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Preparation of Specimens for Metallographic Examination

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Metallography

INDEXObjectives

Introduction

Metallography

Questions

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Objective

To prepare the specimens surfaces to be examined for their microstructure study by the microscope .

To learn and to gain experience in the preparation of metallographic specimens.

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Introduction Metallography is basically the study of the structures and

constitution of metals and alloys, using metallurgical microscopes and magnifications, so that the physical and mechanical properties of an alloy can be related to its observed microstructure.

It provides information about the specimen under investigation, including the size and shape of the grains (crystallites), the presence of micro defects (such as segregation, hair cracks, and nonmetallic inclusions), and the nature and distribution of secondary phases.

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MetallographyCutting

Grounding – emery paper (240, 300, 400, 600)

Polishing (0.5, 0.1, 0.05μ)

Etching –Nitol/ Kellers Solution

Microscopy

Groundinggrinding is a subset of cutting, as grinding is a true

metal cutting process.

Abrasive :The use of Premium SiC abrasive paper is the most efficient and practical technique for grinding metallic metallographic specimens. Although many qualities of silicon carbide are readily available, only the premium grade SiC powder provides the most consistent results and highest grinding rates.

Each grain of abrasive functions as a microscopic single-point cutting edge and shears a tiny chip that is analogous to what would conventionally be called a "cut" chip (turning, milling, drilling, tapping, etc.) 8

Soft non-ferrous metals - Initial grinding is recommended with 320 grit SiC

abrasive paper followed by 320 400, 600 and , 800 grit SiC paper. because These materials are relatively soft they do not easily break down the SiC paper.

The initial grinding with 320 grit is generally sufficient for minimizing initial deformation and yet maintaining adequate removal rates.

For extremely soft materials such as tin, lead and zinc it is also recommended that the abrasive paper be lightly coated with a paraffin wax. The wax reduces

the tendency of the SiC abrasive to embed into the soft specimen.

Grinding Soft non-ferrous metals

Ferrous metals - are relatively easy to grind with the depth of deformation being a major consideration.

120 grit SiC abrasives provide a good initial start with subsequent use of 240 or 320, 400, 600 and 800 grit SiC.

Super alloys - are generally of moderate hardness but have extremely stable elevated temperature characteristics and corrosion resistance.

the procedures for preparing super alloys is very similar to that for most non-ferrous metals.

Grinding Ferrous metals

Sample Preparation

The purpose of this practice is to understand how to prepare and interpret metallographic samples systematically.

Gather information about chemical composition, heat treatment, processing, phase diagram.

Cut representative sample.

Mount sample, grind and polish.

Examine un etched sample.

Etch lightly and examine again.

Etch further if necessary.

Compare with microstructure expected from equilibrium phase diagram

Spheroidal Graphite Cast Iron

Abrasive Material

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Materials used for the abrading particles are: garnet: commonly used in woodworkingemery: commonly used to abrade or polish metalaluminium oxide: perhaps most common in widest variety of grits; can be used on metal (i.e. body shops) or woodsilicon carbide: available in very coarse grits all the way through to micro-grits, common in wet applicationsalumina-zirconia: (an aluminium oxide–zirconium oxide alloy), used for machine grinding applicationschromium oxide: used in extremely fine micron grit (micrometre level) papersceramic aluminum oxide: used in high pressure applications, used in both coated abrasives, as well as in bonded abrasives.

Polishing is the process of creating a smooth and shiny surface by rubbing it or using a chemical action, leaving a surface with a significant reflection

Aluminum Oxide(0.5, 0.1, 0.05μ)

Polishing

PROCESS

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The specimen must1. Be free from scratches, stains and others imperfections which

tend to mark the surface.

2. Reveal no evidence of chipping due to brittle inter metallic

compounds and phases.

3. Be free from all traces of disturbed metal.

4. The specimen has to be grounded with the help of abrasive papers.

5. Polishing enhances the surface and makes it suitable to observe its

grain structure under Microscope.

Grain

The micro structure of many metallic or ceramic materials consists of many grains.

A grain

is portion of the materials within which the arrangement of the atoms is nearly identical but the orientation or crystal structure of atoms

are different.

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A Grain BoundaryThe surface that separates the individual grains is a

narrow zoon in which the atoms are properly spaced.

One method of controlling the properties of a material is by controlling the grain size.

aluminum oxide and titanium dioxide (Al2O3 . TiO2)

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Surface Defects

Surface defects are the boundaries, or plans, that separate a material into regions.

Each region may have the same crystal structure but different orientation

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Industrial etching

The surface is than exposed to chemical attack or ETCHING, with grain boundaries being attacked more aggressively than reminder of the grain to reveal the microstructure.

Light from an optical microscope is reflected or scattered from the sample surface depending how the surface is etched

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Etching:

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Surface Finish and Surface Roughness (Original Magnification 50x)

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REFERENCES http://sembach.com/uploads/images/brevier/bild18.gif

http://www.springerimages.com/img/Images/Springer/JOU=11661/VOL=2011.42/ISU=9/ART=688/MediaObjects/MEDIUM_11661_2011_688_Fig26_HTML.jpg

http://www.springerimages.com/img/Images/Springer/JOU=11661/VOL=2011.42/ISU=11/ART=749/MediaObjects/MEDIUM_11661_2011_749_Fig1_HTML.jpg

http://www.sfsa.org/tutorials/uplock/images/Grains.Jpg

http://ars.els-cdn.com/content/image/1-s2.0-S0043164804002364-gr11.jpg

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