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Presentation Part-II Guide for R&M welding engineers to plot and understand the iron-iron carbide phase diagram and easy way to remember different phase names with it’s property.

Presentation by Biranchi Narayan Biswal. Regional Manager Ador Fontech Ltd

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Allotropic forms of Iron At atmospheric pressure, three  allotropic  forms of  iron  exist: alpha  iron  ( α- Fe ), gamma  iron  ( γ- Fe ), and delta  iron  ( δ- Fe ).

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15390C
14000C
9100c
7680C
Liquid
(delta) Fe - B.c.c
7 (gamma) Fe - EC.C
Non Magnetic
Non a(alpha) Fe- B.C.C
a (alpha) Fe - B.C.C
Magnetic

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Peritectic , at 1490 deg.C, with low wt% C  alloys (almost no engineering importance). Eutectic , at 1147 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  steels .

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1600
15380C
1400
1200
1000
14930C
13940C
y, Austenite
Fesc
4 30
Fesc
Pearlite
Eutectoid Fe,c
12.14
Fesc
Eutectoid
11470C
Austenite ledeburite
and cementite
7270C
Pearlite
Proeutectoid
9120C
800
0.76
0.022
600
a, Ferrite
Pearlite and
Ferrite
Pearlite and
Cementite
Cementite and
ledeburite
Cementite (Fe3C)
400
(F
3
Cemenbte, pearlite and
transform«i ledeburite
4
2
Hyper-el ta:toid
e u tatoid
STEEL
5
6
6.
mposition (wt% C)
CAST IRON
e3C

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Important Terms in I RON-CARBON   DIAGRAM

Eutectic eutectoid Pearlite and  Cementine Austenite Ferrite

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Ferrite region of  Fe-Fe Carbide  diagram

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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.

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Average properties   are:  	 Tensile  strength =  40,000   psi;

=  40  % in 2   in; Rockwell  C 0   or Rockwell  B   90

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Pearlite  is the eutectoid mixture  containing 0.80 % 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).

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Pearlite  	 Average properties are:  	 Tensile   strength	 =  120,000   psi;

=	20 %	in 2 in.; =		Rockwell C 20, Rock-well B  95-100, or BHN 250-300.

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Austenite  is an interstitial solid solution of  Carbon dissolved in     (F.C.C.) iron. Maximum solubility is 2.0 % C at 1130°C. High formability, most of heat treatments  begin with this single phase . It is normally not stable at room  temperature. But, under certain conditions it  is possible to obtain austenite at room  	 temperature .

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Austenite  	 Average properties   are:

=	 150,000   psi; =	 10 percent in  2   in.;

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Cementite  or iron carbide, is  very hard,  brittle  intermetallic compound of iron &  carbon, as Fe 3 C, contains 6.67 % C. It is the   hardest structure   that appears on the  diagram, exact melting point unknown. Its crystal structure is orthorhombic. It is has low tensile strength (approx. 5,000 psi),  but high compressive strength.

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mixture	of	austenite	and  cementite.  	 It contains 4.3 percent C and is  formed at 1147°C.

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