1. The Science and Engineering
of Materials, 4th ed
Donald R. Askeland – Pradeep P. Phulé
Chapter 12 – Ferrous Alloys
1
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.
2
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
3
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.)
5
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.
6
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.)
8
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
steel’s 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 steel’s carbon
content.
10
11. Example 12.1 SOLUTION
The first way is to heat the steel to a temperature just below
the A1 temperature and hold for a long time. The steel
overtempers and large Fe3C spheres form in a ferrite matrix.
We then estimate the amount of ferrite and cementite and
calculate the carbon content using the lever law. If we measure
16% Fe3C using this method, the carbon content is:
( x − 0.0218)
% Fe3C = ×100 = 16 or x = 1.086%
(6.67 − 0.0218)
A better approach, however, is to heat the steel above
the Acm to produce all austenite. If the steel then cools slowly,
it transforms to pearlite and a primary microconstituent. If,
when we do this, we estimate that the structure contains 95%
pearlite and 5% primary Fe3C, then:
6.67 - x
% Pearlite = × 100 = 95 or x = 1.065%
6.67 − 0.77
11
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.
12
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.)
15
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.
16
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.
18
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.
19
24. Example 12.3 SOLUTION
1. Austenitize the steel at 770 + (30 to 55) = 805oC to
825oC, holding for 1 h and obtaining 100% γ.
2. Quench the steel to 600oC and hold for a minimum of 10 s.
Primary ferrite begins to precipitate from the unstable
austenite after about 1.0 s. After 1.5 s, pearlite begins to
grow, and the austenite is completely transformed to
ferrite and pearlite after about 10 s. After this treatment,
the microconstituents present are:
(0.77 − 0.5)
Primary α = × 100 = 36%
(0.77 − 0.0218)
(0.5 − 0.0218)
Pearlite = × 100 = 64%
(0.77 − 0.0218)
3. Cool in air-to-room temperature, preserving the equilibrium
amounts of primary ferrite and pearlite. The microstructure
and hardness are uniform because of the isothermal anneal.
24
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.)
26
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.
27
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
1. Austenitize above the A3 temperature of 770oC for 1 h.
An appropriate temperature may be 770 + 55 = 825oC.
2. Quench rapidly to room temperature. Since the Mf is
about 250oC, martensite will form.
3. Temper by heating the steel to 440oC. Normally, 1 h will
be sufficient if the steel is not too thick.
4. Cool to room temperature.
29
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.
37
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.
42
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.
48
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.
50
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.
53
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.
54
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.)
55
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.
56
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.
60
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.
62
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
63
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.)
66
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
treatments—such as annealing, cold working, or quench and
tempering—the 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.
67
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
68
74. Figure 12.37 The heat treatments for ferritic and
pearlitic malleable irons.
74
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).
75
