Exp.5: Preparation of Specimens for Metallographic Examination

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Preparation of Specimens for Metallographic
Examination

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Metallography
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INDEX

• Objectives
• 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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Metallography

• Cutting
• Grounding emery paper (240, 300, 400, 600)
• Polishing (0.5, 0.1, 0.05µ)
• Etching Nitol/ Kellers Solution
• Microscopy

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Grounding

• grinding 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.)

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Grinding Soft non-ferrous metals

• 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.

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Grinding 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.

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• 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

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Spheroidal Graphite Cast Iron

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Abrasive Material

• Materials used for the abrading particles are
• garnet commonly used in woodworking
• emery commonly used to abrade or polish metal
• aluminium oxide perhaps most common in widest
  variety of grits can be used on metal (i.e. body
  shops) or wood
• silicon carbide available in very coarse grits
  all the way through to micro-grits, common in wet
  applications
• alumina-zirconia (an aluminium oxidezirconium
  oxide alloy), used for machine grinding
  applications
• chromium oxide used in extremely fine micron
  grit (micrometre level) papers
• ceramic aluminum oxide used in high pressure
  applications, used in both coated abrasives, as
  well as in bonded abrasives.

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Polishing

• 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)

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PROCESS

• 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.

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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 Boundary

• The 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.

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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
Sample material Etchant Composition Remarks
Carbon steel (usually 2) (nitric acid) HNO3 1-5 ml Ethyl alcohol 100ml Few seconds (15 Sec)
Carbon steel Picric Acid Picric acid 4g Ethyl alcohol 100ml Few seconds (15 Sec)
Aluminum Hydrofluoric acid HF (conc.) 0.5ml H2O 99.5ml Swab for 15 sec.
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Surface Finish and Surface Roughness
(Original Magnification 50x)
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REFERENCES

• http//sembach.com/uploads/images/brevier/bild18.g
  if
• http//www.springerimages.com/img/Images/Springer/
  JOU11661/VOL2011.42/ISU9/ART688/MediaObjects/M
  EDIUM_11661_2011_688_Fig26_HTML.jpg
• http//www.springerimages.com/img/Images/Springer/
  JOU11661/VOL2011.42/ISU11/ART749/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-S00431
  64804002364-gr11.jpg