Metallography Ppt.pptx 5421o

Metallography

Metallography • Microscopic examination of a metal specimen. • It helps in determining: – – – – –

Grain size & shape Distribution of various phases Inclusions Mechanical and thermal treatment and Expected behavior

• Prepared specimen should have flat, scratch free, mirror finish surface. • The steps involved are: – Selection of Specimen – Mounting the Specimen

Selection of Specimen • The metal may not be homogeneous either in composition or crystal structure. • Some metals show directionality (eg. Wrought iron has slag fiber in the direction of rolling). • Defects are included in the face to be polished. • Specimen can be cut using, a hacksaw or abrasive cut off wheel.

Mounting the Specimen For •   easy holding of very thin/irregular samples while grinding and polishing: Mounting using simple mechanical clamping devices Mounting on polymer base: The specimen is kept on a flat surface in an open box which is filled with one powder and two liquids which are polymerized without application of pressure. The mass sets into a hard solid in about 20 mts.  Mounting on Bakelite base: The specimen is placed and bakelite powder is filled in the cylinder of mounting press which is heated to 150°C and pressure up to 25 is applied. Mounting on Lucite base: Same as above. The

Grinding • A fairly flat surface is to be obtained using a course file or emery belt. • The file can be kept flat on table and the specimen is moved back and forth. • This removes nicks & burrs and scratches made by hacksaw, cut off wheel etc. • Preferably the specimen should be washed after each stage of grinding/polishing.

Intermediate & Fine Grinding • This is done on a series of emery paper of progressively finer grades. • At each stage, the specimen is to be turned through 900 so that the scratches of the previous operation are at right angles to the new scratches produced. • Grinding should be done on at least four paper of grades 220, 320, 400 & 600 (from course to fine). • Grinding can be done wet or dry. Wet method on water proof base is preferred. • The back and forth movement should be straight and parallel and only light pressures are to be used.

Fine Polishing • Fine polishing removes the ridges by burnishing action. • This is done with the help of a rotating wheel, covered with a special cloth (velvet, billiard cloth etc) and charged with abrasive particles such as gamma aluminium oxide (for Fe and Cu based alloys) • The specimen is held on the rotating disc without application of force and given a slight circular motion by wrist. • Water is constantly supplied on to the disc. • Polishing can be stopped when the specimen has developed a scratch free mirror like finish.

Etching • • • • •

Only chemical treatment. The polished specimen is washed free from any dirt or grease using soap solution or heated spirit. It is then immersed in the appropriate chemical for few seconds and immediately washed again in running water. Preferably the specimen is dried using warm air blown by a hair drier.   Etching is done for the following reasons. – Etching, by chemical exposes the structure beneath. – Grain boundaries are seen as dark lines when observed through microscope.

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The common etchants used are: – – – – –

Nitric acid + ethyl alcohol – nital – for iron & steels, CI Picric acid + ethyl alcohol – picral – CI Alkaline sodium picrate – distinguishes ferrite & cementite Mixed acids & glycerol – for Ni-Cr alloys

Observing using the microscope • The specimen is arranged below the objective lines in such a way that the axis of the instrument is perpendicular to the prepared surface. • The surface is brought into focus by coarse adjustment and then by fine adjustment. • The lowest magnification is to be used first and then move to successively higher magnifications.

Macroscopic Observation

Macroscopic Observation

Microscopic Observation

Microscopic Observation

Microscopic Observation

Metallurgical Microscope

SEM Images

SEM Images

X Ray Diffraction

Bragg’s Law

Field Ion Emission