Iron-carbon diagram
Carbon plays a dominant role in the structure formation of iron. Iron (Fe) and carbon (C) form a cubic space lattice or 3D lattice. The corner points of this lattice contain iron atoms.
The carbon atoms can take up two positions. They are either surface-centred in the middle of each cube surface and the result is called gamma mixed crystal (top animation). In the case of alpha mixed crystal the carbon atom is spatially-centred inside the cute. Delta mixed crystal plays a secondary role, but it also has spatially-centred carbon atoms, however, it is only used in high-alloyed steels.
Carbon is the most important alloying element in iron. The quantity of carbon contained in the iron is decisive with regard to the hardness of the material and therefore its subsequent usability. At this point it is important to note that far more carbon can be absorbed in the gamma mixed crystal. Iron and carbon form a chemical compound called cementite (Fe3C).
Some of the micro constituents of Iron and Steel are Austenite, Ferrite, Cementide, Pearlite, Bainite, Martenite, Troostite, Sorbite and Ledeburite.
Austenite (γ-iron): It is solid solution of ferrite iron carbide in gamma iron which is formed when steel contains carbon up to 1.8% at 1130°C. On cooling below 723°C it starts transforming into pearlite and ferrite. Austenite is non-magnetic and soft. It exists in FCC crystal structure.
Ferrite: It is a BCC iron phase with very limited solubility of carbon. The solubility of carbon in ferrite is 0.08% at 723°C. Ferrite does not harden when cooled rapidly. It is very soft and highly magnetic. At room temperature ferrite contains maximum 0.0025% C only.
Cementide: Cementide is actually Fe3C, which contains 6.67%C by weight, which is extremely hard and brittle in nature. Cementide increases gradually with increase in carbon percentage. it is magnetic at below 200°C. Cementide contains orthorhombic crystal structure.
Pearlite: Pearlite is a combination of ferrite and 13% of Cementide. Steel with 0.8% carbon is wholly Pearlite, less than 0.8% carbon is wholly Pearlite, less than 0.8% is hypo eutectoid contains ferrite and Pearlite and is soft. More than 0.8% is hyper eutectoid steel which contains Pearlite and Cementide which is hard and brittle. It is having a pearl like lusture when viewed through microscope.
Bainite: Bainite is the product of isothermal decomposition of Austenite and it cannot be produced by continuous cooling Banite is aggregated of ferrite and carbide. Also it is tougher.
Martenite: This is obtained by rapid cooling of Austenite. It is extremely hard and posses articular needle like structure. It is magnetic and has carbon content up to 2%. It is extremely hard and brittle. The decomposition of Austenite below 320°C starts the formation of Martensite.
Troostite: Troostite differs from Pearlite only in the degree of fitness of structure and carbon content. It is produced by transformation of tempered Martensite. Troosite is weaker than Martensite.
Sorbite: Sorbite micro structure constitute a mixture of ferrite and finely divided cementide produced on tempering martensite above 450°C. Pearlite, Troostite and Sorbite all are ferrite cementide mixture having a lamellar structure.
Ledeburite: Ledeburite is the product of eutectic reaction. Thus Ledebruite is a euctectic mixture; consists of alternative layers of Austenite and Cementide. It contains 4.3% carbon and is formed at about 1130°C
The Iron–Iron Carbide (Fe–Fe3C) Phase Diagram
This is one of the most important alloys for structural applications. The diagram Fe—C is simplified at low carbon concentrations by assuming it is the Fe—Fe3C diagram. Concentrations are usually given in weight percent. The possible phases are:
For their role in mechanical properties of the alloy, it is important to note that:
- a-ferrite (BCC) Fe-C solution
- g-austenite (FCC) Fe-C solution
- d-ferrite (BCC) Fe-C solution
- liquid Fe-C solution
- Fe3C (iron carbide) or cementite. An intermetallic compound.
For their role in mechanical properties of the alloy, it is important to note that:
HEATING PURE IRON
to its melting point and then allowing it to cool slowly results in an idealized time-temperature relationship (.1). As the iron cools, some discontinuation or temperature arrests are observed.
to its melting point and then allowing it to cool slowly results in an idealized time-temperature relationship (.1). As the iron cools, some discontinuation or temperature arrests are observed.
These discontinuations are caused by physical changes of iron crystals. Some of the temperatures at which these changes take place are important for heat treatment of gears.
The first arrest at 1540 °C (2800 °F) marks the temperature at which the iron freezes or solidifies. Immediately after freezing, the iron atoms are arranged in what is termed the body-centered cubic (bcc) pattern. In this crystal structure, an iron atom is located at each of the eight corners and
one in the center (Fig. 2.a). This form of iron is known as delta (d) iron. Then, at 1400 °C (2550 °F) (A4, Fig. 1), iron undergoes an allotropic transformation, that is, rearrangement of atoms in the crystal. The new crystal structure becomes face-centered cubic (fcc) with an iron atom at
each of the eight corners and also with an atom in the center of the six faces instead of one in the center of the cube (Fig. 2.b). This form is known as gamma (g) iron. At 910 °C (1670 °F) (A3, Fig. 1), iron undergoes another allotropic transformation and reverts to the bcc system.
This structure, which is crystallo graphically the same as delta iron, is stable at all temperatures below the A3 point (Fig. 1) and is known as alpha (a) iron. The next arrest at 770 °C (1420 °F) (A2, Fig. 1) is not caused by any allotropic change. It marks the temperature at which iron becomes ferromagnetic and is therefore termed the magnetic transition. Above this temperature, iron is non-magnetic.
These various temperature arrests on the cooling of iron are caused by evolutions of heat. On heating, the arrests occur in reverse order and are caused by absorption of heat. The critical points also may be detected by sudden changes in other physical properties, for instance, expansivity or
electrical conductivity.
Alloys of Iron and Carbon
Steels are basically alloys of iron and carbon. The properties of iron and, hence, the steel are affected markedly as the percentage of carbon varies.
An iron-carbon phase diagram represents the relationship between temperatures, compositions, and crystal structures of all phases that may be formed by iron and carbon. Thus, it is felt some knowledge of the iron-carbon phase diagram is helpful for better understanding of gear heat
treatment. A portion of this diagram for alloys ranging up to 6.7% of carbon is reproduced in Fig. 3; the upper limit of carbon in cast iron is usually not in excess of 5%. The left-hand boundary of the diagram represents pure iron, and the right-hand boundary represents the compound
iron carbide, Fe3C, commonly called cementite.
The beginning of freezing of the various iron-carbon alloys is given by the curve ABCD, termed the liquidus curve. The ending of freezing is given by the curve AHJECF, termed the solidus curve. The freezing point of iron is lowered by the addition of carbon (up to 4.3%), and the resultant alloys freeze over a temperature range instead of at a constant temperature as does pure iron metal. The alloy containing 4.3% carbon, called the eutectic alloy of iron and cementite, freezes at a constant temperature as
indicated by the point C. This temperature is 1130 °C (2065 °F), considerably below the freezing point of pure iron.
Carbon has an important effect on the transformation temperatures (critical points) of iron. It raises the A4 (Fig. .1) temperature and lowers the A3 (Fig. 1) temperature. The effect on the A3 (Fig. 1) temperature is significant in the heat treatment of carbon and low-alloy steels, while that on the A4 (Fig. 1) is important in the heat treatment of certain high-alloy steels.
It is possible for solid iron to absorb or dissolve carbon, the amount being dependent on the crystal structure of the iron and its temperature. The body-centered (alpha or delta) iron can dissolve only small amounts of carbon, whereas the face-centered (gamma) iron can dissolve a considerable amount, the maximum being about 2.0% at 1130 °C (2065 °F) (Fig.3). This solid solution of carbon in gamma iron is termed austenite. The solid solution of carbon in delta iron is termed delta ferrite,
and the solid solution of carbon in alpha iron is termed alpha ferrite or, more simply, ferrite.
The mechanism of solidification of iron-carbon alloys, especially those containing less than 0.6% carbon, is rather complicated and is of no importance in the heat treatment of carbon steels. It is sufficient to know that all iron-carbon alloys containing less than 2.0% carbon steel immediately or soon after solidification consist of the single-phase austenite.
The part of the iron-carbon diagram that is of concern with the heat treatment of steel is reproduced on an expanded scale in Fig.4. Regardless of the carbon content, steel exists as austenite above the line GOSE. Steel of composition S (0.80% of carbon) is designated as “eutectoid” steel, and those with lower or higher carbon as “hypoeutectoid” and “hypereutectoid,” respectively.
A eutectoid steel, when cooled at very slow rates from temperatures within the austenitic region, undergoes no change until the temperature denoted by PSK is reached. At this temperature of 720 °C (1330 °F), also known as the A1 temperature, the austenite transforms completely to an aggregate of ferrite and cementite. This aggregate is also known as pearlite. The A1 temperature is, therefore, frequently referred to as the pearlite point. Because the A1 temperature involves the transformation of austenite to pearlite (which contains cementite, Fe3C), pure iron does not possess an A1 transformation. Theoretically, iron must be alloyed with a minimum of 0.03% of carbon before the first minute traces of pearlite can be formed on cooling (point P). If the steel is held at a temperature just below A1 (either during cooling or heating), the carbide in the pearlite tends to coalesce into globules or spheroids. This phenomenon is known as spheroidization, which later helps to form martensitic steel structure.
Hypoeutectoid steels (less than 0.80% carbon), when slowly cooled from temperatures above the A3, begin to precipitate ferrite when the A3 (GOS) line is reached. As the temperature drops from the A3 to A1, the precipitation of ferrite increases progressively, and the amount of the remaining austenite reaches eutectoid composition and, upon further cooling, transforms completely into pearlite. The lower the carbon content, the higher the temperature at which ferrite begins to precipitate
and the greater the amount in the final crystal structure.
Hypereutectoid steels (more than 0.80% of carbon), when slowly cooled from temperatures above the line SE (Acm), begin to precipitate cementite when the Acm line is reached. As the temperature drops from the Acm to A1, the precipitation of cementite increases progressively, and the amount of the remaining austenite decreases accordingly, its carbon content having been depleted. At the A1 temperature, the remaining austenite reaches eutectoid composition and upon further cooling transforms completely into pearlite. The higher the carbon content, the higher the temperature at which cementite begins to precipitate and the greater the amount in the final crystal structure.
The temperature range between the A1 and A3 is called the critical transformation range. Theoretically, the critical points in any steel should occur at about the same temperatures on either heating or cooling very slowly. Practically, however, they do not because the A3 and A1 points are
affected slightly by the rate of heating but tremendously by the rate of cooling. Thus, rapid rates of heating raise these points only slightly, but
rapid rates of cooling lower the temperatures of transformation considerably. To differentiate between the critical points on heating and cooling, the small letters “c” (for chauffage, from the French, meaning heating) and “r” (for refroidissement, from the French, meaning cooling) are
added. The terminology of the critical points thus becomes Ac3, Ar3, Ac1, Ar1, and so on. The letter “e” is used to designate the occurrence of the points under conditions of extremely slow cooling on the assumption that this represents equilibrium conditions (“e” for equilibrium), for instance,
the Ae3, Ae1, and Aecm.
It is important to remember that the iron-carbon phase diagram represents transformation and crystal structures in slowly cooled steel under equilibrium conditions. Any departure from equilibrium conditions, as by rapid cooling, changes transformation characteristics, forming
different grades of heat treated steels.
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