Materials Science and Engineering Handbook Part 8 pdf

Cold drawn 650 94 550 80 12 35 187
Hot rolled 585 85 325 47 15 40 170
1146
Cold drawn 650 94 550 80 12 35 187
Hot rolled 635 92 350 50.5

15 35 187
1151
Cold drawn 705 102 595 86 10 30 207
Low-alloy steels
(b)

Normalized at 870 °C (1600 °F) 834 121 558 81 22.0 63 248
1340
Annealed at 800 °C (1475 °F) 703 102 434 63 25.5 57 207
Normalized at 870 °C (1600 °F) 889 129 600 87 19.7 57 262
3140
Annealed at 815 °C (1500 °F) 690 100 420 61 24.5 51 197
Normalized at 870 °C (1600 °F) 670 97 435 63 25.5 59.5 197
Annealed at 865 °C (1585 °F) 560 81 460 67 21.5 59.6 217
4130
Water quenched from 855 °C (1575 °F) and
tempered at 540 °C (1000 °F)
1040

151 979 142 18.1 63.9 302
Normalized at 870 °C (1600 °F) 1020

148 655 95 17.7 46.8 302
Annealed at 815 °C (1500 °F) 655 95 915 60 25.7 56.9 197
4140

Water quenched from 845 °C (1550 °F) and
tempered at 540 °C (1000 °F)
1075

156 986 143 15.5 56.9 311
Normalized at 870 °C (1600 °F) 1160

168 731 106 11.7 30.8 321
Annealed at 830 °C (1525 °F) 731 106 380 55 20.2 40.2 197
4150
Oil quenched from 830 °C (1525 °F) and tempered
at 540 °C (1000 °F)
1310

190 1215

176 13.5 47.2 375
4320
Normalized at 895 °C (1640 °F) 793 115 460 67 20.8 51 235
Annealed at 850 °C (1560 °F) 580 84 425 62 29.0 58 163
Normalized at 870 °C (1600 °F) 1282

186 862 125 12.2 36.3 363
Annealed at 810 °C (1490 °F) 745 108 470 68 22.0 50.0 217
4340
Oil quenched from 800 °C (1475 °F) and tempered
at 540 °C (1000 °F)
1207

175 1145

166 14.2 45.9 352
Normalized at 955 °C (1750 °F) 515 75 350 51 32.5 69.4 143
4419
Annealed at 915 °C (1675 °F) 450 65 330 48 31.2 62.8 121
Normalized at 900 °C (1650 °F) 570 83 365 53 29.0 66.7 174
4620
Annealed at 855 °C (1575 °F) 510 74 370 54 31.3 60.3 149
Normalized at 860 °C (1580 °F) 758 110 485 70 24.0 59.2 229
4820
Annealed at 815 °C (1500 °F) 685 99 460 67 22.3 58.8 197
Normalized at 870 °C (1600 °F) 793 115 470 68 22.7 59.2 229
Annealed at 830 °C (1525 °F) 570 83 290 42 28.6 57.3 167
5140
Oil quenched from 845 °C (1550 °F) and tempered
at 540 °C (1000 °F)
972 141 841 122 18.5 58.9 293
Normalized at 870 °C (1600 °F) 869 126 530 77 20.7 58.7 255
Annealed at 825 °C (1520 °F) 675 98 360 52 22.0 43.7 197
5150
Oil quenched from 830 °C (1525 °F) and tempered
at 540 °C (1000 °F)
1055

159 1000

145 16.4 52.9 311
Normalized at 855 °C (1575 °F) 1025

149 650 94 18.2 50.7 285

Annealed at 815 °C (1495 °F) 724 105 275 40 17.2 30.6 197
5160
Oil quenched from 830 °C (1525 °F) and tempered
at 540 °C (1000 °F)
1145

166 1005

146 14.5 45.7 341
Normalized at 870 °C (1600 °F) 938 136 615 89 21.8 61.0 269
Annealed at 815 °C (1500 °F) 670 97 415 60 23.0 48.4 197
6150
Oil quenched from 845 °C (1550 °F) and tempered
at 540 °C (1000 °F)
1200

174 1160

168 14.5 48.2 352
Normalized at 915 °C (1675 °F) 635 92 360 52 26.3 59.7 183
8620
Annealed at 870 °C (1600 °F) 540 78 385 56 31.3 62.1 149
Normalized at 870 °C (1600 °F) 650 94 425 62 23.5 53.5 187
Annealed at 845 °C (1550 °F) 565 82 370 54 29.0 58.9 156
8630
Water quenched from 845 °C (1550 °F) and
tempered at 540 °C (1000 °F)
931 135 850 123 18.7 59.6 269
Normalized at 870 °C (1600) 1025

149 690 100 14 45.0 302
Annealed at 795 °C (1465 °F) 715 104 385 56 22.5 46.0 212
8650
Oil quenched from 800 °C (1475 °F) and tempered
at 540 °C (1000 °F)
1185

172 1105

160 14.5 49.1 352
Normalized at 870 °C (1600 °F) 931 135 605 88 16.0 47.9 269
Annealed at 815 °C (1500 °F) 696 101 415 60 22.2 46.4 201
8740
Oil quenched from 830 °C (1525 °F) and tempered
at 540 °C (1000 °F)
1225

178 1130

164 16.0 53.0 352
Normalized at 900 °C (1650 °F) 931 135 580 84 19.7 43.4 269
Annealed at 845 °C (1550 °F) 779 113 485 70 21.7 41.1 229
9255
Oil quenched from 885 °C (1625 °F) and tempered
at 540 °C (1000 °F)
1130

164 924 134 16.7 38.3 321
Normalized at 890 °C (1630 °F) 910 132 570 83 18.8 58.1 269 HRB

9310
Annealed at 845 °C (1550 °F) 820 119 450 65 17.3 42.1 241 HRB

Ferritic stainless steels
(b)

Annealed bar 483 70 276 40 30 60 150
405
Cold drawn bar 586 85 483 70 20 60 185
409
Annealed bar 450 65 240 35 25 . . . 75 HRB
Annealed bar 517 75 310 45 30 65 155
430
Annealed and cold drawn 586 85 483 70 20 65 185
Annealed bar 515 75 310 45 30 50 160
442
Annealed at 815 °C (1500 °F) and cold worked 545 79 427 62 35.5 79 92 HRC
Annealed bar 550 80 345 50 25 45 86 HRB
446
Annealed at 815 °C (1500 °F) and cold drawn 607 88 462 67 26 64 96 HRB
Martensitic stainless steels
(b)

Annealed bar 515 75 275 40 35 70 82 HRB
403
Tempered bar 765 111 585 85 23 67 97 HRB
Oil quenched from 980 °C (1800 °F); tempered at
540 °C (1000 °F); 16 mm (0.625 in.) bar
1085

158 1005

146 13 70 . . .
410
Oil quenched from 980 °C (1800 °F); tempered at 40
°C (104 °F); 16 mm (0.625 in.) bar
1525

221 1225

178 15 64 45 HRB
Annealed bar 795 115 620 90 20 60 235
Cold drawn bar 895 130 795 115 15 58 270
414
Oil quenched from 980 °C (1800 °F); tempered at
650 °C (1200 °F)
1005

146 800 116 19 58 . . .
Annealed bar 655 95 345 50 25 55 195
420
Annealed and cold drawn 760 110 690 100 14 40 228
Annealed bar 860 125 655 95 20 55 260
Annealed and cold drawn 895 130 760 110 15 35 270
Oil quenched from 980 °C (1800 °F); tempered at
650 °C (1200 °F)
831 121 738 107 20 64 . . .
431
Oil quenched from 980 °C (1800 °F); tempered at 40
°C (104 °F)

1435

208 1140

166 17 59 45 HRC
Annealed bar 760 110 450 65 14 25 97 HRB
Annealed and cold drawn bar 860 125 690 100 7 20 260
440C
Hardened and tempered at 315 °C (600 °F) 1970

285 1900

275 2 10 580
Austenitic stainless steels
(b)

Annealed 760 110 380 55 52 . . . 87 HRB
50% hard 1035

150 760 110 12 . . . 32 HRC
Full hard 1275

185 965 140 8 . . . 41 HRC
201
Extra hard 1550

225 1480

215 1 . . . 43 HRC
Annealed bar 515 75 275 40 40 . . . . . .

Annealed sheet 655 95 310 45 40 . . . . . .
202
50% hard sheet 1030

150 760 110 10 . . . . . .
Annealed 725 105 275 40 60 70 . . .
50% hard 1035

150 655 95 54 61 . . .
301
Full hard 1415

205 1330

193 6 . . . . . .
Annealed strip 620 90 275 40 55 . . . 80 HRB
302
25% hard strip 860 125 515 75 12 . . . 25 HRC
Annealed bar 585 85 240 35 60 70 80 HRB
Annealed bar 620 90 240 35 50 55 160
303
Cold drawn 690 100 415 60 40 53 228
Annealed bar 585 85 235 34 60 70 149
Annealed and cold drawn 690 100 415 60 45 . . . 212
304
Cold-drawn high tensile 860 125 655 95 25 . . . 275
305
Annealed sheet 585 85 260 38 50 . . . 80 HRB
308
Annealed bar 585 85 205 30 55 65 150

309
Annealed bar 655 95 275 40 45 65 83 HRB
Annealed sheet 620 90 310 45 45 . . . 85 HRB
310
Annealed bar 655 95 275 40 45 65 160
314
Annealed bar 689 100 345 50 45 60 180
Annealed sheet 580 84 290 42 50 . . . 79 HRB
Annealed bar 550 80 240 35 60 70 149
316
Annealed and cold-drawn bar 620 90 415 60 45 65 190
317
Annealed sheet 620 90 275 40 45 . . . 85 HRB

Annealed bar 585 85 275 40 50 . . . 160
Annealed sheet 620 90 240 35 45 . . . 80 HRB
Annealed bar 585 85 240 35 55 65 150
321
Annealed and cold-drawn bar 655 95 415 60 40 60 185
330
Annealed sheet 550 80 260 38 40 . . . . . .
Annealed bar 585 85 290 42 45 . . . 80 HRB
Annealed sheet 655 95 275 40 45 . . . 85 HRB
Annealed bar 620 90 240 35 50 65 160
347
Annealed and cold drawn bar 690 100 450 65 40 60 212
384
Annealed wire 1040 °C (1900 °F) 515 75 240 35 55 72 70 HRB
Maraging steels
(b)

Annealed 965 140 655 95 17 75 30 HRC
Aged bar 32 mm (1.25 in.) 1844

269 1784

259 11 56.5 51.8
HRC
18Ni(250)

Aged sheet 6 mm (0.25 in.) 1874

272 1832

266 8 40.8 50.6
HRC
Annealed 1034

150 758 110 18 72 32 HRC
Aged bar 32 mm (1.25 in.) 2041

296 2020

293 11.6 55.8 54.7
HRC
18Ni(300)

Aged sheet 6 mm (0.25 in.) 2169

315 2135

310 7.7 35 55.1
HRC
Annealed 1140

165 827 120 18 70 35 HRC
Aged bar 32 mm (1.25 in.) 2391

347 2348

341 7.6 33.8 58.4
HRC
18Ni(350)

Aged sheet 6 mm (0.25 in.) 2451

356 2395

347 3 15.4 57.7
HRC
Source: Ref 1
(a)
All values are estimated minimum values; type 1100 series steels are rated on the basis of 0.10% max Si or coarse-grain melting practice; the
mechanical properties shown are expected minimums for the sizes ranging from 19 to 31.8 mm (0.75 to 1.25 in.).
(b)
Most data are for 25 mm (1 in.) diam bar.

In the selection process, what is required for one application may be totally inappropriate for another application. For
example, steel beams for a railway bridge require a totally different set of properties than the steel rails that are attached
to the wooden ties on the bridge deck. In designing the bridge, the steel must have sufficient strength to withstand

substantial applied loads. In fact, the designer will generally select a steel with higher strength than actually required.
Also, the designer knows that the steel must have fracture toughness to resist the growth and propagation of cracks and
must be capable of being welded so that structural members can be joined without sacrificing strength and toughness. The
steel bridge must also be corrosion resistant. This can be provided by a protective layer of paint. If painting is not
allowed, small amounts of certain alloying elements such as copper and chromium can be added to the steel to inhibit or
reduce corrosion rates. Thus, the steel selected for the bridge would be a high-strength low-alloy (HSLA) structural steel
such as ASTM A572, grade 50 or possibly a weathering steel such as ASTM A588. A typical HSLA steel has a ferrite-
pearlite microstructure as seen in Fig. 2 and is microalloyed with vanadium and/or niobium for strengthening.
(Microalloying is a term used to describe the process of using small additions of carbonitride forming elements titanium,
vanadium, and niobium to strengthen steels by grain refinement and precipitation hardening.)

Fig. 2 Microstructure of a typical HSLA structural steel (ASTM A572, grade 50).
2% nital + 4% picral etch.
200×
On the other hand, the steel rails must have high strength coupled with excellent wear resistance. Modern rail steels
consist of a fully pearlitic microstructure with a fine pearlite interlamellar spacing, as shown in Fig. 3. Pearlite is unique
because it is a lamellar composite consisting of 88% soft, ductile ferrite and 12% hard, brittle cementite (Fe
3
C). The hard
cementite plates provide excellent wear resistance, especially when embedded in soft ferrite. Pearlitic steels have high
strength and are fully adequate to support heavy axle loads of modern locomotives and freight cars. Most of the load is
applied in compression. Pearlitic steels also have relatively poor toughness and cannot generally withstand impact loads
without failure. The rail steel could not meet the requirements of the bridge builder, and the HSLA structural steel could
not meet the requirements of the civil engineer who designed the bridge or the rail system.

Fig. 3 Microstructure of a typical fully pearlitic rail steel showing the charac
teristic fine pearlite interlamellar
spacing. 2% nital + 4% picral etch. 500×
A similar case can be made for the selection of cast irons. A cast machine housing on a large lathe requires a material
with adequate strength, rigidity, and durability to support the applied load and a certain degree of damping capacity in

order to rapidly attenuate (dampen) vibrations from the rotating parts of the lathe. The cast iron jaws of a crusher require a
material with substantial wear resistance. For this application, a casting is required because wear-resistant steels are very
difficult to machine. For the machine housing, gray cast iron is selected because it is relatively inexpensive, can be easily
cast, and has the ability to dampen vibrations as a result of the graphite flakes present in its microstructure. These flakes
are dispersed throughout the ferrite and pearlite matrix (Fig. 4). The graphite, being a major nonmetallic constituent in the
gray iron, provides a tortuous path for sound to travel through the material. With so many flakes, sound waves are easily
reflected and the sound dampened over a relatively short distance. However, for the jaw crusher, damping capacity is not
a requirement. In this case, an alloy white cast iron is selected because of its high hardness and wear resistance. The white
cast iron microstructure shown in Fig. 5 is graphite free and consists of martensite in a matrix of cementite. Both of these
constituents are very hard and thus provide the required wear resistance. Thus, in this example the gray cast iron would
not meet the requirements for the jaws of a crusher and the white cast iron would not meet the requirements for the lathe
housing.

Fig. 4 Microstructure of a gray cast iron with a ferrite-pearlite matrix.
4% picral etch. 320×. Courtesy of A.O.
Benscoter, Lehigh University

Fig. 5
Microstructure of an alloy white cast iron. White constituent is cementite and the darker constituent is
martensite with some retained austenite. 4% picral etch. 250×. Courtesy of A.O. Benscoter, Lehigh University

References cited in this section
1.

Engineering Properties of Steel, P.D. Harvey, Ed., American Society for Metals, 1982
2.

G. Krauss, Principles of the Heat Treatment of Steel, American Society for Metals, 1980

Effects of Composition, Processing, and Structure on Properties of Irons and Steels

Bruce L. Bramfitt, Homer Research Laboratories, Bethlehem Steel Corporation

Role of Microstructure
In steels and cast irons, the microstructural constituents have the names ferrite, pearlite, bainite, martensite, cementite,
and austenite. In most all other metallic systems, the constituents are not named, but are simply referred to by a Greek
letter ( , , , etc.) derived from the location of the constituent on a phase diagram. Ferrous alloy constituents, on the
other hand, have been widely studied for more than 100 years. In the early days, many of the investigators were
petrographers, mining engineers, and geologists. Because minerals have long been named after their discoverer or place
of origin, it was natural to similarly name the constituents in steels and cast irons.
It can be seen that the four examples described above have very different microstructures: the structural steel has a ferrite
+ pearlite microstructure; the rail steel has a fully pearlitic microstructure; the machine housing (lathe) has a ferrite +
pearlite matrix with graphite flakes; and the jaw crusher microstructure contains martensite and cementite. In each case,
the microstructure plays the primary role in providing the properties desired for each application. From these examples,
one can see how material properties can be tailored by microstructural manipulation or alteration. Knowledge about
microstructure is thus paramount in component design and alloy development. In this section, each microstructural
constituent will be described with particular reference to the properties that can be developed by appropriate manipulation
of the microstructure through deformation (e.g., hot and cold rolling) and heat treatment. Further details about these
microstructural constituents can be found in Ref 2, 3, 4, 5, and 6.
Ferrite
A wide variety of steels and cast irons fully exploit the properties of ferrite. However, only a few commercial steels are
completely ferritic. An example of the microstructure of a fully ferritic, ultralow carbon steel is shown in Fig. 6.

Fig. 6 Microstructure of a fully ferritic, ultralow carbon steel. Marshalls etch + HF, 300×. Courtesy of
A.O.
Benscoter, Lehigh University
Ferrite is essentially a solid solution of iron containing carbon or one or more alloying elements such as silicon,
chromium, manganese, and nickel. There are two types of solid solutions: interstitial and substitutional. In an interstitial
solid solution, elements with small atomic diameter, for example, carbon and nitrogen, occupy specific interstitial sites in
the body-centered cubic (bcc) iron crystalline lattice. These sites are essentially the open spaces between the larger iron
atoms. In a substitutional solid solution, elements of similar atomic diameter replace or substitute for iron atoms. The two

types of solid solutions impart different characteristics to ferrite. For example, interstitial elements like carbon and
nitrogen can easily diffuse through the open bcc lattice, whereas substitutional elements like manganese and nickel
diffuse with great difficulty. Therefore, an interstitial solid solution of iron and carbon responds quickly during heat
treatment, whereas substitutional solid solutions behave sluggishly during heat treatment, such as in homogenization.
According to the iron-carbon phase diagram (Fig. 7(a)), very little carbon (0.022% C) can dissolve in ferrite ( Fe), even
at the eutectoid temperature of 727 °C (1330 °F). (The iron-carbon phase diagram indicates the phase regions that exist
over a wide carbon and temperature range. The diagram represents equilibrium conditions. Figure 7(b) shows an
expanded iron-carbon diagram with both the eutectoid and eutectic regions.) At room temperature, the solubility is an
order of magnitude less (below 0.005% C). However, even at these small amounts, the addition of carbon to pure iron
increases the room-temperature yield strength of iron by more than five times, as seen in Fig. 8. If the carbon content
exceeds the solubility limit of 0.022%, the carbon forms another phase called cementite (Fig. 9). Cementite is also a
constituent of pearlite, as seen in Fig. 10. The role of cementite and pearlite on the mechanical properties of steel is
discussed below.

Fig. 7(a) Iron-carbon phase diagram showing the austenite ( Fe) and ferrite (
Fe) phase regions and
eutectoid composition and temperature. Dotted lines represent iron-
graphite equilibrium conditions and solid
lines represent iron-cementite equilibrium conditions. Only the solid lines are important wit
h respect to steels.
Source: Ref 2

Fig. 7(b) Expanded iron-carbon phase diagram showing both the eutectoid (shown in Fig. 7(a)
) and eutectic
regions. Dotted lines represent iron-graphite equilibrium conditions and solid lines represent iron-
cementite
equilibrium conditions. The solid lines at the eutectic are important to white cast irons and the dotted lines
are
important to gray cast irons. Source: Ref 2

Fig. 8 Increase in room-temperature yield strength of iron with small additions of carbon. Source: Ref 7


Fig. 9 Photomicrograph of an annealed low-carbon sheet steel with grain-boundary cementite.
2% nital + 4%
picral etch. 1000×

Fig. 10 Photomicrograph of pearlite (dark constituent) in a low-carbon steel sheet.
2% nital + 4% picral etch.
1000×
The influence of solid-solution elements on the yield strength of ferrite is shown in Fig. 11. Here one can clearly see the
strong effect of carbon on increasing the strength of ferrite. Nitrogen, also an interstitial element, has a similar effect.
Phosphorus is also a ferrite strengthener. In fact, there are commercially available steels containing phosphorus for
strengthening. These steels are the rephosphorized steels (type 1211 to 1215 series). Compositions and mechanical
property data for these steels can be found in Tables 1 and 5.

Fig. 11 Influence of solid-solution elements on the changes in yield stress of low-carbon ferritic steels.
Source:
Ref 5
In Fig. 11, the substitutional solid solution elements of silicon, copper, manganese, molybdenum, nickel, aluminum, and
chromium are shown to have far less effect as ferrite strengtheners than the interstitial elements. In fact, chromium,
nickel, and aluminum in solid solution have very little influence on the strength of ferrite.
In addition to carbon (and other solid-solution elements), the strength of a ferritic steel is also determined by its grain size
according to the Hall-Petch relationship:
y
=
o
+ k
y
d

-1/2


(Eq 1)
where
y
is the yield strength (in MPa),
o
is a constant, k
y
is a constant, and d is the grain diameter (in mm).
The grain diameter is a measurement of size of the ferrite grains in the microstructure, for example, note the grains in the
ultralow carbon steel in Fig. 6. Figure 12 shows the Hall-Petch relationship for a low-carbon fully ferritic steel. This
relationship is extremely important for understanding structure-property relationships in steels. Control of grain size
through thermomechanical treatment, heat treatment, and/or microalloying is vital to the control of strength and toughness
of most steels. The role of grain size is discussed in more detail later in this article.

Fig. 12 Hall-Petch relationship in low-carbon ferritic steels. Source: Ref 8
There is a simple way to stabilize ferrite, thereby expanding the region of ferrite in the iron-carbon phase diagram, namely
by the addition of alloying elements such as silicon, chromium, and molybdenum. These elements are called ferrite
stabilizers because they stabilize ferrite at room temperature through reducing the amount of solid solution (austenite)
with the formation of what is called a -loop as seen at the far left in Fig. 13. This iron-chromium phase diagram shows
that ferrite exists up above 12% Cr and is stable up to the melting point (liquidus temperature). An important fully ferritic
family of steels is the iron-chromium ferritic stainless steels. These steels are resistant to corrosion, and are classified as
type 405, 409, 429, 430, 434, 436, 439, 442, 444, and 446 stainless steels. These steels range in chromium content from
11 to 30%. Additions of molybdenum, silicon, niobium, aluminum, and titanium provide specific properties. Ferritic
stainless steels have good ductility (up to 30% total elongation and 60% reduction in area) and formability, but lack
strength at elevated temperatures compared with austenitic stainless steels. Room-temperature yield strengths range from
170 to about 440 MPa (25 to 64 ksi), and room-temperature tensile strengths range from 380 to about 550 MPa (55 to 80
ksi). Table 5 lists the mechanical properties of some of the ferritic stainless steels. Type 409 stainless steel is widely used

for automotive exhaust systems. Type 430 free-machining stainless steel has the best machinability of all stainless steels
other than that of a low-carbon, free-machining martensitic stainless steel (type 416).

Fig. 13 Iron-chromium phase diagram. Source: Ref 9
Another family of steels utilizing a ferrite stabilizer ( -loop) are the iron-silicon ferritic alloys containing up to about
6.5% Si (carbon-free). These steels are of commercial importance because they have excellent magnetic permeability and
low core loss. High-efficiency motors and transformers are produced from these iron-silicon electrical steels (aluminum
can also substitute for silicon in them).
Over the past 20 years or so, a new breed of very-low-carbon fully ferritic sheet steels has emerged for applications
requiring exceptional formability (see Fig. 6). These are the interstitial-free (IF) steels for which carbon and nitrogen are
reduced in the steelmaking process to very low levels, and any remaining interstitial carbon or nitrogen is tied up with
small amounts of alloying elements (e.g., titanium or niobium) that form preferentially carbides and nitrides. These steels
have very low strength, but are used to produce components that are difficult or impossible to form from other steels.
Very-low-carbon, fully ferritic steels (0.001% C) are now being manufactured for automotive components that harden
during the paint-curing cycle. These steels are called bake-hardening steels, and have controlled amounts of carbon and
nitrogen that combine with other elements, such as titanium and niobium, during the baking cycle (175 °C, or 350 °F, for
30 min). The process is called aging, and the strength derives from the precipitation of titanium/niobium carbonitrides at
the elevated temperature.
Another form of very-low-carbon, fully ferritic steel is motor lamination steel. The carbon is removed from these steels
by a process known as decarburization. The decarburized (carbon-free) ferritic steel has good permeability and
sufficiently low core loss (not as low as the iron-silicon alloys) to be used for electric motor laminations, that is, the
stacked steel layers in the rotor and stator of the motor.
As noted previously, a number of properties are exploited in fully ferritic steels:
• Iron-silicon steels: Exceptional electrical properties
• Iron-chromium steels: Good corrosion resistance
• Interstitial-free steels: Exceptional formability
• Bake-hardening steels: Strengthens during paint cure cycle
• Lamination steels: Good electrical properties
Pearlite
As the carbon content of steel is increased beyond the solubility limit (0.02% C) on the iron-carbon binary phase diagram,

a constituent called pearlite forms. Pearlite is formed by cooling the steel through the eutectoid temperature (the
temperature of 727 °C in Fig. 7(a) and 7(b)) by the following reaction:
Austenite cementite + ferrite


(Eq 2)
The cementite and ferrite form as parallel plates called lamellae (Fig. 14). This is essentially a composite microstructure
consisting of a very hard carbide phase, cementite, and a very soft and ductile ferrite phase. A fully pearlitic
microstructure is formed at the eutectoid composition of 0.78% C. As can be seen in Fig. 3 and 14, pearlite forms as
colonies where the lamellae are aligned in the same orientation. The properties of fully pearlitic steels are determined by
the spacing between the ferrite-cementite lamellae, a dimension called the interlamellar spacing, , and the colony size.
A simple relationship for yield strength has been developed by Heller (Ref 10) as follows:
y
= -85.9 + 8.3 (
-1/2
)


(Eq 3)
where
y
is the 0.2% offset yield strength (in MPa) and is the interlamellar spacing (in mm). Figure 15 shows Heller's
plot of strength versus interlamellar spacing for fully pearlitic eutectoid steels.

Fig. 14 SEM micrograph of pearlite showing ferrite and cementite lamellae. 4% picral etch. 10,000×


Fig. 15 Relationship between pearlite interlamellar spacing and yield strength for eutectoid steels. Source:
Ref
10

It has also been shown by Hyzak and Bernstein (Ref 11) that strength is related to interlamellar spacing, pearlite colony
size, and prior-austenite grain size, according to the following relationship:
YS = 52.3 + 2.18(
-1/2
) - 0.4( ) - 2.88(d
-1/2
)


(Eq 4)
where YS is the yield strength (in MPa), d
c
is the pearlite colony size (in mm), and d is the prior-austenite grain size (in
mm). From Eq 3 and 4, it can be seen that the steel composition does not have a major influence on the yield strength of a
fully pearlitic eutectoid steel. There is some solid-solution strengthening of the ferrite in the lamellar structure (see Fig.
11).
The thickness of the cementite lamellae can also influence the properties of pearlite. Fine cementite lamellae can be
deformed, compared with coarse lamellae, which tend to crack during deformation.
Although fully pearlitic steels have high strength, high hardness, and good wear resistance, they also have poor ductility
and toughness. For example, a low-carbon, fully ferritic steel will typically have a total elongation of more than 50%,
whereas a fully pearlitic steel (e.g., type 1080) will typically have a total elongation of about 10% (see Table 5). A low-
carbon fully ferritic steel will have a room-temperature Charpy V-notch impact energy of about 200 J (150 ft · lbf),
whereas a fully pearlitic steel will have room-temperature impact energy of under 10 J (7 ft · lbf). The transition
temperature (i.e., the temperature at which a material changes from ductile fracture to brittle fracture) for a fully pearlitic
steel can be approximated from the following relationship (Ref 11):
TT = 217.84 - 0.83( ) - 2.98(d
-1/2
)

(Eq 5)
where TT is the transition temperature (in °C).
From Eq 5, one can see that both the prior-austenite grain size and pearlite colony size control the transition temperature
of a pearlitic steel. Unfortunately, the transition temperature of a fully pearlitic steel is always well above room
temperature. This means that at room temperature the general fracture mode is cleavage, which is associated with brittle
fracture. Therefore, fully pearlitic steels should not be used in applications where toughness is important. Also, pearlitic
steels with carbon contents slightly or moderately higher than the eutectoid composition (called hypereutectoid steels)
have even poorer toughness.
From Eq 4 and 5, one can see that for pearlite, strength is controlled by interlamellar spacing, colony size, and prior-
austenite grain size, and toughness is controlled by colony size and prior-austenite grain size.
Unfortunately, these three factors are rather difficult to measure. To determine interlamellar spacing, a scanning electron
microscope (SEM), or a transmission electron microscope (TEM) is needed in order to resolve the spacing. Generally, a
magnification of 10,000× is adequate, as seen in Fig. 14. Special statistical procedures have been developed to determine
an accurate measurement of the spacing (Ref 12). The colony size and especially the prior austenite grain size are very
difficult to measure and require a skilled metallographer using the light microscope or SEM and special etching
procedures.
Because of poor ductility/toughness, there are only a few applications for fully pearlitic steels, including railroad rails and
wheels and high-strength wire. By far, the largest tonnage application is for rails. A fully pearlitic rail steel provides
excellent wear resistance for railroad wheel/rail contact. Rail life is measured in millions of gross tons (MGT) of travel
and current rail life easily exceeds 250 MGT. The wear resistance of pearlite arises from the unique morphology of the
ferrite-cementite lamellar composite where a hard constituent is embedded into a soft-ductile constituent. This means that
the hard cementite plates do not abrade away as easily as the rounded cementite particles found in other steel
microstructures, that is, tempered martensite and bainite, which will be discussed later. Wear resistance of a rail steel is
directly proportional to hardness. This is shown in Fig. 16, which indicates less weight loss as hardness increases. Also,
wear resistance (less weight loss) increases as interlamellar spacing decreases, as shown in Fig. 17. Thus, the most
important microstructural parameter for controlling hardness and wear resistance is the pearlite interlamellar spacing.
Fortunately, interlamellar spacing is easy to control and is dependent solely on transformation temperature.

Fig. 16 Relationship between hardness and wear resistance (weight loss) for rail steels. Source: Ref 13

Fig. 17 Relationship be
tween pearlite interlamellar spacing and wear resistance (weight loss) for rail steels.
Source: Ref 13
Figure 18 shows a continuous cooling transformation (CCT) diagram for a typical rail steel. A CCT diagram is a time
versus temperature plot showing the regions at which various constituents ferrite, pearlite, bainite, and martensite form
during the continuous cooling of a steel component. Usually several cooling curves are shown with the associated start
and finish transformation temperatures of each constituent. These diagrams should not be confused with isothermal
transformation (IT or TTT) diagrams, which are derived by rapidly quenching very thin specimens to various
temperatures, and maintaining that temperature (isothermal) until the specimens begin to transform, partially transform,
and fully transform, at which time they are quenched to room temperature. An IT diagram does not represent the
transformation behavior in most processes where steel parts are continuously cooled, that is, air cooled, and so forth.

Fig. 18 A CCT diagram of a typical rail steel (composition: 0.77% C, 0.95% Mn, 0.22% Si, 0.014%
P, 0.017%
S, 0.010% Cr). Source: Ref 14
As shown in Fig. 18, the pearlite transformation temperature (indicated by the pearlite-start curve, P
s
) decreases with
increasing cooling rate. The hardness of pearlite increases with decreasing transformation temperature. Thus, in order to
provide a rail steel with the highest hardness and wear resistance, one must cool the rail from the austenite at the fastest
rate possible to obtain the lowest transformation temperature. This is done in practice by a process known as head
hardening, which is simply an accelerated cooling process using forced air or water sprays to achieve the desired cooling
rate (Ref 15). Because only the head of the rail contacts the wheel of the railway car and locomotive, only the head
requires the higher hardness and wear resistance.
Another application for a fully pearlitic steel is high-strength wire (e.g., piano wire). Again, the composite morphology of
lamellar ferrite and cementite is exploited, this time during wire drawing. A fully pearlitic steel rod is heat treated by a
process known as patenting. During patenting, the rod is transformed at a temperature of about 540 °C (1000 °F) by
passing it through a lead or salt bath at this temperature. This develops a microstructure with a very fine pearlite
interlamellar spacing because the transformation takes place at the nose of the CCT diagram, that is, at the lowest possible

pearlite transformation temperature (see Fig. 18). The rod is then cold drawn to wire. Because of the very fine
interlamellar spacing, the ferrite and cementite lamellae become aligned along the wire axis during the deformation
process. Also, the fine cementite lamella tend to bend and deform as the wire is elongated during drawing. The resulting
wire is one of the strongest commercial products available; for example, a commercial 0.1 mm (0.004 in.) diam wire can
have a tensile strength in the range of 3.0 to 3.3 GPa (439 to 485 ksi), and in special cases a tensile strength as high as 4.8
GPa can be obtained. These wires are used in musical instruments because of the sound quality developed from the high
tensile stresses applied in stringing a piano and violin and are also used in wire rope cables for suspension bridges.
Ferrite-Pearlite. The most common structural steels produced have a mixed ferrite-pearlite microstructure. Their
applications include beams for bridges and high-rise buildings, plates for ships, and reinforcing bars for roadways. These
steels are relatively inexpensive and are produced in large tonnages. They also have the advantage of being able to be
produced with a wide range of properties. The microstructure of typical ferrite-pearlite steels is shown in Fig. 19.

Fig. 19 Microstructure of typical ferrite-pearlite structural steels at two different carbon contents.
(a) 0.10% C.
(b) 0.25% C. 2% nital + 4% picral etch. 200×
In most ferrite-pearlite steels, the carbon content and the grain size determine the microstructure and resulting properties.
For example, Fig. 20 shows the effect of carbon on tensile and impact properties. The ultimate tensile strength steadily
increases with increasing carbon content. This is caused by the increase in the volume fraction of pearlite in the
microstructure, which has a strength much higher than that of ferrite. Thus, increasing the volume fraction of pearlite has
a profound effect on increasing tensile strength.

Fig. 20 Mechanical properties of ferrite-pearlite steels as a function of carbon content. Source: Ref 2

However, as seen in Fig. 20, the yield strength is relatively unaffected by carbon content, rising from about 275 MPa (40
ksi) to about 415 MPa (60 ksi) over the range of carbon content shown. This is because yielding in a ferrite-pearlite steel
is controlled by the ferrite matrix, which is generally considered to be the continuous phase (matrix) in the microstructure.
Therefore, pearlite plays only a minor role in yielding behavior.
From Fig. 20, one can also see that ductility, as represented by reduction in area, steadily decreases with increasing
carbon content. A steel with 0.10% C has a reduction in area of about 75%, whereas a steel with 0.70% C has a reduction
in area of only 25%. Percent total elongation would show a similar trend, however, with values much less than percent

reduction in area.
Much work has been done to develop empirical equations for ferrite-pearlite steels that relate strength and toughness to
microstructural features, for example, grain size and percent of pearlite as well as composition. One such equation for
ferrite-pearlite steels under 0.25% C is as follows (Ref 16):
YS = 53.9 + 32.34 (Mn) + 83.2(Si)

+ 354.2(N
f
) + 17.4(d
-1/2
)

(Eq 6)
where Mn is the manganese content (%), Si is the silicon content (%), N
f
is the free nitrogen content (%), and d is the
ferrite grain size (in mm). Equation 6 shows that carbon content (percent pearlite) has no effect on yield strength, whereas
the yield strength in Fig. 20 increases somewhat with carbon content. According to Eq 6, manganese, silicon, and nitrogen
have a pronounced effect on yield strength, as does grain size. However, in most ferrite-pearlite steels nitrogen is quite
low (under 0.010%) and thus has minimal effect on yield strength. In addition, as discussed below, nitrogen has a
detrimental effect on impact properties.
The regression equation for tensile strength for the same steels is as follows (Ref 16):
TS = 294.1 + 27.7(Mn) + 83.2(Si)

+ 3.9(P) + 7.7(d
-1/2
)

(Eq 7)
where TS is the tensile strength (in MPa) and P is pearlite content (%). Thus, in distinction to yield strength, the

percentage of pearlite in the microstructure plays an important role on tensile strength.
Toughness of ferrite-pearlite steels is also an important consideration in their use. It has long been known that the
absorbed energy in a Charpy V-notch test is decreased by increasing carbon content, as seen in Fig. 21. In this graph of
impact energy versus test temperature, the shelf energy decreases from about 200 J (150 ft · lbf) for a 0.11% C steel to
about 35 J (25 ft · lbf) for a 0.80% C steel. Also, the transition temperature increases from about -50 to 150 °C (-60 to 300
°F) over this same range of carbon content. The effect of carbon is due mainly to its effect on the percentage of pearlite in
the microstructure. This is reflected in the regression equation for transition temperature below (Ref 16):
TT = -19 + 44(Si) + 700( )

+ 2.2(P) - 11.5(d
-1/2
)

(Eq 8)

Fig. 21 Effect of carbon content in ferrite-pearlite steels on Charpy V-
notch transition temperature and shelf
energy. Source: Ref 17