Thermodynamics of Condensed Phases
2.1 THERMODYNAMICS OF METALS AND ALLOYS
2.1.3 The Iron– Carbon Phase Diagram*
Iron and its alloys continue to be the most widely utilized metallic systems for construc- tion. The process by whichiron-based alloys, includingsteels, are produced from iron
HISTORICAL HIGHLIGHT Iron was the first metal for the masses.
As far back as five thousand years ago, metalworkers must have somehow gotten their furnaces especially hot on a day when they had charged the furnace with some iron-rich rock, maybe blood-red hematite.
Perhaps it was a consistent blast from the sea that made the difference. From this they obtained a silver-gray metal that they had never seen before. It was iron. One of the oldest known iron implements is a dagger blade from 1350 B.C.
The downside of iron is that wrenching it from ore requires a temperature hundreds of degrees higher than that needed to extract copper or tin. In fact, the melting temper- ature was too high for ancient furnaces, so early iron-workers got their metal by beating it out of a solid “bloom,” a conglomeration of metal and rocky slag that forms when the ore is heated to temperatures high enough to loosen things up in the ore but not high enough to melt iron.
The earliest iron turned out to be no bet- ter than bronze. Often it was worse, espe- cially for weaponry. Compared to copper and bronze, it was a slightly crazy metal, with unpredictably varying qualities, so a smith never quite knew what to expect from a fin- ished iron implement or weapon — often they came out too soft or too brittle.
The reason for this fickleness is the spe- cial role that carbon atoms play in the prop- erties of iron-based metals, including steel.
No one knew in the early days of iron that carbon atoms from the burning fuel in their bloomery and smithing furnaces were actu- ally becoming part of the metal’s internal anatomy and changing its properties. In the smelting of copper or tin, carbon’s role is to remove oxygen without remaining in the metal. The iron that smiths wrought from the blooms and then formed into weapons and implements absorbed varying amounts of carbon. Too little carbon resulted in a con- sistency closer to that of pure iron, which is soft and malleable. A man wielding a pure iron sword would be no match for an oppo- nent swinging a bronze sword. Too much carbon in the iron, however, yielded a brittle metal that could shatter like pottery.
Most often, smiths ended up with iron containing somewhere around 1 to 3 percent carbon by weight. When lucky, they would get somewhere between .1 and 1 percent carbon. When that happened, the metal underwent a dramatic personality change. It became much harder and tougher, and it could hold a much sharper edge. When a smith managed to keep the carbon content within this range, he made steel.
Source: Stuff, I. Amato, pp. 28–29.
ore involves the reduction of iron oxide by means of carbon to form a product known as pig iron, which contains approximately 4% carbon. As a result, the iron–carbon phase diagram is an industrially important one. The compounds formed between iron and carbon are sufficiently complex in their microstructures and properties to warrant intensive investigation and more detailed description. However, the iron–carbon phase diagram is really just another binary phase diagram, and, as such, we have all the tools we need to fully understand it.
The iron–carbon phase diagram at low weight percentages of carbon is shown in Figure 2.8. Actually, the phase boundary at 6.69 wt% carbon represents the compound iron carbide, Fe3C, known as cementite, so that the phase diagram in Figure 2.8 is more appropriately that of Fe3C–C.
The allotropy of elemental iron plays an important role in the formation of iron alloys. Upon solidification from the melt, iron undergoes two allotropic transformations (see Figure 2.9). At 1539◦C, iron assumes a BCC structure, called delta-iron (δ-Fe).
Upon further cooling, this structure transforms to the FCC structure at 1400◦C, resulting ingamma-iron(γ-Fe). The FCC structure is stable down to 910◦C, where it transforms back into a low-temperature BCC structure, alpha-iron (α-Fe). Thus,δ-Fe andα-Fe are actually the same form of iron, but are treated as distinct forms due to their two different temperature ranges of stability.
Carbon is soluble to varying degrees in each of these allotropic forms of iron.
The solid solutions of carbon in α-Fe, γ-Fe, and δ-Fe are called, respectively, fer- rite, austenite, and δ-ferrite. So, for example, the single-phase region labeled asγ in
1700
1500 d + L
L + Fe3C
g + Fe3C
a + Fe3C g + L d + g
d + g 0.090.17
0.53 1495°C
1148°C
723°C 0.020.80
1300
1100
900
700
500
0 1 2 3 4 5 6 6.69
(Fe) (Fe3C)
912
a 1538
1394 d
g
L
2.11 4.30
Fe3C
Wt. % C
T(°C)
Figure 2.8 The Fe– C phase diagram (low weight % C). From K. M. Ralls, T. H. Courtney, and J. Wulff, Introduction to Materials Science and Engineering. Copyright 1976 by John Wiley & Sons, Inc. This material is used by permission of John Wiley & Sons, Inc.
THERMODYNAMICS OF METALS AND ALLOYS 161
1539°
1400°
T (°C)
910° 768°
Time a-Iron, BCC non-magnetic
g-Iron, FCC d-Iron, BCC
a-Iron, BCC magnetic
g a d g Liquid Solid, d
Figure 2.9 Cooling curve for pure iron. Reprinted, by permission, from Committee on Metal- lurgy,Engineering Metallurgy, p. 245. Copyright1957 by Pitman Publishing.
Figure 2.8 could be (and is in many diagrams) labeled as the austenite phase region.
Similarly, the α and δ phase can be called the ferrite and δ-ferrite regions. Keep in mind, however, that these single-phase regions are simply a solid solution between one of the allotropic forms of elemental iron and carbon. Note that the ferrite region is particularly limited in both temperature and composition, as is δ-ferrite, and that austenite has a particularly large range of stability at higher temperatures. All alloys with carbon contents between 0.5 and 2 wt% C solidify as austenite. Alloys contain- ing more than 2% carbon are subject to a eutectic transformation that forms austenite and cementite. At 0.8% carbon, what has solidified as austenite undergoes a eutectoid transformation at 723◦C to an intimate mixture of ferrite and cementite. This mixture is given a special name,pearlite. Recall that phases need not be continuous, and such is the case in pearlite where the cementite (Fe3C) forms lamellar structures within the austenite phase, which can also have some carbon dissolved in it (see Figure 2.10).
The eutectoid transformation is an important one not only for this specific carbon composition, but for classifying all types of steels.Carbon steels have carbon contents between 0.1 and 1.5 wt%. Those with carbon contents less than 0.8% are termed hypoeutectoid steels, and those with greater than 0.8% C are called hypereutectoid steels. Further classifications of steels are given in Table 2.4.
When austenite is cooled under more rapid conditions, a compound calledbainite is produced. Bainite is a nonequilibrium product that is similar to pearlite, but consists of a dispersion of very small Fe3C particles between the ferrite plates. Bainite formation is favored at a high degree of supercooling from the austenite phase, whereas pearlite forms at low degrees of supercooling, or more equilibrium cooling.
Finally, if an austenite steel is rapidly quenched, martensite steel can form. Steel martensite possesses a body-centered tetragonal structure (see Figure 2.11), which is
1000
900
800
727°C
700 b
a x
Fe3C
a + Fe3C
g + Fe3C
600
500
400 1100
0 1.0
Composition (wt % C)
Temperature (°C)
2.0 a
a g
a+g
g g
g g
x′
Figure 2.10 Schematic representation of the microstructures for a eutectoid transformation in the Fe– C system. Reprinted, by permission, from W. Callister,Materials Science and Engineer- ing: An Introduction, 5th ed., p. 277. Copyright2000 by John Wiley & Sons, Inc.
Table 2.4 Classification of Ferrous Alloys
Component (wt%) Classification Examples
C<0.01 Iron
0.1<C<1.5 Carbon steel 1000 series — ferrite/pearlite
2000 series — ferrite/pearlite or bainite 3000 series — martensitic 4000 series
2<C<4.5 Cast iron Gray iron
White iron
Various Alloy steels Ni, Mn, Cu, Co, C, N — austenite
stabilizers
Cr, Mo, Si, W, V, Sn, Cb, Ti — ferrite stabilizers
Cr>12 Stainless steels 200/300 — austenitic 400 — ferritic 400/500 — martensitic
THERMODYNAMICS OF METALS AND ALLOYS 163
a c
= Fe atoms
= C atom sites
a
Figure 2.11 The body-centered tetragonal unit cell of steel martensite. From K. M. Ralls, T. H. Courtney, and J. Wulff,Introduction to Materials Science and Engineering. Copyright 1976 by John Wiley & Sons, Inc. This material is used by permission of John Wiley & Sons, Inc.
0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 Wt.% C
c parameter
a parameter 0.305
0.300
0.295
0.290
0.285
0.280
Lattice parameter (nm)
Figure 2.12 Variation of tetragonality in steel martensite with axial ratio. From K. M. Ralls, T. H. Courtney, and J. Wulff,Introduction to Materials Science and Engineering. Copyright 1976 by John Wiley & Sons, Inc. This material is used by permission of John Wiley & Sons, Inc.
only slightly distorted from the BCC structure of austenite. The distortion arises from interstitial carbon atoms that occupy one set of preferred octahedral interstitial sites.
During the martensitic transformation upon rapid cooling, carbon atoms residing at equivalent octahedral sites in BCC change position slightly to only one (along the c-axis) of the three possible sets of octahedral sites in the body-centered tetragonal structure, thus creating the distortion. The extent of tetragonal distortion, as measured by the axial ratio, increases with increasing carbon content (see Figure 2.12). The trans- formation process from austenite to martensite is very rapid, because it involves only a distortion of the BCC unit cell, as opposed to the BCC–FCC transition to pearlite or bainite, which is a diffusion-controlled process. This type of diffusionless transfor- mation is termed amartensitic transformationand can occur in alloys other than steel.
Temperature Zone III II I
Commercial cast iron range Fe3C
Desulfurized 0.8
Wt.% C
4.3 C
Fast cool Moderate Slow cool Moderate Slow cool I
II III
I II III
γ + L γ + L γ + L γ + L γ + L
γ + Fe3C γ + Gf
γ + Gr γ + Gr
α + Gr
γ + Gn γ + Gn γ + Gf
P + Gf
P + Gr
P + Gn
α + Gf α + Gn
P + Fe3C
White cast iron Pearlitic gray cast iron
Pearlitic ductile cast iron
Ferritic ductile cast iron Reheat; hold in
zone II 30+ hours
Fast cool Slow cool II
III
Ferritic malleable
cast iron Pearlitic
malleable cast iron
3.0
Ferritic gray cast iron
Figure 2.13 Microstructures obtained by varying thermal treatments in cast irons (Gf =graphite flakes; Gr=graphite rosettes; Gn=graphite nodules; P=pearlite). From K. M. Ralls, T. H. Courtney, and J. Wulff,Introduction to Materials Science and Engineering.
Copyright 1976 by John Wiley & Sons, Inc. This material is used by permission of John Wiley & Sons, Inc.
THERMODYNAMICS OF CERAMICS AND GLASSES 165
As you can see, the process by which the iron–carbon alloy is processed and solid- ified is just as important as the overall stoichiometry. Although a discussion regarding phase transformations is more the realm of kinetic processes, it is nonetheless pertinent to summarize here the types of important ferrous alloys, particularly those in thecast iron categories. This is done in Figure 2.13.
As indicated in Table 2.4, cast irons contain more than 2% carbon by weight, compared to steels which usually have less than 1.5% carbon. Cast irons also differ microstructurally from steels in that a separate graphite phase typically exists. Graphite is seldom found in steels because the solid-state transformations that give rise to it are so slow that transformations yielding cementite always predominate. In cast irons, however, graphite can form directly from eutectic solidification and by solid-state trans- formations. Whether carbon is present as graphite or is tied up in cementite in cast irons depends on whether the solidification and cooling processes are carried out under close to equilibrium conditions, which favor graphite formation, or under highly nonequilib- rium conditions, which favor Fe3C formation. There is an analogy here with austenite decomposition that we have already described: Pearlite forms under near-equilibrium cooling conditions in preference to the less stable bainite, which forms under rapid cooling conditions. As shown in Figure 2.13, slow cooling rates favor graphite forma- tion from the eutectic reaction. The graphite phase continues to grow with decreasing temperature as the carbon content of the iron-rich phase (γ or α) decreases. Under these conditions, the phases present in zones I, II, and III are austenite and liquid, austenite and graphite, and ferrite and graphite, respectively. Under normal conditions, the graphite is present as flakes, but if the melt is desulfurized (where sulfur is an impurity in the ore), the graphite will be present as nodules. The normal structure is called ferritic gray cast iron, and that for the desulfurized material is called ferritic ductile cast iron.
Under moderate cooling rates, graphite still forms in the eutectic reaction, but Fe3C is one product of the eutectoid reaction. The final structure, consisting of locally inter- connected graphite flakes dispersed in a pearlitic matrix, is called pearlitic gray cast iron or, if desulfurized,pearlitic ductile cast iron. White cast irons are formed under rapid cooling, and have similar structures to steel insofar as there is no graphite present.
Thus, austenite and Fe3C are present in zone II, and pearlite and Fe3C are present in zone III. White casts can be converted into more usable structures by heating into zone II for fairly long time periods. This treatment causes graphite, in the shape of rosettes, to precipitate and the cementite is eliminated. Keep in mind that the cooling classifications of “slow,” “medium,” and “rapid,” are arbitrary and that commercial processes can produce materials with a mixture of the various microstructures shown in Figure 2.13. We will discuss transformation processes and the effect of heating and cooling rates on them in Chapter 3.
2.2 THERMODYNAMICS OF CERAMICS AND GLASSES