In this article we will discuss about:- 1. Introduction to Cast Iron 2. Effects of Elements on Cast Iron 3. Types.
Introduction to Cast Iron:
Alloys of iron and carbon having carbon content more than 2.11%. Generally all cast irons have carbon content between 3 to 4% when Fe-Fe3C diagram is extended to 100% C. Graphite is a more stable phase as compared to cementite.
Slow cooling rate and silicon content of more than 1% promote graphitization.
Effects of Elements on Cast Iron:
i. Vanadium (0.01-0.5%):
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Increases hardness strength and machinability.
ii. Copper (0.02-2.5%):
Promotes formation of graphite.
iii. Molybdenum (0.15-0.9%):
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To improve tensile strength, hardness to casting and also improves toughness, fatigue strength machinability, hardenability.
iv. Chromium (0.15-0.9%):
Acts as a carbide stabilizer, increases strength, hardness and wear-resistant.
v. Nickel (0.25-2.0%):
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Aids fusibility and fluidity but induces brittleness.
vi. Phosphorus (1.0%):
Aids fusibility and fluidity but induces brittleness.
vii. Manganese (0.75%):
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Harden the cast iron by encouraging the formation of carbide.
viii. Sulphur (0.1%):
Makes hard and brittle
ix. Silicon (2.5%):
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Promotes the formation of free graphite > 4% which makes the iron soft and causes brittleness machinable.
x. 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.
Types of Cast Iron (CI):
1. Gray Cast Iron:
Carbon content (2.5-4% wt C).
Silicon content (1-3% wt).
It exists in the form of flakes which are surrounded by α-ferrites or pearlite matrix. Because of graphite flakes, the fractured surface take grey appearances.
It is weak in tension as tips of graphite flakes are pointed and serve as points of stress concentration.
Strength and ductility are higher in compression, thus high compressive strength.
It has excellent vibration damping characteristic, high wear resistance, and high fluidity, at casting temperature shrinkage is very low.
Low ductility and impact strength compared with steel.
It possess lowest melting point of the ferrous alloy.
It possess high fluidity and hence cast into complex slopes on thin section.
Due to graphite lubrication quality grey cast iron easily mechanical.
Applications:
Castings M/c Bases, cylinder lines, cylinder blocks, clutch plates, pistons, sanipry works, and tunnel segment underground pipes.
2. Mottled Cast Iron (Composition):
Iron = 93.5%, Graphite = 1.75%, Combined carbon = 1.75%, the remaining is impurities.
Properties:
(i) Mixture of the two states, grey cast iron and white cast iron.
(ii) Strength and hardness vary according to the ratio of free carbon to combined carbon.
(iii) Less tendency to rust than grey iron.
(iv) Fluidity is good.
(v) Hard and brittle.
Uses:
(i) Used for manhole covers and pipes.
3. Ductile (Nodular) Cast Iron:
When a small amount of magnesium or cerium is added to gray iron before casting a different microstructure and set of mechanical properties are produced in this microstructure.
Graphite exist in the form of nodules or sphere like particles instead of flakes.
This is called nodular CI.
The matrix phase surrounding is generally pearlite. This CI has more strength and ductility as compared to gray CI (380 – 480 MPa strength) and ductilities (as elongation) form 10% to 20%.
Composition:
(3.5 – 4%) C + (2 – 2.8%) Si + (0.05%) Mg + (< 20%) Ni.
Application:
Valve pump bodies, high strength gears, Pinion rollers, slides, cranks shafts, machine components etc.
4. While Cast Iron:
When the cooling rates of cast iron are high and silicon content is below 1%, most of the carbon exists as cementite instead of graphite.
This alloy is called white CI. Fractured surface of this alloy is white in appearance.
It is also called chilled CI as cooling rates are very high.
It is extremely hard and brittle and almost impossible to machine.
It has a very limited application where wear resistance is required without ductility e.g. Rollers of rolling mills.
5. Malleable Cast Iron:
When white CI is heated to a temperature of 800 – 900° C for a prolonged time period and in neutral atmosphere (to prevent oxidation), it causes decomposition of cementite to graphite in the form of clusters or russets surrounded by a ferrite or pearlite matrix. It’s Strength and ductility increases.
Composition:
(2.3 – 2.7% C) + (1 – 1.5% Si) + (<0.55% Mn)
Applications:
Connecting Rods, Transmission gears, differential cases, flanges marine and heavy duty services.
Properties:
(i) It possesses high yield strength
(ii) It can be hammered and rolled to different shapes
(iii) It has high Young’s modulus and low coefficient of thermal expansion
(iv) It possesses good wear resistance and vibration damping capacity
(v) It has shrinkage of 1.5mm/100mm
(vi) It is soft, tough and easily machined
Uses:
(i) Brake pedals,
(ii) Tractor springs,
(iii) Hangers,
(iv) Washing machine parts
(v) Agriculture implements,
(vi) Universal joint yoke,
(vii) Automotive crankshafts,
(viii) Rail road, etc.
6. Wrought Iron:
Extremely low carbon content (0.02%). It has a fibrous structure. It is ductile and has good forming qualities & has very high resistance to corrosion.
It produce in less quantity because of high cost.
Applications:
Pipe making, engine bolts and rivets, chains and crane hooks, forging applications etc.