Iron-Carbon Alloy Status Chart

Iron forms the chemical compound Fe ₃ C cementite with carbon. Since metal alloys based on iron with a carbon content of up to 5% are used in practice, a part of the state diagram from pure iron to cementite is practically interesting ^[1] . Since cementite is a metastable phase, the corresponding diagram is also called metastable (solid lines in the figure).

For gray cast irons and graphitized steels , the stable part of the iron – graphite (Fe – Gr) diagram is considered, since it is graphite that is the stable phase in this case. Cementite is released from the melt much faster than graphite, and in many steels and white cast irons it can exist for a long time, despite metastability. In gray cast iron, graphite necessarily exists.

The thin dashed lines in the figure show the lines of stable equilibrium (i.e., with the participation of graphite), where they differ from the lines of metastable equilibrium (with the participation of cementite), and the corresponding points are indicated by a dash. The phase and point designations in this diagram are given in accordance with an unofficial international agreement.

The following phases exist in the iron - carbon system: liquid phase, ferrite , austenite , cementite, graphite .

Liquid phase . In a liquid state, iron dissolves carbon well in any proportions. with the formation of a homogeneous liquid phase.

Ferrite - A solid solution of carbon incorporation in α-iron with a body-centered cubic lattice .

Ferrite has a variable, depending on the temperature, the maximum solubility of carbon: minimum - 0.006% at room temperature (point Q), maximum - 0.02% at a temperature of 700 ° C (point P). Carbon atoms are located in the center of the face or (which is crystallometrically equivalent) in the middle of the edges of the cube, as well as in lattice defects ^[2] .

Above 1392 ° C, there is high temperature ferrite with an extreme solubility of carbon of about 0.1% at a temperature of about 1500 ° C (point H).

The properties of ferrite are close to those of pure iron. It is soft ( Brinell hardness - 130 HB) and plastic, ferromagnetic (in the absence of carbon) to the Curie point - 770 ° C.

Austenite (γ) is a solid solution of carbon incorporation in γ-iron with a face-centered cubic lattice.

Carbon atoms occupy a place in the center of a face-centered cubic cell . The ultimate solubility of carbon in austenite is 2.14% at a temperature of 1147 ° C (point E). Austenite has a hardness of 200-250 HB, plastic, paramagnetic . Upon dissolution of other elements in austenite or in ferrite, the properties and temperature boundaries of their existence change ^[3] .

Cementite (Fe ₃ C) - a chemical compound of iron with carbon ( iron carbide ), with a complex rhombic lattice, contains 6.67% carbon. It is hard (over 1000 HB), and very fragile. Cementite is a metastable phase and, with prolonged heating, spontaneously decomposes with the release of graphite .

In iron-carbon alloys, cementite as a phase can be released under various conditions:

• primary cementite (released from the liquid),
• secondary cementite (released from austenite),
• tertiary cementite (from ferrite),
• cementite is eutectic and
• eutectoid cementite.

Primary cementite is released from the liquid phase in the form of large plate crystals. Secondary cementite is separated from austenite and is arranged in a grid around austenite grains (after eutectoid transformation, they will become perlite grains). Tertiary cementite is released from ferrite and in the form of small inclusions is located at the boundaries of ferrite grains ^[4] .

Eutectic cementite is observed only in white cast irons. Eutectoid cementite has a lamellar form and is an integral part of perlite . Cementite can stand out in the form of small spheres with a special spheroidizing annealing or quenching with high tempering. The shape, size, quantity and location of cementite inclusions affects the mechanical properties of alloys, which allows in practice for each concrete application of the alloy to achieve the optimal combination of hardness, strength, resistance to brittle fracture, etc. ^[5]

Graphite is a phase consisting only of carbon with a layered hexagonal lattice. The density of graphite (2.3 g / cm ³ ) is much lower than the density of all other phases (about 7.5–7.8 g / cm ³ ) and this complicates and slows down its formation, which leads to the release of cementite during faster cooling. The formation of graphite reduces shrinkage during crystallization, graphite acts as a lubricant during friction, reducing wear, and helps to disperse the energy of vibrations.

Graphite has the form of large crab-shaped (curved lamellar) inclusions (ordinary gray cast iron ) or spheres ( high-strength cast iron ).

Graphite is necessarily present in gray cast irons and their varieties - high-strength cast irons. Graphite is also present in some steel grades - in the so-called graphitized steels.

The ACD line is a liquidus line showing the temperatures of the onset of solidification (end of melting) of steels and white cast irons. At temperatures above the line, ACD is a liquid alloy. The AECF line is a solidus line showing the temperature of the end of solidification (start of melting).

Austenite is crystallized from the liquid alloy along the AS liquidus line (at temperatures corresponding to the AS line), and cementite , called primary cementite, is crystallized from the liquidus line CD. At point C at 1147 ° C and a content of 4.3% carbon from a liquid alloy, austenite and primary cementite crystallize simultaneously, forming a eutectic called ledeburite . At temperatures corresponding to the solidus line AE, alloys with a carbon content of up to 2.14% finally solidify with the formation of an austenite structure. On the EC solidus line (1147 ° C), alloys with a carbon content of 2.14 to 4.3% finally solidify with the formation of ledeburite eutectic . Since austenite precipitated from the liquid alloy at higher temperatures, therefore, such alloys after solidification will have an austenite + ledeburite structure.

On the CF solidus line (1147 ° С), alloys with a carbon content of 4.3 to 6.67% finally solidify also with the formation of ledeburite eutectic. Since cementite (primary) was precipitated from a liquid alloy at higher temperatures, therefore, such alloys after solidification will have a structure - primary cementite + ledeburite ^[6] .

In the ACEA region, between the line of the AC liquidus and the AEC solidus, there will be a liquid alloy + austenite crystals. In the CDF region, between the line of the liquidus CD and the solidus CF, there will be a liquid alloy + cementite crystals (primary). The transformations that occur during the solidification of alloys are called primary crystallization. As a result of primary crystallization in all alloys with a carbon content of up to 2.14%, a single-phase structure is formed - austenite. Alloys of iron and carbon, in which an austenitic structure is obtained as a result of primary crystallization under equilibrium conditions, are called steels.

Alloys with a carbon content of more than 2.14%, in which, during crystallization, a ledeburite eutectic is formed, are called cast irons. In the system under consideration, almost all carbon is in a bound state, in the form of cementite. The break of such cast irons is light, shiny (white break), therefore, such cast irons are called white ^[4] .

In iron-carbon alloys, transformations also occur in the solid state, called secondary crystallization and characterized by GSE, PSK, PQ lines. The GS line shows the beginning of the conversion of austenite to ferrite (upon cooling). Therefore, in the GSP region there will be an austenite + ferrite structure.

The SE line shows that with decreasing temperature, the solubility of carbon in austenite decreases. So, at 1147 ° C, 2.14% carbon can dissolve in austenite, and 0.8% at 727 ° C. With a decrease in temperature in steels with a carbon content of 0.8 to 2.14%, excess carbon is released from austenite in the form of cementite, called secondary. Therefore, below the SE line (up to a temperature of 727 ° С), the steel has the structure: austenite + cementite (secondary). In cast irons with a carbon content of 2.14 to 4.3% at 1147 ° C, in addition to ledeburite, there is austenite, from which secondary cementite will also precipitate upon lowering the temperature. Therefore, below the EC line (up to a temperature of 727 ° С), white cast iron has the structure: ledeburite + austenite + cementite secondary.

The PSK line (727 ° C) is the eutectoid transformation line. On this line, in all iron-carbon alloys, austenite decomposes, forming a structure that is a mechanical mixture of ferrite and cementite and called perlite . Below 727 ° C, iron-carbon alloys have the following structures.

• Steels containing carbon less than 0.8% have a ferrite + perlite structure and are called hypereutectoid steels.
• Steel with a carbon content of 0.8% has a perlite structure and is called eutectoid steel.
• Steels with a carbon content of 0.8 to 2.14% have a cementite + perlite structure and are called hypereutectoid steels.
• White cast irons with a carbon content of 2.14 to 4.3% have a structure of perlite + secondary cementite + ledeburite and are called pre-eutectic cast irons.
• White cast iron with a carbon content of 4.3% has the structure of ledeburite and is called eutectic cast iron.
• White cast irons with a carbon content of 4.3 to 6.67% have a primary cementite + ledeburite structure and are called hypereutectic cast irons ^[5] .

The PQ line shows that with decreasing temperature, the solubility of carbon in ferrite decreases from 0.02% at 727 ° C to 0.006% at room temperature. When cooled below 727 ° C, excess carbon is released from ferrite in the form of cementite, called tertiary. In most alloys of iron with carbon, tertiary cementite in the structure can be ignored due to its very small amounts. However, in low-carbon steels under conditions of slow cooling, tertiary cementite is released along the boundaries of ferrite grains (Fig. 76). These precipitates reduce the plastic properties of steel, especially the ability to cold stamping ^[5] .

• Kuzmin B.A., Samokhotsky A.I., Kuznetsova T.N. Metallurgy, metallurgy and structural materials. - Moscow: Higher School, 1971. - 352 p.
• Zimmerman R., Gunther K. Metallurgy and materials science. - Ref. edition. Per. with him .. - Moscow: Metallurgy, 1982. - 480 p.