The IronCarbide diagram.docx

IRON CARBON PHASE DIAGRAM

Content

Phase Cooling curve of pure iron solid phases in fe-c diagram DIFFERENT TEMP.ZONE DIFFERENT PHASE COMPOSITION conclusion

IRON CARBON PHASE DIAGRAM Phase A phase diagrams is a type of graph used to show the equilibrium conditionsbetween the thermodynamically-distinct phases; or to show what phases are present in the material system at various T, p, and compositions

IRON CARBON PHASE DIAGRAM Iron –carbon phase diagram shows temp. & composition at which different constituent of iron-carbon alloys are stable these constituent are austenite ferrite & cementite (iron carbide)

Cooling curve of pure iron iron first solidifies at 2802°F, it is in B.C.C. (body-centered cubic) δ (delta) form or δ feritte . As it is cooled, it changes to F.C.C. nonmagnetic γ iron or austenite , B.C.C. α ion – nonmagnetic and finally to B.C.C. α iron or ferrite – magnetic. Originally, nonmagnetic α iron was called β iron until x-ray studies showed no change in lattice structure at 1414°F.

solid phases in fe-c diagram four solid phases in iron carbon diagram    

α iron: ferrite, ferritic steel γ iron: austenite, austenitic steel Fe3C cementite δ Ferrite. This is a solid solution of carbon in iron and has a BCC crystal structure. The maximum solubility or C in Fe is 0,09% at 1495oC

Austenite Austenite has FCC (cubic face centered) crystal structure, permitting high solubility of carbon – up to 2.06% at 2097 ºF (1147 ºC). Austenite does not exist below 1333 ºF (723ºC) and maximum carbon concentration at this temperature is 0.83%.

α-ferrite solid solution of carbon in α-iron.α-ferrite has BCC crystal structure and low solubility of carbon – up to 0.025% at 1333 ºF (723ºC). α-ferrite exists at room temperature

Cementite Also known as iron carbide, is a chemical compound of iron and carbon, with the formula Fe3C (or Fe2C:Fe). By weight, it is 6.67% carbon and 93.3% iron. It has an orthorhombic crystal structure.[1] This intermetallic compound is metastable, itremains as a compound indefinitely at room T, butdecomposes (very slowly, within several years)into α-Fe and C (graphite) at 650 - 700 °C Cementite is iron carbide with the formula Fe3C, and an orthorhombic crystal structure. It is a hard, brittle material, essentially a ceramic in its pure form. It forms directly from the melt in the case of white cast iron. In carbon steel, it either forms from austenite during cooling or from martensite during tempering. Cementite contains 6.67% Carbon by weight; thus above that carbon content in the Fe-C phase system, the alloy is no longer steel or cast iron, as all of the available iron is contained in cementite. Cementite mixes with ferrite, the other

product of austenite, to form lamellar structures called pearlite and bainite. Much larger lamellae, visible to the naked eye, make up the structure of Damascus steel. Fe3C is also known as cohenite, particularly when found mixed with nickel and cobalt carbides in meteorites.

DIFFERENT TEMP.ZONE Different temp. zones in fe-c phase diagram are s follows Upper critical temperature (point) ACM Upper critical temperature (point) A3 Lower critical temperature (point) A1 Magnetic transformation temperature A2

Upper critical temperature (point) ACM Is the temperature, below which cementite starts to form as a result of ejection from austenite in the hypereutectoid alloys.

Upper critical temperature (point) A3 Is the temperature, below which ferrite starts to form as a result of ejection from austenite in the hypoeutectoid alloys.

Lower critical temperature (point) A1 Is the temperature of the austenite-to-pearlite eutectoid transformation. Below this temperature austenite does not exist.

Magnetic transformation temperature A2 Is the temperature below which α-ferrite is ferromagnetic

DIFFERENT PHASE COMPOSITION Hypoeutectoid steels Cast irons Hypereutectoid steels Eutectoid steel

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Hypoeutectoid steels (carbon content from 0 to 0.83%) consist of primary (proeutectoid) ferrite (according to the curve A3) and pearlite. Eutectoid steel (carbon content 0.83%) entirely consists of pearlite. Hypereutectoid steels (carbon content from 0.83 to 2.06%) consist of primary (proeutectoid)cementite (according to the curve ACM) and pearlite. Cast irons (carbon content from 2.06% to 4.3%) consist of proeutectoid cementite C2 ejected from austenite according to the curve ACM , pearlite and transformed ledeburite (ledeburite in which austenite transformed to pearlite).

Phase transformation Eutectoid When the solution above the transformation point is solid, rather than liquid, an analogous eutectoid transformation can occur. For instance, in the iron-

carbon system, the austenite phase can undergo a eutectoid transformation to produce ferrite and cementite,

Peritectic Peritectic transformations are also similar to eutectic reactions. Here, a liquid and solid phase of fixed proportions react at a fixed temperature to yield a single solid phase in the iron-carbon system. It resembles an inverted eutectic, with the δ phase combining with the liquid to produce pure austenite at 1,495 °C (2,723 °F) and 0.17% carbon. eutectic eutectic mixture is a mixture of two or more phases at a composition that has a lowest melting point & where the phases crystallize from molten solution at this temp.

Conclusion

Iron carbon phase diagram can be said as the map of steel & cast iron. By changing the carbon we can get different varieties of steel & cast iron. Also by controlling the heating & cooling rate of the component we can get metals whose physical properties are different