MME 294 Experiment 1 Metallography Sessional

Background The properties of a metal are a direct consequence of the microstructural features of that metal. These structural patterns vary both with the metals themselves, and with their manufacturing processes.

Background The identification of the microstructural features of a metal is important. The metallurgical microscope is a major tool that is used for the identification of these features of a material. Metallography is the study of the physical structure and components of metals, typically using microscopy. Ceramic and polymeric materials may also be prepared using metallographic techniques, hence the terms ceramography, plastography and, collectively, materialography.

Introduction to metallography The term metallography was first introduced in the early 1700’s Sorby marked the first systematic attempt to discover by the aid of the microscope the distinguishing structural characteristics of different samples of iron and steel. In the beginning preparation of specimens was very labour intensive, preparing a specimen took 5 weeks of polishing.

Metallographic Sample Preparation Need for preparation: to produce a distortion-free, polished surface capable of revealing the true microstructure. A typical preparation sequence and basic equipment for an average laboratory would include: Selection of the sample Sectioning: to produce a manageable size sample Rough Grinding: to remove surface deposits or level irregular surfaces. Mounting: to provide a safe means of holding the specimen and protect its edges from rounding.

Metallographic Sample Preparation Fine Grinding: to systematically abrade the specimen with a series of grits of decreasing coarseness. Polishing: to remove the remaining scratches and produce the smooth lustrous surface required for microscopic examination. Etching: to develop the microstructure not normally visible in the as-polished condition. Inclusions and certain types of defect identification should be performed prior to application of the etchant. Microscopy: to realize the ultimate goal of specimen preparation to observe, analyse, and record the true microstructure of the material. Other tests, such as microhardness, provide additional information as required by the authority requesting the services.

SELECTION AND IDENTIFICATION OF SPECIMEN Only a small piece of material can be placed on the platform of a metallurgical microscope and only a plane or flat section of it can be observed under it. A metallographer should always exercise careful judgement in selecting the section to be observed. With respect to purpose of study the metallographic examination may be divided into three classifications Routine studies Study of failures Research studies

SELECTION AND IDENTIFICATION OF SPECIMEN Routine Studies: A large body of metal may not be homogeneous either in composition or in structure. Specimens from locations that are most likely to reveal the maximum variations within the material under study should be chosen. Sometimes more than one specimen may be necessary to adequately represent the material. For example, for the examination of the structure of a casting, specimens should be taken from the zones wherein maximum segregation might be expected to occur as well from the zones where segregation should be at a minimum.

SELECTION AND IDENTIFICATION OF SPECIMEN Study of Failures: For the identification of the causes of premature failures, the test specimens should be taken as closely as possible to the fracture or to the initiation of the failure. In many cases, specimens should also be taken from a sound area for a comparison of structure and properties. Research Studies: The nature of the study dictates the location of test specimens, orientation etc. Sampling in such a case is usually more extensive than in routine examinations.

SECTIONING OR CUTTING OF METALLOGRAPHIC SPECIMENS The object of sectioning is to extract a specimen of suitable size from the parent metal. Sectioning can have a drastic effect on subsequent preparation techniques. On some occasions the damage is so great that it is impossible to reveal the true structure by grinding and polishing. Heat generated during sectioning has not been dissipated. In cutting metallographic specimens from the main body of the material, care must be exercised to ensure that the structure of the metal is not changed.

The common techniques for sectioning An Abrasive Cut-off Wheel A sample for metallographic examination may be cut from a larger object with a hacksaw. An abrasive cut-off wheel provides the best method of removing a sample from an object. This often produces a surface ready for fine grinding. Depending on the material to be cut, wheels of different compositions may be needed. The hardness and ductility of the material influence the choice of cut-off wheel.

Fault Cause Remedy Wheel does not cut or cutting ceases after a short time Incorrect abrasive Wheel has become blunted Use alternative abrasive or softer grade of wheel Wheel wears rapidly Wheel is too soft Use harder grade of wheel Wheel breaks Excessive cutting force Sample moved during cutting Wheel not clamped securely Reduce cutting pressure. Clamp sample more securely. Tighten wheel more securely.

Sections of Metallographic Samples The specimen for microscopic examination should be in proper orientation. For example, if grain flow or distortion is important, a cross section of the part may not show the elongated grains; only a slice parallel to the direction of rolling would adequately reveal elongated grains from rolling. The locations of surfaces examined should always be given in reporting results and in any illustrative micrographs. Longitudinal Section Transverse Section

Sections of Metallographic Samples The specimens for metallographic examination are generally not more than about 12 - 25 mm square or 12 - 25 mm in diameter, if the material is round. The height of the specimen should be no greater (15 mm in height) than necessary for convenient handling during polishing. Edge Preparation A small bevel or chamfer should be ground on each edge of the surface to be polished to avoid a sharp edge. Bevelling would remove the very areas that you must examine and would thus make the job worthless. For example, examination of a case hardened job, or checking the condition of a galvanized or electrodeposited surface layer, or when looking for decarburisation on the outer layers of steel. When a mounting material such as plastic is available to imbed the specimen the outer edge of the plastic mount should then be bevelled rather than the metal itself. Specimen Bevelling Surface to be Polished and Observed Bevel

MOUNTING Mounting is an encapsulating process that facilitates further processing. The reasons for mounting can be summarised as follows: the specimen is too small or of awkward shape for ease of handling in subsequent stages of the preparation. to support the outermost edge of the specimens surface to prevent damage or rounding during the subsequent grinding/polishing operations. the specimen is of delicate nature Bulk samples usually do not require mounting.

NECESSITY FOR POLISHING AND ETCHING A microscope requires an extremely flat, scratch-free surface on the material to be observed. The angle between a reflected beam and the normal to the reflecting surface is the same as the angle between the incident beam and the normal to that surface. It is also known that something becomes visible to an observer only when light reflected from that thing reaches his/her eyes. Scratch-free horizontal surface incident light reflected back to the observer Flat inclined surface incident light reflected away

NECESSITY FOR POLISHING AND ETCHING Metallic objects are opaque to light. These objects are, therefore, observed under reflected light. If the surface of the metallic object is not perfectly flat or scratch-free light reflected from that surface will not reach the eyes of the observer. A cut surface may be considered to be composed of a large number of very small inclined surfaces. As a result such a surface will not reflect light to the eyes of the observer. That is why the surface of a metallic object has to be made perfectly flat and scratch-free for observation under a metallurgical microscope. Such a surface is generally produced by careful grinding or polishing with consecutively finer abrasives.

NECESSITY FOR POLISHING AND ETCHING A perfectly flat surface, on the other hand, reflects all beams incident on it. A polished surface, therefore, appears bright under a microscope and thus no useful information can be obtained. To reveal the microstructural features of metals and alloys, the polished surface is usually treated with an etchant. An etchant is a very dilute solution of an acid or alkali in alcohol, water or a suitable solvent The phases present in a metallic object vary in composition and energy. When in contact with an etchant, the essential requirements for the formation of a cell are satisfied and corrosion occurs. This etchant acts at varying rates on different parts of the structure depending on local variations in composition within the material being etched. Differential corrosion is a term used to describe this kind of varying chemical action that creates relief on the surface being etched. This produces contrast as some of the incident rays are directed in other directions.

MECHANICAL PREPARATION Mechanical preparation is the most common method of preparing metallographic samples for microscopic examination. Abrasive particles are used in successively finer steps to remove material from the surface of the specimen in order to produce in it a surface that is perfectly flat and scratch-free when viewed under a microscope. According to the roughness of the ground surface the grinding operation is classified as coarse (or rough) grinding, or as fine grinding. Depending on whether the grinding operation is performed in air or in a liquid (water, oil, etc) the process is called dry or wet grinding.

MECHANICAL PREPARATION The first step towards obtaining a perfectly flat and scratch-free surface is to rough grind the face of the specimen on a grinding wheel.

MECHANICAL PREPARATION The samples are then ground manually by grinding on a series of emery papers of progressively finer grade. Emery is a natural abrasive containing 55 – 75 per cent Al2O3 (corundum) the balance is iron oxide (magnetite) and has Mohs hardness of 8.0. The emery papers are made by attaching hard abrasive particles onto papers by suitable glue. In the laboratories these papers are usually laid on a piece of flat glass and held in wooden frames

MECHANICAL PREPARATION In some laboratory the paper containing the coarsest paper is designated as No. 3. Subsequent papers are designated as No. 2, 1, 1/0, 2/0, 3/0 and 4/0. Each frame should be reserved for only one grade of abrasive paper, and each should be isolated from adjacent papers of different grades, by a suitable distance. When all the marks on the surface being polished are running in one direction and all others have been removed, the operator should clean the specimen and his hands and then proceed to the next finer paper. During grinding on the next finer paper, the operator should hold the specimen in such a way that the new, finer set of scratches will be approximately perpendicular to the existing set of scratches. Appearance of Specimen Surface at Successive Stages of Grinding

MECHANICAL PREPARATION The process is continued until grinding on the finest paper (the 4/0 step) is completed. During grinding, the specimen should not be pressed too hard against the abrasive paper, because the heat generated due to friction may change the original structure of the specimen. The specimen should be held flat against the abrasive paper. A Modern Wet-grinding Deck

MECHANICAL PREPARATION At this point it should be emphasized that there are no short cuts to the preparation of a first class specimen. It is quite useless to leave noticeable scratches on the surface of a specimen and hope that these will disappear with polishing or etching. Although considerable time and energy may have been spent grinding, polishing and etching the surface of a specimen, the failure to observe simple precautions will result in crisscross lines that will show up under the microscope, signifying the presence of un-removed scratches. Should this be the case, it will be necessary to go back and start all over again.

Criss-cross Lines due to Improper Grinding

General Rules for Mechanical Preparation If a material is prepared for the first time the samples should be examined after every step under the microscope. This makes it easier to see when preparation defects occur. Each set of polishing scratches must be removed completely, before proceeding to the next finer abrasive. The hands of the operator and the specimen must be cleaned properly between each and every polishing step. . The specimen must not tilt during grinding or polishing. Keep preparation times as short as possible. Unnecessarily long preparation time waste consumables and may even damage the sample, for example with edge rounding and relief. New polishing cloths or grinding discs may need to be “run in” for a short time, or dressed or cleaned before use to give the best results. Old polishing cloths should be thoroughly cleaned before polishing.

POLISHING Like grinding, polishing smoothens the surface of the specimen. The difference between grinding and polishing consists in quality of the surface attainable. In polishing this is always better than grinding. Several techniques of polishing are used, which can be divided into chemical, mechanical or electrolytic methods. Fig. 2.14: Micrograph Showing Scratches Resulting from Insufficient Polishing or Contamination from a Coarser Grit (b) the same Specimen Polished Correctly (a) (b)

POLISHING Chemical polishing is a very simple method by which the ground surface is immersed in an electrolyte. The electrolyte serves as a polishing media. The specimen is moved in the electrolyte for some time. The duration depends on the nature of the metal and the electrolyte. The chemical attack removes the unevenness, and a smooth surface without deformations will result. The mechanical polishing process must be regarded as a sort of very fine machining operation, the material being removed as chips. Alumina (Al2O3) or diamond powders are most common abrasives in polishing. In contrast to the grinding media, the polishing media are used as suspension in water or as pastes, in some cases also as dry powders. They are applied to a suitable polishing surface, usually a cloth, and so are not fixed to a support as are the grinding abrasives.

CLEANING An important requirement in all metallographic work is cleanliness. The impurities originally sticking to the specimen, such as oil, grease, dust etc, and also all the residues of grinding and polishing media from the preceding preparation stages must be removed. All these remains may have harmful effects during the etching and lead to false interpretations during examination under the microscope. The remains of the cleaning medium itself (e.g. water) should also be removed. Therefore, every cleaning procedure includes drying.

CLEANING The simplest cleaning method is rinsing in water with subsequent drying, but ultrasonic cleaning is much more efficient. By this method impurities can be removed from the finest cracks and voids, as found in porous or cracked specimens or between specimens and the mounting medium. In an ultrasonic cleaner the ultrasound waves pass on into the cleaning solvent in the tank and guarantee an effective cleaning of the samples which cannot at all or only insufficiently be cleaned in the usual way. An Ultrasonic Cleaner

ETCHING Mainly aqueous or alcoholic solutions of acids, bases or salts serve as etching mediums. The etching conditions, i.e. the composition of the etching medium, the temperature and the etching time can be varied. On the basis of more than one hundred years of experience there are now thousands of chemical etching recipes summed up in many manuals. For pure metals and single phase alloys, a potential difference exists between grains of different orientations, between grain boundaries and grain interiors, between impurity phases and the matrix. For multiphase alloys, a potential difference also exists between the various phases present. These potential differences alter the rate of attack, thus revealing the microstructure when chemical etchants are used. During etching, the more electropositive (anodic) phase is attacked while the more electronegative (cathode) phase is not attacked appreciably.

ETCHING Before etching it must be ensured that the specimen is clean and dry. In general wiping the surface with moist cotton under running water is adequate, although ultrasonic cleaning, especially if cracks or pores are present, is preferable. The specimen must be dry because the presence of moisture can readily affect the chemical behaviour of some etchants, particularly those in alcoholic solutions such as nital and picral. For etching, a small amount of the proper etchant is poured into a dish. The dry and clean specimen is then immersed, facedown, in the solution. The surface of the specimen should be kept completely covered with the etchant throughout the etching period The progress of the etching action, as it turns the shiny mirror into a slightly cloudy mirror, should be watched carefully and closely. The time required to etch a specimen varies with different metals. Some metals, such as bronzes, can be etched in a few seconds while some stainless steels may take much longer time.

Method Description and Remarks Immersion etching The specimen surface is immersed in the etching fluid. Most common method. Drop etching A drop of the etchant is placed on the specimen. Methods used with expensive etching solutions Wash etching The specimen surface is rinsed with the etching solution. Used in case of big specimens Alternative-immersing etching The specimen is immersed alternately in two solutions.

Characteristics and Uses Type of Etchant Composition Characteristics and Uses Nital 2 ml HNO3 98 ml Alcohol General etchant for irons and steels. For pure iron and wrought iron the concn of HNO3 may be raised to 5 ml. Also suitable for ferritic gray cast irons and black-heart malleable irons. Picral 4 g Picric acid 96 ml Alcohol The most suitable reagent for all cast irons, with the exception of alloy and completely ferritic cast irons. Alkaline sodium picrate 2 g Picric acid 25 g NaOH 100 ml. water Its main use is to distinguish between ferrite and cementite. The latter is stained black, but ferrite is not attacked.

General Rules for Etching Time of etching should be as short as possible. An under-etched specimen can be etched again but a over-etched samples must be polished again. Etchants should be free of moisture, especially if the solvent in them is alcohol. Etchant should be rinsed-off immediately after etching has made the surface of the specimen slightly cloudy Immediately after rinsing off the etchant, the etched surface should be dried with a powerful, dry puff of air. The polished, or polished and etched, surface of the specimen should never be touched with fingers.