30079_02.pdf

CHAPTER 2

STEEL

Robert J. King

U.S. Steel Group, USX Corporation

Pittsburgh, Pennsylvania

2.1 METALLOGRAPHY AND HEAT

TREATMENT

2.4.5

Austempering

2.4.6

Normalizing

2.4.7

Annealing

2.2 IRON-IRON CARBIDE PHASE

DIAGRAM

2.4.8

Isothermal Annealing

2.4.9

Spheroidization Annealing

2.2.1

Changes on Heating and

Cooling Pure Iron

2.2.10 Process Annealing

2.4.11 Carburizing

2.2.2

Changes on Heating and

Cooling Eutectoid Steel

2.4.12 Nitriding

2.2.3

Changes on Heating and

Cooling Hypoeutectoid Steels 20

2.5 CARBON STEELS

2.5.1

Properties

2.2.4

Changes on Heating and

Cooling Hypereutectoid Steels 20

2.5.2

Microstructure and Grain

Size

2.2.5

Effect on Alloys on the

Equilibrium Diagram

2.5.3

Microstructure of Cast Steels 33

2.5.4

Hot Working

2.2.6

Grain Size— Austenite

2.5.5

Cold Working

2.2.7

Microscopic-Grain-Size

Determination

2.5.6

Heat Treatment

2.5.7

Residual Elements

2.2.8

Fine- and Coarse-Grain

Steels

2.6 DUAL-PHASESHEETSTEELS

2.2.9

Phase Transformations —

Austenite

2.7 ALLOYSTEELS

2.2.10 Isothermal Transformation

Diagram 21

2.2.11 Pearlite 23

2.2.12 Bainite 23

2.2.13 Martensite 23

2.2.14 Phase Properties— Pearlite 23

2.2.15 Phase Properties — Bainite 23

2.2.16 Phase Properties — Martensite 23

2.2.17 Tempered Martensite

2.7.1

Functions of Alloying

Elements

2.7.2

Thermomechanical

Treatment

2.7.3

High-Strength Low-Alloy

(HSLA) Steels

2.7.4

AISI Alloy Steels

2.7.5

Alloy Tool Steels

2.7.6

Stainless Steels

2.2.18 Transformation Rates

2.7.7

Martensitic Stainless Steels

2.2.19 Continuous Cooling

2.7.8

Ferrite Stainless Steels

2.7.9

Austenitic Stainless Steels

2.3 HARDENABILITY

2.7.10 High-Temperature Service,

Heat-Resisting Steels

2.4 HEAT-TREATINGPROCESSES

2.7. 1 1 Quenched and Tempered

Low-Carbon Constructional

Alloy Steels

2.4. 1

Austenitization

2.4.2

Quenching

2.4.3

Tempering

2.7.12 Maraging Steels

2.4.4

Martempering

2.7.13 Silicon-Steel Electrical

Sheets

Reprinted from Kirk-Othmer Encyclopedia of Chemical Technology, 3rd ed., Wiley, New York, 1983,

Vol. 21, by permission of the publisher.

Mechanical Engineers' Handbook, 2nd ed., Edited by Myer Kutz.

2.1 METALLOGRAPHY AND HEAT TREATMENT

The great advantage of steel as an engineering material is its versatility, which arises from the fact

that its properties can be controlled and changed by heat treatment. 1 ' 3 Thus, if steel is to be formed

into some intricate shape, it can be made very soft and ductile by heat treatment; on the other hand,

heat treatment can also impart high strength.

The physical and mechanical properties of steel depend on its constitution, that is, the nature,

distribution, and amounts of its metallographic constituents as distinct from its chemical composition.

The amount and distribution of iron and iron carbide determine the properties, although most plain

carbon steels also contain manganese, silicon, phosphorus, sulfur, oxygen, and traces of nitrogen,

hydrogen, and other chemical elements such as aluminum and copper. These elements may modify,

to a certain extent, the main effects of iron and iron carbide, but the influence of iron carbide always

predominates. This is true even of medium-alloy steels, which may contain considerable amounts of

nickel, chromium, and molybdenum.

The iron in steel is called ferrite. In pure iron-carbon alloys, the ferrite consists of iron with a

trace of carbon in solution, but in steels it may also contain alloying elements such as manganese,

silicon, or nickel. The atomic arrangement in crystals of the allotrophic forms of iron is shown in

Fig. 2.1.

Cementite, the term for iron carbide in steel, is the form in which carbon appears in steels. It has

the formula Fe 3 C, and consists of 6.67% carbon and 93.33% iron. Little is known about its properties,

except that it is very hard and brittle. As the hardest constituent of plain carbon steel, it scratches

glass and feldspar but not quartz. It exhibits about two-thirds the induction of pure iron in a strong

magnetic field.

Austenite is the high-temperature phase of steel. Upon cooling, it gives ferrite and cementite.

Austenite is a homogeneous phase, consisting of a solid solution of carbon in the y form of iron. It

forms when steel is heated above 79O 0 C. The limiting temperatures for its formation vary with

composition and are discussed below. The atomic structure of austenite is that of y iron, fee; the

atomic spacing varies with the carbon content.

When a plain carbon steel of ~ 0.80% carbon content is cooled slowly from the temperature

range at which austenite is stable, ferrite and cementite precipitate together in a characteristically

lamellar structure known as pearlite. It is similar in its characteristics to a eutectic structure but, since

it is formed from a solid solution rather than from a liquid phase, it is known as a eutectoid structure.

At carbon contents above and below 0.80%, pearlite of ~ 0.80% carbon is likewise formed on slow

cooling, but excess ferrite or cementite precipitates first, usually as a grain-boundary network, but

occasionally also along the cleavage planes of austenite. The excess ferrite or cementite rejected by

the cooling austenite is known as a proeutectoid constituent. The carbon content of a slowly cooled

steel can be estimated from the relative amounts of pearlite and proeutectoid constituents in the

microstructure.

Bainite is a decomposition product of austenite consisting of an aggregate of ferrite and cementite.

It forms at temperatures lower than those where very fine pearlite forms and higher than those at

which martensite begins to form on cooling. Metallographically, its appearance is feathery if formed

Fig. 2.1 Crystalline structure of allotropic forms of iron. Each white sphere represents an atom

of (a) a and 8 iron in bcc form, and (b) y iron, in fee (from Ref. 1).

in the upper part of the temperature range, or acicular (needlelike) and resembling tempered marten-

site if formed in the lower part.

Martensite in steel is a metastable phase formed by the transformation of austenite below the

temperature called the M s temperature, where martensite begins to form as austenite is cooled con-

tinuously. Martensite is an interstitial supersaturated solid solution of carbon in iron with a body-

centred tetragonal lattice. Its microstructure is acicular.

2.2 IRON-IRON CARBIDE PHASE DIAGRAM

The iron-iron carbide phase diagram (Fig. 2.2) furnishes a map showing the ranges of compositions

and temperatures in which the various phases such as austenite, ferrite, and cementite are present in

slowly cooled steels. The diagram covers the temperature range from 60O 0 C to the melting point of

iron, and carbon contents from O to 5%. In steels and cast irons, carbon can be present either as iron

carbide (cementite) or as graphite. Under equilibrium conditions, only graphite is present because

iron carbide is unstable with respect to iron and graphite. However, in commercial steels, iron carbide

is present instead of graphite. When a steel containing carbon solidifies, the carbon in the steel usually

solidifies as iron carbide. Although the iron carbide in a steel can change to graphite and iron when

the steel is held at ~ 90O 0 C for several days or weeks, iron carbide in steel under normal conditions

is quite stable.

The portion of the iron-iron carbide diagram of interest here is that part extending from O to

2.01% carbon. Its application to heat treatment can be illustrated by considering the changes occurring

on heating and cooling steels of selected carbon contents.

Iron occurs in two allotropic forms, a or 8 (the latter at a very high temperature) and y (see Fig.

2.1.) The temperatures at which these phase changes occur are known as the critical temperatures,

and the boundaries in Fig. 2.2 show how these temperatures are affected by composition. For pure

iron, these temperatures are 91O 0 C for the a-y phase change and 1390° for the y-8 phase change.

2.2.1 Changes on Heating and Cooling Pure Iron

The only changes occurring on heating or cooling pure iron are the reversible changes at —910 0 C

from bcc a iron to fee y iron and from the fee 8 iron to bcc y iron at ~1390°C.

2.2.2 Changes on Heating and Cooling Eutectoid Steel

Eutectoid steels are those that contain 0.8% carbon. The diagram shows that at and below 727 0 C the

constituents are a-ferrite and cementite. At 60O 0 C, the a-ferrite may dissolve as much as 0.007%

carbon. Up to 727 0 C, the solubility of carbon in the ferrite increases until, at this temperature, the

Fig. 2.2 Iron-iron carbide phase diagram (from Ref. 1).

ferrite contains about 0.02% carbon. The phase change on heating an 0.8% carbon steel occurs at

727 0 C which is designated as A 1 , as the eutectoid or lower critical temperature. On heating just above

this temperature, all ferrite and cementite transform to austenite, and on slow cooling the reverse

change occurs.

When a eutectoid steel is slowly cooled from —738 0 C, the ferrite and cementite form in alternate

layers of microscopic thickness. Under the microscope at low magnification, this mixture of ferrite

and cementite has an appearance similar to that of a pearl and is therefore called pearlite.

2.2.3 Changes on Heating and Cooling Hypoeutectoid Steels

Hypoeutectoid steels are those that contain less carbon than the eutectoid steels. If the steel contains

more than 0.02% carbon, the constituents present at and below 727 0 C are usually ferrite and pearlite;

the relative amounts depend on the carbon content. As the carbon content increases, the amount of

ferrite decreases and the amount of pearlite increases.

The first phase change on heating, if the steel contains more than 0.02% carbon, occurs at 727 0 C.

On heating just above this temperature, the pearlite changes to austenite. The excess ferrite, called

proeutectoid ferrite, remains unchanged. As the temperature rises further above A 1 , the austenite

dissolves more and more of the surrounding proeutectoid ferrite, becoming lower and lower in carbon

content until all the proeutectoid ferrite is dissolved in the austenite, which now has the same average

carbon content as the steel.

On slow cooling the reverse changes occur. Ferrite precipitates, generally at the grain boundaries

of the austenite, which becomes progressively richer in carbon. Just above A 1 , the austenite is sub-

stantially of eutectoid composition, 0.8% carbon.

2.2.4 Changes on Heating and Cooling Hypereutectoid Steels

The behavior on heating and cooling hypereutectoid steels (steels containing >0.80% carbon) is

similar to that of hypoeutectoid steels, except that the excess constituent is cementite rather than

ferrite. Thus, on heating above A 1 , the austentie gradually dissolves the excess cementite until at the

A cm temperature the proeutectoid cementite has been completely dissolved and austenite of the same

carbon content as the steel is formed. Similarly, on cooling below A cm , cementite precipitates and

the carbon content of the austenite approaches the eutectoid composition. On cooling below A 1 , this

eutectoid austenite changes to pearlite and the room-temperature composition is, therefore, pearlite

and proeutectoid cementite.

Early iron-carbon equilibrium diagrams indicated a critical temperature at ~768°C. It has since

been found that there is no true phase change at this point. However, between —768 and 79O 0 C there

is a gradual magnetic change, since ferrite is magnetic below this range and paramagnetic above it.

This change, occurring at what formerly was called the A 2 change, is of little or no significance with

regard to the heat treatment of steel.

2.2.5 Effect of Alloys on the Equilibrium Diagram

The iron-carbon diagram may, of course, be profoundly altered by alloying elements, and its appli-

cation should be limited to plain carbon and low-alloy steels. The most important effects of the

alloying elements are that the number of phases that may be in equilibrium is no longer limited to

two as in the iron-carbon diagram; the temperature and composition range, with respect to carbon,

over which austenite is stable may be increased or reduced; and the eutectoid temperature and com-

position may change.

Alloying elements either enlarge the austenite field or reduce it. The former include manganese,

nickel, cobalt, copper, carbon, and nitrogen and are referred to as austenite formers.

The elements that decrease the extent of the austenite field include chromium, silicon, molyb-

denum, tungsten, vanadium, tin, niobium, phosphorus, aluminum, and titanium; they are known as

ferrite formers.

Manganese and nickel lower the eutectoid temperature, whereas chromium, tungsten, silicon,

molybdenum, and titanium generally raise it. All these elements seem to lower the eutectoid carbon

content.

2.2.6 Grain Size—Austenite

A significant aspect of the behavior of steels on heating is the grain growth that occurs when the

austenite, formed on heating above A 3 or A cm , is heated even higher; A 3 is the upper critical tem-

perature and A c m is the temperature at which cementite begins to form. The austenite, like any metal

composed of a solid solution, consists of polygonal grains. As formed at a temperature just above

A 3 or A cm , the size of the individual grains is very small but, as the temperature is increased above

the critical temperature, the grain sizes increase. The final austenite grain size depends, therefore, on

the temperature above the critical temperature to which the steel is heated. The grain size of the

austenite has a marked influence on transformation behavior during cooling and on the grain size of

the constituents of the final microstructure. Grain growth may be inhibited by carbides that dissolve

slowly or by dispersion of nonmetallic inclusions. Hot working refines the coarse grain formed by

reheating steel to the relatively high temperatures used in forging or rolling, and the grain size of

hot-worked steel is determined largely by the temperature at which the final stage of the hot-working

process is carried out. The general effects of austenite grain size on the properties of heat-treated

steel are summarized in Table 2.1.

2.2.7 Microscopic-Grain-Size Determination

The microscopic grain size of steel is customarily determined from a polished plane section prepared

in such a way as to delineate the grain boundaries. The grain size can be estimated by several methods.

The results can be expressed as diameter of average grain in millimeters (reciprocal of the square

root of the number of grains per mm 2 ), number of grains per unit area, number of grains per unit

volume, or a micrograin-size number obtained by comparing the microstructure of the sample with

a series of standard charts.

2.2.8 Fine- and Coarse-Grain Steels

As mentioned previously, austenite-grain growth may be inhibited by undissolved carbides or non-

metallic inclusions. Steels of this type are commonly referred to as fine-grained steels, whereas steels

that are free from grain-growth inhibitors are known as coarse-grained steels.

The general pattern of grain coarsening when steel is heated above the critical temperature is as

follows: Coarse-grained steel coarsens gradually and consistently as the temperature is increased,

whereas fine-grained steel coarsens only slightly, if at all, until a certain temperature known as the

coarsening temperature is reached, after which abrupt coarsening occurs. Heat treatment can make

any type of steel either fine or coarse grained; as a matter of fact, at temperatures above its coarsening

temperature, the fine-grained steel usually exhibits a coarser grain size than the coarse-grained steel

at the same temperature.

Making steels that remain fine grained above 925 0 C involves the judicious use of deoxidation

with aluminum. The inhibiting agent in such steels is generally conjectured to be a submicroscopic

dispersion of aluminum nitride or, perhaps at times, aluminum oxide.

2.2.9 Phase Transformations—Austenite

At equilibrium, that is, with very slow cooling, austenite transforms to pearlite when cooled below

the A 1 temperature. When austenite is cooled more rapidly, this transformation is depressed and

occurs at a lower temperature. The faster the cooling rate, the lower the temperature at which trans-

formation occurs. Furthermore, the nature of the ferrite-carbide aggregate formed when the austenite

transforms varies markedly with the transformation temperature, and the properites are found to vary

correspondingly. Thus, heat treatment involves a controlled supercooling of austenite, and in order

to take full advantage of the wide range of structures and properties that this treatment permits, a

knowledge of the transformation behavior of austenite and the properties of the resulting aggregates

is essential.

2.2.10 Isothermal Transformation Diagram

The transformation behavior of austenite is best studied by observing the isothermal transformation

at a series of temperatures below A 1 . The transformation progress is ordinarily followed metallo-

graphically in such a way that both the time-temperature relationships and the manner in which the

microstructure changes are established. The times at which transformation begins and ends at a given

temperature are plotted, and curves depicting the transformation behavior as a function of temperature

are obtained by joining these points (Fig. 2.3) Such a diagram is referred to as an isothermal trans-

formation (IT) diagram, a time-temperature-transformation (TTT) diagram, or, an S curve. 4

Table 2.1 Trends in Heat-Treated Products

Property

Coarse-grain Austenite

Fine-grain Austenite

Quenched and Tempered Products

Hardenability

Increasing

Decreasing

Toughness

Decreasing

Increasing

Distortion

Less

Quench cracking

Less

Internal stress

Higher

Lower

Annealed or Normalized Products

Machinability

Rough

finish

Better

Inferior

Fine

finish

Inferior

Better

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