Full Explanation Of Heat Treatment Of Steel

1. HEAT TREATMENT OF STEEL
Presentation Submitted by :-
Tushar Devaji Khodankar
(BE18S02F010)

2. IRON-CARBON PHASE DIAGRAM
Figure 1: Phase diagram
for iron-carbon system,
up to about 6.67%
carbon.
  - Ferrite (Alpha iron)
  - Austenite (Gamma
iron)
  - Ferrite (Delta iron)
 Fe3C – Cementite
(Iron Carbide)

3. EXPLANATION ON IRON-CARBON
PHASE DIAGRAM
• Vertical line (y-axis) – temperature from 0oC ~ 1538oC
• Horizontal line (x-axis) – carbon content from 0% ~ 6.67%
• At 723oC :-
- where pure iron, steel and cast iron looses their
magnetism
- Plain carbon steel up to 0.86%C changes from ferrite
and pearlite to austenite and ferrite
- Plain carbon steel with 0.86%C changes from pure
pearlite to austenite
- Other steel or cast iron with more than 0.86%C
changes from pearlite and cementite to austenite and
cementite

4. SOLUBILITY LIMITS OF CARBON
IN IRON
• Ferrite phase can dissolve only about 0.022% carbon at 723C
(1333F)
• Austenite can dissolve up to about 2.1% carbon at 1130C (2066F)
• The difference in solubility between alpha and
gamma provides opportunities for
strengthening by heat treatment
• Pure Iron melt at 1536oC
• Cast Iron melt at 1145oC

5. • FIGURE 2: The iron-
iron-carbide phase
diagram. Because of the
importance of steel as
an engineering
material, this diagram
is one of the most
important phase
diagrams.
Eutectoid
Eutectic
Eutectic and Eutectoid Compositions
To study the structure changes during heat treatment process.

6. CONSTITUENT IN STEEL
1. Ferrite
•  and -phase with the BCC
lattice

7. 2. AUSTENITE
• The -phase with the FCC
lattice.

8. 3. CEMENTITE
• Name given to iron carbide
• having fixed composition Fe3C.
• Cementite is a hard and brittle
substance, influencing on the
properties of steels and cast irons

9. 4. PEARLITE
• fine mixture of ferrite
and cementite structure
• forming as a result of
decomposition of
austenite at slow cooling
conditions
• the black lamellae are
the Fe3C parts; their
thickness is a few µm.
• The name comes from
the pearl-like luster of
this material.
Microstructure of pearlite in 1080 steel, formed from austenite
of eutectoid composition. In this lamellar structure, the lighter
regions are ferrite, and the darker regions are carbide.
Magnification: 2500X.

10. 5. MARTENSITE
• In steel, under rapid cooling, so that
equilibrium is prevented, austenite
transforms into a nonequilibrium phase
called martensite, which is hard and
brittle
• A unique phase consisting of an
iron-carbon solution whose composition
is the same as the austenite from which
it was derived
• Face-centered cubic (FCC) structure
of austenite is transformed into
body-centered tetragonal (BCT)
structure of martensite
• The extreme hardness of martensite
results from the lattice strain created by
carbon atoms trapped in the BCT
structure, thus providing a barrier to
slip

11. WHAT IS HEAT TREATMENT
?
Various metallurgical process that involves heating
and cooling.
• It is done in furnace so that the environment is in
controlled
• The heating process should take place slowly in
order to minimize distortion
• It is performed to effect structural changes in a
material, which in turn affect its mechanical
properties
• Example:quenching, annealing, normalizing,
tempering, surface hardening, etc

12. HEAT TREATMENT IN
MANUFACTURING
• Heat treatment operations are performed on
metal workparts at various times during their
manufacturing sequence
• To soften a metal for forming prior to
shaping
• To relieve strain hardening that occurs
during forming
• To strengthen and harden the metal near
the end of the manufacturing sequence

13. Hardening – heat treating operation necessary to
impart hardness to any component.
• This treatment consist of:-
1. Heating – to selected temperature (Austenitizing
temperature - 723oC
2. Holding – at Austenitizing temperature
3. Cooling / quenching – at fast rate in order to get
desired hardness
• In order to get required mechanical properties such as
tensile strength, ductility, elasticity, etc, tempering
process is done to the part.

14. • The steel hardening process involved:-
1. Heating – 27.8oC to 55.6oC above upper
critical temperature
( 750.8oC ~ 778.6oC)
2. Quenching – 27.8oC to 55.6oC above
upper critical temperature
( 750.8oC ~ 778.6oC)
2. Tempering

15. QUENCHING
It is a process where
• metal is heated at temperature above the
transformation temperature ,
• and then plunged quickly into the quenching
medium and
• submerged until they are cooled. (will cause
metal to be hardened – formation of martensite).

16. HEAT TREATMENT TO FORM
MARTENSITE
(QUENCHING)
Consists of two steps:
1. Austenitizing - heating the steel to a
sufficiently high temperature for a long enough
time to convert it entirely or partially to
austenite
2. Quenching - cooling the austenite rapidly
enough to avoid passing through the nose of the
TTT curve

17. FOUR STAGES OF QUENCHING
1. Vapor formation stage. 2. Vapor covering stage..
3. Vapor discharge stage4. Slow cooling stage.

18. QUENCHING MEDIA AND COOLING
RATE
• Various quenching media are used to affect cooling
rate
• Brine -salt water, usually agitated (fastest cooling rate)
• Still fresh water
• Oil
• Air (slowest cooling rate)
• Molten salt
• Sand and etc..
• The faster the cooling, the more likely are internal
stresses, distortion, and cracks in the product

19. QUENCHING MEDIUM
= liquid / medium which the metals is plunged
during quenching process
• Water Quenching
- The most commonly used
- Inexpensive, convenient to use
- Normally used for low carbon steel
- Provide very rapid cooling / drastic quench
- This will cause internal stresses, distortion or cracking
- Normally done at room temperature for best result

20. • Oil Quenching
- Gentler than water
- Normally used for critical parts
- Less chance for internal stresses, distortion or
cracking
- Hardness normally less than water quench
- Oil is heated slightly above room temperature (38oC ~
66oC) for best result
• Air Quenching
- Less drastic compared to water and oil quenchant
- The slowest cooling rate
- Less chance for internal stresses, distortion or
cracking
- Strength and hardness will not be high
- Normally used for special alloy steel which contain
chromium and molybdenum

21. • Brine/ Salt Water Quenching
= salt water (5~10 % salt in water), usually agitated
- Similar effect to water quench
- Faster cooling rate
- Quenching action will be more drastic
• Polymer Quenching
= water + glycol polymer, usually agitated
cooling rate is in between water and oil
- Quenching action will be more drastic
- Less corrosion than water quenching
- Less fire hazard

22. • Cryogenic Quenching
= deep freezing
- This is done in order to make sure no retained of
austenite during quenching
- Usually done at room temperature

23. WHAT IS ISOTHERMAL
DIAGRAM (IT) ?
• It is a graph of temperature vs time for the process
of cooling metal.
• It is used to predict the final structure of metal
• It is also called TTT diagram (Time-Temperature-
Transformation), C curves, or S curves
• Limitations:
- IT diagram does not plot carbon percentage

24. BASIC OF IT DIAGRAM
Log
scale

25. IT / TTT Curve - Region of Transformation

26. The TTT curve, showing transformation of austenite into other phases
as function of time and temperature for a composition of about 0.80%
C steel. Cooling trajectory shown yields martensite.
Time-Temperature-Transformation Curve

27. WHAT IS TEMPERING?
= Treatment involves heating and soaking at a temperature
below the eutectoid for about one hour, followed by slow
cooling @
= Process of reheating steel after hardening to a
temperature which is below the lower transformation
temperature, followed by any rate of cooling.
• The heat treatment applied to martensite to reduce
brittleness, increase toughness, and relieve stresses
• Results in precipitation of very fine carbide particles from
the martensite iron-carbon solution, gradually
transforming the crystal structure from BCT to BCC
• New structure is called tempered martensite

28. PURPOSE OF TEMPERING
• Tempering is done to a steel that has been quenched which
maybe hard and brittle
• Therefore, tempering is done in order to reduce brittleness,
increase toughness, and relieve stresses
• Tempering also will reduce chance of distortion
• By removing internal stresses, tempered material is more
ductile and it will give better impact resistance, easily machined
and cold work

29. EFFECTS OF TEMPERING
HARDNESS Decreased
STRENGTH Decreased
TOUGHNESS Increased
BRITLLENESS Decreased
DUCTILITY Increased
INTERNAL STRESS Decreased
DISTORTION Reduced
CRACKING Reduced
MACHINABILITY Improved
FORMABILITY Improved

30. TYPES OF TEMPERING
2 Techniques
Martempering
Austempering

31. MARTEMPERING
• Result= tempered
martensite
• Advantage:
- Reduce chances of
cracking and
distortion
• Disadvantage:
- Time consuming
and inconvenient

32. AUSTEMPERING
• Result= Bainite
• Advantage:
- Less internal stress
- less cracking and
distortion tendencies
- Tougher and more ductile
- Better impact resistance
• Disadvantage:
- Limited to thin part only

33. FURNACE
• They are 2 types of furnace used,
- Batch Furnace
- Continous Furnace
• Heating system fuels used
- gas
- oil
- electricity

34. BATCH FURNACE VS.
CONTINUOUS FURNACE
• Batch furnaces
• Heating system in an insulated chamber, with a door for
loading and unloading
• Production in batches
• Example: - box furnace
- Pit furnace
- Bell furnace
- Elevator furnace
• Continuous furnaces
• Generally for higher production rates
• Mechanisms for transporting work through furnace
include rotating hearths and straight-through conveyors

35. 1. HORIZONTAL TYPES
FURNACE
• It is also called box furnace
(rectangular box)
• Source of heat:- gas or
electricity
• Relatively easy to construct in
any size,
• easily insulated and thermally
efficient
• However, difficult to heat-treat
long, slender work because of
the sagging or warping will
occur.

36. 2. VERTICAL PIT FURNACE
• cylindrical chambers sunk into
the floor with a door on top
• can be swung aside to permit
the work to be lowered into
the furnace.
• Thus, long work pieces are
less likely to warp.
• used for small parts

37. 3. BELL FURNACE
• The heating elements are
contained within a bottomless bell
that is lowered over the work.
• An airtight inner shell is used to
contain a protective atmosphere
during the heating and cooling
cycles and thus to reduce tarnish
or oxidation.
• The furnace unit can be lifted off
and transferred to another batch,
• the inner shell retaining the
controlled atmosphere during
cooling.
• An insulated cover can be placed
over the heated work if slower
cooling is desired,

38. 4. ELEVATOR TYPE
FURNACE
• It is a modification of the bell
furnace (the bell is stationary and
the work is raised up into it by
means of a movable platform that
forms the bottom of the furnace.)
• There are three vertical positions.
• Middle position, the work is loaded
onto the platform elevator.
• In the upper position, the work is
in the bell furnace
• Lower position, it is in a quench
tank.
• Used for work piece that must be
quenched as soon as possible after
being removed from the furnace.

39. 5. CONTINUOUS FURNACES
• They are used for large
production runs
• It is a steady flow of work
pieces is moved through the
furnace by some type of
conveyor or push mechanism.
• It often ending that the work
piece falls into a quench tank
to complete the treatment.
• It is convenient when a single
workstation is used to load
and unload the furnace.
• This furnaces, is designed to
employ artificial gas
atmospheres.

40. 6. SALT BATH FURNACES
• Use liquid heating medium
instead of gas,
• Electrically conductive salt is
heated by passing a current
between two electrodes suspended
in the bath.
• The electrical currents also cause
the bath to circulate and thus to
maintain uniform temperature.
• Non conductive salt bath can be
heated by some form of immersion
heater or the containment vessels
can be externally fired.
• The molten salt serves as a
uniform source of heat
• This is to prevent scaling or
decarburization.

41. 7. LEAD POT FURNACES
• It is a similar furnace to
salt bath furnace
• molten lead replaces salt
as the heat transfer

42. 8. FLUIDIZED BED
FURNACES
• These furnaces consist of a bed of
dry, inert particles, such as
aluminum oxide, which are heated
and fluidized (suspended) in a
stream of flowing gas introduced
into the bed become engulfed in
the particles, which then radiate
uniform heat.
• Temperature and atmosphere can
be altered quickly, and high heat
transfer rates, high thermal
efficiency, and also low fuel
consumption.
• Thus, one furnace can be used for
nitriding, stress relieving,
carburizing, carbonitriding,
annealing and hardening.

43. 9. INDUCTION HEATING
Application of electromagnetically
induced energy supplied by an
induction coil to an electrically
conductive workpart
• Widely used for brazing,
soldering, adhesive curing, and
various heat treatments
• When used for steel hardening
treatments, quenching follows
heating
• Cycle times are short, so process
lends itself to high production

44. Typical induction heating setup. High frequency alternating
current in a coil induces current in the workpart to effect
heating.
Induction Heating

45. SURFACE HARDENING
• Thermo chemical treatments applied to steels in which
the composition of the part surface is altered by adding
various elements
• Often called case hardening
• Most common treatments are carburizing, nitriding, and
carbonitriding
• Commonly applied to low carbon steel parts to achieve a
hard, wear-resistant outer shell while retaining a tough
inner core

46. SURFACE HARDENING
PROCESS
1. Pack carburising
2. Gas carburising
3. Liquid carburising
4. Nitriding
5. Boronizing
6. Carbo – nitriding
7. Cyaniding
8. Flame hardening
9. Induction Hardening
10. Laser hardening

47. OUTLINE OF HEAT TREATMENT PROCESSES FOR
SURFACE HARDENING
TABLE 4.1
Process Metals hardened Element added to
surface
Procedure General Characteristics Typical applications
Carburizing Low-carbon steel
(0.2% C), alloy
steels (0.08–0.2%
C)
C Heat steel at 870–950 °C (1600–1750 °F)
in an atmosphere of carbonaceous gases
(gas carburizing) or carbon-containing
solids
(pack carburizing). Then quench.
A hard, high-carbon surface is
produced. Hardness 55 to 65
HRC. Case depth < 0.5–1.5 mm
( < 0.020 to 0.060 in.). Some
distortion of part during heat
treatment.
Gears, cams, shafts,
bearings, piston pins,
sprockets, clutch plates
Carbonitriding Low-carbon steel C and N Heat steel at 700–800 °C (1300–1600 °F)
in an atmosphere of carbonaceous gas
and ammonia. Then quench in oil.
Surface hardness 55 to 62 HRC.
Case depth 0.07 to 0.5 mm
(0.003 to 0.020 in.). Less
distortion than in
carburizing.
Bolts, nuts, gears
Cyaniding Low-carbon steel
(0.2% C), alloy
steels (0.08–0.2%
C)
C and N Heat steel at 760–845 °C (1400–1550 °F)
in a molten bath of solutions of cyanide
(e.g., 30% sodium cyanide) and other
salts.
Surface hardness up to 65 HRC.
Case depth 0.025 to 0.25 mm
(0.001 to 0.010 in.). Some
distortion.
Bolts, nuts, screws, small
gears
Nitriding Steels (1% Al,
1.5% Cr, 0.3%
Mo), alloy steels
(Cr, Mo), stainless
steels, high-speed
tool steels
N Heat steel at 500–600 °C (925–1100 °F)
in an atmosphere of ammonia gas or
mixtures of molten cyanide salts. No
further treatment.
Surface hardness up to 1100 HV.
Case depth 0.1 to 0.6 mm (0.005
to 0.030 in.) and 0.02 to 0.07
mm (0.001
to 0.003 in.) for high speed steel.
Gears, shafts, sprockets,
valves, cutters, boring
bars, fuel-injection pump
parts
Boronizing Steels B Part is heated using boron-containing gas
or solid in contact with part.
Extremely hard and wear
resistant surface. Case depth
0.025– 0.075 mm (0.001–
0.003 in.).
Tool and die steels
Flame hardening Medium-carbon
steels, cast irons
None Surface is heated with an oxyacetylene
torch, then quenched with water spray or
other quenching methods.
Surface hardness 50 to 60 HRC.
Case depth 0.7 to 6 mm (0.030
to 0.25 in.). Little distortion.
Gear and sprocket teeth,
axles, crankshafts, piston
rods, lathe beds and
centers
Induction
hardening
Same as above None Metal part is placed in copper induction
coils and is heated by high frequency
current, then quenched.
Same as above Same as above

48. ANNEALING &
NORMALIZING
Heating and soaking metal at suitable temperature for a certain
time, and slowly cooling
• Reasons for annealing:
• Reduce hardness and brittleness
• Alter microstructure to obtain desirable mechanical
properties
• Soften metals to improve machinability or
formability
• Recrystallize cold worked metals
• Relieve residual stresses induced by shaping

49. DEFINITION
• Normalizing
Heating the material above the upper
transformation temperature and then cooling it
slowly at room temperature
• Annealing
Heating above the upper transformation
temperature or some other high temperature,
and left to soak at that temperature for a
period of time and cooling slowly in the oven
where the temperature is slowly lowered.

50. THE DIFFERENT PURPOSES OF
ANNEALING & NORMALIZING WITH
QUENCHING
ANNEALING AND
NORMALIZING
QUENCHING
Soften Hardens
Weakens the Material Strengthens
Causes Ductility Causes brittleness
Removes Internal Stress Causes internal stress
Removes Distortion Trends Causes distortion
Removes Cracking Trends Causes cracking
Is a Slow Cooling Process In a fast cooling process

51. PURPOSES OF ANNEALING AND
NORMALIZING
Material that has softer and less strong characteristics
are:-
• Easy to machine (machineability)
• Easy to form (forming ability)
• To relieve internal stresses
• To refine the crystal structure.

52. THANK YOU
!!