The iron-carbon system can have the following phases:
- liquid phase - solution carbon in iron;
- solid solution of carbon in gamma iron;
- solid solution of carbon in alpha iron (delta gland);
- carbon in the form of graphite;
- iron carbide Fe3C, which is more often called cementite.
Cementite is present even in relatively slowly cooled alloys: long holding at elevated temperatures is required, to decompose cementite into iron and graphite. For this reason iron status diagram-carbon has two types: stable chart iron-carbon and metastable diagram iron-cementite. They are also called "cementite system" and "graphite system".
Iron-carbon state diagram
Double status diagram iron-carbon, which combines both options - stable and metastable – shown in the figure. Dotted lines refer to stable iron-carbon diagram, solid - denote a metastable iron-cementite diagram. Note, that "disagreements" between two diagrams - stable and unstable - arise only along those lines, where the phase stands out, carbon-rich: carbon itself in the form of graphite or carbon-rich cementite.
Iron-cementite metastable diagram
The metastable diagram is also called "cementite". Difference from stable ("Graphite") diagrams appear when carbon is released - in the form of graphite or cementite.
As shown in the double diagram, lattices of allotropic forms of iron (d, γ and α or delta, gamma and alpha) serve as a "solvent" for the formation of respectively solid δ-, c- and α-solutions of carbon in iron.
When very low carbon iron crystallizes, then the crystals of the solid delta solution stand out along the liquidus AB line and the solidus AN line. The solid delta solution has a body-centered cubic (OCK) lattice - exactly the same as the solid alpha solution. At the maximum temperature of the solid delta solution 1490 ˚C carbon content is 0,1 % (point H). At a temperature 1490 ˚С a peritectic reaction occurs between the supersaturated solid δ-solution and the liquid phase, containing 0,5 % carbon (point B). As a result, a solid γ-solution of carbon in iron is formed. He contains 0,18 % carbon (point I).
If the carbon content is higher, than 0,5 %, then the solid γ-solution crystallizes directly from the liquid phase (along the liquidus line of the BC and the line of solidus IE). At a temperature 1130 ˚C limited solubility of carbon in γ-iron is about 2,0 % (point E). Decrease in temperature below 1130 ˚С leads to a decrease in the solubility of carbon in γ-iron along the line ЕS. At a temperature 723 ˚C carbon solubility is 0,8 % (point S). ES line corresponds to precipitation of iron carbide (cementite) γ-iron.
With an increase in the carbon content, the temperature, at which the gamma lattice transforms into an alpha lattice decreases and this transformation occurs in the temperature range, corresponding to the GS and GP lines.
The alpha phase GS line intersects with the iron carbide ES line at point S. Point S is a eutectic point with the following coordinates: 723 ˚С by temperature and 0,80 % by carbon content. At this point, a solid alpha solution is simultaneously released from the gamma solution of the eutectoid composition (ferrite) and iron carbide (cementite).
The lattice of the solid alpha solution is identical to the lattice of the solid delta solution. At eutectoid temperature 723 ˚C solid alpha solution contains 0,02 % carbon (point P). Further cooling leads to a decrease in the solubility of carbon in alpha-iron and at room temperature it is only small fractions of a percent (point D).
When the carbon content is between 2,0 to 4,3 %, crystallization begins with the release of a gamma solution along the BC line. Increase in carbon content higher 4,3% leads to the precipitation of iron carbide along the line CD.
Isolation of an additional primary phase in alloys with a carbon content higher 2,0 % accompanied by eutectic crystallization of gamma solution and carbon carbide at point C, whose coordinates are - 1130 ° C and 4,3 % carbon.
The MO line is associated with magnetic transformation, that is, the transition from ferromagnetic to paramagnetic state of iron.
Stable iron-graphite diagram
This diagram is also called "graphite". At very low cooling rates, carbon in the form of graphite can crystallize directly from the liquid phase. In this case, a eutectic mixture of austenite and graphite is formed instead of the eutectic of austenite and cementite. As shown in the diagram, the dashed lines, which belong to the iron-graphite system, pass at higher temperatures, than the corresponding lines of the iron-cementite system. This indicates a greater stability and closeness to complete equilibrium of the iron-graphite system.. This conclusion is also supported by the fact, that heating high-carbon steels with a high content of cementite leads to its decomposition: cementite -> iron + graphite.
At moderate cooling rates, part of the alloy can crystallize according to the "graphite" phase diagram, and the other part - according to the "cementite" state diagram.
The lines of phase equilibrium in the diagrams of both systems can shift depending on the specific cooling rates.. The most noticeable shift can be seen for the carbon emission lines in solid gamma solution.. Therefore, the diagram is truly correct only if the alloys are cooled very slowly..