Phase Diagram of Carbon Steel and Methods of Steel heat treatment.

2. Iron & Steel
If you had to pick a few technologies that have had a
tremendous effect on modern society, the refining of
iron and steel would have to be somewhere near the
top of the list. Iron and steel show up in a huge array of
modern products. Cars, tractors, bridges, trains (and
their rails), tools, skyscrapers, guns, ships; all depend on
iron and steel to make them strong and inexpensive.
Iron is so important that primitive societies are
measured by the point at which they learn how to refine
iron and enter the iron age!

4. Fe-C phase diagram or Fe-Fe3C phase diagram?????

5. The hardness of plain carbon steel increases
progressively with the increase in carbon
content, so that generally the low and medium
carbon steel are used for structural and
constructional work, whilst the high carbon
steels are used for the manufacture of tools and
other components where hardness and wear-
resistance are necessary.

6. Type of
steel
% Carbon Uses
Dead mild 0.05-0.15 Chains, stampings, rivets, wire, seam
welded pipes.
Mild 0.1-0.3 Structural steels, screws, machine parts,
gears, shafts, levers.
Medium
Carbon
0.3-0.6 Connecting rods, shafts, axles, high
tensile tubes, rotors, loco tyres, rails,
wire ropes.
High
Carbon
0.6-0.9 Drop hammer dies, screw drivers,
hammers, cable wire, dies, punches, rock
drills and some hand tools
Tool Steels 0.9-1.4 Springs, axes, knives, dies, drills, milling
cutters, ball bearings, lathe tools, saws,
razors, machine parts where resistance

8.  Pure Iron:
 At room temperature – 912 0C; Ferrite; α Iron; BCC.
 At 912 – 1394 0C; Austenite; γ Iron; FCC.
 At 1394 – 1538 0C; δ Iron; BCC.
 At >1538 0C; melts.
 In the Iron Carbon phases diagram, three phases
are of importance:
 Austenite; γ.
 Ferrite; α.
 Cementite; Fe3C.

9. Ferrite Austenite

10.  Peritectic at 0.16%C and 1493 0C:
δ (0.1%C)+ L(0.51%C) ↔ γ (0.16%C)
 Eutectic at 4.3%C and 1147 0C:
L (4.3%C)↔ γ (2%C) + Fe3C(6.7%C)
 Eutectoid at 0.76%C and 727 0C:
γ (0.76%C) ↔ α (0.022%C) + Fe3C(6.7%C)
Pearlite

11. Alloy of eutectoid composition(0.76%C)

13. Hypoeutectoid Alloys

14. Hypereutectoid Alloys

15.  <0.006%C: ferritic and classed as commercially
pure iorn.
 0.006-0.8%C: ferrite + pearlite
 0.8%C: pearlite.
 0.8-2.0%C: cementite + pearlite

16. For a 99.65 wt% Fe-O.35 wt% C alloy at a
temperature just below the eutectoid,
 determine the following:
 (a) The fractions of total ferrite and cementite
phases.
 (b) The fractions of the proeutectoid ferrite and
pearlite.
 (c) The fraction of eutectoid ferrite.

17. Alloying/Impurity elements
 Effect of alloying elements on the eutectoid
temperature:

18.  Effect of alloying elements on the eutectoid
composition:

19.  Manganese: soluble in ferrite and austenite, also
forms a stable carbide. Improves strength and
toughness. Should not exceed 0.3% in high carbon
steels because of tendency to induce quench cracks
particularly during water quenching.
 Silicon: imparts fluidity to steels intended for the
manufacture of castings, and is present in such steels in
amounts up to 0.3%. In high carbon steels, silicon must
be kept low, because of its tendency to render
cementite unstable and liable to decompose into
graphite and ferrite.

20.  Phosphorus: soluble in steel to almost 1%. In excess
of this amount, the brittle phosphide Fe3P is
precipitated. Has a considerable hardening effect on
steel but it must be well controlled because of the
brittleness it imparts.
 Nitrogen: Can combine with iron to form iron
nitride or remain dissolved interstitially after
solidification. It causes serious embrittlement and
renders the steel unsuitable for severe cold work.

21.  Sulphur: the most deleterious impurity commonly
present in steel. Tends to form the brittle sulphide; FeS.
Usually precipitate at the crystals grain boundaries. It
has a low melting point which causes the steel to
crumble during hot working. Being brittle at low
temperature, causes the steel unsuitable for cold
working.
To nullify the effects of the sulphur present an excess
of manganese is therefore added during deoxidation.
Thus the sulphur forms manganese oxide which is
insoluble in molten steel and some is lost in slag.

22.  A hypoeutectoid plain carbon steel which was
slow-cooled from austenitic region to room
temperature contains 9.1% eutectoid ferrite.
Assuming no change in structure on cooling
from just below the eutectoid temperature to
room temperature, what is the carbon content of
the steel.

24. Phase Transformations in Metals
 Simple Diffusion-Dependent Transformations:
there is no change in either the number or composition of
the phases present (solidification of a pure metal,
allotropic transformations, and, recrystallization and grain
growth.
 Diffusion-Dependent Transformation with Alteration in
phase compositions / number of phases present:
the final microstructure ordinarily consists of two phases.
The eutectoid reaction,
 Diffusionless Transformation: (displacive transformation)
a metastable phase is produced (martensite transformation)

25. Displacive, Diffusionless Diffusive
Atoms move over distances ≤
interatomic spacing.
Atoms move by making and
breaking interatomic bonds and
by minor “shuffling”.
Atoms move one after another
in precise sequence
(“military” transformation).
Speed of transformation ≈
velocity of lattice vibrations
through crystal (essentially
independent of temperature);
transformation can occur at
temperatures as low as 4 K.
Atoms move over distances of 1
to 106 interatomic spacings.
Atoms move by thermally
activated diffusion from site to
site.
Atoms hop randomly from site
to site (although more hop
“forwards” than “backwards”)
(“civilian” transformation).
Speed of transformation
depends strongly on temperature;
transformation does not occur
below 0.3 Tm to 0.4 Tm .

26. Displacive, Diffusionless Diffusive
Extent of transformation
(volume transformed) depends
on temperature only.
Composition cannot change
(because atoms have no time to
diffuse, they stay where they are).
Always specific crystallographic
relationship between old phase
and parent lattice.
Extent of transformation
depends on time as well as
temperature.
Diffusion allows compositions
of individual phases to change in
alloyed systems.
Sometimes have
crystallographic relationships
between phases.

27.  If you take a piece of 0.8% carbon steel “off the
shelf” and measure its mechanical properties
(hardness, tensile strength and ductility).
 Then test a piece that has been heated to red
heat and then quenched into cold water.
Property As received heated to red heat and
then quenched into cold
water
Hardness(GPa) 2 9
Tensile
strength(MPa)
600 Limited by
brittleness
Elongation % 10 ≈0

28. Phase Transformation kinetics
 Nucleation: the formation of very small (often
submicroscopic) particles, or nuclei, of the new phase,
which are capable of growing.
 Two types of nucleation: homogeneous and
heterogeneous.
 The homogeneous type: nuclei of the new phase
form uniformly throughout the parent phase.
 The heterogeneous type: nuclei form preferentially
at structural inhomogeneities, such as, insoluble
impurities, grain boundaries, dislocations, and so
on.

29. Because solids usually
contain high-energy defects
(like dislocations, grain
boundaries and surfaces) new
phases usually nucleate
heterogeneously;
homogeneous nucleation,
which occurs in
defect-free regions, is rare.

31.  Growth: in which the nuclei increase in size.
Some volume of the parent phase disappears.
The transformation reaches completion if
growth of these new phase particles is allowed
to proceed until the equilibrium fraction is
attained.

32.  Rate of Transformation: the fraction of reaction
that has occurred is measured as a function of
time, while the temperature is maintained
constant.
 Transformation progress is usually ascertained
by either
 microscopic examination.
 measurement of some physical property the
magnitude of which is distinctive of the new phase.

33.  The fraction of transformation y is a function of time t
as follows:
Avrami Equation
 
n
kt
y 

 exp
1

34.  The rate of a transformation r:
the reciprocal of time required for the
transformation to proceed halfway to completion,
t0.5, or:
5
.
0
1
t
r 





 

RT
Q
A
r exp

35. •Supercooling
•Superheating
•Metastable phase

36. γ (0.76%C) ↔ α (0.022%C) + Fe3C(6.7%C)
Pearlite
 Temperature plays an important role in the rate of the
austenite-to-pearlite transformation.
MICROSTRUCTURAL AND PROPERTY
CHANGES IN IRON-CARBON ALLOYS

37. Isothermal Transformation Diagram

38.  Isothermal Transformation Diagrams
 Time-Temperature-Transformation
 TTT Diagram

39. FCC → BCC transformation in iron: the time–temperature–
transformation (TTT) diagram.

41.  This rate-temperature behavior is in apparent
contradiction to what stated earlier which stipulates that
rate increases with increasing temperature.
 The transformation rate is controlled by the rate of
pearlite nucleation, and nucleation rate decreases with
rising temperature (less supercooling).

44. Coarse pearlite Fine pearlite

45. Mechanical Behaviour of pearlite
 Cementite is much harder but more brittle than ferrite.
 Thus, increasing the fraction of Fe3C in a steel alloy
while holding other microstructural elements constant
will result in a harder and stronger material.

47.  The layer thickness of each of the ferrite and
cementite phases in the microstructure also
influences the mechanical behavior of the material.
 Fine pearlite is harder and stronger than coarse
pearlite.
 A large degree of adherence between the two phases
across the boundary: the strong and rigid cementite
phase severely restricts deformation of the softer ferrite
phase in the regions adjacent to the boundary; thus the
cementite may be said to reinforce the ferrite.
 Phase boundaries serve as barriers to dislocation motion
in much the same way as grain boundaries.
 Coarse pearlite is more ductile than fine
pearlite.

49. BAINITE
 Other microconstituents that are products of the
austenitic transformation are found to exist at these
lower temperatures. One of these microconstituents is
called bainite.
 Depending on transformation temperature, two general
types of bainite have been observed: upper bainite and
lower bainite.
 Like pearlite, the microstructure of each of these
bainites consists of ferrite and cementite phases;
however, their arrangements are different from the
alternating lamellar structure found in pearlite.

51.  Upper Bainite :For temperatures between
approximately 300 and 540 0C, bainite forms as a series
of parallel laths (i.e., thin narrow strips) or needles of
ferrite that are separated by elongated particles of the
cementite phase.

52.  Lower Bainite At lower temperatures between
about 200 and 300 0C. The ferrite phase exists as thin
plates, and narrow cementite particles (as very fine rods
or blades) form within these ferrite plates.

53.  Pearlitic and Bainitic transformations are really
competitive with each other, and once some
portion of an alloy has transformed to either
pearlite or bainite, transformation to the other
microconstituent is not possible without
reheating to form austenite.
 Because bainitic steels have a finer structure (i.e.,
smaller Fe3C particles in the ferrite matrix), they
are generally stronger and harder than pearlitic
ones.
 Yet they exhibit a desirable combination of
strength and ductility.

54. SPHEROIDITE
 The Fe3C phase appears as sphere-like particles
embedded in a continuous α phase matrix.
 This transformation has occurred by additional
carbon diffusion with no change in the
compositions or relative amounts of ferrite and
cementite phases.
 The driving force for this transformation is the
reduction in α - Fe3C phase boundary area.

56.  If a steel alloy having either pearlitic or bainitic
microstructures is heated to, and left at, a
temperature below the eutectoid for a
sufficiently long period of time-for example, at
about 700°C for between 18 and 24 h.

57.  Alloys containing pearlitic microstructures have
greater strength and hardness than do those with
spheroidite.
 Spheroidized steels are extremely ductile, and
they are notably tough.
 This behavior is explained in terms of
reinforcement at, and impedence to, dislocation
motion across the ferrite-cementite boundaries.
 Of all steel alloys, those that are softest and
weakest have a spheroidite microstructure.

59. MARTENSITE
 Martensite is a non-equilibrium single phase structure
that results from a diffusionless transformation of
austenite.
 It may be thought of as a transformation product that is
competitive with pearlite and bainite.
 The martensitic transformation occurs when the
quenching rate is rapid enough to prevent carbon
diffusion. Any diffusion whatsoever will result in the
formation of ferrite and cementite phases.

60.  Large numbers of atoms experience cooperative
movements, in that there is only a slight displacement
of each atom relative to its neighbors.
 the martensitic transformation is independent of
time; it is a function only of the temperature to
which the alloy is quenched or rapidly cooled. A
transformation of this type is termed an
(athermal transformation).

62.  This occurs in such a way that the FCC austenite
experiences a polymorphic transformation to a
body-centered tetragonal (BCT) martensite.
 All the carbon atoms remain as interstitial impurities in
martensite; as such, they constitute a supersaturated
solid solution that is capable of rapidly transforming to
other structures if heated to temperatures at which
diffusion rates become appreciable.
 Many steels, however, retain their martensitic structure
almost indefinitely at room temperature.

64.  Two distinctly different martensitic
microstructures are found in iron–carbon alloys:
lath and lenticular.

65. Lath Martensite
 For alloys containing less than about 0.6 wt% C, the
martensite grains form as long and thin plates ( like
blades of grass) that form side by side and are aligned
parallel to one another.
 These laths are grouped into larger structural entities,
called blocks.

66. Lenticular martensite
 For iron–carbon alloys containing greater than
approximately 0.6 wt% C.
 With this structure the martensite grains take on a
lenticular-like or plate-like appearance.

68. Effect of Alloying elements
 The presence of alloying elements other than
carbon (e.g., Cr, Ni, Mo, and W) may cause
significant changes in the positions and shapes of
the curves in the isothermal transformation
diagrams.
 These changes include
 shifting to longer times the nose of the austenite-to-
pearlite transformation
 the formation of a separate bainite nose.

70.  Using the isothermal transformation diagram for
an iron-carbon alloy of eutectoid composition,
specify the nature of the final microstructure (in
terms of microconstituents present and
approximate percentages) of a small specimen
that has been subjected to the following time-
temperature treatments. In each case assume
that the specimen begins at 760°C and that it
has been held at this temperature long enough
to have achieved a complete and homogeneous
austenitic structure.

71. a) Rapidly cool to 350°C, hold for 104 s, and
quench to room temperature.
b) Rapidly cool to 250°C, hold for 100 s, and
quench to room temperature.
c) Rapidly cool to 650°C, hold for 20 s, rapidly
cool to 400°C, hold for 103 s, and quench to
room temperature.

74. Continuous Cooling Transformation
CCT
 For continuous cooling, the time required for a reaction
to begin and end is delayed.
 Thus the isothermal curves are shifted to longer times
and lower temperatures.

77.  Critical Cooling Rate: the minimum rate of cooling
that will produce a totally martensitic structure.
 Only martensite will exist for quenching rates
greater than the critical.

81. Mechanical Strength of Martensite
 Martensite is the hardest, strongest, and the
most brittle.
 Its hardness is dependent on the carbon
content, up to about 0.6 wt%.
 The hardness and strength are attributed to:
 The effectiveness of the interstitial carbon atoms in
hindering dislocation motion.
 To the relatively few slip systems (along which
dislocations move) for the BCT structure.

82. Effect of Quenching the Austenite
 The cooling rate of a specimen depends on the rate of
heat energy extraction, which is a function of the
characteristics of the quenching medium in contact
with the specimen surface, as well as the specimen size
and geometry.
 Severity of quench:
more rapid cooling → more severe quench.
 Water, Oil, and Air…….?

83. Effect of Volume Change
 Increase in volume (decrease in density) at martensite
formation.
 Effect of specimen size on the phase transformation.
 Thick specimen → Larger variation in % martensite
formed across the cross section → Larger variation in
volume change across the section of the specimen.
 Mass Effect: variation in properties due to the large
size of the structure.
 Large pieces may crack during quenching as a result of
internal stresses.

85. Tempering of Martensite
 Removal of internal stresses, and decreasing brittleness.
 Tempering is to increase toughness.
 Unfortunately is accompanied by some decrease in
hardness.
 Tempering tends to transform unstable martensite back
to stable pearlite.
 It causes the dissolved carbon atoms to participate out
as iron carbide particles.

86.  Tempering is accomplished by heating a martensitic
steel to a temperature below the eutectoid for a
specified time period.
 Normally, tempering is carried out at temperatures
between 250 and 650°C.
 Martensite → Tempered martensite
(BCT, single phase) (α + Fe3C phases)
 The microstructure of tempered martensite consists of
extremely small and uniformly dispersed cementite
particles embedded within a continuous ferrite matrix.

88.  Tempered martensite may be nearly as hard and
strong as martensite, but with substantially
enhanced ductility and toughness.
 The hardness and strength may be explained by
 The large ferrite-cementite phase boundary area per unit
volume that exists for the very fine and numerous
cementite particles which act as barriers to dislocation
motion during plastic deformation (The structure is
similar to the microstructure of spheroidite except that
the cementite particles are much, much smaller).
 The hard cementite phase reinforces the ferrite matrix
along the boundaries.

89.  The increase in toughness and ductility may be
explained that the continuous ferrite phase is also very
ductile and relatively tough.
 The size of the cementite particles influences the
mechanical behavior of tempered martensite:
 Increasing the particle size decreases the ferrite-
cementite phase boundary area and, consequently,
results in a softer and weaker material yet one that is
tougher and more ductile.
 The tempering heat treatment (temperature and time)
determines the size of the cementite particles.

90. The dependence of tensile and yield strength and ductility
on tempering temperature for an alloy steel.

91. The time dependence of hardness at several different temperatures.

92.  At temperatures approaching the eutectoid and after
several hours, the microstructure will have become
spheroiditic, with large cementite spheroids
embedded within the continuous ferrite phase.
 Overtempered martensite is relatively soft and
ductile.

93. Is it possible to produce an iron-carbon alloy of
eutectoid composition that has a minimum
hardness of 75 HRB and a minimum ductility of
35%AR ?
If so, describe the continuous cooling heat
treatment to which the alloy would be subjected
to achieve these properties. If it is not possible,
explain why.
????????

96. Temper Embrittlement
 The tempering of some steels may result in a
reduction of toughness.
 When?
 when the steel is tempered at a temperature above
about 575°C followed by slow cooling to room
temperature.
 when tempering is carried out at between
approximately 375 and 575°C.
 Impurities presence helps in the occurrence of
Temper Embrittlement, even when present in
small concentrations.

97.  Impurities presence plays an important role in the
occurrence of Temper Embrittlement, even when
present in small concentrations. (manganese,
nickel, chromium, antimony, phosphorus, arsenic,
and tin)
 The presence of these alloying elements and
impurities shifts the ductile-to-brittle transition to
significantly higher temperatures.
 The crack propagation of these embrittled
materials is intergranular.
 Intergranular: between granules.
Intragranular: within granules.
 Alloy and impurity elements have been found to
preferentially segregate in these regions.

98. How to avoid Temper Embrittlement?
 Reduce impurities.
 Tempering above 575°C or below 375 °C, followed by
quenching to room temperature.
How to cure Temper Embrittlement?
 The toughness of steels that have been embrittled may
be improved significantly by heating to about 600°C
and then rapidly cooling to below 300°C.

99.  The optimum properties of a steel that has been
quenched and then tempered can be realized only
if, during the quenching heat treatment, the
specimen has been converted to a high content of
martensite
 The formation of any pearlite and/or bainite will
result in other than the best combination of
mechanical characteristics.
 During the quenching treatment, it is impossible
to cool the specimen at a uniform rate throughout-
the surface will always cool more rapidly than
interior regions.

100.  Therefore, the austenite will transform over a range
of temperatures, yielding a possible variation of
microstructure and properties with position within
a specimen.
 The successful heat treating of steels to produce a
predominantly martensitic microstructure
throughout the cross section depends mainly on
three factors:
 the composition of the alloy,
 the type and character of the quenching medium,
 the size and shape of the specimen.

101.  There are a multitude of steels that are
responsive to a martensitic heat treatment, and
one of the most important criteria in the
selection process is hardenability.
 Hardenability curves, may be used to ascertain
the suitability of a specific steel alloy for a
particular application.

102. Hardenability
 Is a term that is used to describe the ability of an
alloy to be hardened by the formation of martensite
as a result of a given heat treatment.
 It is a qualitative measure of the rate at which
hardness drops off with distance into the interior of
a specimen as a result of diminished martensite
content.
 A steel alloy that has a high hardenability is one
that hardens, or forms martensite to a large degree
throughout the entire interior.

103. The Jominy end-quench test

106. Effect of Composition

107. Hardenability curves
for five different
steel alloys, each
containing 0.4 wt% C.
Approximate alloy
compositions (wt%) are
as follows:
4340–1.85 Ni, 0.80 Cr,
and 0.25Mo;
4140–1.0 Cr and 0.20
Mo;
8640–0.55 Ni, 0.50 Cr,
and 0.20 Mo;
5140–0.85 Cr;
1040 is an unalloyed
steel.

111. •The Society of Automotive Engineers (SAE),
The American Iron and Steel Institute (AISI), and
The American Society for Testing and Materials
(ASTM)
•The AISI/SAE designation for these steels is a
four-digit number:
•The first two digits indicate the alloy content;
•The last two, the carbon concentration.
•For plain carbon steels, the first two digits are 1 and 0;
•alloy steels are designated by other initial two-digit
combinations (e.g., 13,41,43).
•The third and fourth digits represent the weight percent
carbon multiplied by 100.

112. A unified numbering system (UNS) is used for
uniformly indexing both ferrous and nonferrous
alloys.
 Each UNS number consists of a single-letter prefix
followed by a five-digit number.
 The letter is indicative of the family of metals to which
an alloy belongs.
 The UNS designation for these alloys begins with a G,
followed by the AISI/SAE number; the fifth digit is a
zero.

114. Effect of Quenching Medium
 The cooling rate of a specimen depends on the
rate of heat energy extraction, which is a
function of the characteristics of the quenching
medium in contact with the specimen surface, as
well as the specimen size and geometry.
 Severity of quench:
more rapid cooling → more severe quench.
 Degree of Agitation ?????

115. The Size & Shape of the specimen
 The rate of cooling for a particular quenching
treatment depends on the ratio of surface area to
the mass of the specimen.
 The larger this ratio, the more rapid will be the
cooling rate and, consequently, the deeper the
hardening effect.
 Irregular shapes with edges and corners are more
amenable to hardening by quenching.

118. ?????
 Determine the hardness profile for a 50 mm (2
in.) diameter cylindrical specimen of 1040 steel
that has been quenched in moderately agitated
water.

120. Thermal Processing of Metals
 Annealing: the material is exposed to an elevated
temperature for an extended time period and then
slowly cooled.
 Precipitation Hardening: the formation of
extremely small uniformly dispersed particles of a
second phase within the original phase matrix to
enhance the strength and hardness.

121. Annealing
 The material is exposed to an elevated temperature
for an extended time period and then slowly cooled
in order to:
 relieve stresses;
 increase softness, ductility, and toughness;
 produce a specific microstructure.

122.  Process Annealing: is a heat treatment that is used
to negate the effects of cold work, that is, to soften
and increase the ductility of a previously strain-
hardened metal. (Recovery, recrystalization and
grain growth).
 Stress Relief Annealing: A heat treatment to relief
the internal stresses that might have formed in the
structure due to:
 plastic deformation processes such as machining and
grinding.
 non-uniform cooling of a piece that was processed or
fabricated at an elevated temperature, such as a weld or
a casting.

123. Annealing of Ferrous Alloys

124. Normalizing
 Normalizing: is used to refine the grains and produce a
more uniform and desirable size distribution.
 Normalizing is accomplished by heating at
approximately 55 to 85°C above the upper critical
temperature, until a complete austenite structure is
formed and then cooling in air.
 Austenitizing treatment ????

125. Full Anneal
 The full anneal is often utilized in low- and medium
carbon steels that will be machined or will experience
extensive plastic deformation during a forming
operation.
 The alloy is austenitized by heating to 15 to 40°C above
the A3 or A1 lines until equilibrium is achieved and is
then furnace cooled.
 The microstructural product of this anneal is uniform
coarse pearlite (in addition to any proeutectoid phase)
that is relatively soft and ductile.

126. Spheroidizing
 To produce a Spheroidized steels that have a
maximum softness and ductility and that are easily
machined or deformed.
 Heating the alloy at a temperature just below the
eutectoid for a time that will ordinarily range
between 15 and 25 h.
 During this annealing there is a coalescence of the
Fe3C to form the spheroid particles.

127. Precipitation Hardening

130.  Between 500°C and 580°C, the 4% Cu alloy is
single phase: the Cu dissolves in the Al to give the
random substitutional solid solution α.
 Below 500°C the alloy enters the two-phase field of
α + CuAl2.
 As the temperature decreases the amount of CuAl2
increases, and at room temperature the
equilibrium mixture is 93 wt% α + 7 wt% CuAl2.
 In slow cooling the driving force for the
precipitation of CuAl2 is small and the nucleation
rate is low.

132.  In order to accommodate the equilibrium amount
of CuAl2 the few nuclei that do form grow into
large precipitates of CuAl2 spaced well apart.
 Moving dislocations find it easy to avoid the
precipitates and the alloy is soft.
 If the cooling rate is high, we produce a much finer
structure. Because the driving force is large the
nucleation rate is high.
 The precipitates, although small, are closely
spaced: they get in the way of moving dislocations
and make the alloy harder.

133.  To age harden our Al–4 wt% Cu alloy we use the
following schedule of heat treatments.
 Solution heat treat at 550°C. This gets all the Cu into
solid solution.
 Cool rapidly to room temperature by quenching into
water or oil. We will miss the nose of the C-curve and
will end up with a highly supersaturated solid solution at
room temperature.
 Hold at 150°C for 100 hours (“age”). The supersaturated
α will transform to the equilibrium mixture of saturated
α + CuAl2. But it will do so under a very high driving
force and will give a very fine (and very strong)
structure.