Metallurgy of Grey Cast Iron
Overview of cast iron
Iron with 1.7 to 4.5% carbon and 0.5 to 3% silicon
Lower melting point and more fluid than steel in
terms of better castability.
Low cost material usually produced by sand casting
A wide range of properties, depending on composition
& cooling rate
Strength
Hardness
Ductility
Thermal conductivity
Damping capacity
Effects of Elements on Cast Iron:
Vanadium (0.01-0.5%) : Increases hardness strength
and machinability.
Copper (0.02-2.5%) : Promotes formation of graphite.
Molybdenum (0.15-0.9%) : To improve tensile strength,
hardness to casting and also improves toughness, fatigue
strength machinability, hardenability.
Chromium (0.15-0.9%) : Acts as a carbide stabilizer,
increases strength, hardness and wear-resistant.
Nickel (0.25-2.0%) : Aids fusibility and fluidity but induces
brittleness.
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Effects of Elements on Cast Iron:
Phosphorus (1.0%) : Aids fusibility and fluidity but
induces brittleness.
Sulphur (0.1%) : Makes hard and brittle
Manganese (0.75%) : Harden the cast iron by
encouraging the formation of carbide.
Silicon (2.5%) : Promotes the formation of free graphite >
4% which makes the iron soft and causes brittleness
machinable.
Carbon : -6.67% Graphite (formed by slow cooling of
carbon) tough iron, easy to machine cementite (rapidly
cool) high tensile strength, difficult to machine element.
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Iron carbon diagram
Liquid
+ L L + Fe3C
Austenite
Carbon
910˚C Cast Iron
Steel + Fe3C
+
723˚C
+ Fe3C
0% 0.8% ~2% ~3%
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Production of cast iron
P i g iron, scrap steel, limestone and
carbon (coke)
Cupola
E l e c t r i c arcfurnace
E l e c t r i c induction furnace
U s u a l l y sand cast, but can be gravity die
cast in reusable graphite moulds
N o t formed, finished by machining
Types of cast iron
G r e y cast iron - carbon as graphite
W h i t e cast iron - carbides, often alloyed
D u c t i l e cast iron
nodular, spheroidal graphite
M a l l e a b l e castiron
C o m p a c t e d graphite cast iron
CG or Vermicular Iron
Effect of cooling rate
Slow cooling favours the formation of graphite &
low lhardness
Rapid cooling promotes carbides with high
hardness
Thick sections cool slowly, while thin sections
cool quickly
Sand moulds cool slowly, but metal chills can be
used to increase cooling rate & promote white iron
Grey cast iron
F l a k e graphite in a matrix of pearlite,
ferrite or martensite
W i d e range of applications
L o w ductility - elongation 0.6%
G r e y cast iron forms when
Cooling is slow, as in heavy sections
High silicon or carbon
Gray Cast Irons contain silicon, in addition tocarbon, as a primary alloy.
Amounts of manganese are also added toyield the desired microstructure.
Generally the graphite exists in the form of flakes, which are surrounded by an
a-ferrite or Pearlite matrix. Most Gray Irons are hypoeutectic, meaning they
have carbon equivalence (C.E.) of less than 4.3. Carbon content (2.5-
4% wt C). Silicon content (1-3% wt).
Gray cast irons are comparatively weakand brittle in tension due toits
microstructure; the graphite flakes have tips which serve as points of stress
concentration. Strength and ductility are much higher under compression
loads.
Typical properties
Depend strongly on casting shape & thickness
Low strength, A48 Class 20, Rm 120 MPa
High carbon, 3.6 to 3.8%
Kish graphite (hypereutectic)
High conductivity, high damping
High strength, A48 Class 60, Rm 410 MPa
Low carbon, (eutectic composition)
Graphite form
Uniform
Rosette
Superimposed (Kish and
normal)
Interdendritic random
Interdendritic preferred
orientation
Matrix structure
P e a r l i t e or ferrite
Tr a n s f o r m a t i o n is to ferrite when
Cooling rate is slow
High silicon content
High carbon equivalence
Presence of fine undercooled graphite
Properties of grey cast iron
M a c h i n e a b i l i t y is excellent
D u c t i l i t y is low (0.6%), impact resistance
low
D a m p i n g capacity high
T h e r m a l conductivity high
D r y and normal wear properties excellent
Applications
Engines
Cylinder blocks, liners,
B r a k e drums, clutch plates
P r e s s u r e pipe fittings
M a c h i n e r y beds
F u r n a c e parts, ingot and glass molds
