SREE CHAITANYA COLLEGE OF ENGINEERING: KARIMNAGAR

DEPARTMENT OF MECHANICAL ENGINEERING

EXPERIMENT NO -3

STUDY OF MICROSTRUCTURE OF CASTIRONS

AIM: Toidentify the different phases and to draw the microstructure of different cast irons.

APPARATUS & MATERIALS:

Metallurgical microscope, specimen: Spheroidal Graphite cast iron, Grey cast Iron.

THEORY: Cast irons are iron carbon alloys in which carbon content varies from 2 to 6.67%. Cast irons that contain carbon percentage between 2 to 4.3% are called hypoeutectic cast irons. If carbon content of cast iron is 4.3%, it is called Eutectic cast irons. If the carbon content is above 4.3%, it is called hypereutectic cast iron.

Types of cast iron:

Depending on the form of carbon, cast iron are divided into (a) White cast iron (b) Grey cast iron (c) Malleable cast iron (d) Spheroidal cast iron (e) Chilled cast iron.

a) White cast iron:

The composition of white cast iron is C = 2 to 3%, Si = 1 to 1.65%, Mn = 0.4 to 0.8%, P = 0.15%,S=0.5%. In White cast iron most of the carbon is present in combined form as cementite. This is obtained by rapidly cooling the cast iron from molten state. These are hard and wear resistance. These are used only for hard and wear resistance applications and also used as raw material to produce malleable iron. At room temperature microstructure of hypo eutectic cast iron consists of dendrite areas of transformed austenite in a matrix transformed lideburite. At room temperature microstructure of eutectic cast iron consists of cementite and Pearlite. At room temperature microstructure of hypereutectic C.I consists of dendrites of primary cementite in the matrix of transformed lideburite. It is used for rollers for crushers, brake shoes etc. subjected to heavy wear. The major use of white cast iron is produce malleable cast iron.

b) Grey cast iron:-

The composition of grey cast iron is C=3.2 to 3.5%,Si = 1.3 to 2.3%, Mn= 0.5to 0.7%,p =0.1 to 1%,S = 0.1%.In grey cast iron carbon is present as graphite flakes. They contain more carbon and silicon content than white cast irons. It is a low melting alloy having good cast ability, machinability and good damping capacity. The tendency of carbon to form graphiteflakes is due to increase in carbon and silicon decrease in the cooling rate. At room temperature the microstructure of grey cast iron consists of graphite flakes and Pearlite. In grey cast iron the presence of graphite flakes provides lubrication effect to reduce friction. (So it is particular suitable for machine tool beds and slides.)

c)Malleable cast iron:

The composition of malleable cast iron is C=2 to 2.8%,Si = 0.7 to 1.4%, Mn= 0.4 to 0.6%,p =0.2% (max) ,S = 0.1%(max).It is produced by heating white cast iron to 9000C to 10000C for about 50 hrs followed by slow cooling to room temperature. On heating white cast iron, cementite structure and tend to decompose into ferrite and tempered carbon. The lubrication of action graphite imparts high machinability to malleable cast iron. Malleable cast iron is tough, strong, and shock resistant. At room temperature the microstructure of malleable cast iron consists of rosettes of tempered carbon graphite in the matrix of Pearlite. Malleable cast iron is used in Automobile parts, man whole covers, railroad equipment’s, gears, cams, pipefittings and electrical switchgear casting.

d)Spheroid graphite cast iron : (Nodular cast iron or ductile cast iron )

The composition of Spheroidal graphite cast iron C = 3 to 3.5%,Si 2 to 2.5%,Mn = 0.15 to 0.6%, P = 0.025 to 0.04%(max),S = 0.015 to 0.04%(max), Mg = 0.015 to 0.1%. Spheroidal graphite cast iron is an iron carbon alloy having a structure composed of nodules (spheroids) of graphite formed directly during the process of solidification and embedded in matrix of steel. The formation of spherical graphite is due to addition of Mg for hypoeutectic cast iron and cerium of hypereutectic cast iron. Due to spherodisation tensile strength, ductility, and toughness are improved. The graphite in spherical shape reduces stress concentration effect.SG iron is widely uses for crankshafts, hydraulic cylinders, values, cylinder heads, connecting rods and high-pressure pipes, because of good compression and corrosion resistance. It is also used for stream plants and main applications.

e)Chilled cast iron :-

The composition of chilled cast iron is C = 3.3 to 3.5%, Si = 2 to 2.5%. This type of cast iron shows white structure of surface and gray structure in the centre. Due to this, the good properties of white cast iron (hardness and wear resistance) and grey cast iron (machinability, damping capacity and notch sensitivity) can be combined together in a chilled casting. Adjusting the white cast iron and then cooling it rapidly to room temperature produce it, rapid cooling promotes hard. Chilled cast iron is used in making dies and rolls for crushing, road rollers.

PROCEDURE:

  1. The eye piece tube caps are removed from the microscope and a pair of eye pieces of required magnification is inserted into it.
  2. The mechanical stage is then lowered. so that there is sufficient space between the objective lens and the stage, for the specimen to fit without coming to close to the lens to avoid scratching of the lens.
  3. Now, the specimen is placed on the stage and is centred under the lens.
  4. The lowest total magnification lens combination (objective and eyepiece) is adjusted.
  5. The microscope light is now turned on.
  6. The stage is now raised slowlytowards the objective lens by using the coarse adjustment screw.
  7. The specimen and objective lens are moved closer until a spot of light appears on the surface of the specimen. The specimen is repositioned, if necessary, so that the light falls on its centre.
  8. The gap between the specimen and the lens is reduced until, the specimen is near almost, but not quite touches the lens.
  9. The coarse adjustment is used, until the field of view reaches a maximum illumination. At this time the microstructure comes into focus.
  10. The fine adjustment knob is turned in a direction, which brightens the view until the structure comes into sharp focus..
  11. Using the mechanical stage movements, the etched surface is explored by adjusting the focus, if necessary.
  12. The above procedure is repeated by changing the magnification of objective lens until a clear microstructure of the specimen is observed.
  13. The microstructure of the given specimen is now sketched
  1. The microscope light is now turned off.

PRECAUTIONS:

  1. The glass lens of the objective or eyepiece should not be touched with fingers.
  2. Cover the microscope with a dust cover after use.
  3. Proper gap should be maintained between the specimen and the objective lens so that scratches are not formed on the lens of the microscope or on the surface of the specimen.

OBSERVATION:

GREY CAST IRON

As Polished: Magnification: 150XAs Etched: Magnification: 150X

Etchant: Nital Solution

The specimen is a Annealed Cast Iron containing carbon 3.8% and silicon 2.6%. After casting the specimen is annealed in a furnace at 8000C for 1 Hour and cooled. The polished (not etched) specimen shows free carbon in the form of flakes (like worms or snakes). The cast iron will have carbon content more than 2%. The carbon in cast iron will be in two forms.

  1. Combined carbon (as carbides namely cementite)
  2. Free carbon as graphites.

The microstructure shows free carbon as graphite flakes and the matrix in etched condition is pearlite and ferrite.

Magnification: x

MICROSTRUCTURE OF SPECIMEN

Specimen Material :

Etching reagent :

Etching time:

Composition:

Description of microstructure :

Heat treatment/Mechanical working:

OBSERVATION:

SPHEROIDAL GRAPHITIC CAST IRON

As Polished: Magnification: 150XAs Etched: Magnification: 150X

Etchant: Nital Solution

The specimen is an spheroidal graphitic cast- iron. The grey cast iron being poured in the molten stage is the molten stage is modularised by the addition of noulating agent of inoculants. This agent acts as a nuclear to graphite to form as spherules. This type of cast iron is also known as Nodular Iron or ductile Iron due to it improved mechanical properties. The composition is same as that of cast iron but free graphite are in the form of nodules. The microstructure shows graphite nodules in a matrix of ferrite and pearlite with majority of ferrite content. Unlike malleable Iron the S.G. Iron will have well formed nodules.

Magnification: x

MICROSTRUCTURE OF SPECIMEN

Specimen Material :

Etching reagent :

Etching time:

Composition:

Description of microstructure :

Heat treatment/Mechanical working:

METALLURGY LAB