Lab1 Microscopic Examination 3f2p20

UNIVERSITY OF TECHNOLOGY, JAMAICA School Of Engineering

Name: Lorenzo Thompson I.D.#: 0906455 Programme: Mechanical Engineering Course: Engineering Materials Lab Experiment #1: Microscopic Examination of Metals Date: September , 2012

Abstract: Proper preparation of metallographic specimens to determine microstructure and content requires that a rigid step-by-step process be followed. In sequence, the steps include sectioning, mounting, course grinding, fine grinding, polishing, etching and microscopic examination. Specimens must be kept clean and preparation procedure carefully followed in order to reveal accurate microstructures. Each student will prepare and examine a brass, steel or aluminium sample for metallographic examination. Additionally, a brass, steel or aluminium sample with different composition, a eutectoid steel sample and a eutectoid Pb-Sn sample will be examined under the metallographic microscope. Photographs will be taken of the samples and the ASTM grain size number determined for each of the specimens if applicable.

Table of Contents Nomenclature ………………………………………………………………………………………… 1

Objective …………………………………………………………………………………………………2

Theory …………………………………………………………………………………………………….2

Apparatus ……………………………………………………………………………………………….3

Procedure ……………………………………………………………………………………………….4

Result ……………………………………………………………………………………………………..5

Discussion ………………………………………………………………………………………………6

Conclusion ……………………………………………………………………………………………..6

Reference ……………………………………………………………………………………………….7

Nomenclature : 

Abrasive Paper: a hard wear-resistance material that is used to grind, cut or wear away other materials.

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Ductile: property that refers to the malleability of the metal or material. Grain: an individual crystal in a polycrystalline metal or ceramic.

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Etchant: acidic chemical used to cut into the unprotective surfaces of exposed metal.

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Grain boundary: this is the interface separating two ading grains having different crystallographic orientation, size or character.

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Grain size: the average grain diameter as determine from a random cross section.

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Grinding: the act or continuously cutting, or ‘wearing’ away the surface of material to produce a smooth surface or surface with less abrasions.

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Hardness: property of metal that measure its resistivity to deformation, scratches or abrasions.

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Toughness: a measure of the amount of energy the material absorbs as it fracture.

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Mild steel: carbon steel with a maximum of about 0.25% carbon.

Objective: To examine the microscopic structure of metals and to identify the microstructure of typical ferrous metals.

Theory: The details of the structure of most metals are not readily visible to the naked eye but may be seen with the aid of a metallurgical microscope. In modern metallurgical laboratories, image analysis systems are utilized to analyse microstructures. The image is displayed on a screen and can be digitally saved to a disc. Operations such as volume fraction of phases, grain size distribution and grains per unit area can also be analysed. Ferrous: In metallurgy these metals which are primarily made of iron and other metals. Non-ferrous: In metallurgy, a non-ferrous metal is any metal that is not ferrous, including alloys, that does not contain iron in appreciable amounts Phase diagram: A phase is a type of chart used to show conditions at which thermodynamically distinct phases can occur at equilibrium. phase diagram. The percentage of carbon present and the temperature define the phase of the iron carbon alloy and therefore its physical characteristics and mechanical properties. The percentage of carbon determines the type of the ferrous alloy: iron, steel or cast iron

The iron–iron carbide (Fe–Fe3C)

Apparatus: Metallurgical equipments for grinding, polishing and etching. Diagrams showing the grinder and polisher, Emery cloth and paper

Diagram showing metallurgical microscope used for observation of the specimen

Diagram showing mounting press and mounting powders

Procedure: For examination of the specimen a small cut was made and a plane surface free from any irregularities was prepared by a method of grinding, this was performed by starting with emery paper no.1 and going on successively finer grades such as 240, 320, 400 and 600. It is important that the specimen must be cleaned during transfer to successively finer abrasive papers to prevent carry-over of coarse abrasive particles. The surface was lubricated with water to keep it cool and to remove the grinding products. The sample was moved forward and backward on the paper until the whole surface is covered with unidirectional scratches. It was then washed with running water to remove debris associated with the grade of paper used and then grounded on a next finer paper such that the scratches produced are at right angles to those formed by the previous paper. This enables you to easily decide when the scratches from the coarser paper have been completely removed. This procedure is repeated through the range of papers available. The specimen was then polished and this was done using rotating wheels covered with a cloth impregnated with a very fine abrasive compound until a mirror finish surface was obtained. At this stage a microscopic examination was done to check for the presence of any cracks, seams, non-metallic inclusions. The specimen was examined in the “as polished” condition to assess the quality of polishing and to observe any features showing contrast. After the microscopic examination the specimen was then etched by a proper etchant to reveal the crystalline structure of the specimen. The grain boundaries are then attacked at a greater rate than the proper grain due to higher energy content of the grain boundaries. Etching was performed by swabbing the etchant on the specimen. After etching, the mount is thoroughly rinsed in running water. Then alcohol is sprayed over the surface and the specimen was then examined again.

Result

Figure 1 showing specimen after grinding and polishing

Figure 4 showing the etched surface of the specimen

Figure 2 showing the etching of the specimen Figure 5 showing etched surface of the specimen when wiped

Figure 3 showing the unetched surface of the specimen

Discussion: Upon viewing the un-etched, polished surface of the metals under the metallurgical microscope, we saw what appeared to be a perfect surface with little indentations or scratches. We also saw the absence of grain boundaries (figure 3). But after applying the etchant to the surface of the metal and then viewing under the microscope again, the grain boundaries were clearly seen (figure 4 & 5). This change was a result of the etchant attacking the surface of the metal, resulting in the discolouration of the metal. The microstructure of the specimen is Austenite. Having an austenite microstructure the material will exhibit certain characteristics. Characteristics such as high ductility, high carbon equivalent, high strain hardening capacity and high mechanical strength. It is important to know the processing history of the specimen because it will help to give a better understanding of the microstructure of the metal. For example, if the metal was heated and quenched before, then we would know that the metal would be very brittle and have a Martensite. Having this knowledge, we would know what type of structure to get when observing the metal under the microscope.

Conclusion: Examination of the microstructure of a metal cannot be done with the naked eye. It must be carried out with the use of a metallurgical microscope. But even with a microscope an etchant is needed to make the grain boundaries visible. Upon examining the specimen, it was found to have an Austenite microstructure.

Reference: Stuers, Inc., Stuers Metallographic News, special issue on sample preparation, Structure 4.1, 1982. American Society for Metals, Metals Handbook, 8th ed., Vol. 7, Atlas of Microstructure of Industrial Alloys, ASM, Metals Park, OH, 1972. B. J. Kestel, Polishing Methods for Metallic and Ceramic TEM Specimens, ANL80-120, Argonne National Laboratory, Argonne, IL, 1981. U. Linde and W. U. Kopp, Structures, 2, 9 (1981).