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The Science and Engineering of Materials, 4th ed Donald R. Askeland

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Title: The Science and Engineering of Materials, 4th ed Donald R. Askeland


1
The Science and Engineering of Materials, 4th
edDonald R. Askeland Pradeep P. Phulé
  • Chapter 12 Ferrous Alloys

2
Objectives of Chapter 12
  • Discuss how to use the eutectoid reaction to
    control the structure and properties of steels
    through heat treatment and alloying.
  • Examine two special classes of ferrous alloys
    stainless steels and cast irons.

3
Chapter Outline
  • 12.1 Designations and Classification of
    Steels
  • 12.2 Simple Heat Treatments
  • 12.3 Isothermal Heat Treatments
  • 12.4 Quench and Temper Heat Treatments
  • 12.5 Effect of Alloying Elements
  • 12.6 Application of Hardenability

4
Chapter Outline (Continued)
  • 12.7 Specialty Steels
  • 12.8 Surface Treatments
  • 12.9 Weldability of Steel
  • 12.10 Stainless Steels
  • 12.11 Cast Irons

5
Figure 12.1 (a) In a blast furnace, iron ore is
reduced using coke (carbon) and air to produce
liquid pig iron. The high-carbon content in the
pig iron is reduce by introducing oxygen into the
basic oxygen furnace to produce liquid steel. An
electric arc furnace can be used to produce
liquid steel by melting scrap. (b) Schematic of a
blast furnace operation. (Source www.steel.org.
Used with permission of the American Iron and
Steel Institute.)
6
Section 12.1
Designations and Classification of Steels
  • Designations - The AISI (American Iron and Steel
    Institute) and SAE (Society of Automotive
    Engineers) provide designation systems for steels
    that use a four- or five-digit number.
  • Classifications - Steels can be classified based
    on their composition or the way they have been
    processed.

7
Figure 12.2 (a) The eutectoid portion of the
Fe-Fe3C phase diagram. (b) An expanded version of
the Fe-C diagram, adapted from several sources.
8
Figure 12.3 Electron micrographs of (a) pearlite,
(b) bainite, and (c) tempered martensite,
illustrating the differences in cementite size
and shape among these three microconstituents (?
7500). (From The Making, Shaping, and Treating of
Steel, 10th Ed. Courtesy of the Association of
Iron and Steel Engineers.)
9
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10
Example 12.1
Design of a Method to Determine AISI
Number
An unalloyed steel tool used for machining
aluminum automobile wheels has been found to work
well, but the purchase records have been lost and
you do not know the steels composition. The
microstructure of the steel is tempered
martensite, and assume that you cannot estimate
the composition of the steel from the structure.
Design a treatment that may help determine the
steels carbon content.
11
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12
Section 12.2
Simple Heat Treatments
  • Process Annealing Eliminating Cold Work A
    low-temperature heat treatment used to eliminate
    all or part of the effect of cold working in
    steels.
  • Annealing and Normalizing Dispersion
    Strengthening Annealing - A heat treatment used
    to produce a soft, coarse pearlite in steel by
    austenitizing, then furnace cooling. Normalizing
    - A simple heat treatment obtained by
    austenitizing and air cooling to produce a fine
    pearlitic structure.
  • Spheroidizing Improving Machinability
    Spheroidite - A microconstituent containing
    coarse spheroidal cementite particles in a matrix
    of ferrite, permitting excellent machining
    characteristics in high-carbon steels.

13
Figure 12.4 Schematic summary of the simple heat
treatments for (a) hypoeutectoid steels and (b)
hypereutectoid steels.
14
Figure 12.5 The effect of carbon and heat
treatment on the properties of plain-carbon
steels.
15
Figure 12.6 The microstructure of spheroidite,
with Fe3C particles dispersed in a ferrite matrix
(? 850). (From ASM Handbook, Vol. 7, (1972), ASM
International, Materials Park, OH 44073.)
16
Example 12.2
Determination of Heat Treating Temperatures
Recommend temperatures for the process annealing,
annealing, normalizing, and spheroidizing of
1020, 1077, and 10120 steels.
17
Figure 12.2 (a) The eutectoid portion of the
Fe-Fe3C phase diagram. (b) An expanded version of
the Fe-C diagram, adapted from several sources.
18
Example 12.2 SOLUTION From Figure 12.2, we find
the critical A1, A3, or Acm, temperatures for
each steel. We can then specify the heat
treatment based on these temperatures.
19
Section 12.3
Isothermal Heat Treatments
  • Austempering - The isothermal heat treatment by
    which austenite transforms to bainite.
  • Isothermal annealing - Heat treatment of a steel
    by austenitizing, cooling rapidly to a
    temperature between the A1 and the nose of the
    TTT curve, and holding until the austenite
    transforms to pearlite.

20
Figure 12.7 The austempering and isothermal
anneal heat treatments in a 1080 steel.
21
Figure 12.8 The TTT diagrams for (a) a 1050 and
(b) a 10110 steel.
22
Example 12.3
Design of a Heat Treatment for an Axle
A heat treatment is needed to produce a uniform
microstructure and hardness of HRC 23 in a 1050
steel axle.
Figure 12.8 The TTT diagrams for (a) a 1050 and
(b) a 10110 steel.
23
Figure 12.2 (a) The eutectoid portion of the
Fe-Fe3C phase diagram. (b) An expanded version of
the Fe-C diagram, adapted from several sources.
24
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25
Figure 12.9 Producing complicated structures by
interrupting the isothermal heat treatment of a
1050 steel.
26
Figure 12.10 Dark feathers of bainite surrounded
by light martensite, obtained by interrupting the
isothermal transformation process (? 1500). (ASM
Handbook, Vol. 9 Metallography and Microstructure
(1985), ASM International, Materials Park, OH
44073.)
27
Section 12.4
Quench and Temper Heat Treatments
  • Retained austenite - Austenite that is unable to
    transform into martensite during quenching
    because of the volume expansion associated with
    the reaction.
  • Tempered martensite - The microconstituent of
    ferrite and cementite formed when martensite is
    tempered.
  • Quench cracks - Cracks that form at the surface
    of a steel during quenching due to tensile
    residual stresses that are produced because of
    the volume change that accompanies the
    austenite-to-martensite transformation.
  • Marquenching - Quenching austenite to a
    temperature just above the MS and holding until
    the temperature is equalized throughout the steel
    before further cooling to produce martensite.

28
Figure 12.11 The effect of tempering temperature
on the mechanical properties of a 1050 steel.
29
Example 12.4
Design of a Quench and
Temper Treatment
A rotating shaft that delivers power from an
electric motor is made from a 1050 steel. Its
yield strength should be at least 145,000 psi,
yet it should also have at least 15 elongation
in order to provide toughness. Design a heat
treatment to produce this part.
  • Example 12.4 SOLUTION
  • Austenitize above the A3 temperature of 770oC for
    1 h. An appropriate temperature may be 770 55
    825oC.
  • Quench rapidly to room temperature. Since the Mf
    is about 250oC, martensite will form.
  • Temper by heating the steel to 440oC. Normally, 1
    h will be sufficient if the steel is not too
    thick.
  • Cool to room temperature.

30
Figure 12.12 Retained austenite (white) trapped
between martensite needles (black) (?
1000). (From ASM Handbook, Vol. 8, (1973), ASM
International, Materials Park, OH 44073.)
31
Figure 12.13 Increasing carbon reduces the Ms and
Mf temperatures in plain-carbon steels.
32
Figure 12.14 Formation of quench cracks caused
by residual stresses produced during quenching.
The figure illustrates the development of
stresses as the austenite transforms to
martensite during cooling.
33
Figure 12.15 The marquenching heat treatment
designed to reduce residual stresses ands quench
cracking.
34
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35
Figure 12.16 The CCT diagram (solid lines) for a
1080 steel compared with the TTT diagram (dashed
lines).
36
Figure 12.17 The CCT diagram for a low-alloy,
0.2 C Steel.
37
Section 12.5
Effect of Alloying Elements
  • Hardenability - Alloy steels have high
    hardenability.
  • Effect on the Phase Stability - When alloying
    elements are added to steel, the binary Fe-Fe3C
    stability is affected and the phase diagram is
    altered.
  • Shape of the TTT Diagram - Ausforming is a
    thermomechanical heat treatment in which
    austenite is plastically deformed below the A1
    temperature, then permitted to transform to
    bainite or martensite.
  • Tempering - Alloying elements reduce the rate of
    tempering compared with that of a plain-carbon
    steel.

38
Figure 12.18 (a) TTT and (b) CCT curves for a
4340 steel.
39
Figure 12.19 The effect of 6 manganese on the
stability ranges of the phases in the eutectoid
portion of the Fe-Fe3C phase diagram.
40
Figure 12.20 When alloying elements introduce a
bay region into the TTT diagram, the steel can be
ausformed.
41
Figure 12.21 The effect of alloying elements on
the phases formed during the tempering of steels.
The air-hardenable steel shows a secondary
hardening peak.
42
Section 12.6
Application of Hardenability
  • Jominy test - The test used to evaluate
    hardenability. An austenitized steel bar is
    quenched at one end only, thus producing a range
    of cooling rates along the bar.
  • Hardenability curves - Graphs showing the effect
    of the cooling rate on the hardness of
    as-quenched steel.
  • Jominy distance - The distance from the quenched
    end of a Jominy bar. The Jominy distance is
    related to the cooling rate.

43
Figure 12.22 The set-up for the Jominy test used
for determining the hardenability of a steel.
44
Figure 12.23 The hardenability curves for
several steels.
45
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46
Example 12.5
Design of a Wear-Resistant Gear
A gear made from 9310 steel, which has an
as-quenched hardness at a critical location of
HRC 40, wears at an excessive rate. Tests have
shown that an as-quenched hardness of at least
HRC 50 is required at that critical location.
Design a steel that would be appropriate.
Figure 12.23 The hardenability curves for
several steels.
47
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48
Example 12.5 SOLUTION From Figure 12.23, a
hardness of HRC 40 in a 9310 steel corresponds to
a Jominy distance of 10/16 in. (10oC/s). If we
assume the same Jominy distance, the other steels
shown in Figure 12.23 have the following
hardnesses at the critical location 1050 HRC
28 1080 HRC 36 4320 HRC 31 8640 HRC 52 4340
HRC 60 In Table 12-1, we find that the 86xx
steels contain less alloying elements than the
43xx steels thus the 8640 steel is probably less
expensive than the 4340 steel and might be our
best choice. We must also consider other factors
such as durability.
49
Figure 12.24 The Grossman chart used to
determine the hardenability at the center of a
steel bar for different quenchants.
50
Example 12.6
Design of a Quenching Process
Design a quenching process to produce a minimum
hardness of HRC 40 at the center of a 1.5-in.
diameter 4320 steel bar.
51
Figure 12.24 The Grossman chart used to
determine the hardenability at the center of a
steel bar for different quenchants.
52
Figure 12.23 The hardenability curves for
several steels.
53
Example 12.6 SOLUTION Several quenching media are
listed in Table 12-2. We can find an approximate
H coefficient for each of the quenching media,
then use Figure 12.24 to estimate the Jominy
distance in a 1.5-in. diameter bar for each
media. Finally, we can use the hardenability
curve (Figure 12.23) to find the hardness in the
4320 steel. The results are listed below.
The last three methods, based on brine or
agitated water, are satisfactory. Using an
unagitated brine quenchant might be least
expensive, since no extra equipment is needed to
agitate the quenching bath. However, H2O is less
corrosive than the brine quenchant.
54
Section 12.7 Specialty
Steels
  • Tool steels - A group of high-carbon steels that
    provide combinations of high hardness, toughness,
    or resistance to elevated temperatures.
  • Secondary hardening peak - Unusually high
    hardness in a steel tempered at a high
    temperature caused by the precipitation of alloy
    carbides.
  • Dual-phase steels - Special steels treated to
    produce martensite dispersed in a ferrite matrix.
  • Maraging steels - A special class of alloy steels
    that obtain high strengths by a combination of
    the martensitic and age-hardening reactions.

55
Figure 12.25 Microstructure of a dual-phase
steel, showing islands of light martensite in a
ferrite matrix (? 2500). (From G. Speich,
Physical Metallurgy of Dual-Phase Steels,
Fundamentals of Dual-Phase Steels, The
Metallurgical Society of AIME, 1981.)
56
Section 12.8
Surface Treatments
  • Selectively Heating the Surface - Rapidly heat
    the surface of a medium-carbon steel above the A3
    temperature and then quench the steel.
  • Case depth - The depth below the surface of a
    steel at which hardening occurs by surface
    hardening and carburizing processes.
  • Carburizing - A group of surface-hardening
    techniques by which carbon diffuses into steel.
  • Cyaniding - Hardening the surface of steel with
    carbon and nitrogen obtained from a bath of
    liquid cyanide solution.
  • Carbonitriding - Hardening the surface of steel
    with carbon and nitrogen obtained from a special
    gas atmosphere.

57
Figure 12.26 (a) Surface hardening by localized
heating. (b) Only the surface heats above the A1
temperature and is quenched to martensite.
58
Figure 12.27 Carburizing of a low-carbon steel
to produce a high-carbon, wear-resistant surface.
59
Example 12.7
Design of Surface-Hardening Treatments for
a Drive Train
Design the materials and heat treatments for an
automobile axle and drive gear (Figure 12.28).
Figure 12.28 Sketch of axle and gear assembly
(for example 12.7).
60
Example 12.7 SOLUTION The axle might be made from
a forged 1050 steel containing a matrix of
ferrite and pearlite. The axle could be
surface-hardened, perhaps by moving the axle
through an induction coil to selectively heat the
surface of the steel above the A3 temperature
(about 770oC). After the coil passes any
particular location of the axle, the cold
interior quenches the surface to martensite.
Tempering then softens the martensite to improve
ductility. Carburize a 1010 steel for the gear.
By performing a gas carburizing process above the
A3 temperature (about 860oC), we introduce about
1.0 C in a very thin case at the surface of the
gear teeth. This high-carbon case, which
transforms to martensite during quenching, is
tempered to control the hardness. This
high-carbon case, which transforms to martensite
during quenching, is tempered to control the
hardness.
61
Section 12.9 Weldability of Steel
Figure 12.29 The development of the heat-affected
zone in a weld (a) the structure at the maximum
temperature, (b) the structure after cooling in a
steel of low hardenability, and (c) the structure
after cooling in a steel of high hardenability.
62
Example 12.8
Structures of Heat-Affected Zones
Compare the structures in the heat-affected zones
of welds in 1080 and 4340 steels if the cooling
rate in the heat-affected zone is 5oC/s. Example
12.8 SOLUTION The cooling rate in the weld
produces the following structures 1080 100
pearlite 4340 Bainite and martensite The high
hardenability of the alloy steel reduces the
weldability, permitting martensite to form and
embrittle the weld.
63
Section 12.10 Stainless
Steels
  • Stainless steels - A group of ferrous alloys that
    contain at least 11 Cr, providing extraordinary
    corrosion resistance.
  • Categories of stainless steels
  • Ferritic Stainless Steels
  • Martensitic Stainless Steels
  • Austenitic Stainless Steels
  • Precipitation-Hardening (PH) Stainless Steels
  • Duplex Stainless Steels

64
Figure 12.30 (a) The effect of 17 chromium on
the iron-carbon phase diagram. At low-carbon
contents, ferrite is stable at all temperatures.
(b) A section of the iron-chromium-nickel-carbon
phase diagram at a constant 18 Cr-8 Ni. At
low-carbon contents, austenite is stable at room
temperatures.
65
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66
Figure 12.31 (a) Martensitic stainless steel
containing large primary carbides and small
carbides formed during tempering (? 350). (b)
Austenitic stainless steel (? 500). (From ASM
Handbook, Vols. 7 and 8, (1972, 1973), ASM
International, Materials Park, OH 44073.)
67
Example 12.9
Design of a Test to Separate
Stainless Steels
In order to efficiently recycle stainless steel
scrap, we wish to separate the high-nickel
stainless steel from the low-nickel stainless
steel. Design a method for doing this. Example
12.9 SOLUTION Performing a chemical analysis on
each piece of scrap is tedious and expensive.
Sorting based on hardness might be less
expensive however, because of the different
types of treatmentssuch as annealing, cold
working, or quench and temperingthe hardness may
not be related to the steel composition. The
high-nickel stainless steels are ordinarily
austenitic, whereas the low-nickel alloys are
ferritic or martensitic. An ordinary magnet will
be attracted to the low-nickel ferritic and
martensitic steels, but will not be attracted to
the high-nickel austenitic steel. We might
specify this simple and inexpensive magnetic test
for our separation process.
68
Section 12.11 Cast
Irons
  • Cast iron - Ferrous alloys containing sufficient
    carbon so that the eutectic reaction occurs
    during solidification.
  • Eutectic and Eutectoid reaction in Cast Irons
  • Types of cast irons
  • Gray cast iron
  • White cast iron
  • Malleable cast iron
  • Ductile or nodular, cast iron
  • Compacted graphite cast iron

69
Figure 12.32 Schematic drawings of the five types
of cast iron (a) gray iron, (b) white iron, (c)
malleable iron, (d) ductile iron, and (e)
compacted graphite iron.
70
Figure 12.33 The iron-carbon phase diagram
showing the relationship between the stable
iron-graphite equilibria (solid lines) and the
metastable iron-cementite reactions (dashed
lines).
71
Figure 12.34 The transformation diagram for
austenite in a cast iron.
72
Figure 12.35 (a) Sketch and (b) photomicrograph
of the flake graphite in gray cast iron (x 100).
73
Figure 12.36 The effect of the cooling rate or
casting size on the tensile properties of two
gray cast irons.
74
Figure 12.37 The heat treatments for ferritic and
pearlitic malleable irons.
75
Figure 12.38 (a) White cast iron prior to heat
treatment (? 100). (b) Ferritic malleable iron
with graphite nodules and small MnS inclusions in
a ferrite matrix (? 200). (c) Pearlitic malleable
iron drawn to produce a tempered martensite
matrix (? 500). (Images (b) and (c) are from
Metals Handbook, Vols. 7 and 8, (1972, 1973), ASM
International, Materials Park, OH 44073.) (d)
Annealed ductile iron with a ferrite matrix (?
250). (e) As-cast ductile iron with a matrix of
ferrite (white) and pearlite (? 250). (f)
Normalized ductile iron with a pearlite matrix (?
250).
76
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77
Figure 12.17 (Repeated for Problem 12.20) The CCT
diagram for a low-alloy, 0.2 C steel.
78
Figure 12.23 (Repeated for Problem 12.54) The
hardenability curves for several steels.
79
Figure 12.30b (Repeated for Problem 12.48) (b) A
section of the iron-chromium-nickel-carbon phase
diagram at a constant 18 Cr-8 Ni. At
low-carbon contents, austenite is stable at room
temperature.
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