Iron carbon equilibrium diagram
Orthor Orthorhom hombic bic Fe C. Iron Iron at
blu
Fe-C equilibrium diagram ●
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The structural form of pure iron at room temperature is called ferrite or -iron. Ferrite is soft and ductile. Since ferrite has a body-centred cubic structure, the inter-atomic spaces are small and pronouncedly oblate, and cannot readily accommodate even a small carbon atom. Therefore, solubility of carbon in ferrite is very low, of the order of 0.006% at room temperature. The maximum carbon content in ferrite is 0.05% at 723 ° C.
In addition to carbon, a certain amount of silicon, manganese and phosphorous may be found in ferrite. ● The face-centred modification of iron is called austenite or -iron. It is the stable form of pure iron at temperatures between 910°C and 1400°C. At its stable temperature austenite is soft and ductile and consequently, is well suited for manufacturing processes. face-centred cubic structure of iron has larger ● The inter-atomic spacing than in ferrite. Even so, in FCC structure the interstices are barely large enough to accommodate carbon atoms, and lattice strains are produced. As a result, not all the interstitial sites can be filled at any one time. ●
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The maximum solubility is only 2% of carbon at 11 30°C. Above 1400°C, austenite is no longer the most stable form of iron, and the crystal structure changes back to a body-centred cubic phase called delta iron. This is the same phase as the -iron except for its temperature range. The solubility of carbon in -ferrite is small, but it is appreciably larger than In -ferrite, because of higher temperature. The maximum solubility of carbon in &iron is 0.1% at 1490°C.
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In iron-carbon alloys, carbon in excess of the solubility limit must form a second phase, which is called iron carbide or cementite. Iron carbide has the chemical composition of . This does not mean that iron carbide forms molecules of but simply that the crystal lattice contains iron and carbon atoms in a three -to- one ratio. The compound has an orthorhombic unit cell with twelve iron atoms and four carbon atoms per cell, and thus has a carbon content of 6.67%. As compared to austenite and ferrite, cementite being an inter-metallic compound, is very hard and brittle. The presence of iron carbide with ferrite in steel greatly increases the strength of steel.
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In the reaction, the simultaneous formation of ferrite and cementite from austenite results at the temperature of 723° C and composition of 0.80% carbon. There are nearly 12% of iron carbide and slightly more than 88% of ferrite in the resulting mixture. Since the ferrite and cementite are formed simultaneously, they are intimately mixed. Characteristically, the mixture is lamellar, i.e., it is composed of alternate layers of ferrite and cementite. This micro-structure is called pearlite which is very important in iron and steel technology, because it can be formed in almost all steels by means of suitable heat treatments.
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The alloy containing 0.80% of carbon is called the eutectoid steel. Upon cooling the eutectoid steel below 723°C, all of the austenite is transformed into pearlite. Alloys with less than 0.80% C are ar e called hypo-eutectoid steels and those with higher composition are called hyper-eutectoid steels.
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Fe-Fe3C phase diagram is characterized by five individual phases,: α –ferrite (BCC) Fe-C solid solution, γ-austenite (FCC) Fe-C solid solution, δ-ferrite (BCC) Fe-C solid solution, Fe 3C (iron carbide) or cementite - an inter-metallic compound and liquid Fe-C solution and four invariant reactions: peritectic reaction at 1495 oC and 0.16%C, δ-ferrite + L↔ γ-iron (austenite) monotectic reaction 1495 oC and 0.51%C, L ↔ L + γ-iron (austenite) eutectic reaction at 1147 oC and 4.3 %C, L ↔ γ-iron + Fe3C (cementite) [ledeburite] eutectoid reaction at 723 oC and 0.8%C, γ-iron ↔ α – ferrite + Fe3C (cementite) [pearlite]
Three- phase reactions ●
Suffix – ic denotes at least one liquid phase is there
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Suffix – oid all phases involve are solid
IES-2004 Consider the following temperature ranges:
1. Room temperature
2. 0 to 910°C
3. 910°C to 1400 oC point
4. 1400oC to below melting
In which of the above temperature ranges ferrite with body centered cubic structure structure is indicated in, the Fe-Fe 3C phase diagram? (a)1, 2 and 4
(b)2, 3 and 4
(c)1 and 3
(d) 2 and 3
IES-1992 The microstructure composition of pearlite for a Fe 3C diagram consists of (a) Carbon Carbon dissolved dissolved in alpha iron iron having a body body cantered cubic structure. (b) Carbon dissolved in in gama iron having a face cantered cubic structure. (c) A mixture mixture of body-cantere body-cantered d alpha iron iron and face-entered gamma iron (d) Carbon dissolved in in body-cantered alpha alpha iron and an Fe, Fe3C.
IES 2011 Liquid + solid (1) on cooling converting solid (2) reaction is known as: (a) Eutectoid reaction (b) Eutectic reaction (c) Peritectic reaction (d) Peritectioid reaction
IES 2010 Iron-carbon equilibrium diagram
(a) Correlates the microstructure and properties of steel and cast iron (b) Indicates the phase changes occurring during heating and cooling (c) Is made by plotting carbon percentage along X-axis and temperature along Yaxis. (d) All of the above
IES 2010 Pearlite phase in an iron-carbide phase diagram is
(a) Eutectic phase (b) Hypoeutectic mixture (c) Eutectoidal mixture (d) Hypereutectic phase
IES-1995 Match List I with List II and select the correct answer using the codes given below the lists: List I (Name of Material) List II (% Carbon Range) A. Hypo Hypo-e -eu utec tectoi toid ste steel 1. 4.3 4.3 - 6.67 6.67 B. Hype Hyperr-eu euttect ectoid oid stee steell 2. 2.0 2.0 - 4.3 4.3 C. Hypo Hypo-e -eu utec tectic tic cast cast iron iron 3. 0.8 0.8 - 2.0 D. Hyper-eutectic cast iron ron 4. 0.008 - 0.8 Codes:A B C D A B C D (a) 4 3 2 1 (b) 1 3 2 4 (c) 4 1 2 3 (d) 1 2 3 4
IES-2004 An iron-carbon binary alloy has 0.5% C by weight. What is this alloy called?
(a) Eutect Eutectoid oid alloy alloy (b)Eutectic alloy (c) Hypo-eutec Hypo-eutectoid toid alloy alloy (d) Hypereutec Hypereutectoid toid alloy alloy
IES-1995 Eutectoid reaction occurs at
(a) (a) 600° 600°C C (b)723°C (c) (c) 1147 1147°C °C (d)1493°C
IES-2005 The eutectoid of carbon in iron, above lower critical temperature, when cooled, results in:
(a) Ferrite Ferrite and and austen austenite ite (b) Ferrite Ferrite and cementite cementite (c) Cementite Cementite and austen austenite ite (d)Ferrite, cementite and austenite
GATE-1992 Match the terms used in connection with heat-treatment of steel with the micro structural/physical characteristics: Terms Characteristics (A) Pearlite Pearlite (P)Extremel (P)Extremely y hard and brittle phase (B) Martensite Martensite (Q)Cementi (Q)Cementite te is finely finely dispersed dispersed in ferrite ferrite (C) Austenite Austenite (R)Alternat (R)Alternatee layers layers of cementite cementite and ferrite ferrite (D) Eutect Eutectoid oid (S)Can (S)Can exist exist only above above 723°C 723°C (T)Pertaining to state of equilibrium between three solid phases (U)Pertaining to state of equilibrium between one liquid and two solid phase Codes: A B C D A B C D (a) R P S T (b) R S P T (c) T R P S (d) T R S P
IES-2006 Match List-I (Fe-Fe3C Phase Diagram Characteristic) with List-II (Phase) and select the correct answer using the code given below the Lists: List-I List-II A. Alph Alphaa (α (α)) iron iron 1. iron iron B. Iron Iron carbid carbidee having having crysta crystall lattice lattice 2.Eute 2.Eutectic ctic with 3 iron and 1 carbon atom C. BCC pure pure allotro allotrope pe of iron iron is stable stable 3.Ferr 3.Ferrite ite between 1388 °C and is melting point at 1535°C 4.Cementite Codes:A B C A B C (a) 4 2 3 (b) 3 4 1 (c) 4 2 1 (d) 3 1 2
IES-2002 Match List I with List II and select the correct answer: List I (Phase diagram) diagram) List II (Characteristic) (Characteristic) A. Isomorphous system 1.One liquid decomposes into another liquid and solid B.Eutectic system 2.One liquid and another solid combine to form a new solid C.Peri eritecti ctic sy system 3.Two me metals are are co completely ely so soluble in in liquid state And completely insoluble in solid state D.Mo D.Mono note tect ctic ic sys syste tem m 4.Tw 4.Two o meta metals ls,, solu solubl blee in sol solid id and and liq liqui uid d state Codes: A B C D A B C D (a) 2 3 4 1 (b) 4 1 2 3 (c) 2 1 4 3 (d) 4 3 2 1
IES-1999 In a eutectic system, two elements are completely
(a) Insoluble Insoluble in solid solid and liquid liquid states states (b) Soluble Soluble in liquid liquid state (c) Soluble Soluble in in solid solid state state (d) Insoluble Insoluble in liquid liquid state
IES-1993 Eutectic reaction for iron-carbon system occurs at
(a) (a) 600° 600°C C
(b)723°C
(c) (c) 1147 1147°C °C
(d)1493°C
IES-2000 During peritectic solidification, one liquid
(a) Combines Combines with one solid solid to form form a second second new solid (b)Solidifies into two different solids (c) Forms Forms one one soli solid d (d) Forms one solid solid and another another liquid
IES 2007 Which one of the following is the correct statement? Pearlite in iron-carbon system is a (a) Phase consisting consisting of ferrite and cementite at room temperature (b) Mechanical mixture of ferrite and cementite at room temperature (c) Eutectic mixture ferrite and cementite cementite at room temperature (d) All the above above three are correct correct
IES-2005 Increase of ferrite phase in steel st eel increases:
(a) Stre Streng ngth th
(b)Hardness (c) Duct Ductil ilit ity y
(d)Brittleness
IES-2005 A 60 C-plain carbon steel has, approximately:
(a) 75% of pearlit pearlitee and 25% 25% of ferrite ferrite (b) 25% of pearlite pearlite and 75% of ferrite ferrite (c) 75% of cement cementite ite and and 25% of ferrite ferrite (d)75% of pearlite and 25% of cementite
IES-2000 Pearlite consists of
(a) 6.67% C and 93.33% 93.33% ferrite ferrite (b) 13% Fe and 87% 87% cementit cementitee (c) 13% C and and 87% ferrit ferritee (d) 13% cementit cementitee and 87% ferrite ferrite
IES-1997 A given steel test specimen is studied under metallurgical microscope. Magnification used is 100 X. In that different phases are observed. One of them is Fe3C. The observed phase Fe 3C is also known as
(a) Ferri Ferrite te
(b)Cementite
(c) Aust Austen enit itee
(d)Martensite
IES-1995 Which one of the following sets of constituents is expected in equilibrium cooling of a hypereutectoid steel from austenitic state?
(a) Ferrite Ferrite and and pearlite pearlite (b) Cementite Cementite and and pearlite pearlite (c) Ferrite Ferrite and and bainite bainite (d) Cementite Cementite and martensit martensitee
IES-2001 Martensite is a super-saturated solution of carbon in
(a) Alph Alphaa iron iron (b) Beta Beta iron iron (c) Gamm Gammaa iron iron (d) Delta Delta iron iron
1. Eutectic ( Fe-C) ● ●
(L1→S1+S2) Two materials are completely soluble in liquid state and completely insoluble in solid state.
2. Peritectic ( Fe-C) ● ●
(L+S1→S2) One Liquid and another solid combine to form a new solid.
3. Monotectic FeO2-SiO2 ●
(L1→S1+L2)
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One Liquid decomposes into another liquid and solid.
4. Syntectic Na-Zn ●
(L1+L2→S1)
5. Eutectoid Fe-C ●
(S1→S2+S3)
6. Peritectoid Cu-Al ●
S1 + S2→S3
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Isomorphus system: In a binary system when there is complete inter-solubility between components in all phases, the system is isomorphus. isomorphus.
Ex: ●
Cu-Ni, Al2O3-Cr 2O3, NiO-MgO
Azeotropic system: Some of isomorphus binary system, the liquidus touches, the solid tangentially at a minimum temperature which is lower then milting temperature of either of the two components. Au-Cu, Au-Ni
IES 2007 Match List I with List II and select the correct answer using the code given below the Lists: List I List II (Name of the Invariant (Invariant Reaction during Reaction) cooling) A. Monotectic 1.LIQUID….SOLID1 + SOLID2 B. Eutectic 2.LIQUID1…..LIQUID2 + SOLID C. Eutectoid 3.SOLID1…..SOLID1 + SOLID2 D. Peritectic 4.LIQUID + SOLID1…..SOLID2 Code: A B C D A B C D (a) 3 1 2 4 (b) 2 4 3 1 (c) 3 4 2 1 (d) 2 1 3 4
IES-2004 Consider the following lead-tin phase diagram given below: For which one of the following alloy compositions, the alloy will have the lowest melting point at 185 oC
(a) 20% Sn and 80% 80% Pb by weight weight (b) 60% Sn and 40% 40% Pb by weight weight (c) 97% Sn and 3% Pb by weight weight (d) 40% Sn and 60% Pb byweight
Lever Rule ●
At a point in a phase diagram, phases present and their composition (tie-line method) along with relative fraction of phases (lever rule) can be computed. Relative amount of liquid and solid phases is given respectively by:
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Therefore it is not restricted to solid phases only.
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IES-2008 Assertion (A): Lever Rule can be applied to determine relative amounts of phases present at any temperature. Reason (R): Lever Rule is restricted to estimate relative phases, only if they are solid phases. (a) Both A and and R are true and R is the correct correct explana explanation tion of A (b) Both A and and R are true true but R is NOT NOT the correct correct explanation of A (c) A is true true but but R is is false false (d) A is fals falsee but R is true true
Example ●
In a binary system of A and B, if a liquid of 35% A (65% B) is co-exists with a solid of 75% A (25% B), for an overall composition of 40% A, the fraction of the liquid is given by
GATE-2016 In a binary system of A and B, a liquid of 20% A (80% B) is coexisting with a solid of 70% A (30%B). For an overall composition having 40% A, the fraction of solid is
(a) 0.40
(b) 0.50
(c) 0.60
(d) 0.75
Fe-C alloy classification ●
Fe-C alloys are classified according to wt.% C present in the alloy for technological convenience as follows: ●
Commercial pure irons
% C < 0.008
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Low-carbon/mild steels
0.008 - %C - 0.3
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Medium carbon steels
0.3 - %C - 0.8
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High-carbon steels
0.8- %C - 2.11
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Cast irons
2.11 < %C
IES-2003 According to Gibbs' phase rule, the number of degrees of freedom of an eutectic point in a binary system is
(a)1 (b)2 (c)0 (d)3
The End
