Metallographic specimen preparation


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Metallographic specimen preparation for metallographic observation is explained in detail. All the steps involved with detailed images help to understand the subject thoroughly.

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Metallographic specimen preparation

  2. 2. SAMPLE PREPARATION ▪ Consists of five major steps ▪ Sectioning ▪ Mounting (optional) ▪ Grinding ▪ Polishing ▪ Etching
  3. 3. SECTIONING ▪ Sectioning is the most important step in preparing specimens for physical or microscopic analysis ▪ Microstructure should not be altered, but practically hot and cold working accompany most sectioning methods. ▪ The damage to the specimen during sectioning depends on ▪ the material being sectioned ▪ the nature of the cutting device ▪ the cutting speed and feed rate ▪ the amount and type of coolant used Depth of deformation in different metals due to cutting method
  4. 4. SECTIONING METHODS ▪ Fracturing ▪ Breaking specimens with blows of a hammer or by steadily applying pressure ▪ Location of the fracture can be controlled by nicking or notching the material ▪ Not recommended, because it rarely follows desired directions and damage from fracturing can mask inherent features ▪ Also lengthy coarse grinding may be required to obtain a flat surface ▪ Shearing ▪ Low-carbon sheet steel and other thin, soft materials can be cut to size by shearing ▪ The area affected by shearing must be removed by grinding
  5. 5. SECTIONING METHODS ▪ Sawing ▪ Using hacksaws, band saws, and wire saws ▪ Hand-held hacksaws or band saws generally do not generate enough frictional heat to alter the microstructure ▪ Saw-cut surfaces are rough, and coarse grinding is required to obtain a flat surface ▪ Electric discharge machining (EDM) ▪ Electric discharge machining (EDM), or spark machining, is a process that uses sparks in a controlled manner to remove material from a conducting workpiece in a dielectric fluid like kerosene ▪ The material is removed from the sample in the form of microscopic craters
  6. 6. SAWING hacksaw Band saw Wire saw
  7. 7. EDM EDM machine EDM schematic
  8. 8. SECTIONING METHODS Abrasive cutting ▪ Abrasive cutting is the most widely used method of sectioning ▪ Conventional abrasive cutting using consumable wheels is fast, accurate, and economical and most popular ▪ The quality of the cut surface obtained is often superior to that obtained by other methods ▪ Consumable Abrasive Cutting ▪ Abrasive cutting is the sectioning of material using a relatively thin rotating disk composed of abrasive particles supported by a suitable medium ▪ Silicon carbide is preferred for cutting non-ferrous metals and non metals. ▪ Alumina (Al2O3) is recommended for ferrous metals
  9. 9. SECTIONING METHODS ▪ Cutoff wheels with grit sizes from 60 to 120 are recommended for sectioning metallographic specimens ▪ Abrasive-wheel sectioning can produce damage to a depth of 1 mm (0.04 in.) ▪ Control of cutting speed, wheel pressure, and coolant application minimizes damage ▪ Non consumable Abrasive Cutting ▪ Diamond that has been crushed, graded, chemically cleaned, and properly sized is attached to a metal wheel using resin, vitreous, or metal bonding. ▪ Metal-bonded rimlock wheels consist of metal disks with hundreds of small notches (containing many diamond particles) uniformly cut into the periphery ▪ Continuous-rim resin-bonded wheels consist of diamond particles attached by resin bonding to the rim of a metal core suitable for cutting very hard metallics.
  10. 10. ABRASIVE CUTTING Abrasive cutting machine Consumable abrasive wheel Non consumable abrasive wheel
  11. 11. MOUNTING ▪ Small or oddly shaped specimens are mounted to facilitate easy handling during preparation and examination ▪ Standard mounts usually measure 25 mm (1 in.), 32 mm (1.25 in.), or 38 mm (1.5 in.) in diameter ▪ Bakelite and diallyl phthalate are thermosetting resins which are most widely used as moulding material ▪ Transparent methyl methacrylate, polystyrene, polyvinyl chloride (PVC)are some of the thermoplastic resins used in moulding ▪ Both requires heat and pressure during molding. Thermosetting molds can be ejected from the mould at the moulding temperature, while thermoplastic resins must be cooled to ambient temperature under pressure
  12. 12. MOUNTING Mounted specimen Hot mounting press machine
  13. 13. GRINDING ▪ To prepare the cut surface suitably for metallographic examination as optically flat, reflective, smooth and scratch free ▪ Machining ▪ involves the use of tools having cutting edges of controlled shape like sawing, lathe turning, milling, and filing ▪ used only for the preliminary stages of preparation ▪ Grinding and abrasion ▪ uses abrasive particles whose projecting points act as the cutting tools ▪ the abrasive particles are cemented together into a block whose exposed surface is the working surface
  14. 14. GRINDING ▪ Examples are abrasive cutoff wheels, grinding wheels, abrasive laps, and abrasive stones ▪ in another type, a layer of abrasive particles is cemented onto a cloth or paper backing, creating coated abrasive products ▪ Emery papers are typical example for this kind ▪ Grinding employ high surface speeds were heating of the surface layers may occur ▪ Abrasion uses low surface speeds, hence no significant heating takes place
  15. 15. GRINDING WHEELS Mechanized grinding wheels
  16. 16. Typical metallographic preparation procedures ▪ Employ a sequence of machining or grinding stages of increasing fineness, were fineness refers to the use of finer grades of abrasive to produce finer grooves or scratches in the surface ▪ Then sequence of abrasive processes of increasing fineness , Followed by polishing until the desired surface finish has been achieved ▪ Section surface is rubbed by hand against the working surface of an abrasive (usually those coated with silicon carbide ) paper supported on a flat backing surface using successively finer grades of abrasive paper, usually to the finest available ▪ In Mechanized processes The abrasive paper or cloth is attached to the surface of a wheel that is rotated at a comparatively low speed in a horizontal plane. The specimen is held against the working surface of a wheel and rotated slowly in a direction opposite that of the wheel.
  17. 17. POLISHING ▪ Manual polishing is done by rotating specimen by hand against a cloth that has been charged with a fine abrasive and an appropriate liquid, and then has been stretched across a flat backing surface ▪ Diamond, alumina (Al2O3), and magnesium oxide (MgO) are the abrasives most commonly used for polishing ▪ Mechanized processes are less time consuming and laborious than manual operations ▪ The paper or cloth is attached to the surface of a wheel that is rotated at a comparatively low speed in a horizontal plane ▪ The specimen is held against the working surface of a wheel and rotated slowly in a direction opposite that of the wheel
  18. 18. ELECTROLYTIC POLISHING ▪ used widely in the metallography of stainless steels, copper alloys, aluminum alloys, magnesium, zirconium, and other metals that are difficult to polish by conventional mechanical methods ▪ Electrolytic polishing can completely remove all traces of worked metal remaining from mechanical grinding and polishing operations used in specimen preparation ▪ Rough surface is made the anode of a suitable electrolytic cell, and preferential solution of the "hills" or ridges on a rough surface takes place ▪ In molded sample, only the portion of the specimen to be polished should be in contact with the electrolyte ▪ High surface finish can be obtained
  19. 19. ELECTROLYTIC POLISHING 1. Electrolyte 2. Cathode 3. Workpiece to polish (Anode) 4. Particle moving from the workpiece to the cathode 5. Surface before polishing 6. Surface after polishing Principle Machine
  20. 20. ETCHING ▪ ETCHING is used in metallography primarily to reveal the microstructure of a specimen under the optical microscope ▪ It is procedure for achieving contrast in the microstructure ▪ Nondestructive Etching is a type of etching which do not alter the surface of the microsection ▪ Optical Etching uses special illumination techniques to obtain contrast ▪ These optical etching techniques are dark-field illumination, polarized light microscopy, phase contrast microscopy, and differential interference contrast
  21. 21. ETCHING ▪ Destructive Etching induces surface damages. They are classified as 1. Electrochemical (Chemical) Etching ▪ Chemical etching ▪ Oldest and most commonly applied technique ▪ Etchant reacts with the specimen without the use of an external current supply ▪ Etching proceeds by selective dissolution according to the electrochemical characteristics of the constituents ▪ Proper selection of etchant and etching time is the most important criteria which are determined experimentally and is given by ASTM E407 (Standard Practice for Microetching Metals and Alloys) ▪ Microstructure obtained is decided by etchant and etching time
  22. 22. Behavior of different etchants on microstructure Low carbon steel Etched with 2% Nital Etched with 4% Picral Etched with Beraha’s reagent
  23. 23. SOME COMMONLY USED ETCHANTS NAME COMPOSITION USE Nital 2 mL HNO3 and 98 mL ethanol or methanol For carbon steels; gives maximum contrast between pearlite and a ferrite or cementite network Picral 4 g picric acid, 100 mL ethanol or methanol For all grades of carbon steels Beraha's reagent 3 g K2S2O5, 10 g Na2S2O3, and 100 mL H2O Colours ferrite grains Aqua regia 3 parts HCl + 1 part HNO3 Used with austenitic grades to reveal grain structure, outline ferrite and σ phase Glyceregia 3 parts glycerol, 2-5 parts HCl, 1 part HNO3 Popular etch for all stainless grades
  24. 24. ▪ Heat Tinting ▪ The polished specimen is heated in an oxidizing atmosphere ▪ Coloration of the surface takes place at different rates according to the reaction characteristics of different elements ▪ The observed interference colours allow the differentiation of phases and grains ▪ Electrolytic Etching ▪ In electrolytic (anodic) etching, electrical potential is applied to the specimen using an external circuit ▪ During electrolytic etching, positive metal ions leave the specimen surface and diffuse into the electrolyte ▪ Voltage is reduced to approximately one tenth the potential required for electropolishing and then continuing electrolysis for a few seconds Electrochemical (Chemical) Etching cont.
  25. 25. Basic laboratory setup for electrolytic etchingMicrograph produced by Heat Tinting etching
  26. 26. 2. Physical etching ▪ leaves the surface free of chemical residues and offers advantages where electrochemical etching is difficult ▪ Ion etching and thermal etching are two types of physical etching ▪ Ion etching ▪ Ion etching, or cathodic vacuum etching, produces structural contrast by selective removal of atoms from the specimen surface ▪ Done by using high energy ions, such as argon, accelerated by voltages of 1 to10 kV. Individual atoms are removed at various rates, depending on their atomic number, their bonding state, and the crystal orientation of the individual grains ETCHING
  27. 27. ▪ Thermal etching ▪ used in high-temperature microscopy and to etch polished surfaces of ceramic materials ▪ Thermal etching is also partially based on atoms leaving the material surface as a result of additional energy obtained by heating Physical Etching cont.
  28. 28. Microstructure of the ZnO, after thermal etching at 1150 °C, for 1 h, in airSchematic of ion etching process
  29. 29. REFERENCE ▪ ASM Metals Hand Book, 9th edn, Vol 9, Metallography and Microstructures, ASM, Metals Park, (1983),