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

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EXAMPLE 12-9 Heat Treatment to Generate Bainite Microstructure

Excellent combinations of hardness, strength, and toughness are obtained from

bainite. One heat treatment facility austenitized an eutectoid steel at 750



quenched and held the steel at 250



C for 15 min, and ﬁnally permitted the steel

to cool to room temperature. Was the required bainitic structure produced?

SOLUTION

Let’s examine the heat treatment using Figure 12-17. After heating at 750



the microstructure is 100% g. After quenching to 250



C, unstable austenite

remains for slightly more than 100 s, when ﬁne bainite begins to grow. After

15 min, or 900 s, about 50% ﬁne bainite has formed and the remainder of the

steel still contains unstable austenite. As we will see later, the unstable austenite

transforms to martensite when the steel is cooled to room temperature and

the ﬁnal structure is a mixture of bainite and hard, brittle martensite. The heat

treatment was not successful! The heat treatment facility should have held the

steel at 250



C for at least 10

s, or about 3 h.

12-11 The Martensitic Reaction and Tempering

Martensite is a phase that forms as the result of a di¤usionless solid-state trans-

formation. In this transformation there is no di¤usion and, hence, it does not follow

the Avrami transformation kinetics. The growth rate in martensitic transformations

(also known as displacive or athermal transformations) is so high that nucleation be-

comes the controlling step. The phase that forms upon the quenching of steels was

named ‘‘martensite’’ by Floris Osmond in 1895 in honor of German metallurgist Adolf

Martens. Similar martensitic phase transformations occur in other systems as well.

Cobalt, for example, transforms from a FCC to a HCP crystal structure by a slight

shift in the atom locations that alters the stacking sequence of close-packed planes. Be-

cause the reaction does not depend on di¤usion, the martensite reaction is an athermal

transformation—that is, the reaction depends only on the temperature, not on the time.

The martensite reactio n often proceeds rapidly, at speeds approaching the velocity of

sound in the material.

Many other alloys such as Cu-Zn-Al and Cu-Al-Ni and Ni-Ti show martensitic

phase transformations. These transformations can also be driven by the application of

mechanical stress. Other than martensite that forms in certain type of steels, the Ni-Ti

alloy, known as nitinol (which stands for Nickel Titanium Naval Ordinance Labo-

ratory, developed by the U.S. Naval Ordinance Laboratory in the 1940s) is perhaps the

best-known example of alloys that make use of martensitic phase transformations.

These materials can remember their shape (i.e. shape memory e¤ect) and are known as

shape-memory alloys (SMAs).

Martensite in Steels In steels with less than about 0.2% C, the FCC austenite trans-

forms to a supersaturated BCC martensite structure. In higher carbon steels, the mar-

tensite reaction occurs as FCC austenite transforms to BCT (body centered tetragonal)

martensite. The relationship between the FCC austenite and the BCT martensite [Fig-

ure 12-21(a)] shows that carbon atoms in the 1/2, 0, 0 type of interstitial sites in the

C H A P T E R 12 Dispersion Stren gthening by Phase Transfor mations and Heat Treatment380

FCC cell can be trapped during the transformation to the body-centered structure,

causing the tetragonal structure to be produced. As the carbon content of the steel

increases, a greater number of carbon atoms are trapped in these sites, thereby increas-

ing the di¤erence between the a- and c-axes of the martensite [Figure 12-21(b)].

The steel must be quenched, or rapidly cooled, from the stable austenite region to

prevent the formation of pearlite, bainite, or primary microconsti tuents. The martensite

reaction begins in an eutectoid steel when austenite cools below 220



C, the marten-

site start (M

) temperature (Figure 12-17). The amount of martensite increases as

the temperature decreases. When the temperature passes below the martensite ﬁnish

temperature (M

), the steel should contain 100% martensite. At any intermediate

temperature, the amount of martensite does not change as the time at that temperature

increases.

Owing to the conservation of mass, the composition of martensite must be the same

as that of the austenite from which it forms. There is no long-range di¤usion during the

transformation that can change the composition. Thus, in iron-carbon alloys, the initial

austenite composition and the ﬁnal martensite composition are the same. The following

example illustrates how heat treatment is used to produce a dual phase steel.

Figure 12-21 (a) The unit cell of BCT martensite is related to the FCC austenite unit cell.

(b) As the percentage of carbon increases, more interstitial sites are filled by the carbon atoms

and the tetragonal structure of the martensite becomes more pronounced.

EXAMPLE 12-10

Design of a Heat Treatment for a Dual Phase Steel

Unusual combinations of properties can be obtained by producing a steel

whose microstructure contains 50% ferrite and 50% martensite; the martensite

provides strength and the ferrite provides ductility and toughness. Design a

heat treatment to produce a dual phase steel in which the composition of the

martensite is 0.60% C.

12-11 The Martensitic Reaction and Tempering 381

SOLUTION

To obtain a mixture of ferrite and martensite, we need to heat-treat a hypo-

eutectoid steel into the a þ g region of the phase diagram. The steel is then

quenched, permitting the g portion of the structure to transform to martensite.

The heat treatment temperature is ﬁxed by the requirement that the mar-

tensite contain 0.60% C. From the solubility line between the g and the a þ g

regions, we ﬁnd that 0.60% C is obtained in austenite when the temperature is

about 750



C. To produce 50% martensite, we need to select a steel that gives

50% austenite when the steel is held at 750



C. If the carbon content of the steel

is x, then:

% g ¼

ðx  0:02Þ

ð0:60  0:02Þ



 100 ¼ 50 or x ¼ 0:31% C

Our ﬁnal design is:

1. Select a hypoeutectoid steel containing 0.31% C.

2. Heat the steel to 750



C and hold (perhaps for 1 h, depending on the thick-

ness of the part) to produce a structure containing 50% ferrite and 50%

austenite, with 0.60% C in the austenite.

3. Quench the steel to room temperature. The austenite transforms to marten-

site, also containing 0.60% C.

Figure 12-22

The effect of carbon content on the

hardness of martensite in steels.

Properties of Steel Martensite Martensite in steels is very hard and brittle, just like

ceramics. The BCT crystal structure has no close-packed slip planes in which dis-

locations can easily move. The martensite is highly supersaturated with carbon, since

iron normally contains less than 0.0218% C at room temperature, and martensite con-

tains the amount of carbon present in the steel. Finally, martensite has a ﬁne grain size

and an even ﬁner substructure within the grains.

The structure and properties of steel martensites depend on the carbon content

of the alloy (Figure 12-22). When the carbon content is low, the martensite grows in

a ‘‘lath’’ shape, composed of bundles of ﬂat, narrow plates that grow side by side

C H A P T E R 12 Dispersion Stren gthening by Phase Transfor mations and Heat Treatment382

[Figure 12-23(a)]. This martensite is not very hard. At a higher carbon content, plate

martensite grows, in which ﬂat, narrow plates grow individually rather than as bundles

[Figure 12-23(b)]. The hardness is much greater in the higher carbon, plate martensite

structure, partly due to the greater distortion, or large c/a ratio, of the crystal structure.

Tempering of Steel Martensite Martensite is not an equilibrium phase. This is why it

does not appear on the Fe-Fe

C phase diagram (Figure 12-11). When martensite in a

steel is heated below the eutectoid temperature, the thermodynamically stable a and

C phases precipitate. This process is called tempering. The decomposition of

martensite in steels causes the strength and hardness of the steel to decrease while the

ductility and impact properties are improved (Figure 12-24). Note that the term tem-

pering here is di¤erent from the term we used for tempering of silicate glasses. In both

Figure 12-23 (a) Lath martensite in low-carbon steel (80). (b) Plate martensite in high-

carbon steel (400). (From ASM Handbook, Vol. 8, (1973), ASM International, Materials

Park, OH 44073.)

Figure 12-24

Effect of tempering temperature on

the properties of an eutectoid steel.

12-11 The Martensitic Reaction and Tempering 383

tempering of glasses and tempering of steels, however, the key result is an increase in

the toughness of the material.

At low tempering temperatures, the martensite may form two transition phases—

a lower carbon martensite and a very ﬁne nonequilibrium e-carbide, or Fe

2:4

C. The steel

is still strong, brittle, and perhaps even harder than before tempering. At higher tem-

peratures, the stable a and Fe

C form and the steel becomes softer and more ductile.

If the steel is tempered just below the eutectoid temperature, the Fe

C becomes very

coarse and the dispersion-strengthening e¤ect is greatly reduced. By selecting the

appropriate tempering temperature, a wide range of properties can be obtained. The

product of the tempering process is a microconstituent called tempered martensite

(Figure 12-25).

Martensite in Other Systems The characteristics of the martensite reaction are di¤er-

ent in other alloy systems. For example, martensite can form in iron-based alloys that

contain little or no carbon by a transformation of the FCC crystal structure to a BCC

crystal structure. In certain high-manganese steels and stainless steels, the FCC struc-

ture changes to a HCP crystal structure during the martensite transformation. In addi-

tion, the martensitic reaction occurs during the transformation of many polymorphic

ceramic materials, including ZrO

, and even in some crystalline polymers. Thus the

terms martensitic reaction and martensite are rather generic. In the context of steel

properties, microstructure, and heat treatment, the term ‘‘martensite’’ refers to the hard

and brittle bct phase obtained upon the quenching of steels.

The properties of martensite in other alloys are also di¤erent from the properties

of steel martensite. In titanium alloys, BCC titanium transforms to a HCP martensite

structure during quenching. However, the titanium martensite is softer and weaker than

the original structure. The martensite that forms in other alloys can also be tempered.

The martensite produced in titanium alloys can be reheated to permit the precipitation

of a second phase. Unlike the case of steel, however, the tempering process increases,

rather than decreases, the strength of the titanium alloy.

SUMMARY

V Solid-state phase transformations, which have a profound e¤ect on the structure

and properties of a material, can be controlled by proper heat treatments. These

heat treatments are designed to provide an optimum distribution of two or more

phases in the microstructure. Dispersion strengthening permits a wide variety of

structures and properties to be obtained.

Figure 12-25

Tempered martensite in steel (500). (From

ASM Handbook, Vol. 9, Metallography and

Microstructure (1985), ASM International

Materials Park, OH 44073.)

C H A P T E R 12 Dispersion Stren gthening by Phase Transfor mations and Heat Treatment384

V Age hardening, or precipitation hardening, is one powerful method for controlling

the optimum dispersion strengthening in many metallic alloys. In age hardening,

a very ﬁne widely dispersed coherent precipitate is allowed to precipitate by a heat

treatment that includes (a) solution treating to produce a single-phase solid sol-

ution, (b) quenching to retain that single phase, and (c) aging to permit a precip-

itate to form. In order for age hardening to occur, the phase diagram must show

decreasing solubility of the solute in the solvent as the temperature decreases.

V The most widely used eutectoid reaction occurs in producing steels from iron-

carbon alloys: Either pearlite or bainite can be produced as a result of the eutectoid

reaction in steel. In addition, primary ferrite or primary cementite may be present,

depending on the carbon content of the alloy. The trick is to formulate a micro-

structure that consists of a right mix of metal-like phases that are tough and

ceramic-like phases that are hard and brittle.

V Factors that inﬂuence the mechanical properties of the microconstituent produced

by the eutectoid reaction include (a) the composition of the alloy (amount of

eutectoid microconstituent), (b) the grain size of the original solid, the eutectoid

microconstituent, and any primary microconstituents, (c) the ﬁneness of the struc-

ture within the eutectoid microconstituent (interlamellar spacing), (d) the cooling

rate during the phase transformation, and (e) the temperature at which the trans-

formation occurs (the amount of undercooling).

V A martensitic reaction occurs with no long-range di¤usion. Again, the best known

example occurs in steels:



The amount of martensite that forms depends on the temperature of the

transformation (an athermal reaction).



Martensite is very hard and brittle, with the hardness determined primarily

by the carbon content.



The amount and composition of the martensite are the same as the austenite

from which it forms.

V Martensite can be tempered. During tempering, a dispersion-strengthened structure

is produced. In steels, tempering reduces the strength and hardness but improves

the ductility and toughness.

GLOSSARY

Age hardening A special dispersion-strengthening heat treatment. By solution treatment,

quenching, and aging, a coherent precipitate forms that provides a substantial strengthening

e¤ect. Also known as precipitation hardening.

Artificial aging Reheating a solution-treated and quenched alloy to a temperature below the

solvus in order to provide the thermal energy required for a precipitate to form.

Athermal transformation When the amount of the transformation depends only on the temper-

ature, not on the time (same as martensitic transformation or displacive transformation).

Austenite The name given to the FCC crystal structure of iron.

Avrami relationship Describes the fraction of a transformation that occurs as a function of time.

This describes most solid-state transformations that involve di¤usion, thus martensitic trans-

formations are not described.

Bainite A two-phase microconstituent, containing ferrite and cementite, that forms in steels that

are isothermally transformed at relatively low temperatures.

Glossary 385

Cementite The hard, brittle ceramic-like compound Fe

C that, when properly dispersed, pro-

vides the strengthening in steels.

Coherent precipitate A precipitate whose crystal structure and atomic arrangement have a

continuous relationship with the matrix from which the precipitate is formed. The formation of

coherent precipitate provides excellent disruption of the atomic arrangement in the matrix and

provides excellent strengthening.

Displacive transformation A phase transformation that occurs via small displacements of atoms

or ions and without di¤usion. Same as athermal or martensitic transformation.

Ferrite The name given to the BCC crystal structure of iron that can occur as a or d. This is not

to be confused with ceramic ferrites which are magnetic materials.

Guinier-Preston (GP) zones Tiny clusters of atoms that precipitate from the matrix in the early

stages of the age-hardening process. Although the GP zones are coherent with the matrix, they

are too small to provide optimum strengthening.

Interfacial energy The energy associated with the boundary between two phases.

Isothermal transformation When the amount of a transformation at a particular temperature

depends on the time permitted for the transformation.

Martensite A metastable phase formed in steel and other materials by a di¤usionless, athermal

transformation.

Martensitic transformation A phase transformation that occurs without di¤usion. Same as

athermal or displacive transformation. These occur in steels, Ni-Ti and many ceramic materials.

Natural aging When a coherent precipitate forms from a solution-treated and quenched age-

hardenable alloy at room temperature, providing optimum strengthening.

Pearlite A two-phase lamellar microconstituent, containing ferrite and cementite, that forms in

steels cooled in a normal fashion or isothermally transformed at relatively high temperatures.

Shape-memory alloys (SMAs) Certain materials which develop microstructures that, after being

deformed, can return the material to its initial shape when heated (e.g. Ni-Ti alloys).

Solution treatment The ﬁrst step in the age-hardening heat treatment. The alloy is heated above

the solvus temperature to dissolve any second phase and to produce a homogeneous single-phase

structure.

Strain energy The energy required to permit a precipitate to ﬁt into the surrounding matrix

during nucleation and growth of the precipitate.

Supersaturated solid solution The solid solution formed when a material is rapidly cooled from

a high-temperature single-phase region to a low-temperature two-phase region without the second

phase precipitating. Because the quenched phase contains more alloying element than the sol-

ubility limit, it is supersaturated in that element.

Tempering A low-temperature heat treatment used to reduce the hardness of martensite by

permitting the martensite to begin to decompose to the equilibrium phases. This leads to in-

creased toughness.

TTT diagram The time-temperature-transformation diagram describes the time required at any

temperature for a phase transformation to begin and end. The TTT diagram assumes that the

temperature is constant during the transformation.

Widmanstðtten structure The precipitation of a second phase from the matrix when there is a

ﬁxed crystallographic relationship between the precipitate and matrix crystal structures. Often

needle-like or plate-like structures form in the Widmanstðtten structure.

C H A P T E R 12 Dispersion Stren gthening by Phase Transfor mations and Heat Treatment386

PROBLEMS

Section 12-1 Nucleation and Growth in Solid-

State Reactions

12-1 How is the equation for nucleation of a phase in

the solid state di¤erent from that for a liquid to

solid transformation?

12-2 Determine the constants c and n in Equation 12-2

that describe the rate of crystallization of poly-

propylene at 140



C. (See Figure 12-26.)

Section 12-2 Alloys Strengthened By Exceeding

the Solubility Limit

12-3 What are the di¤erent ways by which a second

phase can be made to precipitate in a two-phase

microstructure?

12-4 Explain, when cooled slowly, why it is that the

second phase in Al-4% Cu alloys nucleates and

grows along the grain boundaries. Is this usu-

ally desirable?

12-5 What properties of the precipitate phase are

needed for precipitation hardening? Why?

Section 12-3 Age or Precipitation Hardening

12-6 What is the principle of precipitation hardening?

12-7 What is a supersaturated solution? How do we

obtain supersaturated solutions during precip-

itation hardening? Why is the formation of a

supersaturated solution necessary?

12-8 Suppose that age hardening is possible in the

Al-Mg system. (See Figure 12-8.)

(a) Recommend an artiﬁcial age-hardening heat

treatment for each of the following alloys,

and

(b) compare the amount of the b precipitate that

forms from your treatment of each alloy.

(i) Al-4% Mg (ii) Al-6% Mg

(iii) Al-12% Mg

reveals that little strengthening occurs as a

result of the heat treatment. Which of the re-

quirements for age hardening is likely not

satisﬁed?

12-9 An Al-2.5% Cu alloy is solution-treated,

quenched, and overaged at 230



C to produce a

stable microstructure. If the y precipitates as

spheres with a diameter of 9000 A and a density

of 4.26 g/cm

, determine the number of precip-

itate particles per cm

Section 12-4 Applications of Age-Hardened

Alloys

12-10 Why is precipitation hardening an attractive

mechanism of strengthening for aircraft mate-

rials?

12-11 Why are most precipitation-hardened alloys

suitable only for relatively low-temperature ap-

plications?

Section 12-5 Microstructural Evolution in Age or

Precipitation Hardening

12-12 Explain the three basic steps encountered dur-

ing precipitation hardening.

Section 12-6 Effects of Aging Temperature and

Time

12-13 What is aging? Why is this step needed in pre-

cipitation hardening?

12-14 What do the terms ‘‘natural aging’’ and ‘‘artiﬁ-

cial aging’’ mean?

Figure 12-26 The effect of temperature on the

crystallization of polypropylene (for Problem 12-2).

Figure 12-8 (Repeated for Problem 12-8) Portion of

the aluminum-magnesium phase diagram.

Problems 387

12-15 In the plane ﬂown by the Wright brothers, how

was the alloy precipitation strengthened?

Section 12-7 Requirements for Age Hardening

12-16 Can all alloy compositions be strengthened

using precipitation hardening? Can we use this

mechanism for the strengthening of ceramics,

glasses, or polymers?

12-17 A conductive copper wire is to be made. Would

you choose precipitation hardening as a way of

strengthening this wire? Explain.

12-18 Figure 12-27 shows a hypothetical phase dia-

gram. Determine whether each of the following

alloys might be good candidates for age hard-

ening, and explain your answer. For those

alloys that might be good candidates, describe

the heat treatment required, including recom-

mended temperatures.

(a) A-10% B (b) A-20% B (c) A-55% B

(d) A-87% B (e) A-95% B

Section 12-8 Use of Age-Hardenable Alloys at

High Temperatures

12-19 Why is it that certain aluminum (not nickel-

based) alloys strengthened using age hardening

can lose their strength on welding?

12-20 Would you choose a precipitation-hardened

alloy to make an aluminum alloy baseball bat?

Section 12-9 The Eutectoid Reaction

12-21 Write down the eutectoid reaction in Fe-Fe

system.

12-22 Compare and contrast eutectic and eutectoid

reactions.

12-23 Deﬁne the following terms: ferrite, austenite,

pearlite, and cementite.

12-24 The pearlite microstructure is similar to a

ceramic-metal nanocomposite. True or False.

Comment.

12-25 What is the di¤erence between a microconstitu-

ent and a phase?

12-26 For an Fe-0.35% C alloy, determine

(a) the temperature at which austenite ﬁrst be-

gins to transform on cooling,

(b) the primary microconstituent that forms,

present at 728



(d) the composition and amount of each phase

present at 726



C, and

(e) the composition and amount of each mi-

croconstituent present at 726



12-27 For an Fe-1.15% C alloy, determine

(a) the temperature at which austenite ﬁrst

begins to transform on cooling,

(b) the primary microconstituent that forms,

present at 728



(d) the composition and amount of each phase

present at 726



C, and

(e) the composition and amount of each mi-

croconstituent present at 726



12-28 A steel contains 8% cementite and 92% ferrite at

room temperature. Estimate the carbon content

of the steel. Is the steel hypoeutectoid or hyper-

eutectoid?

12-29 A steel contains 18% cementite and 82% ferrite

at room temperature. Estimate the carbon con-

tent of the steel. Is the steel hypoeutectoid or

hypereutectoid?

12-30 A steel contains 18% pearlite and 82% primary

ferrite at room temperature. Estimate the carbon

content of the steel. Is the steel hypoeutectoid or

hypereutectoid?

12-31 A steel contains 94% pearlite and 6% primary

cementite at room temperature. Estimate the

carbon content of the steel. Is the steel hypo-

eutectoid or hypereutectoid?

12-32 A steel contains 55% a and 45% g at 750



Estimate the carbon content of the steel.

12-33 A steel contains 96% g and 4% Fe

C at 800



Estimate the carbon content of the steel.

12-34 A steel is heated until 40% austenite, with a

carbon content of 0.5%, forms. Estimate the

Figure 12-27 Hypothetical phase diagram (for

Problem 12-18).

C H A P T E R 12 Dispersion Stren gthening by Phase Transfor mations and Heat Treatment388

temperature and the overall carbon content of

the steel.

12-35 A steel is heated until 85% austenite, with a

carbon content of 1.05%, forms. Estimate the

temperature and the overall carbon content of

the steel.

Section 12-10 Controlling the Eutectoid

Reaction

12-36 Why are the distances between lamellae formed

in an eutectoid reaction typically separated by

distances smaller than those formed in eutectic

reactions?

12-37 Compare the interlamellar spacing and the yield

strength when an eutectoid steel is isothermally

transformed to pearlite at

(a) 700



C, and

(b) 600



12-38 Why is it that a eutectoid steel exhibits di¤erent

yield strengths and % elongations, depending

upon if it was cooled slowly or relatively fast?

12-39 What is a TTT diagram?

12-40 Sketch and label clearly di¤erent parts of a TTT

diagram for a plain-carbon steel with 0.77%

carbon.

12-41 On the TTT diagram what is the di¤erence be-

tween the g and g

phases?

12-42 How is it that bainite and pearlite do not appear

in the Fe-Fe

C diagram? Are these phases or

microconstituents?

12-43 Why is it that we cannot make use of TTT dia-

grams for describing heat treatment proﬁles in

which samples are getting cooled over a period

of time (i.e., why are TTT diagrams suitable for

only following isothermal transformations)?

12-44 What is bainite? Why do steels containing bain-

ite exhibit higher levels of toughness?

12-45 An isothermally transformed eutectoid steel is

found to have a yield strength of 410 MPa. Es-

timate

(a) the transformation temperature, and

(b) the interlamellar spacing in the pearlite.

12-46 Determine the required transformation temper-

ature and microconstituent if an eutectoid steel

is to have the following hardness values:

(a) HRC 38 (b) HRC 42

12-47 Describe the hardness and microstructure in an

eutectoid steel that has been heated to 800



C for

1 h, quenched to 350



C and held for 750 s, and

ﬁnally quenched to room temperature.

12-48 Describe the hardness and microstructure in an

eutectoid steel that has been heated to 800



quenched to 650



C, held for 500 s, and ﬁnally

quenched to room temperature.

12-49 Describe the hardness and microstructure in an

eutectoid steel that has been heated to 800



quenched to 300



C and held for 10 s, and ﬁnally

quenched to room temperature.

12-50 Describe the hardness and microstructure in an

eutectoid steel that has been heated to 800



quenched to 300



C and held for 10 s, quenched

to room temperature, and then reheated to

400



C before ﬁnally cooling to room temper-

ature again.

12-51 A steel containing 0.3% 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, composition, and hardness of any

martensite that forms when the heating temper-

ature is:

(a) 728



C (b) 750



C (d) 850



Section 12-11 The Martensitic Reaction and

Tempering

12-52 What is the di¤erence between solid-state phase

transformations such as the eutectoid reaction

and the martensitic phase transformation?

12-53 What is the di¤erence between isothermal and

athermal transformations?

Figure 12-28 The eutectoid portion of the Fe-Fe

phase diagram (for Problems 12-51, 12-56, 12-57,

and 12-58).

Problems 389

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

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