Cast Iron Polishing

Cast Irons : Optical Microscopy

Preparation of microscopic specimens

Polishing :

• Obtain a flat, semi polished surface by abrasive cutting
• Grind on suitable belt for reducing surface finish
• Intermediate grind on series of emery papers of decreasing grit size ‘O’, ‘OO’ and ‘OOO’
• Lap on suitable abrasive lapping wheel

Care during grinding
• Apply moderate pressure, to avoid distorsion and to prevent overheating
• Clean specimen thoroughly between changing grades
• Turn specimen at right angles
• Protect Edges

Final aim is to produce a flat, highly polished, scratch free surface.

The polishing process is directional, which results in draging out inclusions, graphite particles and locally abrade away material immediate adjacent to the particles leading to formation of ‘Comet Tails’. Rotating the specimen in counter to the rotation of the lap during final polishing operation effectively changes the direction of polishing and prevents formation of comet tails.

Electrolytic polishing - Not suitable for cast irons

• as specimen gets etched also and an unetched surface is preferred for graphite shape observation.
• complete or partial removal of inclusions / graphite particles and
• non suitability of mounted specimen due to staining.

Special precautions for cast iron specimen preparation

Due to difficulties to retain graphite particles

• Prepare by grinding on usual three papers 0, 00, 000, with prolonged grinding on a well worn sheet of 00 paper. Final grinding on 000 paper glazed with either graphite or soap stone.

• Graphite particles are more prone to dislodge if the polishing cloth is deep piles. Final polishing is best carried out on napless cloth such as fine silk.

• Polishing abrasive should be preferably levigated alumina instead of heavy magnesium oxide.

• The cloth should be kept damp, but not wet.

• Polishing should proceed in one direction only. Rotating specimen will quickly dislodge graphite particles.

• The polished surface should be frequently examined microscopically and excess polishing should be avoided.

Etching

The purpose of metallographis examination is to determine the true structural characteristics of the specimen of interest. It is therefore necessary that the various components of the microstructure be delineated with preciseness and extreme clarity.

This is usually achieved by subjecting the polished surface of the specimen to the chemical action of some reagent under carefully controlled conditions.