metallographic specimen preparation
DESCRIPTION
Metallographic specimen preparation for metallographic observation is explained in detail. All the steps involved with detailed images help to understand the subject thoroughly.TRANSCRIPT
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METALLOGRAPHIC
SPECIMEN PREPARATIONMuhammed Labeeb
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SAMPLE PREPARATION
▪ Consists of five major steps
▪ Sectioning
▪ Mounting (optional)
▪ Grinding
▪ Polishing
▪ Etching
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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
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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
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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
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SAWING
hacksaw Band saw Wire saw
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EDM
EDM machine
EDM schematic
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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
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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.
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ABRASIVE CUTTING
Abrasive cutting machine
Consumable abrasive wheel
Non consumable abrasive wheel
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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
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MOUNTING
Mounted specimen Hot mounting press machine
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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
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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
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GRINDING WHEELS
Mechanized grinding wheels
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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.
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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
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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
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ELECTROLYTIC POLISHING
1. Electrolyte2. Cathode3. Workpiece to polish (Anode)4. Particle moving from the workpiece to the cathode5. Surface before polishing6. Surface after polishing
Principle Machine
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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
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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
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Behavior of different etchants on microstructure
Low carbon steel
Etched with 2% Nital Etched with 4% Picral Etched with Beraha’s reagent
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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
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▪ 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.
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Basic laboratory setup for electrolytic etchingMicrograph produced by Heat Tinting etching
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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
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▪ 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.
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Microstructure of the ZnO, after thermal etching at 1150 °C, for 1 h, in airSchematic of ion etching process
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REFERENCE
▪ ASM Metals Hand Book, 9th edn, Vol 9, Metallography and Microstructures, ASM, Metals Park, (1983),