is a chemical compound that contains the group C≡N. This group, known as the
cyano group, consists of a carbon atom triple-bonded to a nitrogen atom.
The sample can be quenched immediately after carburization.
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.
If steel is heated in an environment of cracked ammonia it picks up nitrogen.
Nitrogen like carbon forms interstitial solid solution with iron. If it is present in excess it forms
nitride (Fe4N). It is extremely hard and brittle.

28. Nascent nitrogen that forms at the surface of steel as ammonia comes in contact
with Fe.
This diffuses into iron lattice and form nitride as and when the amount of
nitrogen in steel exceeds its solubility limit.
The presence of alloying elements having high affinity for nitrogen increases
nitrogen pick up.
The formation of nitride within the matrix results in a substantial increase in the
hardness of steel.
The preferred thickness of the hardened layer is around 20 micro meter.
The hardness of the nitride layer is usually in the range of 1000‐2000Hv.
Nitriding of steel is carried out only after it has been hardened and tempered. It
is the last heat treatment given to steel.

29. 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.

37. Vapor Deposition
• Physical Vapor Deposition (PVD)
– Thermal PVD
– Sputter Deposition
– Ion plating
• Chemical Vapor Deposition (CVD)

38. Physical Vapor Deposition – Thermal
PVD
• Thermal PVD – also called Vacuum Deposition
– Coating material (typically metal) is evaporated by melting
in a vacuum
– Substrate is usually heated for better bonding
– Deposition rate is increased though the use of a DC current
(substrate is the anode so it attracts the coating material)
– Thin ~0.5 mm to as thick as 1 mm.

39. Physical Vapor Deposition – Sputter Deposition
• Vacuum chamber is usually backfilled with Ar gas
• Chamber has high DC voltage (2,000-6,000 V)
• The Ar becomes a plasma and is used to target the
deposition material. The impact dislodges atoms from the
surface (sputtering), which are then deposited on the
substrate anode
• If the chamber is full of oxygen instead of Ar, then the
sputtered atoms will oxidize immediately and an oxide will
deposit (called reactive sputtering)

40. Physical Vapor Deposition – Ion Plating
• Combination of thermal PVD and sputtering
• Higher rate of evaporation and deposition
• TiN coating is made this way (Ar-N2
atmosphere)
– The gold looking coating on many cutting tools to
decrease the friction, increase the hardness and
wear resistance

41. Chemical Vapor Deposition
• Deposition of a compound (or element) produced by a
vapor-phase reduction between a reactive element and gas
– Produces by-products that must be removed from the process as
well
• Process typically done at elevated temps (~900ºC)
– Coating will crack upon cooling if large difference in thermal
coefficients of expansion
– Plasma CVD done at 300-700ºC (reaction is activated by plasma)
• Typical for tool coatings
• Applications
– Diamond Coating, Carburizing, Nitriding, Chromizing, Aluminizing
and Siliconizing processes
– Semiconductor manufacturing

43. Plasma Nitriding
Plasma nitriding, also known as ion nitriding or glow discharge nitriding, is a gas nitriding
process enhanced by a plasma discharge on the part to be nitrided.
The plasma is gas that when exposed to an electrical potential is ionized and glows. The
parts to be nitrided are connected as a cathode and the furnace walls are the anode.
They are supplied with a potential between 0.3 and 1KV. Particles are accelerated and hit
the cathode (work piece) transferring all their kinetic energy and heating it.
For gas particles to have enough kinetic energy to transfer, they need to have a
considerable large mean free path and in this way gain speed for collision with the
substrate before colliding with another gas particle.
This is why this process and mostly all plasma processes work under vacuum as a measure
to increase the mean free path of the accelerated particles.
The pressure used for plasma nitriding is normally between 100 – 1000Pa. Other authors
suggest a narrower range between 50 – 500Pa.
This pressure is considered as a rough vacuum since there are other processes that use
much higher vacuum values.

44. The chemical reaction when ammonia dissociated was explained in the preceding section.
In the case of plasma nitriding, the process gases are introduced separately.
One combination that is often used is Nitrogen + Hydrogen. Argon is also used in the initial
stages as a plasma sputtering gas for surface cleaning of the substrate to be nitrided.
The voltage drop occurs in what is called the plasma sheath which is a positive charged
area where ions are accelerated towards the cathode and have their highest kinetic
energies.

45. During ion nitriding three reactions will occur at the surface of the material being treated.
In the first reaction, iron and other contaminants are removed from the surface of the
work by an action known as sputtering or by a reducing reaction with hydrogen.
The impact of hydrogen or argon ions bombarding the work surface dislodges the
contaminant that will be extracted by the vacuum system. The removal of these
contaminants allows the diffusion of nitrogen into the surface
During the second reaction, and as a result of the impact of the sputtered ion atoms, case
formation begins at the work surface of iron nitrides.
Sputtered Fe + N = FeN
During the third reaction, a breakdown of the FeN begins under the continuous sputtering
from the plasma.
This one causes the instability of the FeN which begins to break down into the e phase
followed by the g’ phase and a iron-nitrogen compound zone

46. Surface Treatment
Type
Concepts and Applications of the process
Electroplating
A method of forming metallic coatings (plating films) on subject metal surfaces submerged in solutions containing ions by
utilizing electrical reduction effects. Electoplating is employed in a wide variety of fields from micro components to large
products in information equipment, automobiles, and home appliances for ornamental plating, anti-corrosive plating, and
functional plating.
Electroless Plating
A plating method that does not use electricity. The reduction agent that replaces the electricity is contained in the plating
solution. With proper re-processing, virtually any material such as paper, fabrics, plastic and metals can be plated, and the
distribution of the film thickness is more uniform, but slower than electroplating. This is different from chemical plating by
substitution reaction.
Chemical Process
(Chemical Coating)
The process creates thin films of sulfide and oxide films by chemical reactions such as post zinc plating chromate treatment,
phosphate film coating (Parkerizing), black oxide treatments on iron and steels, and chromic acid coating on aluminum. It is
used for metal coloring, corrosion protection, and priming of surfaces to be painted to improve paint adhesion.
Anodic Oxidation
Process
This is a surface treatment for light metals such as aluminum and titanium, and oxide films are formed by electrolysis of the
products made into anodes in electrolytic solutions. Because the coating (anodizing film) is porous, dyeing and coloring are
applied to be used as construction materials such as sashes, and vessels. There is low temperature treated hard coating
also.
Hot Dipping
Products are dipped in dissolved tin, lead, zinc, aluminum, and solder to form surface metallic films. It is also called
Dobuzuke plating and Tempura plating. Familiar example is zinc plating on steel towers.
Vacuum Plating
Gasified or ionized metals, oxides, and nitrides in vacuum chambers are vapor deposited with this method. Methods are
vacuum vapor deposition, sputtering, ion plating, ion nitriding, and ion implantation. Titanium nitride is of gold color.
Painting
There are spray painting, electrostatic painting, electrodeposition painting, powder painting methods, and are generally
used for surface decorations, anti-rusting and anti-corrosion. Recently, functional painting such as electro-conductive
painting, non-adhesive painting, and lubricating painting are in active uses.
Thermal Spraying
Metals and ceramics (oxides, carbides, nitrides) powders are jetted into flames, arcs, plasma streams to be dissolved and
be sprayed onto surfaces. Typically used as paint primer bases on larger structural objects, and ceramic thermal spraying
for wear prevention.
Surface Hardening
This is a process of metal surface alteration, such as carburizing, nitriding, and induction hardening of steel. The processes
improve anti-wear properties and fatigue strength by altering metal surface properties.
Metallic Cementation
This is a method of forming surface alloy layers by covering the surfaces of heated metals and metal diffusion at the same
time. There is a method of heating the pre-plated products, as well as heating the products in powdered form of metal to
be coated.
There are following types of surface treatments.

47. What is meant by Austempering?
Austempering is heat treatment that is applied to ferrous metals, most notably steel and
ductile iron. In steel it produces a bainite microstructure whereas in cast irons it produces
a structure of acicular ferrite and high carbon, stabilized austenite known as ausferrite.
What is Martempering process?
Martempering is a heat treatment for steel involving austenitisation followed by step
quenching, at a rate fast enough to avoid the formation of ferrite, pearlite or bainite to a
temperature slightly above the martensite start (Ms) point.

48. Thank You ....
For your Patient
listening