The document discusses cast iron, emphasizing its composition (2-4.5% carbon, 0.5-3% silicon) and properties, including strength, ductility, and castability. It outlines the impact of cooling rates and composition on microstructure, distinguishing between gray and white cast iron, with gray iron known for its graphite flakes and high wear resistance, while white iron is characterized by its hardness and brittleness. Applications for these materials range from heavy machinery to wear-resistant surfaces, highlighting their industrial utility.

1. MME 214
Metallography
Dr. Md Muktadir Billah
Assistant Professor
MME, BUET

2. Cast Iron
 Iron with 2 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
 Strength
 Hardness
 Ductility
 Thermal conductivity
 Damping capacity

3. Cast Iron

4. Cooling Rate & Composition Effect
 Slow cooling favors formation of graphite
 Rapid cooling promotes carbides with high
hardness
 Tendency to form graphite is regulated by
composition as well
 Graphite formation is promoted by the presence
of silicon in concentrations greater than about 1
wt. %
 Both microstructure and mechanical behavior
depend on composition and heat treatment

5. Fe-Fe3C Phase Diagram

6. Graphite as a Stable Phase

7. Gray Cast iron
 Carbon and silicon contents vary between 2.5
and 4.0 wt. % and 1.0 and 3.0 wt. %
respectively
 For most of these cast irons, the graphite exists
in the form of flakes
 Flakes are normally surrounded by ferrite or
pearlite matrix
 Because of these graphite flakes, a fractured
surface takes on a gray appearance, hence its
name.

8. Gray Cast iron
 Gray irons having different microstructures are
generated by adjustment of composition
and/or by using an appropriate treatment
 Lowering silicon content or increasing cooling
rate may prevent complete dissociation of
cementite to form graphite
 Under these circumstances microstructure
consists of graphite flakes embedded in a
pearlite matrix
 High silicon and faster cooling rate results
ferritic matrix

9. Gray Cast iron

10. Gray Cast iron
 Mechanically, gray iron is comparatively weak and
brittle in tension
 Graphite flakes tip are sharp and pointed, may serve
as points of stress concentration
 Strength & ductility much higher under compression
 Effective in damping vibrational energy
 Base structures for machines and heavy equipment
that are exposed to vibrations
 Gray irons exhibit a high resistance to wear
 High fluidity at casting temp., permits casting with
intricate shapes; low casting shrinkage
 Least expensive of all metallic materials

11. White Cast iron
 For low-silicon cast irons (less than 1.0 wt % Si)
and rapid cooling rates, most of the carbon
exists as cementite instead of graphite
 Fracture surface has white appearance, so
termed white CI
 Thick sections may have only a surface layer of
white iron that was “chilled” during the casting
process
 Gray iron forms at interior regions, which cool
more slowly

12. White Cast iron
 Extremely hard but also very brittle, virtually
unmachinable
 Use is limited to applications that necessitate a
very hard and wear-resistant surface, without a
high degree of ductility
 For example, as rollers in rolling mills
 Generally, white iron is used as an intermediary
in the production of yet another cast iron,
malleable iron

13. White Cast iron