1. Cast Irons
Engr. Waqas Javaid
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2. Overview of Cast Iron
Iron with 2.0 to 4.5% carbon and 0.5 to 3% silicon.
Lower melting point and more fluid than steel (better castability).
Low cost material usually produced by sand casting.
A wide range of properties, depending on composition & cooling rate.
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3. Composition of Cast Iron
A typical cast iron contains
2.0 to 4.5% carbon,
1.0 to 3.0% silicon,
less than 1.0% manganese, and
less than 0.2% sulfur.
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4. Types of Cast Iron Gray cast iron - carbon as graphite
White cast iron - carbides, often alloyed
Ductile cast iron
nodular, spheroidal graphite
Malleable cast iron
Graphite nodules are irregular clusters / tempered graphite
Compacted graphite cast iron
CG or Vermicular Iron
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5. Microstructures of Cast Iron
Gray iron
aFe and graphite flakes
Nodular iron
aFe and graphite spheres
White iron
cementite and pearlite
Malleable iron
a Fe and tempered graphite flakes
low melting point, castable, cheap; however, can be brittle.
Compacted Graphite
Iron
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6. Gray Cast Iron Gray cast iron forms when
Cooling is slow, as in heavy sections
High silicon or carbon,
Mn = 0.4 to 1.0 %
Flake graphite in a matrix of pearlitic, ferrite or martensite
Properties depend strongly on casting shape & thickness
Low ductility - elongation 0.6%
High conductivity, high damping
Wide range of applications
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7. Properties of Gray Cast Irons
Excellent compressive strength (compressive strength is typically 3-4 times tensile strength),
Excellent machinability (graphite acts to break up the chips and lubricate contact surfaces),
Excellent wear resistance (graphite flakes self-lubricate), and
Out­standing sound and vibration-damping capacity (graphite flakes absorb transmitted energy).
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8. Properties of Gray Cast Irons
Good corrosion resistance and the enhanced fluidity due to high silicon contents
Thermal conductivity high
The formation of the lower-density graphite reduces the amount of shrinkage, making possible the production of more complex iron castings
The pointed edges of the flakes act as preexisting notches or crack initiation sites, giving the material a characteristic brittle nature resulting low impact resistance
Ductility is low (0.6%)
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9. Properties of Gray Cast Irons
The size, shape, and distribution of the graphite flakes have a considerable effect on the overall properties of gray cast iron.
For maximum strength, small, uniformly distributed flakes are preferred.
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10. Applications of Gray Cast Irons
Engines
Cylinder blocks, liners,
Transmission housing
Brake drums, clutch plates
Pressure pipe fittings, Machinery beds
Furnace parts, ingot and glass moulds
Machine tool basis & large equipment parts – subjected to compressive load & vibration
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11. Ductile or SG Iron
Also known as spheroidal graphite (SG), and nodular graphite iron
Far better ductility than gray cast iron
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12. Properties of SG Iron good ductility, high strength
Strength higher than gray cast iron
Ductility up to 6% as cast or 20% annealed
Low cost
Simple manufacturing process makes complex shapes
Machineability better than steel
Good toughness, wear resistance,
low-melting-point castability,
up to a 10% weight reduction compared to steel makes ductile iron an attractive engineering material
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13. Applications of SG Iron
Automotive industry 55% of ductile iron in USA
Crankshafts, front wheel spindle supports, steering knuckles, disc brake callipers
Pipe and pipe fittings (joined by welding)
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14. White Cast Iron
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15. White Cast Iron White fracture surface
No graphite, because carbon forms Fe3C or more complex carbides. Features promoting the formation of cementite over graphite are:
low Carbon equivalent (1.8 to 3.6% carbon),
0.5 to 1.9% Silicon,
0.25 to 0.8% Manganese, &
rapid cooling
very hard, brittle & abrasion resistant.
Often alloyed
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16. Uses of White Cast Iron Products such as gates, fences,
parts of stove are manufactured by using white cast iron.
In addition it is also used to manufacture malleable cast iron
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17. Malleable Iron
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18. Malleable Iron
Produced by heat treatment (in the range of 900C) of white cast iron- cementite dissociate into its component element.
Carbon converts into graphite nodules of irregular clusters (also referred to as Clump or popcorn graphite).
Similar properties to ductile iron.
depending on the type of heat treatment, various types of malleable iron can be produced.
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19. Properties of Malleable Iron
Similar to ductile iron
Good shock resistance
Good ductility
Good machineability
Corrosion resistance
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20. Applications of Malleable Iron
Similar applications to ductile iron
Malleable iron is better for thinner castings
Ductile iron better for thicker castings >40mm
Vehicle components
Power trains, frames, suspensions and wheels
Steering components, transmission and differential parts, connecting rods
Railway components
Pipe fittings
products such as door keys, gear wheel, and crank levers
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21. Compacted Graphite Cast Iron (CGI)
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23. Compacted Graphite Cast Iron (CGI)
CGI is produced by addition of an Mg-Ce-Ti
CGI is characterized by a graphite structure that is intermediate to the flake graphite of gray iron and the nodular graphite of ductile iron, and
possesses some of the desirable properties and characteristics of each.
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24. Properties of CGI
Properties of CGI bridge the gap between Gray and Ductile irons
Strength, stiffness, and ductility are greater than those of gray iron,
While,
castability, machinability, thermal conductivity, and damping capacity all exceed those of ductile.
Impact and fatigue properties are good.
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25. Typical Properties of Pearlitic Gray, Compacted Graphite, and Ductile Cast Irons
Property
Gray
CGI
Ductile
Tensile strength (MPa)
250
450
750
Elastic modulus (Gpa)
105
145
160
Elongation (%)
1.5 5
Thermal conductivity (w/mk) 48 37 28
Relative damping capacity (Gray = 1) 1
0.35
0.22
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27. Fracto-Graphs of Cast Irons
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28. Welding Weldability of cast iron is low and depends on
the material type,
thickness,
complexity of the casting, and
on whether machinability is important
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29. Weldability White cast iron - not weldable
Small attachments only
Grey cast iron - low weldability
Welding largely restricted to salvage and repair
Ductile and malleable irons - good weldability (inferior to structural steel)
Welding increasingly used during manufacture
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