METALLOGRAPHIC EXAMINATION OF METALS AND ALLOYS EXPERIMENT # 2 Instructor: M.Yaqub

MICROSTRUCTURE Metals consists of small particles called atoms. Atoms after combination forms molecules. Combination of molecules or atoms is called grain. A grain is a portion of the material within which the arrangement of the atoms are identical.

The orientation of the grain may different but arrangement of atoms is same. Grains are separated by boundaries called grain boundaries. within the grain distance b/w two atoms is same.

Near the boundaries, too close means compression, far apart means tension. Microstructure of a metals consists of many grain. Grains are very small, can not be seen by eyes. Microscope is required. For seeing grain for any metal, few operation has to perform on metal to prepare the sample, this technique is called METALLOGRAPLY.

METALLOGPRAPHY It consists of following operations. SECTIONING: Cut a small piece from parent material by fracturing, hacksawing or abrasive cutting. MOUNTING: If sample is small, first it is mounted before further operation. This can be done by Mechanical Mount, Compression Mount, or Cold Mount. In lab we will use Compression Mount.

GRINDING: Grinding is perform to make surface smooth. POLISHING: Polishing produces mirror like surface. ETCHING: A dilute acid is react with the surface of the sample. This operation is called etching. After etching, the grain boundries are visible in microscope.

GRAIN SIZE ASTM METHOD The average diameter of a grain is represented by a number called ASTM number. ASTM number for any metal or alloy can be calculated by using the formula, Where n is ASTM number

Where N* is number of grains in a unit inch2 area photograph of 100 Magnification. It cab be obtained from following formula. Where A* is fixed. For inch system it is (1/100)2 inch2 and in mm 0.0645 mm2. A is the Actual area of the photograph and N is the number of grains in the photograph.