Iron Carbon Phase Diagram
In refining steel from iron ore, the quantity of carbon used must be carefully controlled in order for the steel to have the desired properties. The reason for the strong relationship between steel properties and carbon content can be understood by examining the iron–carbon phase diagram.
Figure 1 shows a commonly accepted iron–carbon phase diagram. One of the unique features of this diagram is that the abscissa extends only to 6.7%, rather than 100%. This is a matter of convention.
In an iron-rich material, each carbon atom bonds with three iron atoms to form iron carbide, Fe3C, also called cementite. Iron carbide is 6.7% carbon by weight. Thus, on the phase diagram, a carbon weight of 6.7% corresponds to 100% iron carbide. A complete iron–carbon phase diagram should extend to 100% carbon. However, only the iron-rich portion, as shown in Figure 1, is of practical significance.
In fact, structural steels have a maximum carbon content of less than 0.3%, so only a very small portion of the phase diagram is significant for civil engineers. The left side of Figure 1 demonstrates that pure iron goes through two transformations as temperature increases.
Pure iron below 912°C has a BCC crystalline structure called ferrite. At 912°C, the ferrite undergoes a polymorphic change to an FCC structure called austenite. At 1394°C, another polymorphic change occurs, returning the iron to a BCC structure. At 1539°C, the iron melts into a liquid. The high- and low-temperature ferrites are identified as δ and α ferrite, respectively.
Since δ ferrite occurs only at very high temperatures, it does not have practical significance for us. Carbon goes into solution with α ferrite at temperatures between 400°C and 912°C. However, the solubility limit is very low, with a maximum of 0.022% at 727°C.
At temperatures below 727°C and to the right of the solubility limit line, α ferrite and iron carbide coexist as two phases. From 727°C to 1148°C, the solubility of carbon in the austenite increases from 0.77% to 2.11%. The solubility of carbon in austenite is greater than in α ferrite because of the crystalline structure of the austenite.
At 0.77% carbon and 727°C, a eutectoid reaction occurs; that is, a solid phase change occurs when either the temperature or carbon content changes. At 0.77% carbon, and above 727°C, the carbon is in solution as an interstitial element, within the FCC structure of the austenite.
A temperature drop to below 727°C, which happens slowly enough to allow the atoms to reach an equilibrium condition, results in a two-phase material, a ferrite and iron carbide. The a ferrite will have 0.022% carbon in solution, and the iron carbide will have a carbon content of 6.7%. The ferrite and iron carbide will form as thin plates, a lamellae structure. This eutectoid material is called pearlite.
At carbon contents less than the eutectoid composition, 0.77% carbon, hypoeutectoid alloys are formed. Consider a carbon content of 0.25%. Above approximately 860°C, solid austenite exists with carbon in solution. The austenite consists of grains of uniform material that were formed when the steel was cooled from a liquid to a solid. Under equilibrium temperature drop from 860°C to 727°C, α ferrite is formed and accumulates at the grain boundaries of the austenite. This is α proeutectoid ferrite.
At temperatures slightly above 727°C, the ferrite will have 0.022% carbon in solution and austenite will have 0.77% carbon. When the temperature drops below 727°C, the austenite will transform to pearlite. The resulting structure consists of grains of pearlite surrounded by a skeleton of α ferrite.
When the carbon content is greater than the eutectoid composition, 0.77% carbon, hypereutectoid alloys are formed. Iron carbide forms at the grain boundaries of the austenite at temperatures above 727°C.
The resulting microstructure consists of grains of pearlite surrounded by a skeleton of iron carbide. The lever rule for the analysis of phase diagrams can be used to determine the phases and constituents of steel.
Figure 2 shows an optical 50 x photomicrograph of a hot-rolled mild steel plate with a carbon content of 0.18% by weight that was etched with 3% nitol. The light etching phase is proeutectoid ferrite and the dark constituent is pearlite. Note the banded structure resulting from the rolling processes. Figure 3. shows the same material as Figure 2, except that the magnification is 400x .
At this magnification, the alternating layers of ferrite and cementite in the pearlite can be seen. The significance of ferrite, pearlite, and iron carbide formation is that the properties of the steel are highly dependent on the relative proportions of ferrite and iron carbide.
Ferrite has relatively low strength but is very ductile. Iron carbide has high strength but has virtually no ductility. Combining these materials in different proportions alters the mechanical properties of the steel. Increasing the carbon content increases strength and hardness but reduces ductility.
However, the modulus of elasticity of steel does not change by altering the carbon content. All of the preceding reactions are for temperature reduction rates that allow the material to reach equilibrium.
Cooling at more rapid rates greatly alters the microstructure. Moderate cooling rates produce bainite, a fine-structure pearlite without a proeutectoid phase. Rapid quenching produces martensite; the carbon is supersaturated in the iron, causing a body center tetragonal lattice structure. Time– temperature transformation diagrams are used to predict the structure and properties of steel subjected to heat treatment.