Reduced Title: Iron-Carbon Phase Diagram: Properties & Changes, Study notes of Engineering

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IRON – CARBON PHASE DIAGRAM

Definition of structures:

 Various phases that appear on the Iron-

Carbon equilibrium phase diagram are as

under:

• Austenite
• Ferrite
• Pearlite
• Cementite
• Martensite
• Ledeburite

Austenite  Average properties are:  Tensile strength = 150,000 psi;  Elongation = 10 percent in 2 in.;  Hardness = Rockwell C 40, approx; and  toughness = high

Definition of structures:

 Ferrite is known as α solid solution.  It is an interstitial solid solution of a small amount of carbon dissolved in α (BCC) iron.  stable form of iron below 912 deg.C.  The maximum solubility is 0. 025 % C at 723 C and it dissolves only 0. 008 % C at room temperature.  It is the softest structure that appears on the diagram.

Definition of structures:

 Pearlite is the eutectoid mixture containing

1. 80 % C and is formed at 723 °C on very slow cooling.  It is a very fine platelike or lamellar mixture of ferrite and cementite.  The white ferritic background or matrix contains thin plates of cementite (dark).

Pearlite Average properties are: Tensile strength = 120 , 000 psi; Elongation = 20 % in 2 in.; Hardness = Rockwell C 20 , BHN- 300

Definition of structures:

 Martensite - a super-saturated solid solution of carbon in ferrite.  It is formed when steel is cooled so rapidly that the change from austenite to pearlite is suppressed.  The interstitial carbon atoms distort the BCC ferrite into a BC-tetragonal structure (BCT).; responsible for the hardness of quenched steel

Definition of structures:

 Ledeburite is the eutectic mixture of austenite and cementite.  It contains 4. 3 percent C and is formed at 1130 °C.

Cooling curve for pure iron

Various Features of Fe-C diagram

Max. solubility of C in ferrite=0.022% Max. solubility of C in %austenite=2%

Three Phase Reactions

 Peritectic, at 1490 deg.C, with low wt% C alloys (almost no engineering importance).  Eutectic, at 1130 deg.C, with 4.3wt% C, alloys called cast irons.  Eutectoid, at 723 deg.C with eutectoid composition of 0.8wt% C, two-phase mixture (ferrite & cementite). They are

Eutectic, Eutectoid, & Peritectic:

• Peritectic - liquid and one solid phase transform to a 2nd solid phase Solid 1 + Liquid ↔ Solid 2  + L ß  Eutectic - liquid transforms to two solid phases L  + 
• Eutectoid – one solid phase transforms to two other solid phases Solid 1 ↔ Solid 2 + Solid 3   + Fe 3 C

Eutectoid reaction

Redistribution of carbon by diffusion Austenite – 0.76 wt% C Ferrite - 0.022 wt% C Cementite - 6.70 wt% C Pearlite y↔α + Fe 3 C

transformation in relation to Fe-C

diagram