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Heat treatment
1. Introduction
The key to improve the material property is to change
its structure at atomic level. This can be achieved
through (i) alloying and (ii) controlled heat and
cooling basically – heat treatment
The basic steps of heat treatment are:
Heat -> Soaking -> Cooling
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Heat treatment
Heat -> Soaking -> Cooling
Temperature
Time of soaking Rate of cooling
Medium of cooling
- Different combinations of the above parameters
- Different compositions of materials and initial
phases of materials
Different heat treatments
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Heat treatment
2. Purpose of heat treatment
(1) Soften the metal prior (before) to shaping;
(2) Relieve (ease) the effects of strain hardening that
occurs during cold forming;
(3) Achieve the final strength and hardness required
in the finished product as one of the end
manufacturing processes.
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Heat treatment
Body heat treatment
Heat treatment
Surface heat treatment
3. Classification of heat treatment processes
(for basics of engineering technology)
(1) Annealing
(2) Martensite formation in steel
(3) Surface hardening
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Heat treatment
4. Annealing
Heat -> Soaking -> Cooling
-Reduce hardness and brittleness
-Alter microstructure for desired mechanical properties
-Soften metals to improve formability
-Recrystalize cold worked (strain hardened) metals
-Relieve stress from shaping
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Heat treatment
4. Annealing
• Full annealing (heating alloy into austenite region
and slow cooling in furnace to form coarse pearlite)
• Normalizing (cooling at faster rates, cool in air
resulted in fine pearlite, high strength and hardness)
• Process Anneal (to allow further cold working of
part)
- Re-crystallization anneal (full recovery of cold
worked metal)
- Recovery Anneal (partial return of grain structure)
• Stress-relief annealing (reduce distortion and
dimensional variations)
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Heat treatment
5. TTT curve – principle for Martensite Formation
- eutectoid composition
- preheat or heat up alloy to austenite.
- austenite to various phases : cooling rate.
Pearlite, P; Bainite, B: alternative forms of
ferrite-carbide mixtures
Martensite, M
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Metals – Phase diagram for Iron and Carbon
Eutectic
Eutectoid
Fe3C
Handouts 2
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5. TTT curve – principle for Martensite Formation
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 Time ----- x- axis; temperature --- y-axis
 Curve interpreted by starting at 0 in austenite region above
A1=1333F
 Proceeding downwards & to the right along a trajectory
 This is for specific composition of C (0.80%)
 Trajectory proceeds through region indicating transformation into
pearlite or bainite
 TTT diagram shows both start s and finish f lines
 Pearlite (ferrite + Carbide) --- shows slow cooling from austenite--
-- it passes through Ps above nose of TTT curve
 Bainite (ferrite + Carbide) ---- rapid cooling to somewhat above
Ms ----- Nose of TTT curve is avoided----- Much slower cooling
to pass through Bs and into ferrite- carbide region
 If cooling occurs at much faster rate, austenite ----- martensite
BCC to BCT
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Heat treatment
5. TTT curve – principle for Martensite Formation
Martensite: hard and brittle
BCT + carbon
Tetragonal
Ms means the temperature M starts to form.
Ms depends on alloyed element; some are lower
than room temperature
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Heat treatment
5. Heat treatment to form Martensite phase
Austenitizing: heat up to a certain temperature to
form 
Quenching: cooling media:
Brine: the fastest;
Air: the slowest
Tempering: Heat up to temperature below eutectoid
soak for one hour & slow cooling
BCT to BCC
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5. Heat treatment to form Martensite phase
Austenitizing – quenching - tempering
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Heat treatment
5. Hardenability
- The relative capacity of a steel to be hardened by
transformation to martensite; hardness penetration
- Hardenability = Hardness ?
- alloying elements increase the hardenability: to make
TTT curve right: to increase the time to start the
transformation for Austenite to Martensite
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6. Surface heat treatment
1) Thermo chemical treatment
2) Composition of part surface altered by addition of
other elements
3) Adding of carbon, nitrogen, or other elements
Carburizing
Nitriding
Carbonitriding
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Surface heat treatment
- Carburizing
Pack carburizing- Pack carbonaceous
materials (charcoal); Very thick hard
outer layer
Gas carburizing- Diffuse Hydrocarbon
fuels (propane in a furnace); thin hard
outer layer
- Low hardness
- Ductile
- Capable of
withstanding stress
Liquid carburizing- Diffuse molten salt
bath containing sodium cyanide,
barium chloride, and other compounds;
medium sized hard outer layer
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Surface heat treatment
Carburizing -> HRC 60, Thickness: 0.025 – 4 mm
Nitriding -> HRC 70, Thickness: 0.025 – 0.05 mm
Carbonitriding -> HRC 70, Thickness: 0.07-0.5 mm
Chromizing and Boronizing -> HRC 70
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7. Heat treatment methods and facilities
1. Heat treatment furnaces
2. Surface hardening methods
- flame hardening
- induction heating
- Laser beam heating
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