cast iron by prof. krishanu biswas
TRANSCRIPT
8/13/2019 Cast Iron by Prof. Krishanu Biswas
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Slides onCAST IRONS
provided by Prof. Krishanu Biswas
for the course MME 330 Phase Equilibria in Materials
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Fe-C Phase Diagram
Stable
Metastable
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CAST IRONS
Grey CI
Ductile CI
White CI
Malleable CI
Alloy CI
Good castability C > 2.4%
Malleabilize
Stress concentration
at flake tips avoided
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White Cast Iron
All C as Fe3C (Cementite)
Microstructure Pearlite + Ledeburite + Cementite
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Grey Cast Iron
Fe-C-Si + (Mn, P, S)
Invariant lines become invariant regions in phase diagram
Si (1.2, 3.5) C as Graphite flakes in microstructure (Ferrite matrix)
< 0.1% retards graphitization; size of Graphite flakes
< 1.25% Inhibits graphitization
[2.4% (for good castability), 3.8 (for OK mechanical propeties)]
3 3 3L ( ) ( )
Ledeburite Pearlite
Fe C Fe C Fe C
Si decreases Eutectivity
Si promotes graphitization ~ effect as cooling rate
Solidification over a range of temperatures permits the nucleation and growth of Graphite
flakes
Change in interfacial energy between /L & Graphite/L brought about by Si
Growth of Graphite along ‘a’ axis
Sieutectoid
C
volume during solidification better castability
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Ductile/Spheroidal Cast Iron
Graphite nodules instead of flakes (in 2D section)
Mg, Ce, Ca (or other spheroidizing) elements are added
The elements added to promote spheroidization react with the solute in
the liquid to form heterogenous nucleation sites
The alloying elements are injected into mould before pouring (George-
Fischer container)
It is thought that by the modification of the interfacial energy the ‘c’ and
‘a’ growth direction are made comparable leading to spheroidal graphite
morphology
The graphite phase usually nucleates in the liquid pocket created by the
proeutectic
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Ductile Iron/Nodular Iron
With Pearlitic matrix
10 m
With Ferritic Matrix With (Ferrite + Pearlite) Matrix
Ferrite Graphite nodules
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Ductile Iron/Nodular Iron
Bull’s Eye
Ferrite
5 m
Pearlite (grey)
Graphite (black) Ferrite (White)
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Malleable Cast Iron
Malleabilize
To Increase DuctilityWhite Cast Iron Malleable Cast Iron
48
3 2 stage heat treatmentFe C (WCI) Graphite Temper Nodules (Malleable Iron)hrs
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Stage I
B: Graphite nucleation at /Cementite interface
(rate of nucleation increased by C, Si)
(Si solubility of C in driving force
for growth of Graphite)
A: Low T structure (Ferrite + Pearlite + Martensite) ( + Cementite)
C: Cementite dissolves C joining growing Graphite plates
• (940-960)C (Above eutectoid temperature)
• Competed when all Cementite Graphite
Spacing between Cementite and Graphite
spacing time (obtained by faster cooling of liquid)
Si t
Time for
Graphitization
in Stage I
Addition of Alloying elements
which increase the nucleation rate of Graphite temper nodules
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Stage II
Slow cool to the lower temperature such that does not form Cementite
C diffuses through to Graphite temper nodules(called Ferritizing Anneal )
Full Anneal in Ferrite + Graphite two phase region
Partial Anneal (Insufficient time in Stage II Graphitization)
Ferrite is partial and the remaining transforms to Pearlite Pearlite + Ferrite + Graphite
If quench after Stage I Martensite (+ Retained Austenite(RA))
(Graphite temper nodules are present in a matrix of Martensite and RA)
• (720-730)C (Below eutectoid temperature)
• After complete graphitization in Stage I Further Graphitization
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Malleable Iron
Ferritic Matrix
Pearlitic Matrix
Fully Malleabilized Iron
Complete Ferritizing Anneal
10 m
Partially Malleabilized Iron
Incomplete Ferritizing Anneal
Pearlite (grey)
Graphite (black)
Ferrite (White)
Ferrite (White)
Graphite (black)
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Growth of Graphite
Growth of Graphite Hunter and Chadwick
Double and Hellawell
Hillert and Lidblom
Growth of Graphite from Screw dislocations
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Alloy Cast Irons
Cr, Mn, Si, Ni, Al
the range of microstructures Beneficial effect on many properties
high temperature oxidation resistance
corrosion resistance in acidic environments
wear/abaration resistance
Alloy Cast Irons
Graphite bearing
Graphite free
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Cr addition (12- 35 wt %)
Excellent resistance to oxidation at high temperatures
High Cr Cast Irons are of 3 types:
12-28 % Cr matrix of Martensite + dispersed carbide
29-34 % Cr matrix of Ferrite + dispersion of alloy carbides
[(Cr,Fe)23C6, (Cr,Fe)7C3]
15-30 % Cr + 10-15 % Ni stable + carbides [(Cr,Fe)23C6, (Cr,Fe)7C3] Ni stabilizes Austenite structure
High Cr
29.3% Cr, 2.95% C
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Ni:
Stabilizes Austenitic structure
Graphitization (suppresses the formation of carbides)
(Cr counteracts this tendency of Ni for graphitization) Carbon content in Eutectic
Moves nose of TTT diagram to higher times easy formation of
Martensite
Carbide formation in presence of Cr increases the hardness of the eutectic
structure Ni Hard Cast Irons (4%Ni, 2-8% Cr, 2.8% C)
Ni-Hard
4%Ni, 2-8% Cr, 2.8% C
Needles of Martensite
Transformation sequence
Crystallization of primary Eutectic liquid + alloy carbide
Martensite
Good abrasion resistance
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Silal Iron (trade name): Alloy CI with 5% Si
Si allows solidification to occur over larger temperature range
promotes graphitization
Forms surface film of iron silicate resistant to acid corrosion
CI with 5 % Si
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Fe-Ni Phase Diagram
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Alloy Cast Irons
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Bull’s
Eye