Faculty of engineeringMechanical Engineering Department

Materials science lab

Experiment # 2Preparation and examination of specimens

Omar Salah Eldin Sulieman

1.Objectives- To learn and to gain experience in the preparation of metallographic specimens.- To examine and analyze the microstructures of metals and metallic alloys.2.IntroductionMetallography consists of the study of the constitution and structure of metals and alloys. Much can be learned through specimen examination with the naked eye, but more refined techniques require magnification and preparation of the material's surface. Optical microscopy is sufficient for general purpose examination; advanced examination and research laboratories often contain electron microscopes (SEM and TEM), x-ray and electron diffract meters and possibly other scanning devices. Incorrect techniques in preparing a sample may result in altering the true microstructure and will most likely lead to erroneous conclusions. It necessarily follows that the microstructure should not be altered. Hot or cold working can occur during the specimen preparation process if the metallurgist is not careful. Expertise at the methods employed to produce high-quality metallographic samples requires training and practice. The basic techniques can be learned through patient persistence in a matter of hours. This module takes the student through the metallographic sample preparation process step-by-step with demonstrations and explanations of sectioning, mounting, course & fine grinding, polishing, etching and microscopic examination.3. Preparation of specimenPreparation of specimen for microscopic examination requires the following steps: 3.1. Choice of the specimenAny specimen should be selected to be representative of the metal piece to be examined also the specimen should have a suitable size to facilitate the following processes.3.2. Cutting Figure (1): choice of the specimen

The first step in preparing a specimen for metallographic or microstructure analysis is to locate area of interest. Sectioning or cutting is the most common technique for obtaining this area of interest. Proper sectioning guarantees minimal microstructural damage. Excessive subsurface damage to the secondary phasesFigure (2): cutting

The appropriate cutting method should be chosen to avoid any changes in the specimen internal structure due to friction. Also, cooling is required to minimize heating due to friction.3.3. Rough grinding The purpose of the grinding step is to remove damage from cutting of the specimen, and to remove material approaching the area of interest, and to eliminate large scratches and obtain a flat surface.After the completion of rough grinding, the specimen should be washed with water.Figure (3): Rough grinding by using rough file

3.4. Fine grinding or sandingThis process requires some equipments to eliminate small scratches resulted from rough grinding process and to get a smooth and level surface.Silicon carbide is used material in sanding equipments it is used in:1 - Emery papers with different grades number (e.g. 220-320-400-600-800-1000-1200) which the grade number indicates the number of particle per square inch. 2- Silicon carbide powder added to a grinding revolving disk.in order to obtain a highly reflection surface that is free from scratches and deformation the specimens must be carefully grinded before they can be examined under the microscope Qualities offers high offers high quality instruments and consumables for achieving this goal . After the completion of fine grinding the specimen should be cleaned with water.Figure (4): Fine grinding or sanding

3.5. PolishingPolishing is the most important step in preparing a specimen for microstructural analysis. It is the step which is required to completely eliminate previous damage.To remove deformation from fine grinding and obtain a surface that is highly reflective, the specimens must be polished before they can be examined under the microscope. Polishing is a complex activity in which factors such as quality and suitability for the cloth, abrasive, polishing pressure; polishing speed and duration need to be taken into account. The quality of the surface obtained after the final polishing depends on all these factors and the finish of the surface on completion of each of the previous stages.The aim of this process is to eliminate fine scratches from the fine scratches resulted from fine grinding process and to obtain a polished and (mirror alike) surface. The main equipments used in this process: Figure (5): Polishing machine

1- Polishing machine2- Polishing clothPolishing materials, liked 3-a) Diamond paste b) Alumina (AL2O3) * After the complication of fine grinding, the specimen should be washed with alcohol and distilled water.3.6 Etching: Microscopic examination of properly polished, unetched specimen will reveal only a few structural features such as inclusions and cracks or other physical imperfection .etching is used to highlight, and sometimes identify microstructural features or phases present. Even in a carefully prepared sample, a surface layer of disturbed metal, resulted from the final polishing stage, is always must be removed. Etchants are usually dilute acid or dilute alkalis in water, alcohol or some other solvent. Etching occurs when the acid or base is placed on the specimen surface because of the difference in the rate of attack of the various phases present and their orientation. The etching process is usually accomplished by merely applying the appropriate solution to the specimen surface for several second to several minutes.- nital ,[a 1-5% Nitric Acid (HNO3) + 95-99% Alcohol mixture ] ,is the etchant commonly utilized with common irons and steels. Nital is dripped onto the specimen. Several seconds to one minute is usually sufficient for proper etching depending on sample and nital concentration. The sample is immediately washed under running water, rinsed with alcohol and dried in an air blast.Do not touch ,wipe or swab the specimen following etching; dry off the rinsing alcohol on the specimen with the air blast and then move on to the microscopic examination stage.- picral [ a 1-5% Picric Acid (HB) + 95-99% Alcohol mixture ] , is the etchant commonly utilized with cast irons alloys . Figure (6): Etching process

4. Result and discussion -We were able through this experience of seeing grains and boundary.-A grain is a crystalline portion of the material within which the arrangement of the atoms is nearly identical.- The orientation of the atom arrangement, or crystal structure, is different for each adjoining grain.- The arrangement of atoms in each grain is identical but the grains are oriented differently - Friction affects the material properties due to high temperature, so the water must be used during the process of fine grinding. - You must use a certain percentage of alcohol in etching solution. Figure (7): grains and boundaryFigure (8): phase structure