This document discusses various heat treatment processes and their effects on metallic materials. It defines heat treatment as a process of heating and cooling metals to achieve specific microstructures and properties. The major goals of heat treatment are outlined, including increasing strength, hardness, ductility, and toughness. Common hardening processes like hardening and tempering are described in detail. The document also discusses softening treatments, factors affecting hardenability, and other property modification processes like solution heat treatment and surface processing methods like electroplating, anodizing, and electroless plating.

1. ADAMA SCIENCE AND TECHNOLOGY UNIVERSITY
SCHOOL OF MECHANICAL, CHEMICAL AND
MATERIALS ENGINEERING
DEPARTMENT OF MECHANICAL ENGINEERING
Manufacturing Engineering II (MEng3202)
Chapter 4: Property Enhancing
and Surface Processing

2. The aim of this chapter is to gain an
understanding of the role of heat treatment
on the development of microstructure and
properties of metallic materials. The
course will highlight a number of
commercially-significant applications
where heat treatment is important.

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

4. Definition of heat treatment
Heat treatment is an operation or combination of
operations involving heating at a specific rate,
soaking at a temperature for a period of time
and cooling at some specified rate. The aim is to
obtain a desired microstructure to achieve
certain predetermined properties (physical,
mechanical, magnetic or electrical).

5. Can also be used to obtain certain
manufacturing objectives like:
– To improve machining & formability,
– To restore ductility
– To recover grain size etc.
– Known as Process Heat Treatment

6. The major objectives are:
 to increase strength, hardness and wear resistance (bulk
hardening, surface hardening)
 to increase ductility and softness (tempering,
recrystallization annealing)
 to increase toughness (tempering, recrystallization
annealing)
 to obtain fine grain size (recrystallization annealing, full
annealing, normalising)
 to remove internal stresses induced by differential
deformation by cold working, non-uniform cooling from high
temperature during casting and welding (stress relief
annealing)

7.  to improve machineability (full annealing and normalising)
 to improve cutting properties of tool steels (hardening and
tempering)
 to improve surface properties (surface hardening, corrosion
resistance-stabilising treatment and high temperature
resistance-precipitation hardening, surface treatment)
 to improve electrical properties (recrystallization,
tempering, age hardening)
 to improve magnetic properties (hardening, phase
transformation)

8. Generally Heat treatment done for one
of the following objective:
– Hardening.
– Softening.
– Property modification.

9. Common Hardening Heat Treatments:
• Direct Hardening
– Heating Quenching Tempering
• Austempering
• Martempering
• Case Hardening
– Case carburizing

10. Hardening Heat Treatment
• Case Hardening (Contd..)
– Case Nitriding
– Case Carbo-nitriding or Cyaniding
– Flame hardening
– Induction hardening etc
• Precipitation Hardening

11. Quenching
• Quenched steel (Martensite)
• Highly stressed condition
• Too brittle for any practical purpose.
Quenching is always followed by tempering to
– Reduce the brittleness.
– Relieve the internal stresses caused by hardening.

12. Tempering
• Tempering means subsequent heating
– to a specific intermediate temperature
– and holding for specific time
• Tempering leads to the decomposition of martensite into ferrite-
cementite mixture
– Strongly affects all properties of steel.
• At low tempering temperature (up to 2000C or 2500C),
– Hardness changes only to a small extent
– True tensile strength increases
– Bending strength increases

13. This may be explained by
• Separation of carbon atom from the martensite lattice
• Corresponding reduction in its stressed state and acicularity
Martensite

14. Higher tempering temperature reduces
– Hardness
– True tensile strength
– Yield point
–While relative elongation and reduction area increases.
•This is due to formation of ferrite and cementite
mixture.

15. Some features of Hardening Heat Treatment
• retained ferrite detrimental to uniform properties – so heating
beyond Ac3 for Hypoeutectoid steel
• retained Cementite is beneficial as it is more hard & wear
resistant than martensite – so heating beyond Ac1, not ACM, for
Hepereutechtoid steel.
• addition of C shifts TTT curve to right and increases hardness
of martensite
• addition of Alloy elements shifts TTT curve to right and
changes the shape
• higher the Alloy% - Higher the stability of M
• higher the degree of super cooling – Higher the amount of

16. Quenching Media
• Quenching media with increased degree of
severity of quenching
– Normal Cooling
– Forced Air or draft cooling
– Oil
– Polymer
–Water and
– Brine

17. Quenching medium depends on
– Material composition
– Weight of job
• Aim is to have a cooling rate just bye-passing
the nose of TTT curve for
– minimum stress
– minimum warping/crack during quenching.

18. Figure: Conventional quenching and tempering process

19. Case Hardening
• Objective is to harden the surface &
subsurface selectively to obtain:
– Hard and wear-resistant surface
– Tough impact resistant core
– The best of both worlds
• Case hardening can be done to all types of
plain carbon steels and alloy steels

20. Case Carburizing
• Heating of low carbon steel in carburizing medium like
charcoal
• Carbon atoms diffuse in job surface
• Typical depth of carburisation; 0.5 to 5mm
• Typical Temperature is about 9500C
• Quenching to achieve martensite on surface and sub-surface
• If needed, tempering to refine grain size and reduce stresses
Types of carburizing
i. Pack carburizing
ii. Gas carburizing
iii. Liquid carburizing

21. Case Nitriding
• Heating of steel containing Al in nitrogen
medium like Nitride salt, Ammonia etc.
• Typical temperature is about 5300C
• Nitrogen atoms diffuse in job surface
• Forms AlN, a very hard & wear resistant
compound on surface & sub-surface
• Typical use is to harden tubes with small wall
thickness like rifle barrel etc.

22. Case Carbo-nitriding
• Heating of low carbon steel containing Al in cynide
medium like cynide salt followed by Quenching
• Typical temperature is about 8500C
• Nitrogen & Carbon atoms diffuse in job
• Typical case depth 0.07mm to 0.5mm
• Forms very hard & wear resistant complex
compounds, on surface & sub-surface
• If needed, tempering to refine grain size and reduce
stresses

23. Induction and Flame Hardening
• Employed for medium & high carbon steel or
alloy steels
• Local heating of the surface only either by
flame or induction current
• Heating to austenizing range, 30 – 500C above
Ac3 (Hypoeutectoid) or Ac1 (Hypereutectoid)
• Quenching in suitable quenching media
• If needed, tempering to refine grain size and reduce
stresses

24. Softening Heat Treatment
• Softening Heat Treatment done to:
–Reduce strength or hardness
–Remove residual stresses
– Restore ductility
– Improve toughness
– Refine grain size
• Necessary when a large amount of cold
working, such as cold-rolling or wire drawing
been performed.

25. Softening Heat Treatment
• Incomplete Annealing
– Stress Relieving
– Process Annealing
– Spherodising
• Full Annealing
• Normalizing

26. Normalizing Vs Annealing
• normalizing considerably cheaper than full annealing
• no added cost of controlled cooling.
• fully annealed parts are uniform in softness (and
machinability)
• Normalized parts, depending on the part geometry,
exhibit non-uniform material properties
• Annealing always produces a softer material than
normalizing.

27. Hardenability
• ability of a metal to respond to hardening
treatment
• for steel, the treatment is Quenching to form
Martensite
• two factors which decides hardenability
– TTT Diagram specific to the composition
– Heat extraction or cooling rate

28. TTT Diagram
• for low carbon steel, the nose is quite close to
temperature axis
• hence very fast cooling rate is required to form
Martensite
– causes much warp, distortion and stress
– Often impossible for thick sections
• Carbon and Alloy addition shifts the nose to
right and often changes the shape

29. Salt bath I
Austenitisation heat treatment
Salt bath II
Low-temperature for isothermal treatment
Sample and fixtures
for dilatometric measurements
Dilatometer equipment
Figure: Equipment’s for Determination of TTT Diagrams

30. Factors affecting cooling rate
• Heating Temperature
• Quenching bath temperature
• Specific heat of quenching medium
• Job thickness
• Stirring of bath to effect heat convection
• Continuous or batch process

31. Hardenability
• Hardenability is quantified as the depth up to which
full hardness can be achieved
• Amount of carbon affects both hardness of martensite
and hardenability
• Type and amount of alloying elements affect mostly
hardenability
• The significance of alloying element is in lowering
cooling rate for lesser distortion and thick section

32. Property Modification Treatment
These heat treatments are aimed either to
• achieve a specific property
• to get rid of a undesired property
Example
– Solution heat treatment
• Refers to taking all the secondary phases into solution by heating and
holding at a specific temperature
• Except martensite, all other phases in steel are diffusion product
• They appear or disappear in the primary matrix by diffusion controlled
process
• Diffusion is Time & Temperature dependent

33. Surface Processing Operations
Electroplating
A method of forming metallic coatings (plating films)
on subject metal surfaces submerged in solutions
containing ions by utilizing electrical reduction effects.
Electroplating 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.

34. Electroplating
Deposit metal on cathode, sacrifice from anode
Anodizing
chrome-plated auto parts
copper-plating
Metal part on anode: oxide+coloring-dye deposited using electrolytic process

35. Electro less 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.

36. 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.

37. 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.

38. 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.

39. Painting
Electrostatic Spray Painting
Spray Painting in BMW plant
Silk screening

40. 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.

41. Thermal spraying
High velocity oxy-fuel spraying
Thermal metal powder spray
Plasma spray
Tungsten Carbide / Cobalt Chromium
Coating on roll for Paper Manufacturing
Industry

42. 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.

43. The surface properties of metals are
typically changed for:
• decoration and/or reflectivity
• improved hardness (to maintain cutting edges and
resistance to damage and wear)
• prevention of corrosion.

44. Industries using surface processing/treatments
The surface treatment of metals and plastics does not itself form
a distinct vertical industry
sector. Surface treatments do not create products; they change the
surface properties of previously formed components or products
for subsequent use. Printed circuit boards might be considered
products but are components manufactured for use in other
products, and are made by a considerable number of
interdependent manufacturing stages. The surface treatment of
metals and plastics is therefore largely a service to many
industries and examples of key customers are given below:

45. • automotive
• food and drink containers
• aerospace • printing
• information systems
• domestic appliances
• telecommunications
• jewellery, spectacles and ornaments
• heavy engineering
• furniture
• construction (building)

46. • clothing
• bathroom fittings
• coinage
• hardware
• medical.
After treatment activities
1. Drying using hot water
After all wet processing operations have been completed, the
work pieces or substrates need to be quickly and effectively dried
in order to avoid staining and corrosion. The simplest method of
drying is by immersing the components in hot water for a few
seconds and then allowing them to dry-off in the air.

47. 2. Drying using hot air
Drying in automated jig plants is most easily accomplished on automatic lines
using hot air. The jigs are placed in a tank-shaped drier at the end of the
process line; the tank has the same dimensions as the vats in the line to fit into
the transporter system. Hot air is evenly recirculated from the top to the
bottom of the tank at temperatures of 60 – 80 °C. Hot air escaping from the
top of the drier tank makes the equipment thermally inefficient. In some
cases, such as the new thick film passivation’s or to reduce drying times, it is
necessary to heat the substrate or work pieces to 80 °C and higher. The
temperature of the air circulating in the tank-shaped driers then needs to be
above 100 ºC. The air is normally heated by circulation or heat-exchangers
using steam or hot oil. Direct heating systems are an alternative, using a
special gas burner with an open gas flame in the circulating air. The burning
gas heats the air directly with an efficiency of nearly 100 % of the energy
input.

48. 3. Drying using air knives
There is a growing use of localized air drying by
means of precision nozzles or ‘air knives’ that is
more energy efficient than hot air tank drying.