Physical Met-I Lab Manuals
Physical Met-I Lab Manuals
LIST OF EXPERIMENTS
1.
AN INTRODUCTION TO METALLOGRAPHY
2.
INTRODUCTION TO METALLURGICAL MICROSCOPE
3.
SAMPLING & CUTTING OF METALLOGRAPHIC SPECIMENS
4.
METALLOGRAPHIC SAMPLE PREPARATION
5.
MACRO EXAMINATION OF FORGED SPECIMENS
6.
MACRO EXAMINATION OF Zn-Al CAST BLOCK
7.
MEASUREMENT OF ASTM GRAIN SIZE ON THE SUPPLIED STEEL SAMPLES
8.
METALLOGRAPHY OF PLAIN CARBON STEELS
9.
METALLOGRAPHY OF CAST IRONS
10. MICROSTRUCTURAL STUDY OF COPPER AND COPPER BASE ALLOYS 11. MICROSTRUCTURAL STUDY OF ALUMINUM AND ALUMINUM BASE ALLOYS
Physical Met-I Lab Manuals
AN INTRODUCTION TO METALLOGRAPHY METALLOGRAPHY: Metallography or microscopy consists of the microscopic study of the structural characteristics of a metal or an alloy. The microscope is by far the most important tool of a metallurgist from both scientific and technical standpoints. It is possible to determine the grain size and the size, shape & distribution of various phases & inclusions which have a great effect on the mechanical properties of the metal. The microstructure will reveal the mechanical & thermal treatments given to the metal, and it may be possible to predict its expected behavior under a given set of conditions.
Physical Met-I Lab Manuals
GRAIN: An individual crystal in a polycrystalline metal or alloy.
GRAIN
GRAIN
GRAIN BOUNDARY: The boundary b/w two grains is known as grain boundary.
GRAIN BOUNDARY
GRAIN BOUNDARY
Physical Met-I Lab Manuals
GRAIN SIZE: For metals, a measure of the areas or volumes of grains in a polycrystalline material, usually expressed as an average when the individual sizes are fairly uniform. Grain sizes are reported in terms of number of grains per unit area or volume, in terms of average diameter, or as a grain-size number derived from area measurements.
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POLYCRYSTALLINE MATERIALS: Those made up of aggregate of individual crystals i.e., all structural metals.
SINGLE CRYSTAL MATERIALS: Those made up of only one crystal e.g., single crystal of silicon. These are produced for special applications.
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PHASE: A physically homogenous and distinct portion of a material system.
BRIGHT PHASE
DARK PHASE
POLY- HASE
SINGLE PHASE
INCLUSIONS: Non-metallic materials in a solid metallic matrix are called inclusions.
INCLUSIONS
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MECHANICAL PROPERTIES: The properties of a material that reveal its elastic and inelastic behavior when force is applied, thereby indicating its suitability for mechanical applications; for example tensile strength, elongation, hardness, toughness, ductility, modulus of elasticity, yield strength etc.
MICROSTRUCTURE: The structure of polished and etched surface of metals as revealed by microscope at magnification greater than ten diameters.
MACROSTRUCTURE: The structure of metals as revealed by examination of the etched surface of polished specimen at a magnification not exceeding ten diameters. Al-Cast block
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Forged sample
Forged sample
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INTRODUCTION TO METALLURGICAL MICROSCOPE Overview: Metallurgical microscope unlike biological microscope works on the principle of reflected light microscopy. The goal of this current exercise is to familiarize the students with the important tool of Physical Metallurgy and get them accustomed with common terminology and principles.
Theory: The metallurgical microscope is an important tool for metallurgists, with the help of which fine structural details can be studied. Since the metallographic specimens are opaque to light, the sample must be illuminated by reflected light. The metallurgical microscope is the therefore based on reflected light principle. It has to be appreciated that the biological microscope works on transmitted light principle.
METALLURGICAL MICROSCOPE
Physical Met-I Lab Manuals
Image Formation: Figure-1 a ray diagram showing the principle of image formation in the microscope. The essential components are two convex lenses, called the objective and eyepiece respectively. The object under examination O, is placed in front of the objective at a distance greater the focal length fo, but less than 2fo. A real inverted and enlarged image I1, is formed at a distance greater than 2fo from the objective on the side remote from the object. The distance between the objective and the eyepiece is made such that the image I1 is at a distance of less than fe from the eyepiece, where fe is the focal length of eyepiece. The eyepiece forms a virtual, upright and enlarged image I 2 of I1, and it is this final image that is seen by the eye. The surface of the specimen is illuminated with light passing vertically down through the objective. The principle of vertical illumination is illustrated in figure-2 where light from a light source is reflected down by a half silvered plane mirror held at an angle 45° to the axis of the microscope. The light then passes through the objective onto the specimen. Light reflected by the specimen is gathered by the objective which passes through the half silvered plane mirror to form the image, which is viewed with the eyepiece.
Figure-1. Ray diagram showing how the image is formed in a compound microscope.
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Figure-2. Vertical illumination of a metallographic specimen.
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Magnification of Microscope: There are two types of lens systems in metallurgical microscope (i) Objective & (ii) Eyepiece. The initial magnifying power of objective & eye-piece is engraved on the lens mount. When a particular combination of objective & eye-piece is used at the proper tube length, the total magnification is equal to the product of the magnifications of the objective & eye-piece.
Functions of Objective & Eye-Piece: Objective lens is used to resolve the microstructural details of the surface of a polished & etched metallic specimen. Eye-piece is used to magnify the resolved details of the microstructure. It does not further resolve the microstructural details.
Resolving Power of Objective: The resolving power of a microscope is usually dictated by the resolving power of objective as the light first enters it and then is passed onto the eyepiece. The resolving power of an objective is defined as its ability to reveal closely adjacent structural details as actually separate and distinct. Quantitatively, following relation gives the resolving power: Resolving power = μsinα/ λ Where, μ = refractive index of the medium in front of the objective. φ = half the angle of aperture. λ = wavelength of length being used. The smallest distance δ that may be resolved is controlled by three factors: 1. The wavelength of the illumination λ 2. The effective aperture of the objective lens 3. Refractive index of the medium b/w the lens & the specimen The following relation is followed: δ = 0.5 λ / Numerical Aperture = λ / 2 µ Sinα
Physical Met-I Lab Manuals
Numerical Aperture = µ Sinα Resolving Power = N.A / λ = µ Sinα / λ 1. Wavelength of illumination: Wavelength has got an inverse relationship with resolving power. So by decreasing the wavelength of light, which is used for specimen surface illumination, we can increase the resolution of microscope. Resolving Power = N.A / λ = µ Sinα / λ We know that visible light is a combination of seven (7) colors. Violet Indigo Blue Green Yellow Orange Red
Violet light has the least wavelength & highest frequency in the visible light range. Red light has the highest wavelength & least frequency in the visible light range. Green light has intermediate value of wavelength & frequency as well. So Violet light gives very good microscopic resolution, while the red light gives the poorest microscopic resolution. But if we use green light, it will give us good resolution. In order to use the lights of different colors, glass filters of different colors are placed in the way of ordinary light of illuminating device. 2. Effective Aperture of Objective Lens: Numerical aperture is the light gathering power of an objective lens Numerical Aperture = µ Sinα It is clear from the above formula that the numerical aperture of objective lens depends upon (i) the refractive index of the medium b/w the objective and the specimen, (ii) the
Physical Met-I Lab Manuals
semi apex angle (α) of the light cone defined by the most oblique rays collected by the lens.
If angle α is more, then more number of light rays from the object will be able to enter the objective lens after reflection and as a result good resolution will be obtained. N.A or the angle α can be increased by using the objective lens of shorter focal length or by decreasing the working distance b/w the objective lens & the specimen. 4. Refractive Index of the Medium B/W the Objective & the Specimen: The normal medium present b/w objective lens and the specimen is air having ref. index = 1. This medium can be replaced with wood oil (ref. index = 1.5) or monobromonaphthalene (ref. index = 1.66). As a result number of rays of reflected light accepted by the front lens of the objective is increased and resolution and contrast are improved (N.A is increased). Actually, these mediums bend the light rays towards objective lens, therefore those light rays which are not going into the objective lens are also bent towards the objective lens and due to this more no. of light rays are accepted by objective lens.
Physical Met-I Lab Manuals
SAMPLING & CUTTING OF METALLOGRAPHIC SPECIMEN
Physical Met-I Lab Manuals
Overview: Cutting is very frequently employed method of sampling metals. The ability of the metallurgist to diagnose the desired problem in metals or to get required features out of a metallic specimen depends strongly upon a well polished cutting practice. A wrongly cut sample may be a complete waste especially in the studies related to fractures and cracks. The goal of this exercise is to have a thorough practice of metal cutting. Though some force is required in cutting but under usual circumstances it is less important that how hard you cut than that how well and accurate you cut.
Procedure: The students shall be provided with different metallic components from which the metallographic specimen shall be taken out by cutting along the most suitable plane. The object of cutting shall be to decide the suitability of a sectioning plane with respect to the requirements in the metallographic examination. The various samples shall include cast, rolled, forged, welded, fractured, etc.
Consideration in Sampling: The plane along which the specimen is to be sectioned is entirely defined by the aim for which the metallographic examination is being conducted. For example in a rolled or forged sample, if the object is to show the grain flow, the longitudinal section should be examined. Cast or annealed metals should be examined over the entire cross section so that differences of structure from outer edges to the interior may be observed. In fractured components if the goal is to determine the cause of failure, the specimen should be cut from that particular region of the failure, where it will reveal maximum amount of information. In carburized and decarburized samples, if the aim is to examine the case depth then special attention should be paid to the examination of the edges of the specimen. Hand hack-saw
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Specimen in vise
Automatic hack-saw
Cutting Process
Abrasive cut-off wheel
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Sampling of a rolled bar
METALLOGRAPHIC SAMPLE PREPARATION
Physical Met-I Lab Manuals
Overview: Sample preparation is more like an art. It involves Grinding, Mounting, Polishing and Etching of a metallic sample to get a metallographic sample suitable to be studied under a metallurgical microscope. Although now-a-days this job has been much simplified by the introduction of mechanical polishing machines, but to become a good metallographer one still needs to start from manual polishing. Students who are new to this exercise will find it hard to polish a good sample, but there are no shortcuts, it will take as much time and efforts as it always take. However once students get accustomed with this exercise, they will find it much easier. This is an extremely important exercise and a sound metallurgist must go through with this crude exercise more than once. But to get expertise in this, one must consider it an art and delicacy.
Procedure: The students shall be provided with different metallic components from which the metallographic samples shall be taken out by cutting along most suitable plane. These samples shall then be prepared for metallographic inspection. The metallographic preparation would constitute first producing a flat, scratch free, mirror like surface and then performing the etching of this surface to obtain a phase contrast. The various steps involved in the preparation of samples are as follows: 1. Rough Grinding: A soft sample may be made flat slowly moving it up and down across the surface of smooth file. The soft or hard sample may be rough ground on a belt sander or abrasive wheel but the specimen must kept cool by frequent dripping in water. Coarsest File
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Coarse File
Fine File
Grinding Operation
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2. Mounting: The specimens which are small or awkwardly shaped should be mounted (embedded in a support or holder) to facilitate the handling during metallographic preparation. Wires, small rods, sheets, thin sections etc. must be appropriately mounted in suitable materials or rigidly clamped in a mechanical mount. For mounting the most commonly used material is bakelite. The procedure for mounting in bakelite is pretty simple. The specimen is put in a cavity and rest of which is filled with uncured bakelite powder. The cavity is then closed and kept at suitable temperature and pressure for a suitable period of time after which the mounted sample is removed from the cavity.
(Mounting)
Mounting Press
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3. Fine Grinding: After mounting, the specimen is fine ground on a series of emery papers of successively finer abrasive sizes. The intermediate polishing operations are usually done dry but lubricant may be used to prevent overheating of samples, smearing of soft metals and also to provide rinsing action to flush away surface removed products so the paper will not be clogged.
4. Final Polishing: The final approximation to a scratch free flat surface is obtained by the use of wet rotating wheel covered with the special cloth that is charged with carefully sized abrasive particles. A wide range of abrasive used for final polishing includes diamond paste, aluminum oxide, chromium oxide and magnesium oxide. The final polishing is normally carried out in two or three steps (i.e. on successively finer abrasive). The time consumed and the success of the polishing depends largely upon the care that was exercised during the previous polishing steps.
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5. Etching: The purpose of etching is to make visible the structural features of the metals and alloys. This is accomplished by way of preferentially attacking certain structural features with the help of suitable chemical reagent. Table – Etching Phenomena conditioned by structure. Etching Phenomena
Causes and notes
Grain boundary etching
Grain boundaries are important local disturbances of crystal lattice and are easily dissolved. The effect is increased because impurities accumulate in the grain boundaries.
Grain surface etching
The crystal surfaces show different reflection on account of reaction products or dull surfaces.
Crystal figure etching (dislocations etching)
Etching of dislocations depends on their field of tension covering several atomic distances. Crystal figure (etching grooves) turn up where the dislocations cut the surface. With cubic metals cubical flats appear.
Stress etching
Shows whether deformed fields are found beside undeformed. (Stress effect figure).
Table – Some common metals and their etchants. Metal
Micro Etchant
Macro Etchant
Iron, Steel and Cast iron
Nital – 1-10 % HNO3 in Methanol
50 ml H2O 50 ml HCl
Aluminum & Alloys
20 % H2SO4 in Water 20 % HF in Water
10-20 g NaOH in 100 ml Water
Copper & Copper-base Alloys
10 % Ferric Chloride in Water 10 % Ferric Nitrate in Water
50 ml HNO3 50 ml H2O
80 ml HNO3 3 ml HF
50-65 % HNO3 in Water
Nickel
Note: Echants for Lead, Zn, Mg, Chromium etc. are available in related books.
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Polished Surface
Etching of single Phase Alloy or Pure Metal
Etching of poly-phase Alloy
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Etching of twins 6. Washing: After the completion of each step of cutting, grinding, rough polishing, intermediate polishing, final polishing and etching, the specimen is washed with water and foam, rinsed in methanol and then dried with a drier.
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MACRO EXAMINATION OF FORGED SPECIMENS Overview: Macroscopic study of the metals involves study of the polished and etched metallic sample at lower resolution usually with the naked eye or with an aid of magnifying glass. It is used to study material flow on macroscopic level after a subsequent mechanical forming operation. The most common application is the macroscopic study of forged samples to reveal metal flow lines. Other applications include the study of the cast components and welded structures. The goal of this exercise is to familiarize the students with polishing, etching and examination of metallic components for macro examination.
Theory: Macro Examination: Macro examination is a useful and convenient method of studying the coarse details of coarse grained structure of the metallic components. The various types of components studied by this method include cast components, forged components and welded structures. Components sectioned along suitable planes are prepared by grinding up to the stage of fine grinding and are then macro etched. The macro etched surface is then examined at low magnification i.e. either with the naked eye or with a magnifying glass. Hot Forgings: Hot forged components are made by deforming a metal block that has been heated to a temperature level where the material has the ability to plastically deform without work hardening. During work hardening the plastic flow of the material follows the contour of the die so as to eventually fill up the entire die cavity. Thus the original grain structure of the metal block is modified such that a grain flow pattern consistent with the direction of the material flow during forging may be observed on suitably sectioned and macro etched sample.
Procedure: In this experiment the students shall be provided with hot forging sectioned along suitable plane. The students shall be required to prepare the surface by fine grinding followed by macro etching. The grain flow pattern shall be examined and interpreted in terms of the pattern of material flow during forging as well as the original structure of the metal block.
Physical Met-I Lab Manuals
Physical Met-I Lab Manuals
MACRO EXAMINATION OF CAST BLOCKS Overview: Macro examination is a useful and convenient method of studying the coarse details of coarse grained structure of the metallic components. It may be used for cast components to reveal chilling, columnar and equiaxed zones. The cast sample is sectioned along suitable plane ground, polished and macro etched and then studied by naked eye or suitably under a magnifying glass.
Theory: Macro Examination: Macro examination is a useful and convenient method of studying the coarse details of coarse grained structure of the metallic components. The various types of components studied by this method include cast components, forged components and welded structures. Components sectioned along suitable planes are prepared by grinding up to the stage of fine grinding and are then macro etched. The macro etched surface is then examined at low magnification i.e. either with the naked eye or with a magnifying glass.
Procedure: The students shall be provided with cast blocks made from Zn and Al. The students shall be required to prepare the surface by fine grinding followed by macro etching. Features like general grain structure, columnar grains (if any) formed along the casting surface and shrinkage porosity / cavities shall be studied. These observations shall then be interpreted to obtain information about the comparative cooling rates in various portions of the casting and the portion last to solidify.
Physical Met-I Lab Manuals
Physical Met-I Lab Manuals
MEASUREMENT OF ASTM GRAIN SIZE ON THE SUPPLIED STEEL SAMPLES
Overview: The metallographic exercise so far involved qualitative study of the metallic samples; however, there are certain applications when an accurate quantitative representation and/or measurement of microstructure is required. The goal of this exercise is to allow the students to grasp an understanding of the technique used to measure the grain size of steel sample using ASTM standard E112. ASTM E112 Grain Numbers start from 00, 0, 1, 2, 3, ……. and so on. This only contains the average grain size in any microstructure; however, there exists another ASTM standard termed as ALA (as large as) which contains the grain number of the largest grain observed on any microstructure.
Procedure: A number of the prepared metallographic specimens having different grain sizes shall be provided to the students. The microstructure of the specimens shall be projected on the projection screen of the microscope at a magnification of 100X. The grain size as viewed on the projection screen of the microscope shall be compared with ASTM standard grain size charts and reported as the ASTM grain size number. ASTM Comparative Method: The ASTM standard grain size charts are indexed from 00 to 10, each index number representing some mean number of grains per square inch at a magnification of 100X according to the following relation: N = 2n - 1 Where, N = mean number of grains per square inch at 100X. n = ASTM grain size number or index. The method is essentially one of the comparisons in which the image of the microstructure projected at a magnification of 100X, or a photograph of the structure at the same magnification is compared with the graded standard grain size charts. By trial and error method a match is secured and the number corresponding to the index number of the chart designates the grain size of steel. Steels showing a mixed grain size are graded in similar manner and it is customary in such cases to report grain size in terms of two numbers.
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Samples
ASTM Grain Size obtained by comparison (n)
Average number of grains per square inch obtained by counting grains on the image at 100X. (N)
Calculated value of n (N = 2n-1)
Sample 1 Sample 2 Sample 3 Table – ASTM Grain Size. ASTM No. (n) (Grain Size Index) 1 2 3 4 5 6 7 8
Number of Grains / in2 at 100 X (N) Mean Range 1 0.5-1.5 2 1.5 – 3 4 3–6 8 6 – 12 16 12 – 24 32 24 – 48 64 48 – 96 128 96 – 192
Physical Met-I Lab Manuals
Physical Met-I Lab Manuals
METALLOGRAPHY OF PLAIN CARBON STEELS Overview: Plain carbon steels are alloys of iron and carbon in which carbon varies from traces to about 2 % by weight. Carbon in the steel forms cementite (Fe 3C), a hard brittle phase, it both strengthens it and renders it amenable to heat treatment. Commercial steels, in addition to carbon, contain elements such as manganese, silicon, sulphur and phosphorus. These elements are always present in all steels. Sulphur and phosphorus are highly detrimental and are treated as undesirable elements and these are usually kept below 0.04 %. The goal of this exercise is to study microstructure of various plain carbon steel classes and provide enough exposure to students so that they may learn to differentiate between these classes in future.
Procedure: The students shall be provided with prepared metallographic samples of steel for microstructural study. The three basic types of steel would be differentiated on the basis of the phase morphology. The phase present in the microstructure shall also be identified. There are three main classes of plain carbon steels according to the percentage of carbon: 1. Low Carbon Steels (0.05 – 0.25 % Carbon) 2. Medium Carbon Steels (0.25 – 0.65 % Carbon) 3. High Carbon Steels (0.65 – 1.7 % Carbon) 1. Low Carbon Steels: Steels with carbon varying from 0.05 to 0.25 percent are referred to as low carbon steels. The microstructure of low carbon steel consists of two phases, black and white. The black phase is pearlite and white phase is ferrite. The quantity of ferrite is more in low carbon steels than pearlite. 2. Medium Carbon Steels: Steels with carbon varying from 0.25 to 0.65 percent are referred to as medium carbon steels. The microstructure of medium carbon steels consist of two phases, black and white. The black phase is pearlite and white phase is ferrite. The quantity of pearlite is more than in low carbon steels. The quantity of pearlite is either nearly equal to ferrite or more than ferrite depending upon carbon contents of steel.
Physical Met-I Lab Manuals
3. High Carbon Steels: Steels with carbon varying from 0.65 to 1.7 percent are referred to as high carbon steels. The microstructure of high carbon steels consists of two phases, black and white. The black phase is pearlite and white phase present along the grain boundaries is cementite. Cementite forms a network along the grain boundaries.
Eutectoid Steels: Eutectoid steels contain 0.8 % carbon. In the phase diagram of steel at 0.8 % carbon, eutectoid reaction occurs and results into a eutectoid mixture of ferrite and cementite known as pearlite. So the microstructure of eutectoid steel consists of pearlite only, which appears black under microscope.
Very low carbon steel
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0.25% Carbon steel
0.5% Carbon steel
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0.8% Carbon steel
1.3% Carbon steel
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METALLOGRAPHY OF CAST IRONS Overview: Cast iron is an important family of iron carbon alloys. These have carbon composition much higher than that is in the range of 2.0 to 6.67 %, with some other alloying elements like 1 to 3 % silicon etc. Due to high carbon composition the extra carbon usually exists as graphite phase or as a high fraction of cementite. Cementite is a hard and brittle phase, while graphite is soft and brittle. This makes cast iron less suitable for structural applications. Yet it is an ideal metal for casting due to its flowability, expansion on solidification, energy damping capacity, large mass etc. Expansion on solidification is due to the volumetric expansion during graphite formation. From foundry point of view, it is one of the ideal metals for the foundrymen. The goal of this exercise is to study microstructure of various cast iron classes and provide enough exposure to students so that they may learn to differentiate between these classes in future.
Procedure: The students shall be provided with prepared metallographic samples of different types of cast irons for micro-structural study. These cast irons would be differentiated on the basis of the phase morphology. The phases present in the microstructure shall also be identified. The main types of cast irons are white, grey, malleable, nodular, chilled and alloyed cast irons. White Cast Irons: White cast iron is quenched from solidus range to obtain pearlite matrix and cementite. They don’t have any free graphite and all the carbon is present in combined form. The microstructure consists of dark etched areas which are primary dendrites of austenite (pearlite), and a white etched interdendritic network of cementite. Gray Cast Irons: Gray cast irons have graphite flakes in ferritic or pearlitic matrix, leading to grey fracture. The graphite flakes appear as dark constituents distributed in ferrite and pearlite or mixed matrix. The size of flakes may vary with the composition and cooling rate. Malleable Cast Irons: Malleable cast irons are obtained by the heat treatment of solidified Gray Irons to change the morphology of graphite flakes into roughly rounded temper carbon. In the microstructure, temper carbon appears as dark irregular rounded constituents distributed in the ferritic or pearlitic matrix.
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Ductile Cast Irons: Nodular or ductile cast irons are alloyed with inoculants like magnesium or cerium to obtain graphite in spherical morphology. The microstructure consists of dark etched regular sphere of graphite in ferric, pearlitic or mixed matrix. Chilled Cast Irons: Chilled cast irons are molded by using chills which create white cast iron in the surface and grey cast iron in the core.
White Cast Iron
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Fully transformed ledeburite in white cast iron
Grey cast iron
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Malleable cast iron
Spheroidal graphite iron
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MICROSTRUCTURAL STUDY OF COPPER AND COPPER BASE ALLOYS
Overview: Copper and its alloys belong to one of the oldest families of metals. The goal of this exercise is to study the microstructures of pure copper, brasses, and bronzes. The students are required to observe the number of phases, their distribution, features like twins and other discontinuities like oxides, blow holes, etc.
Procedure: The students shall be provided with the prepared metallographic samples of copper and copper base alloys and would be required to study the microstructural features such as: Number of phases present and their relative distribution. Annealing twins if any present in alpha grains Any other features like oxides, blow holes, etc. Copper: Pure copper is a single phase material containing grains of alpha phase (FCC Copper). The grains usually contain twins and oxides. The oxides are more clearly visible in unetched specimens, in which dark regions of Copper-Copper Oxide eutectic can be seen. Twins can be examined in etched specimen, in which the contrast is derived from the differential etching of crystals at different orientations. Alpha Brass: Alpha brasses are Cu-Zn alloys, in which Zn is only present as solid solution. The microstructure thus consists of grains of copper rich alpha solid solution. Annealing twins are invariably seen in the alpha grains. Alpha + Beta Brass: Alpha + Beta brasses are the alloys of Cu-Zn in which the amount of Zn is in excess of the limit of solubility of Zn in alpha copper. The microstructure therefore contains grains of both alpha and beta phases, where alpha is the solid solution and beta is an intermetallic compound. Annealing twins may also be seen within alpha grains. Aluminum Bronzes: Aluminum bronzes are copper base alloys containing aluminum as a main alloying element. The usual aluminum content is about 7-10 %. The microstructure essentially consists of alpha grains of copper rich solid solution (FCC) along with some dark etched beta phase present at the grain boundaries.
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Pure tough-pitch copper with oxygen impurity system
Pure copper showing twins
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Alpha brass showing annealing twins
Alpha + Beta brass (Beta phase is dark and alpha phase is bright)
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Aluminum Bronze (Bright phase is alpha & granular dark eutectoid)
Aluminum Bronze (Bright phase is alpha & granular dark eutectoid)
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MICROSTRUCTURAL STUDY OF ALUMINUM AND ALUMINUM BASE ALLOYS
Overview: Aluminum and its alloys are an important family of non-ferrous metals. During this exercise we will study pure aluminum and aluminum silicon alloys. The main goal is not just to familiarize the students with the microstructure of the non-ferrous family but also to provide them skills to study discontinuities like oxides, blow holes, porosity and insoluble particles as these are quite frequent in this family.
Procedure: The students shall be provided with the prepared metallographic samples of aluminum and aluminum base alloys and would be required to study the microstructural features such as: Number of phases present and their relative distribution. The insoluble particles or constituents. Any other features like oxides, blow holes, etc. Pure Aluminum: Commercial purity of aluminum varies from 99.3 % to 99.7 %. A relatively pure aluminum matrix characterizes the structure of unalloyed aluminum. The insoluble constituents seen in commercially pure aluminum are chiefly iron and silicon. Cold rolled structure of pure aluminum shows metal flow around insoluble particles of FeAl 3 (black). Particles are remnants of script like constituents in the ingot that has been fragmented by working. Aluminum Silicon Alloys: The Al-Si system is a simple eutectic composition at 12.6 % Si. The most important commercial binary Al-Si alloys contain 5.3 % Si and 12.6 % Si. Microstructure of 5.3 % Al-Si alloy consists of dendrites of pure aluminum. The spaces between these dendrites are filled with aluminum-silicon eutectic. Microstructure of 12.6 % Al-Si alloy consists of aluminum-silicon eutectic. Aluminum Copper Alloys: Al-4%Cu system is a precipitation hardenable alloy.
Physical Met-I Lab Manuals
The microstructure of Aluminum-5%Cu alloy consists of alpha soild solution of aluminum and copper along-with second phase particles of ө-Phase after precipitation hardening.
Aluminum-4% Cu alloy (Precipitation hardened)
Aluminum-12% Si alloy (Al-Si Eutectic)