The Iron-Iron carbide (Fe-Fe3C) is defined by five individual phases and four invariant reactions. Five phases are- α-ferrite (BCC) Fe-C solid solution, γ-austenite (FCC) Fe-C solid solution, δ -ferrite (BCC) Fe-C solid solution, Fe3C (iron carbide) or cementite – an inter- metallic compound and liquid Fe-C solution. Four invariant reactions are eutectoid, eutectic, monotectic and peritectic.
As shown in figure by left axes, pure iron upon heating exhibits two allotropic changes. One involves α-ferrite of BCC crystal structure transforming to FCC austenite, γ- iron, at 910C. At 1400°C, austenite changes to BCC phase known as δ -ferrite, which finally melts at 1536°C.
Carbon present in solid iron as interstitial impurity, and forms solid solution with ferrites/austenite as depicted by three single fields represented by α, γ and δ. Carbon dissolves least in a-ferrite in which maximum amount of carbon soluble is 0.02% at 723°C.
This limited solubility give the shape and size of interstitial position in BCC α-ferrite. However, carbon present greatly influences the mechanical properties of α-ferrite. α-ferrite can be used as magnetic material below 768°C. Solubility of carbon in γ-iron reaches its maximum, 2.11%, at a temperature of 1147°C.
Higher solubility of carbon in austenite is property of FCC structure and corresponding interstitial sites. Phase transformations involving austenite plays very significant role in heat treatment of different steels. Austenite itself is non-magnetic. Carbon solubility in δ-ferrite is maximum (0.1%) at 1495°C.
As this ferrite exists only at elevated temperature, it is of no commercial importance. Out of these four solid phases, cementite is hardest and brittle that is used in different forms to increase the strength of steels, α-ferrite, on the other hand is softest and act as matrix of a composite material.
Based on %C dissolved in it, a Fe-C solution is classified as- commercial pure irons with less than 0.008%C; steels having %C between 0.008-2.11; while cast irons have carbon in the range of 2.11%-6.67%. Thus commercial pure iron is composed of exclusively α-ferrite, at room temperature.
The presence of Si promotes the formation of graphite instead of cementite.
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Fe-C system constitutes four invariant reactions:
a. Peritectic reaction at 1495°C and 0.16%C, δ-ferrite + L ↔ γ-iron (austenite)
b. Monotectic reaction 1495°C and 0.51 %C, L ↔ L + γ-iron (austenite)
c. Eutectic reaction at 1147°C and 4.3%C, L ↔ γ-iron + Fe3C (cementite) [ledeburite]
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d. Eutectoid reaction at 723°C and 0.8%C, γ-iron ↔ α-ferrite + Fe3C (cementite) [pearlite]
Product phase of eutectic reaction is called ledeburite, while product from eutectoid reaction is called pearlite. During cooling to room temperature, ledeburite transforms into pearlite and cementite.
Pearlite is in reality not a single phase, but a micro-constituent having alternate thin layers of α-ferrite (~88%) and Fe3C cementite (~12%). Steels with less than 0.8%C (mild steels upto 0.3%C, medium carbon steels with C between 0.3%-0.8% i.e. hypo- eutectoid Fe-C alloys) i.e. consists pro-eutectoid α-ferrite in addition to pearlite, while steels with carbon higher than 0.8% (high-carbon steels i.e. hyper-eutectoid Fe-C alloys) consists of pearlite and pro-eutectoid cementite. Phase transformations involving austenite i.e. processes those involve eutectoid reaction are of great importance in heat treatment of steels.
Generally, steels are almost always cooled from the austenitic region to room temperature. During the cooling upon crossing the boundary of the single phase γ-iron, first-pro-eutectoid phase (either α-ferrite or cementite) forms up to eutectoid temperature. With further cooling below the eutectoid temperature, remaining austenite decomposes to eutectoid product called pearlite, mixture of thin layers of α-ferrite and cementite.
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Though pearlite is not a phase, and also not a constituent. The decomposition of austenite to form pearlite occurs by nucleation and growth. Nucleation, usually, occurs heterogeneously and rarely homogeneously at grain boundaries.
When austenite forms pearlite at a constant temperature, the spacing between adjacent lamellae of cementite is nearly constant. For a given colony of pearlite, all cementite plates have a common orientation in space, and it is also Applicable for the ferrite plates.
Growth of pearlite colonies occurs not only by the nucleation of additional lamellae but also through an advance at the ends of the lamellae.
The thickness ratio of the ferrite and cementite layers in pearlite is approximately 8 to 1. However, the actual layer thickness depends on the temperature at which the isothermal transformation is allowed to occur.
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The spacing of the pearlite lamellae has a practical significance because the hardness of the resulting structure depends upon it; the smaller the spacing, the harder the metal.
The growth rate of pearlite is also a strong function of temperature. At temperatures just below the eutectoid, the growth rate increases rapidly with decreasing temperature, reaching a maximum at 600°C, and then decreases again at lower temperatures.
Additions of alloying elements to Fe-C system bring changes (alternations to positions of phase boundaries and shapes of fields) depends on that particular element and its concentration.
Generally all alloying elements causes the eutectoid concentration to decrease, and most of the alloying elements (e.g.: Ti, Mo, Si, W, Cr) causes the eutectoid temperature to increase while some other (e.g.: Ni, Mn) reduces the eutectoid temperature. Thus, alloying additions alters the relative amount of pearlite and pro-eutectoid phase that form.
Fe-C alloys with more than 2.11%C are called cast irons. Phase transformations in cast irons involve formation of pro-eutectic phase on crossing the liquidus. During the further cooling, liquid of eutectic composition decomposes in to mixture of austenite and cementite, known as ledeburite. On further cooling through eutectoid temperature, austenite converted to pearlite.
The room temperature microstructure of cast irons thus consists of pearlite and cementite. Because of presence of cementite (which is hard, brittle and white in color) product is called white cast iron. However, depending on cooling rate and other alloying elements, carbon in cast iron may be present as graphite or cementite. Gray cast iron contains graphite in form of flakes.
These flakes are sharp and act as stress risers. Brittleness arising because of flake shape can be avoided by producing graphite in spherical modules, as in malleable cast iron and SG (spheroidal graphite) cast iron. Malleable cast iron is produced by heat treating white cast iron (Si < 1%) for prolonged periods at about 900°C and then cooling it very slowly.