Cast Iron

ORE Blast Furnace

PIG IRON

CUPOLA FURNACE

CAST IRON

CAST IRONSGrey CI

Ductile CI

White CI

Malleable CI

Alloy CI

Good castability C > 2.4%

Malleabilize

Stress concentration at flake tips avoided

Gray Cast Iron • Gray cast iron is characterized by its graphitic microstructure, which

causes fractures of the material to have a gray appearance.

• It is the most commonly used cast iron and the most widely use cast material base on weight.

• Most cast irons have a chemical composition of 2.5 to 4.0% carbon, 1 to 3% silicon, and the remainder is iron.

• Gray cast iron has less tensile strength and shock resistance than steel.

• Its compressive strength is comparable to low and medium carbon

steel.

Gray Cast Iron

• A low cost material that can be used for many purposes

Characteristics:

1) Gray Iron basically is an alloy of carbon and silicon with iron.

2) It is readily cast into a desired shape in a sand mould.

3) It contains 2.5-3.8% C, 1.1-2.8%Si, 0.4-1%Mn, 0.15% P and 0.10% S.

4) It is marked by the presence of flakes of graphite in a matrix of ferrite, pearlite and austenite.

5) Graphite flakes occupy about 10% of the metal volume

6) Length of flakes may vary from 0.05 mm to 0.1 mm

7) When fractured, a bar of Gray Cast Iron gives gray appearance

8) Gray iron possesses lowest melting point of the ferrous alloys.

9) G.C.I. possesses high fluidity and hence it can be cast into complex shapes and thin sections.

10) It possesses machinability better than steel.

11) It has higher resistance to wear

12) It possesses high vibration damping capacity

13) Gray iron has low ductility and low impact strength as compared with steel.

14) G.C.I. has a solidification range of 2400 – 2000°F

15) It associates low cost combined with hardness and rigidity.

16) G.C.I. possesses high compressive strength

17) G.C.I. possesses excellent casting qualities for producing simple and complex shapes.

Application

1) Machine tools and structures (bed, frame and details)

2) Gas or water pipes for underground purpose

3) Manhole covers

4) Cylinder blocks and heads for I.C. engines

5) Tunnel segments

6) Frames for electric motors

7) Ingot moulds

8) Sanitary Wares

9) Piston Rings

Malleable Cast Iron

Malleable iron: dark graphite rosettes (temper carbon) in an –ferrite matrix

Malleable Cast Iron • Malleable iron starts as a white iron casting, that is then heat treated at

about 900 °C (1,650 °F).

• Graphite separates out much more slowly in this case, so that surface tension has time to form it into spheroidal particles rather than flakes.

• Due to their lower aspect ratio, spheroids are relatively short and far from one another, and have a lower cross section vis-a-vis a propagating crack.

• They also have blunt boundaries, as opposed to flakes, which alleviates

the stress concentration problems faced by grey cast iron. • In general, the properties of malleable cast iron are more like mild steel

Characteristics:

1) M.C.I. is one which can be hammered and rolled to obtain different shapes.

2) M.C.I. is obtained from the hard and brittle white iron through a controlled heat conversion process.

3) (i) A ferritic M.C.I. has Ferrite matrix

(ii) A pearlitic M.C.I. has pearlite matrix

(iii) An alloy M.C.I. contains chromium and nickel and possess high strength and low corrosion resistance.

4) M.C.I. possess high yield strength.

5) High young’s modulus and low coefficient of Thermal expansion.

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)

6) Good wear resistance and vibration damping capacity.

7) Can be used from – 60 to 1200°F

8) Solidification range of 2550-2065°F

9) Low moderate cost.

10)M.C.I. contains 2-3% C, 0.6 -1.3% Si, 0.2 -0.6% Mn, 0.15% P and 0.10% S.

Application

1) Automotive Industry

2) Rail Road

3) Agricultural implements

4) Electrical line Hardware

5) Conveyor chain links

6) Gear case

7) Universal joint yoke

8) Rear axle banjo housing

9) Automotive crankshaft.

Spheroidal Cast Iron

Nodular (ductile) iron: the dark graphite nodules are surrounded by an -ferrite matrix.

Spheroidal Cast Iron

• Nodular or ductile cast iron. Tiny amounts of magnesium or cerium added to these alloys slow down the growth of graphite precipitates by bonding to the edges of the graphite planes.

• Along with careful control of other elements and timing, this allows the carbon to separate as spheroidal particles as the material solidifies.

• The properties are similar to malleable iron, but parts can be cast with larger sections.

Characteristics:

1) Unlike long flakes as in G.S.I., graphite appears as rounded particles, or nodules or spheroids in N.C.I.

2) The spherodizing element when added to melt eliminate sulphur and oxygen (From the melt), which change solidification characteristics and possibly account for the nodulization.

3) Ductile cast iron possesses very good machinablity.

4) Soft annealed grades of S.C.I. can be turned at very high feeds and speeds.

5) The properties of S.C.I. depend upon the metal composition and the cooling rate.

6) Spheroidal or Nodular or Ductile C.I. contains 3.2%-4.2% C, 1.1%-3.5% Si, 0.3% - 0.8% Mn, 0.08% P and 0.2% S.

7) It possesses excellent damping capacity, casteblity and wear resistance.

Mechanical Properties of SG Iron

Application

1) Paper industries machinery

2) I. C. engines

3) Power transmissions equipment

4) Farm implements and tractors

5) Earth moving machinery

6) Valves and fittings

7) Steel mills rolls and mill equipment

8) Pipes

9) Pumps and compressors

White Cast iron

White iron: the light cementite regions are surrounded by pearlite, which has the ferrite–cementite layered structure.

White cast iron

• With a lower silicon content and faster cooling, the carbon in white cast iron precipitates out of the melt as the metastable phase cementite, Fe3C, rather than graphite.

• The cementite which precipitates from the melt forms as relatively large particles, usually in a eutectic mixture, where the other phase is austenite (which on cooling might transform to martensite).

• It is difficult to cool thick castings fast enough to solidify the melt as white cast iron all the way through.

• However, rapid cooling can be used to solidify a shell of white cast iron, after which the remainder cools more slowly to form a core of grey cast iron.

• The resulting casting, called a chilled casting, has the benefits of a hard surface and a somewhat tougher interior.

• White cast iron can also be made by using a high percentage of chromium (Cr) in the iron; Cr is a strong carbide-forming element, so at high enough percentages of chrome, the precipitation of graphite out of the iron is suppressed.

• White iron is too brittle for use in many structural components, but with good hardness and abrasion resistance and relatively low cost.

• It finds use in such applications as the wear surfaces (impeller and volute) of , shell liners and in ball mills and , balls and rings in , and the teeth of a backhoe's digging bucket (although cast medium-carbon martensitic steel is more common for this application).

Characteristics:

1) W.C.I. derives its name from the fact that its freshly broken surface shows a bright white fracture.

2) Unlike gray iron, W.C.I. has almost all its carbon chemically bonded with the iron- as iron carbide, Fe3C which is very hard and brittle constituent.

3) W.C.I. possesses excellent abrasive wear resistance.

4) W.C.I. under normal circumstances is brittle and not machinable.

5) By using a fairly low silicon content, cast iron may be made to solidify as white iron.

6) W.C.I. casting can be made in sand moulds.

7) W.C.I. can also be made on the surface of a gray iron casting provided the material is of special composition.

8) If iron of proper composition is cooled rapidly, the free carbon will go in the combined form and give rise to white iron casting.

9) W.C.I. contains 1.8-3.6% C, 0.5 – 2.0% Si, 0.2- 0.8% Mn, 0.18% P and 0.10% S.

10) The solidification range of W.C.I. is 2550-2065°F

Application

1) For producing malleable iron casting

2) For manufacturing those component parts which require a hard and abrasion resistant material.

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