ME 330 Engineering Materials
Lecture 13
Applications and Processing of Metal Alloys
 Types of Metal Alloys
 Fabrication of Metals
 Thermal Processing of Metals
Read Chapter 11
Chapter 9 - 1
Objectives for Chapter 11
•
•
•
•
•
•
•
•
Have basic understanding of types of steels, relative C content,
strength, and applications.
Know fundamental differences between the types of cast iron.
Describe the purpose of and procedures for process annealing, stress
relief annealing, normalizing, full annealing, and spheroidizing.
Define hardenability and list the class steels that have low
hardenability.
Know how to use the Jominy End Quench Test - see lab on
hardenability.
Explain the procedure to precipitation harden an alloy and what
characteristics on the phase diagram allow the alloy to be hardened by
this mechanism.
Explain the shape of the strength vs time curve for precipitation
hardened alloys in terms of the mechanism of hardening in each stage.
Distinguish between the four types of forming operations.
Distinguish between the four types of casting techniques.
Chapter 9 - 2
Ferrous Alloys
Chapter 9 - 3
Metal Fabrication
Chapter 9 - 4
Heat Treating Steel
Recall:
Temperature °C
1000
g
800
a
a+g
600
a + Fe3C
0
•
g + Fe3C
% Carbon
1%
pearlite
Full Annealing
– Heat to 15-40 °C above phase transition line, hold for
diffusion, and slow cool (usually in a furnace, often
takes number of hours).
– Returns microstructure to coarse pearlite.
– Often performed on steels to be machined or formed to
increase ductility
Chapter 9 - 5
Recall:
Heat Treating Steel
Temperature °C
1000
g
800
a
g + Fe3C
a+g
600
a + Fe3C
0
•
% Carbon
1%
Normalizing
– Heat to the fully austenite region and air cool.
– Fine pearlite microstructure
– Refine grain size and distribution (often done
after a rolling operation)
Chapter 9 - 6
Heat Treating Steel
Recall:
Temperature °C
1000
g
800
a
a+g
600
a + Fe3C
0
•
g + Fe3C
% Carbon
1%
Spheroidizing
– Heat a medium-high carbon steel below the eutectoid
line and hold for several hours
– Forms small Fe3C spherical particles in a matrix.
– For higher carbon steels, even pearlite is often brittle
and difficult to deform
– Spheroidizing minimizes hardness and is highly
Chapter 9 machinable
7
Heat Treating Steel
Recall:
Temperature °C
1000
g
800
a
a+g
600
a + Fe3C
0
•
g + Fe3C
1%
% Carbon
Quenching
– Heat to fully austenitize steel
– Rapidly cool (quench in water, oil, etc)
– No time for carbon to diffuse
– Produces a non-equilibrium microstructure called
martensite
• Very hard, strong, brittle with large internal stresses
Chapter 9 - 8
Heat Treating Steel
Recall:
Temperature °C
1000
g
800
a
g + Fe3C
a+g
600
a + Fe3C
0
•
1%
% Carbon
Tempering
– After quenching to form martensite
– Reheat below eutectoid temperature
– Hold for several hours to precipitate carbides and
relieve residual stresses
– Forms tempered martensite
• Relatively hard, strong, much more ductile
• Often ideal microstructure ...
Chapter 9 - 9
Hardenability
•
Hardness vs. Hardenability
– Hardness – resistance to deformation
– Hardenability – ability to be form martensite
(to be hardened)
•
Formal definition:
– Hardenability – ability of an alloy to be
hardened by formation of martensite due to
given heat treatment
•
High hardenability  martensite forms (i.e.
hardens) far into specimen, not just at surface
Chapter 9 - 10
Jominy End Quench Test
Standard test for measuring hardness of any material
Hardness, HRC
Sample of fixed geometry suspended over water jet
Geometric factors all constant
Only effect on hardness is alloying content
Hardness measured as function of distance from quenched
end
Distance
–
–
–
–
Cooling rate
•
Distance from quenched end
Chapter 9 - 11
Hardness Profiles
•
Cooling rate
Cooling rate at 700 °C (°C/s )
– Highest at quenched end
– Smoothly decreases with
– distance
Microstructure
–
–
–
–
Martensite
Slower
Bainite
Cooling
Fine Pearlite
Course Pearlite
18
9
5.6
3.9
2.8
2
60
Hardness, HRC
•
170 70
8660
50
40
8640
30
•
Often plot cooling rate and distance
•
Increasing carbon content
8630
8620
10
20
30
40
– Maximum possible hardness increases Distance from quenched end, mm
– Higher overall hardness (more hardenable)
Chapter 9 - 12
50
Severity of Quench
•
•
Rate of cooling!
Depends on
– Medium- Fluid in which an austenetized part is plunged
• Most common: air, oil, water
– Agitation - Relative motion of quenching medium during
cooling
• Agitation rate influences cooling rate
– Surface area to volume ratio • All heat removed at surface
• More surface area provides more opportunity to
remove heat
• Irregular shapes with edges have large quenching
surfaces
•
For highly stressed part, want 80% martensite throughout part
– Usually from a “guideline” for specific company
Chapter 9 - 13
Using Cooling Curves
•
•
Normal quenching takes place on radial surface
Can predict hardness across radius of a bar from
Jominy tests.
Chapter 9 - 14
Review: Heat Treatment of Steel
•
•
•
•
•
Annealing- Heat, holding at temperature, gradual
cool
– General term, but also used for specific
processes
Tempering -Heat martensite to get diffusional
transformation:
– martensite (BCT single phase)  tempered martensite (a + Fe3C )
– Still have ultra-fine microstructure, but more
ductile
Process annealing -Reverse effects of cold work;
recovery, recrystallization, but little grain growth
Stress relief - Lower temperature anneal to undo
thermal stress or transformation mismatch
stress
Spheroidizing - Heat pearlite just below eutectoid
temp to produce spheroidal structure; makes
Chapter 9 - 15
steel easier to machine
Surface Hardening Techniques
•
•
Many applications, especially wear
– Strong, hard, wear-resistant surface
– Tough, fracture resistant inner core
– Two different heat treatments - through, then
surface
Methods
– Chose material with steep cooling curve
– Case Hardening - Change chemical composition
of surface
•
•
•
Carburizing
Nitriding
Carbonitriding
– Decremental Hardening – Very localized heat
treatment
•
•
Flame Hardening
Induction Hardening
Chapter 9 - 16
Alloys for Surface Hardness
Cooling rate at 700 °C (°C/s )
170 70
18
9
5.6
3.9
2.8
2
100
4340
80
A
B
Hardness, HRC
50
50
4140
40
8640
30
A
5140
B
1040
10
Prescribe:
Percent Martensite
60
20
30
40
50
Distance from quenched end, mm
Minimum hardness at A
Maximum hardness at B
Must know the cooling rates at each point!
Chapter 9 - 17
Carburizing
•
Hardens steel by causing carbon
to diffuse into the surface.
– Furnace heat to a temperature
at which carbon will diffuse
– Hold until diffusion creates the
proper case depth.
•
Must be a carbon rich
environment
•
For steels with low carbon content
(%C < 0.2)
•
Used extensively on gears and
shafts to harden them yet
maintain the core toughness.
•
Decarburization is opposite
process for high carbon steels softer case
From: Callister p. 92, 224
Chapter 9 - 18
Flame Hardening
•
For steels of hardenable carbon
content (%C  0.4)
•
A high intensity oxy-acetylene flame
is applied to the bring the region of
interest to an austenite
transformation.
•
The interior never reaches high
temperature.
•
The heated region is quenched to
achieve the desired hardness.
•
The depth of hardening can be
increased by increasing the heating
time.
•
In addition, large parts, which will
not normally fit in a furnace, can be
heat-treated
From: Scheer p. 43
Chapter 9 - 19
Precipitation Hardening
•
Form extremely small, dispersed particles of a
second phase
•
Small second phase particles called precipitates
•
Age hardening - strength develops as the alloy
ages
– Time dependent
– There is a maximum attainable effect
•
Different than tempering martensite
– Similar heat treatment procedure
– Different strengthening mechanism
Chapter 9 - 20
Precipitation Hardening
•
•
Primary strengthening mechanism
for
– Aluminum
– Nickel based superalloys
– Titanium
Examples
a Ti in b Ti matrix
– Al/Cu
– Cu/Sn
– Mg/Al
Ni3Al in Ni matrix
From: Socie
Chapter 9 - 21
Phase Diagrams & Treatment

Temperature
L
L+a
L+b
b
a
a+b
Precipitation hardenable alloys have:
 Appreciable maximum solubility
 Decreasing solubility with
temperature
 Composition less than maximum
solubility
Solution Treating:
Heat into the single phase region and
rapidly quench to room temperature to
produce a supersaturated solid solution
Aging:
Composition
Heat to a temperature below the phase
transition to allow time for precipitates
to form
Chapter 9 - 22
Why Precipitation Harden?
•
Look at Aluminum rich side of Al/Cu system
- a is a substitutional solid solution of copper in aluminum
- q is an intermetallic compound CuAl2
- One slow cool in is NOT helpful in strengthening
Coarse q phase weakens the alloy
•
Temperature
°C
700
600
a
500
400
100% a
(95.5% Al, 4.5% Cu)
Coarse q precipitates
At a grain boundaries
q+a
300
200
100
Al
5
10
wt % Cu
time
Chapter 9 - 23
How to Precipitation Harden
•
Two reheating treatments are needed:
–
–
•
Solution treatment
Age hardening
Fine precipitates strengthen & harden material
100% a solid solution
Temperature
°C
700
600
500
a
Equilibrium
Microstructure
400
q+a
300
Fine precipitates in grains
(retained after cooling)
200
100
Al
5
10
wt % Cu
time
Chapter 9 - 24
Overaging
If precipitates get too big, strengthening/hardening is lost
Process sped up by temperature
Some alloys age at room temperature
Why is strength lost?
100% a solid solution
Temperature
•
•
•
•
Fine precipitates in grains
Coarse precipitates
in grains
taging
time
Chapter 9 - 25
Strength Development
•
•
•
•
Fine precipitates (Guinier-Preston zones  q’’ phase) are
coherent with lattice
Deformation of crystal impedes dislocation motion
At optimal aging time, precipitates are dispersed, small, and
coherent.
After optimal aging time, precipitates get too big, incoherent with
matrix.
From: Callister
Chapter 9 - 26
Age vs. Material Properties
•
Must age carefully to avoid
overaging
– Too high temperature
– Too long age time
•
Precipitation rate maximized at
intermediate temperatures
– Near solvus  no driving
force for nucleation
– Low temperatures 
diffusion is slow
Nucleation
T
Diffusion
t
Gm   G * 
N ~ N o exp   exp  
kT
k

T




From: Callister
Chapter 9 - 27
Strengthening Mechanisms
• Dislocation Looping
• Dislocation cutting
From: Hertzberg
Chapter 9 - 28
What is Attractive About
Aluminum?
•
•
Upside
•
– Lightweight (  Al  0.3*  Steel)
– Ductile
– Heat treatable
– Castable
– Corrosion resistant
•
– Good conductor
Downside
– Formability is lower
• Simpler shapes
E Al  31 E Steel
–
– For same stiffness, need
larger section
Longitudinal stiffness(~EA/L)
E St ASt  E Al AAl
AAl 
ESt
E Al
ASt  3* ASt
 Al AAl L   St ASt L
Bending stiffness (~EI)
– Rect. section, constant b
I  bh3
b
3
3
E St bhSt
 E Al bhAl
h Al  3
ESt
h
E Al St
h
 1.44 * hSt
W Al  Al AAl L  Al h Al b


 0.43
WSt  St ASt L  St hSt b
Chapter 9 - 29
Aluminum Designations
•
•
–
–
–
–
Alloying Composition
–
–
Aluminum
1xxx
Aluminum
• Copper
• Manganese
• Silicon
• Magnesium
• Mg and Si
• Zinc
Common alloys:
Tempering Designations
2xxx
3xxx
4xxx
5xxx
6xxx
7xxx
2024-T4 ,
6061-T6,
7075-T6
-O: Annealed
-F: As fabricated
-H1: Strain Hardened
-H2: Strain Hardened/ Partial
Annealed
– -T: Thermally Processed
• -T2: Annealed (cast products)
• -T3: Solution treated, CW,
naturally aged
• -T4: Solution treated, naturally
aged
• -T5: Artificially aged, no
solution treatment
• -T6: Solution treated, artificially
aged
• -T7: Solution treated, over-aged
• -T8: Solution treated, CW,
artificially aged
• -T9: Solution treated, artificially
aged , CW
Chapter 9 - 30
New Concepts & Terms
• Applications of hardness profiles
(Jominy curves)
• Precipitation hardening
– Heat treatment process
– Effect of overaging
– Microstructures
– Effect on strength
• Applications of aluminum
– Know tradeoffs, especially with
steels
– Which alloys are precipitation
hardenable?
Chapter 9 - 31