Iron-Carbon Alloys

iron-based alloys with carbon. Variation of the composition and structure of the alloys yields alloys with various properties, which makes them all-purpose materials.

A distinction is made between pure iron-carbon alloys (with traces of impurities), which are produced in small quantities for research purposes, and industrial iron-carbon alloys, including steels (up to 2 percent carbon) and cast irons (more than 2 percent carbon). The world production of these alloys is hundreds of millions of tons. Industrial iron-carbon alloys contain impurities, which are divided into common types (phosphorus, P; sulfur, S; manganese, Mn; silicon, Si; hydrogen, H; nitrogen, N; and oxygen, O), alloying types (chromium, Cr; nickel, Ni; molybdenum, Mo; tungsten, W; vanadium, V; titanium, Ti; cobalt, Co; and copper, Cu), and modifying types (magnesium, Mg; cerium, Ce; and calcium, Ca). In most cases, the basic system that determines the structure and properties of steels and cast irons is the Fe-C system, the scientific study of which was begun by the Rus-sian metallurgists P. P. Anosov (1831) and D. K. Chernov (1868). Anosov was the first to use a microscope for the study of iron-carbon alloys, and Chernov established their crystal-line nature, observed their dendritic crystallization, and dis-covered in the materials transformations in the solid state. Among the foreign scientists who aided in the construction of the phase diagrams of the Fe-C alloys are F. Osmond (France), W. C. Roberts-Austen (England), H. Roozeboom (Netherlands), and P. Goerens (Germany).

Phase states. The phase states of iron-carbon alloys of various compositions at various temperatures are described by diagrams of the stable equilibrium (Figure l,a) and the metastable equilibrium (Figure l,b). The stable state of iron-carbon alloys includes a liquid solution of carbon in iron (I) ; three solid solutions of carbon in polymorphic modifications of iron (Table 1), the α-solution (α-ferrite), the γ-solution (austenite), and the δ-solution (δ-ferrite); and graphite (G). The metastable state of iron-carbon alloys includes I, the α-, γ-, and δ-solutions, and iron carbide, Fe₃C, or cementite (C). The regions of stability of iron-carbon alloys in one-phase and two-phase states are shown in the diagrams of Figure 1.

Under certain conditions, iron-carbon alloys may contain three phases in equilibrium. The peritectic equilibrium δ γ + I may exist at temperatures HB; the eutectic stable equilibrium γ + I + G, at temperatures E'C'F', the eutectic metastable equilibrium γ + I + C at temperatures ECF; the eutectoid stable equilibrium α + γ + G, at temperatures P'S'K'; and the eutectoid metastable equilibrium α + γ + C, at temperatures PSK. Diagrams (a) and (b) may also be drawn in a single coordinate system (Figure l,c). Such a superimposed diagram graphically characterizes the relative displacement of the equilibrium lines of the same type and facilitates the analysis of iron-carbon alloys that simultaneously contain stable and metastable phases.

The main reason for the appearance in the iron-carbon alloys of a high-carbon metastable phase in the form of cementite is the difficulty of formation of graphite. The formation of graphite in the liquid solution of I and the solid solutions α and γ is related to a virtually complete removal of the iron atoms from the regions of the alloy in which graphite crystals are generated and grow. This removal requires significant atomic shifts. If iron-carbon alloys are cooled slowly or are held for prolonged periods of time at high temperatures, there is time for the atoms of iron to leave the locations in which graphite is formed, with the resulting occurrence of stable states. Accelerated cooling and insufficiently long holding times retard the removal of the low-mobility iron atoms, al-most all of which remain in their original locations, thus leading to the formation and growth of cementite in liquid and solid solutions. The necessary diffusion of carbon atoms, which are highly mobile at high temperatures, does not require long holding times and can take place even during accelerated cooling. In addition to the basic phases, which are shown in the diagrams, industrial iron-carbon alloys also contain small amounts of other phases, whose appearance is caused by the presence of impurities. Sulfides (FeS and MnS), phosphides (Fe₃P), oxides of iron and the impurities (FeO, MnO, A1₂O₃, Cr₂O₃, and TiO₂), nitrides (FeN and A1N), and other nonmetallic phases occur frequently. The dotted lines in the diagrams denote the Curie points, which are observed in iron-carbon alloys in connection with the magnetic transformations of ferrite (768°C) and cementite (210°C).

Structure. The structure of iron-carbon alloys is determined by their composition, the conditions of solidification, and the structural changes in the solid state. Iron-carbon alloys are

Figure 1. Phase diagrams of iron-carbon alloys: (a) stable equilibriums, (b) metastable equilibriums, (c) superimposition of (a) and (b)

divided into steels and cast irons on the basis of their carbon content. Steels with carbon concentrations less than the eutectoid S' and S (Table 2) are called hypoeutectoid; steels

with higher carbon concentrations are called hypereutectoid. Cast irons with carbon concentrations below the eutectoid C' and C are called hypoeutectoid; cast irons with higher carbon concentrations are called hypereutectoid.

The solidification of steels containing up to 0.5 percent carbon begins with the separation of the crystals of the δ-solution, usually in the form of dendrites. At the carbon concentrations of up to 0.1 percent, crystallization ends with the formation of the single-phase structure of the δ-solution. Steels containing 0.1–0.5 percent carbon exhibit the peritectic transformation I + δ → γ after the separation of a certain amount of the 8-solution. In the concentration interval of 0.10–0.16 percent carbon, this peritectic transformation leads to complete solidification, whereas in the interval of 0.16–0.50 percent carbon, crystallization is completed upon cooling to the temperature corresponding to the line JE. In liquid solutions with 0.5–4.26 percent carbon, crystallization begins with the separation of the y-solution, also in the form of dendrites. The steels harden completely in the temperature interval bounded by the lines BC and JE, acquiring a single-phase austenitic structure. However, the hardening of cast irons begins with the separation of excessive (primary) γ-solution and ends with the eutectic decomposition of the residual liquid according to one of the three possible versions: I → γ + G, I → γ + C, or I → γ + G + C. The first version yields the so-called gray cast irons, the second produces white cast irons, and the third produces mottled cast irons. Depending on the conditions of crystallization, graphite separates in the form of branched or spherical inclusions, whereas cementite separates in the form of monolithic plates or in the form of plates intergrown by branched austenite (so-called ledeburite). In iron-carbon alloys containing more than 4.26–4.30 percent carbon, crystallization of the melt that has been supercooled below the line D'C' begins under conditions of slow cooling, with the formation of pri-mary graphite, which may be either branched or spherical. Under conditions of accelerated cooling (with supercooling below the line DC), primary cementite plates are formed. Intermediate cooling rates lead to the separation of both graphite and cementite. The crystallization of hypereutectic cast irons, as well as of hypoeutectic cast irons, ends with the decomposition of the residual liquid into a mixture of the y-solution and high-carbon phases.

The structure of solidified iron-carbon alloys changes substantially upon further cooling. The changes are due to polymorphic transformations of iron, the decreased solubility of carbon in iron, and the graphitization of cementite. The structure may change in the solid state as a result of processes of recrystallization of solid solutions, the spheroidization of crystals (nonequiaxial crystals become equiaxial), and the coalescence of high-carbon phases (some crystals of cementite grow at the expense of others).

Polymorphic transformations. The polymorphic transformations of iron-carbon alloys are related to the transformations of the face-centered cubic γ- Fe lattice (FCC) and the body-centered cubic lattice (BCC) of α- and δ-Fe (FCC ⇋ BCC). The polymorphic transformations in solid solutions may take place in various ways, depending on the conditions of heating and cooling. Small amounts of supercooling and superheating lead to the so-called normal transformation of the iron lattices, which is achieved by random individual transitions of atoms from the initial to the final phase. This type of trans-formation is accompanied by the diffusional redistribution of carbon between the phases. At high rates of cooling or heating, the polymorphic transformations of solid solutions occur in a diffusionless (martensitic) manner. The iron lattice is transformed by a rapid shear mechanism as a result of orderly collective displacements of atoms, without diffusional redistribution of carbon between the phases. For example, tempering of iron-carbon alloys in water leads to the transition of the γ-solution into the α-solution of the same composition. This α-solution, which is supersaturated with carbon, is called martensite. Transformations under intermediate conditions may combine the shear transformation of the iron lattice with the diffusional redistribution of carbon (bainite trans-formation). The structures formed in this case are substantially different. Equiaxial crystals of the solid solution, with a small number of defects, are formed in the first case. In the second and third cases, needle-shaped and platelike crystals containing numerous twins and slip planes are formed. The structure of iron-carbon alloys also varies with changes of the solubility of carbon in α- and γ-iron upon cooling and heating. Cooling causes the super saturation of the solutions with carbon and the separation of crystals of high-carbon phases (cementite and graphite). Upon heating, the high-carbon phases that are present dissolve in the α- and γ-phases.

The generation and growth of crystals of cementite in supersaturated solutions usually take place at a higher rate than the formation of graphite; therefore, iron-carbon alloys are frequently metastable. Depending on supercooling, the cementite separating from the solid solution may have the shape of equiaxial crystals, an intergranular lattice, or plates and needles. In cases of holding at high temperatures, cementite crystals undergo spheroidization. The process of coalescence may also occur. If iron-carbon alloys that contain cementite undergo prolonged holding at high temperatures, graphitization will occur—that is, graphite is generated and grows, and the cementite dissolves. This process is used in the production of articles from graphitized steel and malleable cast iron. An important role in the structural formation of iron-carbon alloys in the solid state is played by the eutectoid decomposition of the γ-solution into the α-solution and the high-carbon phase. At very low degrees of supercooling, ferrite and graphite are formed; with a slight increase in supercooling, ferrite and spheroidized cementite are formed; and then the mixture of ferrite and cementite assumes the platelike structure of pearlite, which becomes finer with in-creasing supercooling. Supercooling of several hundred degrees leads to the suppression of the eutectoid decomposition and the conversion of the γ-solution into martensite. The structure of iron-carbon alloys may vary within wide limits. The main methods for controlling the structure of iron-carbon alloys are changes in the chemical composition, the conditions of solidification and plastic flow, and in the thermal and thermomechanical treatments. The properties of the iron-carbon alloys may also be varied within wide limits by changing the phase composition and the size, shape, distribution, and defect content of the crystals. For example, the most important operational properties of iron-carbon alloys may be varied within the following limits: hardness, from 60 to 800 Brinell units (HB); ultimate strength, from 2 x 10⁴ to 3.5 x 10⁶ newtons per square centimeter, or 2 x 10³ to 3.5 x 10⁵ kilograms-force per sq cm; relative elongation, from 0 to 70 percent.