Formula Sheet: Heat Treatment | Engineering Materials - Mechanical Engineering PDF Download

 Page 1


Heat Treatment of Steels
1. Purpose of Heat Treatment
• Modifymechanicalproperties: hardness,toughness,ductility,strength,wearresistance
• Refine grain size and relieve stresses
• Tailor microstructure for specific applications
2. Common Heat Treatment Processes
Process Description Key Tempera-
tures
Microstructure
Change
Effects
Annealing Heat above critical temperature,
slow cooling
AboveA
3
(hypo-
eutectoid) or A
1
Coarse pearlite/ferrite
formation
Softens steel, im-
proves machinability,
reduces hardness,
relieves stresses
Normalizing Heat above critical temperature,
air cooling
Above A
3
or A
1
Fine pearlite forma-
tion
Refines grain struc-
ture, improves
strength and tough-
ness
Quenching Heat above critical temperature,
rapid cooling (water/oil)
Above A
3
or A
1
Martensite formation
(hard and brittle)
Increases hardness
and strength; induces
residual stresses
Tempering Reheat quenched steel below A
1
150
? C to 700
? C Martensite decom-
poses into tempered
martensite or bainite
Reduces brittleness,
improves toughness
Stress Relieving Heat below A
1
, slow cooling 550
? C to 650
? C Residual stresses re-
duced
Removes stresses
without signifi-
cant microstructure
changes
3. Critical Temperatures for Steel
• A
1
: Eutectoid temperature˜ 727
? C
• A
3
: Upper critical temperature for hypo-eutectoid steels (composition dependent)
1
Page 2


Heat Treatment of Steels
1. Purpose of Heat Treatment
• Modifymechanicalproperties: hardness,toughness,ductility,strength,wearresistance
• Refine grain size and relieve stresses
• Tailor microstructure for specific applications
2. Common Heat Treatment Processes
Process Description Key Tempera-
tures
Microstructure
Change
Effects
Annealing Heat above critical temperature,
slow cooling
AboveA
3
(hypo-
eutectoid) or A
1
Coarse pearlite/ferrite
formation
Softens steel, im-
proves machinability,
reduces hardness,
relieves stresses
Normalizing Heat above critical temperature,
air cooling
Above A
3
or A
1
Fine pearlite forma-
tion
Refines grain struc-
ture, improves
strength and tough-
ness
Quenching Heat above critical temperature,
rapid cooling (water/oil)
Above A
3
or A
1
Martensite formation
(hard and brittle)
Increases hardness
and strength; induces
residual stresses
Tempering Reheat quenched steel below A
1
150
? C to 700
? C Martensite decom-
poses into tempered
martensite or bainite
Reduces brittleness,
improves toughness
Stress Relieving Heat below A
1
, slow cooling 550
? C to 650
? C Residual stresses re-
duced
Removes stresses
without signifi-
cant microstructure
changes
3. Critical Temperatures for Steel
• A
1
: Eutectoid temperature˜ 727
? C
• A
3
: Upper critical temperature for hypo-eutectoid steels (composition dependent)
1
• A
cm
: Upper critical temperature for hyper-eutectoid steels
4. Microstructures
Name Characteristics
Ferrite (a -Fe) Soft, ductile, body-centered cubic (BCC) crystal
structure
Austenite (? -Fe) Face-centered cubic (FCC) structure, stable at
high temperature
Pearlite Lamellar mixture of ferrite and cementite (Fe
3
C)
Cementite (Fe
3
C) Hard, brittle iron carbide
Martensite Supersaturated carbon in body-centered tetrago-
nal (BCT) structure, very hard and brittle
5. Time-Temperature-Transformation (TTT) Diagram
• Shows phase transformations as a function of time at constant temperature
• Determines cooling rates required to form martensite
• Helps design heat treatment cycles
6. Useful Formulas and Relations
• Approximate Critical Cooling Rate
Minimum cooling rate to avoid pearlite and form martensite:
R
c
?
1
t
where R
c
= critical cooling rate, t = time for transformation
• Hardness and Cooling Rate
Hardness increases with faster cooling rate, approximately:
H? log(R)
where H = hardness, R = cooling rate
• Rough Estimate for Annealing Time
To fully anneal, time t can be estimated by:
t =
L
2
p 2
D
where L = thickness of specimen, D = diffusion coefficient of carbon in austenite
2
Page 3


Heat Treatment of Steels
1. Purpose of Heat Treatment
• Modifymechanicalproperties: hardness,toughness,ductility,strength,wearresistance
• Refine grain size and relieve stresses
• Tailor microstructure for specific applications
2. Common Heat Treatment Processes
Process Description Key Tempera-
tures
Microstructure
Change
Effects
Annealing Heat above critical temperature,
slow cooling
AboveA
3
(hypo-
eutectoid) or A
1
Coarse pearlite/ferrite
formation
Softens steel, im-
proves machinability,
reduces hardness,
relieves stresses
Normalizing Heat above critical temperature,
air cooling
Above A
3
or A
1
Fine pearlite forma-
tion
Refines grain struc-
ture, improves
strength and tough-
ness
Quenching Heat above critical temperature,
rapid cooling (water/oil)
Above A
3
or A
1
Martensite formation
(hard and brittle)
Increases hardness
and strength; induces
residual stresses
Tempering Reheat quenched steel below A
1
150
? C to 700
? C Martensite decom-
poses into tempered
martensite or bainite
Reduces brittleness,
improves toughness
Stress Relieving Heat below A
1
, slow cooling 550
? C to 650
? C Residual stresses re-
duced
Removes stresses
without signifi-
cant microstructure
changes
3. Critical Temperatures for Steel
• A
1
: Eutectoid temperature˜ 727
? C
• A
3
: Upper critical temperature for hypo-eutectoid steels (composition dependent)
1
• A
cm
: Upper critical temperature for hyper-eutectoid steels
4. Microstructures
Name Characteristics
Ferrite (a -Fe) Soft, ductile, body-centered cubic (BCC) crystal
structure
Austenite (? -Fe) Face-centered cubic (FCC) structure, stable at
high temperature
Pearlite Lamellar mixture of ferrite and cementite (Fe
3
C)
Cementite (Fe
3
C) Hard, brittle iron carbide
Martensite Supersaturated carbon in body-centered tetrago-
nal (BCT) structure, very hard and brittle
5. Time-Temperature-Transformation (TTT) Diagram
• Shows phase transformations as a function of time at constant temperature
• Determines cooling rates required to form martensite
• Helps design heat treatment cycles
6. Useful Formulas and Relations
• Approximate Critical Cooling Rate
Minimum cooling rate to avoid pearlite and form martensite:
R
c
?
1
t
where R
c
= critical cooling rate, t = time for transformation
• Hardness and Cooling Rate
Hardness increases with faster cooling rate, approximately:
H? log(R)
where H = hardness, R = cooling rate
• Rough Estimate for Annealing Time
To fully anneal, time t can be estimated by:
t =
L
2
p 2
D
where L = thickness of specimen, D = diffusion coefficient of carbon in austenite
2
• Martensite Start Temperature (M
s
) Approximation
M
s
(
? C) = 565- 400× (wt.% C)- 30× (wt.% Mn)- 17× (wt.% Ni)- 12× (wt.% Cr)- 7× (wt.% Mo)
• CCT and TTT Diagrams
Critical cooling rates can be found from Continuous Cooling Transformation (CCT)
diagrams which approximate the start and finish of phase transformations.
3