Askeland D.R., Fulay P.P. Essentials of Materials Science & Engineering

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12-54 Why does the martensite phase not appear on

the Fe-Fe

C phase diagram?

12-55 Compare the mechanical properties of marten-

site, pearlite, and bainite formed from eutectoid

steel composition.

12-56 A steel containing 0.95% C is heated to various

temperatures above the eutectoid temperature,

held for 1 h, and then quenched to room tem-

perature. Using Figure 12-28 determine the

amount and composition of any martensite that

forms when the heating temperature is:

(a) 728



C (b) 750



C (d) 850



12-57 A steel microstructure contains 75% martensite

and 25% ferrite; the composition of the marten-

site is 0.6% C. Using Figure 12-28, determine

(a) the temperature from which the steel was

quenched, and

(b) the carbon content of the steel.

12-58 A steel microstructure contains 92% martensite

and 8% Fe

C; the composition of the martensite

is 1.10% C. Using Figure 12-28, determine

(a) the temperature from which the steel was

quenched, and

(b) the carbon content of the steel.

12-59 A steel containing 0.8% C is quenched to pro-

duce all martensite. Estimate the volume change

that occurs, assuming that the lattice parameter

of the austenite is 3.6 A. Does the steel expand

or contract during quenching?

12-60 Describe the complete heat treatment required

to produce a quenched and tempered eutectoid

steel having a tensile strength of at least

862 MPa. Include appropriate temperatures.

12-61 Describe the complete heat treatment required

to produce a quenched and tempered eutectoid

steel having a HRC hardness of less than 50.

Include appropriate temperatures.

12-62 In eutectic alloys, the eutectic microconstituent

is generally the continuous one, but in the

eutectoid structures, the primary microconstitu-

ent is normally continuous. By describing the

changes that occur with decreasing temperature

in each reaction, explain why this di¤erence is

expected.

12-63 What is the tempering of steels? Why is temper-

ing necessary?

12-64 What phases are formed by the decomposition

of martensite?

12-65 What is tempered martensite?

12-66 If tempering results in the decomposition of

martensite, why should we form martensite in

the ﬁrst place?

12-67 Describe the changes in properties that occur

upon the tempering of an eutectoid steel.

Design Problems

12-68 You wish to attach aluminum sheets to the

frame of the twenty-fourth ﬂoor of a sky-

scraper. You plan to use rivets made of an

age-hardenable aluminum, but the rivets must

be soft and ductile in order to close. After the

sheets are attached, the rivets must be very

strong. Design a method for producing, using,

and strengthening the rivets.

12-69 An age-hardened, Al-Cu bracket is used to

hold a heavy electrical-sensing device on the

outside of a steel-making furnace. Temper-

atures may exceed 200



C. Is this a good de-

sign? Explain. If it is not, design an appro-

priate bracket and explain why your choice is

acceptable.

12-70 You use an arc-welding process to join an

eutectoid steel. Cooling rates may be very high

following the joining process. Describe what

happens in the heat-a¤ected zone of the weld

and discuss the problems that might occur.

Design a joining process that may minimize

these problems.

C H A P T E R 12 Dispersion Stren gthening by Phase Transfor mations and Heat Treatment390

Heat Treatment of Steels and

Cast Irons

Have You Ever Wondered?

9 What is the most widely used engineered material?

9 What makes stainless steels ‘‘stainless’’?

9 What is the difference between cast iron and steel?

9 Are stainless steels magnetic?

9 Is a tin can made out of tin?

Ferrous alloys, which are based on iron-carbon

alloys, include plain-carbon steels, alloy and tool

steels, stainless steels, and cast irons. These

are the most widely used materials in the world.

In the history of civilization, these materials

made their mark by defining the Iron Age. Steels

typically are produced in two ways: by refining

iron ore or by recycling scrap steel.

In producing primary steel, iron ore (proc-

essed to contain 50 to 70% iron oxide, Fe

) is heated in a blast furnace in the pres-

ence of coke (a form of carbon) and oxygen. The

iron oxide is reduced into a crude molten

iron known as hot metal or pig iron. At about

@1600



C, this material contains about 95%

iron; 4% carbon; 0.3 to 0.9% silicon; 0.5% Mn;

391

and 0.025 to 0.05% of sulfur, phosphorus, and

titanium. Slag is a byproduct of the blast furnace

process. It contains silica, CaO, and other im-

purities in the form of a silicate melt.

Because the liquid pig iron contains a large

amount of carbon, oxygen is blown into it in the

basic oxygen furnace (BOF) to eliminate the ex-

cess carbon and produce liquid steel. Steel has a

carbon content up to a maximum of @2%. In the

second method, steel scrap is often melted in

an electric arc furnace. Many alloy and specialty

steels, such as stainless steels, are produced

using electric melting. Steel is one of the few ma-

terials that is nearly 100% recycled. Molten steels

(including stainless steels) often undergo further

refining. The goal here is to reduce the levels of

impurities such as phosphorus, sulfur, etc., and to

bring the carbon to a desired level.

Molten steel made by either route is poured

into molds to produce finished steel castings, or

cast continuously into shapes that are later proc-

essed through metal-forming techniques such as

rolling or forging. In the latter case, the steel

is either poured into large ingot molds or con-

tinuously cast into regular shapes.

All of the strengthening mechanisms discussed

in the previous chapters apply to at least some of the

ferrous alloys. In this chapter, we will discuss how

to use the eutectoid reaction to control the structure

and properties of steels through heat treatment and

alloying. We will also examine two special classes

of ferrous alloys: stainless steels and cast irons.

13-1 Designations and Classification of Steels

The dividing point between ‘‘steels’’ and ‘‘cast irons’’ is 2.11% C, where the eutectic

reaction becomes possible. For steels, we concentrate on the eutectoid portion of the

diagram (Figure 13-1) in which the solubility lines and the eutectoid isotherm are spe-

cially identiﬁed. The A

shows the temperature at which ferrite starts to form on cool-

ing; the A

shows the temperature at which cementite starts to form; and the A

is the

eutectoid temperature.

Almost all of the heat treatments of steel are directed toward producing the mixture

of ferrite and cementite that gives the proper combination of properties. Figure 13-2

shows the three important microconstituents, or arrangements of ferrite and cementite,

that are usually sought. Pearlite is a microconstituent consisting of a lamellar mixture

of ferrite and cementite. In bainite, which is obtained by transformation of austenite at

a large undercooling, the cementite is more rounded than in pearlite. Tempered mar-

tensite, a mixture of very ﬁne and nearly round cementite in a matrix of ferrite, forms

when martensite is reheated following its formation.

Designations The American Iron and Steel Institute (AISI) and Society of Auto-

motive Engineers (SAE) provide designation systems (Table 13-1) that use a four- or

ﬁve-digit number. The ﬁrst two numbers refer to the major alloying elements present,

and the last two or three numbers refer to the percentage of carbon. An AISI 1040 steel

is a plain-carbon steel with 0.40% C. An SAE 10120 steel is a plain-carbon steel con-

taining 1.20% C. An AISI 4340 steel is an alloy steel containing 0.40% C. Note that the

American Society for Testing of Materials (ASTM) has a di¤erent way of classifying

steels. The ASTM has a list of speciﬁcations that describe steels suitable for di¤erent

applications. The following example illustrates the use of AISI numbers.

C HA P T E R 1 3 Heat Treatment of Steels and Cast Irons392

Figure 13-1 (a) An expanded version of the Fe-C diagram, adapted from several sources.

(b) The eutectoid portion of the Fe-Fe

C phase diagram.

13-1 Designations and Classification of Steels 393

Figure 13-2 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.)

TABLE 13-1 9 Compositions of selected AISI-SAE steels

AISI-SAE

Number % C % Mn % Si % Ni % Cr Others

1020 0.18–0.23 0.30 –0.60

1040 0.37–0.44 0.60 –0.90

1060 0.55–0.65 0.60 –0.90

1080 0.75–0.88 0.60 –0.90

1095 0.90–1.03 0.30 –0.50

1140 0.37–0.44 0.70 –1.00 0.08–0.13% S

4140 0.38–0.43 0.75 –1.00 0.15–0.30 0.80–1.10 0.15–0.25% Mo

4340 0.38–0.43 0.60–0.80 0.15–0.30 1.65–2.00 0.70–0.90 0.20–0.300% Mo

4620 0.17–0.22 0.45–0.65 0.15–0.30 1.65–2.00 0.20–0.30% Mo

52100 0.98–1.10 0.25–0.45 0.15–0.30 1.30–1.60

8620 0.18–0.23 0.70–0.90 0.15–0.30 0.40–0.70 0.40–0.60 0.15–0.25% Y

9260 0.56–0.64 0.75–1.00 1.80–2.20

EXAMPLE 13-1

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 car-

bon content.

C HA P T E R 1 3 Heat Treatment of Steels and Cast Irons394

SOLUTION

Assume that there is no access to equipment that would permit you to analyze

the chemical composition directly. Since the entire structure of the steel is a

very ﬁne tempered martensite, we can do a simple heat treatment to produce a

structure that can be analyzed more easily. This can be done in two di¤erent

ways.

The ﬁrst way is to heat the steel to a temperature just below the A

tem-

perature and hold for a long time. The steel overtempers and large Fe

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% Fe

C using this method, the carbon content is:

%Fe

C ¼

ðx  0:0218Þ

ð6:67  0:0218Þ



 100 ¼ 16 or x ¼ 1:086% C

A better approach, however, is to heat the steel above the A

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 Fe

C, then:

% Pearlite ¼

ð6:67  xÞ

ð6:67  0:77Þ



 100 ¼ 95 or x ¼ 1:065% C

The carbon content is on the order of 1.065 to 1.086%, consistent with a 10110

steel. In this procedure, we assumed that the weight and volume percentages of

the microconstituents are the same, which is nearly the case in steels.

Classifications Steels can be classiﬁed based on their composition or the way they

have been processed. Carbon steels contain up to @2% carbon. These steels may also

contain other elements, such as Si (maximum 0.6%), copper (up to 0.6%), and Mn (up

to 1.65%). Decarburized steels contain less than 0.005% C. Ultra-low carbon steels

contain a maximum of 0.03% carbon. They also contain very low levels of other ele-

ments such as Si and Mn. Low-carbon steels contain 0.04 to 0.15% carbon. These low-

carbon steels are used for making car bodies and hundreds of other applications. Mild

steel contains 0.15 to 0.3% carbon. This steel is used in buildings, bridges, piping, etc.

Medium-carbon steels contain 0.3 to 0.6% carbon. These are used in making machi-

nery, tractors, mining equipment, etc. High-carbon steels contain above 0.6% carbon.

These are used in making springs, railroad car wheels, and the like. Note that cast irons

are Fe-C alloys containing about 2 to 4% carbon.

Alloy steels are compositions that contain more signiﬁcant levels of alloying ele-

ments. We will discuss the e¤ect of alloying elements later in this chapter. They improve

the hardenability of steels. The AISI deﬁnes alloy steels as steels that exceed in one or

more of these elements: b1.65% Mn, 0.6% Si, 0.6% Cu. The total carbon content is up

to 1% and the total alloying elements content is below 5%. A material is also an alloy

steel if a deﬁnite concentration of alloying elements, such as Ni, Cr, Mo, Ti, etc., is

speciﬁed. These steels are used for making tools (hammers, chisels, etc.) and also in

making parts such as axles, shafts, and gears.

Certain specialty steels may consist of higher levels of sulfur (>0.1%) or lead

(@0.15–0.35%) to provide machinability. These, however, can not be welded easily.

Recently, researchers have developed ‘‘green steel’’ in which lead, an environmental

13-1 Designations and Classification of Steels 395

toxin, was replaced with tin (Sn) and/or antimony (Sb). Steels can also be classiﬁed

based on their processing. For example, the term ‘‘concast steels’’ refers to continuously

cast steels. Galvanized steels have a zinc coating for corrosion resistance. Similarly, tin-

plated steel is used to make cor rosion-resistant tin cans and other products. Tin is de-

posited using electroplating—a process known as ‘‘continuous web electrodeposition.’’

‘‘E-steels’’ are steels that are melted using an electric furnace, while ‘‘B-steels’’ contain

a small (0.0005 to 0.003%), yet signiﬁcant, concentration of boron. Recently, a ‘‘germ-

resistant’’ coated stainless steel has been developed. Some biomedical products, such as

cardiovascular stents, also make use of stainless steel coated with heprin, a chemical

that makes the stents more biocompatible.

13-2 Simple Heat Treatments

Four simple heat treatments—process annealing, annealing, normalizing, and

spheroidizing—are commonly used for steels (Figure 13-3). These heat treatments are

used to accomplish one of three purposes: (1) eliminating the e¤ects of cold work,

(2) controlling dispersion strengthening, or (3) improving machinability.

Process Annealing—Eliminating Cold Work The recrystallization heat treatment used

to eliminate the e¤ect of cold working in steels with less than about 0.25% C is called a

process anneal. The process anneal is done 80 to 170



C below the A

temperature. The

goal of the process anneal treatment for steels is similar to the annealing of inorganic

glasses in that the main idea is to signiﬁcan tly reduce or eliminate residual stresses.

Annealing and Normalizing—Dispersion Strengthening Steels can be dispersion

strengthened by controlling the ﬁneness of pearlite. The steel is initially heated to

produce homogeneous austenite (FCC g phase), a step called austenitizing. Annealing,

or a full anneal, allows the steel to cool slowly in a furnace, producing coarse

pearlite. In normalizing, the steel is cooled more rapidly in air, producing ﬁne pearlite.

Figure 13-4 shows the typical properties obtained by annealing and normalizing plain-

carbon steels.

Figure 13-3 Schematic summary of the simple heat treatments for (a) hypoeutectoid steels

and (b) hypereutectoid steels.

C HA P T E R 1 3 Heat Treatment of Steels and Cast Irons396

For annealing, austenitizing of hypoeutectoid steels is conducted about 30



C above

the A

, producing 100% g. However, austenitizing of a hypereutectoid steel is done at

about 30



C above the A

, producing austenite and Fe

C. This process prevents the

formation of a brittle, continuous ﬁlm of Fe

C at the grain boundaries that occurs on

slow cooling from the 100% g region. In both cases, the slow furnace cool and coarse

pearlite provide relatively low strength and good ductility.

For normalizing, austenitizing is done at about 55



C above the A

or A

; the steel

is then removed from the furnace and cooled in air. The faster cooling gives ﬁne pearlite

and provides higher strength.

Spheroidizing—Improving Machinability Steels, which contain a large concentration

of Fe

C, have poor machining characteristics. It is possible to transform the morphol-

ogy of Fe

C using spheroidizing. During the spheroidizing treatment, which requires

several hours at about 30



C below the A

, the Fe

C phase morphology changes into

large, spherical particles in order to reduce boundary area. The microstructure, known

as spheroidite, has a continuous matrix of soft, machinable ferrite (Figure 13-5). After

machining, the steel is given a more sophisticated heat treatment to produce the re-

quired properties. A similar microstructure occurs when martensite is tempered just

below the A

for long periods of time. The di¤erence between tempered martensite and

spheroidite is that in formation of spheroidite, martensite is never formed. As noted

before, alloying elements such as Pb and S are also added to improve machinability of

steels and, more recently, lead-free ‘‘green steels’’ that have very good machinability

have been developed.

The following example shows how di¤erent heat treatment conditions can be de-

veloped for a given composition of steel.

Figure 13-4

The effect of carbon and heat

treatment on the properties of

plain-carbon steels.

13-2 Simple Heat Treatments 397

Figure 13-5

The microstructure of spheroidite, with Fe

particles dispersed in a ferrite matrix (850).

(From ASM Handbook, Vol. 7, (1972), ASM

International, Materials Park, OH 44073.)

EXAMPLE 13-2

Determination of Heat Treating Temperatures

Recommend temperatures for the process annealing, annealing, normalizing,

and spheroidizing of 1020, 1077, and 10120 steels.

SOLUTION

From Figure 13-1, we ﬁnd the critical A

, A

,orA

temperatures for each

steel. We can then specify the heat treatment based on these temperatures.

Steel Type 1020 1077 10120

Critical temperatures A

¼ 727



C A

¼ 727



C A

¼ 727



¼ 830



C A

¼ 895



Process annealing 727  (80 to 170)

¼ 557



C to 647



Not done Not done

Annealing 830 þ 30 ¼ 860



C 727 þ 30 ¼ 757



C 727 þ 30 ¼ 757



Normalizing 830 þ 55 ¼ 885



C 727 þ 55 ¼ 782



C 895 þ 55 ¼ 950



Spheroidizing Not done 727  30 ¼ 697



C 727  30 ¼ 697



13-3 Isothermal Heat Treatments

The e¤ect of transformation temperature on the properties of a 1080 (eutectoid) steel

was discussed in Chapter 12. As the isothermal transformation temperature decreases,

pearlite becomes progressively ﬁner before bainite begins to form. At very low temper-

atures, martensite is obtained.

Austempering and Isothermal Annealing The isothermal transformation heat treat-

ment used to produce bainite, called austempering, simply involves austenitizing the

steel, quenching to some temperature below the nose of the TTT curve, and holding at

that temperature until all of the austenite transforms to bainite (Figure 13-6).

Annealing and normalizing are usually used to control the ﬁneness of pearlite.

However, pearlite forme d by an isothermal anneal (Figure 13-6) may give more uniform

properties, since the cooling rates and microstructure obtained during annealing and

normalizing vary across the cross section of the steel. Note t hat the TTT diagrams only

C HA P T E R 1 3 Heat Treatment of Steels and Cast Irons398

P ¼ Pearlite

B ¼ Bainite

M ¼ Martensite

s ¼ Start

f ¼ Finish

Figure 13-6 The austempering and isothermal anneal heat treatments in a 1080 steel.

P ¼ Pearlite

B ¼ Bainite

M ¼ Martensite

s ¼ Start

f ¼ Finish

¼ Unstable gamma phase

Figure 13-7 The TTT diagrams for (a) a 1050 and (b) a 10110 steel.

13-3 Is othermal Heat Treatments 399

Askeland D.R., Fulay P.P. Essentials of Materials Science &amp; Engineering

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Askeland D.R., Fulay P.P. Essentials of Materials Science & Engineering