UNIVERSITI TEKNOLOGI MARA FAKULTI KEJURUTERAAN MEKANIKAL
Program: Bachelor of Mechanical (Hons) Mechanical (EM220) Course : Dynamic and Materials Lab 1 Code : MEC 424
Lab 1 Introduction to Sample Preparation and Optical Opt ical Microscope For Metallographic Observation
Name : Haziman Bin Abdullah I/D Student: 2007126805 Group : EMD4M5 Date : 21 July 2008 Lecture : Pn. Koay Mei Hyie
Lab Session
: 21 JULY 2008 (Date)
Report Submission
: 28 JULY 2008 (Date)
Staff Approval
: (Signature)
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CONTENT: page
1.0) INTRODUCTION AND THEORY
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1.1) Sample Preparation: a) Cutting.
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b) Molding.
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c) Grinding.
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d) Polishing.
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e) Etching.
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1.2) Introduction to Optical Microscope
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2.0) EXPERIMENTAL PROCEDURE
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3.0) RESULT
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4.0) DISCUSION
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5.0) CONCLUSION
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6.0) REFERENCE
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1.0) INTRODUCTION AND THEORY
Metallographic is the study of a materials microstructure. Analysis of a materials microstructure aids in determining if the material has been processed correctly and is therefore a critical step for determining product reliability and for determining why a material failed. The basic steps for proper metallographic specimen preparation include: cutting and sectioning, mounting, grinding, polishing, etching and microscopic analysis. 1.1 Sample Preparation:
In the study of metallic materials it is often to analyze the phase exists and grain size in the structure. Details of the structure of metals are not readily visible through naked eyes, but grain structures in metals may be seen with the aid of microscope. Metal characteristics such as grain sizes, effect of heat treatment, and carbon content of steels may be determined by studying the micrograph. For this purpose, the metal that we use in metallurgical examination must prepare and polished carefully before a good microscopic picture can be seen. There is important to ensure that the surface is totally flat and smooth before seeing it under microscope. Any irregularity will appear as a dark surface and will confuse the observer who attempts to analyze a structure. In order to obtain this smooth flat surface, several preparatory steps are required. (a) Cutting.
- The first step in sample preparation is raw material selection. After the specimen had been selected, it goes to the cutting process. When cutting a specimen from a larger piece of material, care must be taken to ensure that it is representative of the features found in the larger sample, or that it contains all the information required to investigate a feature of interest. One problem is that preparation of the specimen may change the microstructure of the material, for example through heating, chemical attack, or mechanical damage. The amount of damage depends on the method by which the specimen is cut and the material itself.
Abrasive wheel cutter
Low speed precision cutter
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(b) Molding.
- Mounting of specimens is usually necessary to allow them to be handled easily. It also minimizes the amount of damage likely to be caused to the specimen itself. Specimens can be hot mounted (about 150 °C) using a mounting press either in a thermosetting plastic example phenolic resin or a thermosoftening plastic example acrylic resin. If hot mounting will alter the structure of the specimen a cold-setting resin can be used example epoxy, acrylic or polyester resin. Porous materials must be impregnated by resin before mounting or polishing, to prevent grit, polishing media or etching being trapped in the pores and to preserve the open structure of the material. Cold mounting materials are used to encapsulate samples that cannot withstand the heat and pressure of compression mounting, when better flow and penetration of the mounting material is needed or when a large quantity of samples must be encapsulated at once.
Apparatus hot mounting:
Electro-hydraulic specimens mount press
Small specimen mounted in thermosetting plastic
Apparatus cold mounting:
Disposable Mounting Cups
Small specimen mounted in thermosoftening plastic
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(c) Grinding.
- Grinding is required to planar the specimen and to reduce the damage created by sectioning. The planar grinding step is accomplished by decreasing the abrasive grit/ particle size sequentially to obtain surface finishes that are ready for polishing. The specimen is first moved back and forth on the coarse grit paper until all the scratches go in one direction and then the specimen must be thoroughly cleaned before moving to a finer grit. Rough grinding is to remove rough scale and gross imperfections on the surface of sample. Fine grinding is to improve surface until it begin to shine and reflect light slightly.
Roller grinder. (d) Polishing.
- Polishing can categories by two steps, rough polishing and fine polishing. Rough polishing, the purpose of the rough po lishing step is to remove the damage produced during cutting and grinding. Proper rough polishing will maintain specimen flatness and retain all inclusions or secondary phases. By eliminating the previous damage and maintaining the micro structural integrity of the specimen at this step, a minimal amount of time should be required to remove the cosmetic damage at the final polishing step. Fine polishing, the purpose o f final polishing is to remove only surface damage. It should not be used to remove any damage remaining from cutting and planar grinding. If the damage from these steps is not complete, the rough polishing step should be repeated or continued.
Manual polishing a specimen.
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(e) Etching.
- The purpose of etching is to optically enhance micro structural features such as grain size and phase features. Etching selectively alters these micro structural features based on composition, stress, or crystal structure. The most common technique for etching is selective chemical etching and numerous formulations have been used over the years. Other techniques such as molten salt, electrolytic, thermal and plasma etching have also found specialized applications. Chemical etching selectively attacks specific micro structural features. It generally consists of a mixture of acids or bases with oxidizing or reducing agents.
Etching the specimen.
1.2 Introduction to Optical Microscope:
After a mirror-like metal surface has been prepared, now the structure can be observed under optical microscope for analysis. Before interpreting of the structure that had been observe, it is important to understand some basic operations of optical microscope. The optical microscope magnifies images by sending a beam a light through the object as seen in schematic diagram. The condenser lens focuses the light on the sample and the objective lenses magnify the beam, which contains the image to the projector lens so the image can be viewed by the observer. In interpreting the microstructure, it is helpful to consult with the phase diagram and to have some knowledge of the composition and thermal history of the specimen. If both resources are used, phases can be often identified and sequence in which they formed can be traced.
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Illumination in a metallurgical microscope.
2.0) EXPERIMENTAL PROCEDURE:
1. First step in sample preparation is raw material selection. After the specimen had been selected, it goes to the cutting process. Cut the specimen using the recommended machine. 2. Use a mounting press if hot mounting to clothing the specimen and grind any rough surface lightly to make them level. 3. Sand the surface from rough to finer grit paper. Important, grinding process most do in one direction and then the specimen must be thoroughly cleaned before moving to a finer grit. 4. Change to a finer grit and rotate the specimen to show the new lines. Be sure to clean the specimen thoroughly between each grit change and carefully avoid contaminating the felt on the polishing wheel. 5. Take a mounted specimen to the polishing wheel. Turn on the wheel and add a little slurry to the felt surface. Gently press the specimen to the wheel surface and move it opposite direction of the wheel rotation. Move the specimen around the wheel several times and clean with distilled water then check the surface. When it becomes mirror bright, it is ready to etching. 6. Apply the etchant the surface of the specimen using a reagent and count the limitation time. Rinse with water and immediately follow with an alcohol rinse from the wash bottle. This will remove the water and prevent rusting. Allow the alcohol to dry. 7. If the microstructure is not visible, apply the step number 5. 8. Observe each specimen and determine the microstructure and sketch the result. 7
3.0) RESULT:
1) Lens focuses M5/0.10
Ferrite
Martensite
2) Lens focuses M20/040
Ferrite
3) Lens focuses M40/0.65
Ferrite
Pearlite
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4.0) DISCUSION:
Grinding One Direction. The orientation of the specimen can have a significant impact on the preparation results, especially for specimens with coatings. In general, when grinding and polishing materials with coatings the brittle component should be kept in compression. In other words, for brittle coatings the direction of the abrasive should be through the coating and into the substrate. Conversely, for brittle substrates with ductile coatings, the direction of the abrasive should be through the brittle substrate into the ductile coating. Specimen Microstructure. Grinding process cannot change or affect the microstructure specimen because when the grinding process, firstly we are use low grid size for abrasive paper to remove rough scale and gross imperfections on the surface of sample. After that change the high grid size when the surface specimen clean to improve the surface until it begins to shine and reflect light slightly before go to polishing process. Purpose Etching the Specimen. The purpose of etching is to optically enhance microstructural features such as grain size and phase features. Etching selectively alters these microstructural features based on composition, stress, or crystal structure. The most common technique for etching is selective chemical etching and numerous formulations have been used over the years. Different Phase Exist In Iron-Iron Carbide during M icrostructures Changes. All materials exist in gaseous, liquid, or solid form (usually referred to as a phase), depending on the conditions of state. State variables include compositions, temperature, pressure, magnetic field, electrostatic field, gravitational field, and so on. The term "phase" refers to that region of space occupied by a physically homogeneous material. However, there are two uses of the term: the strict sense normally used by scientists and the somewhat looser sense normally used by materials engineers. In a phase diagram, however, each single-phase field (phase fields are discussed in a following section) is usually given a single label, and engineers often find it convenient to use this label to refer to all the materials lying within the field, regardless of how much the physical properties of the materials continuously change from one part of the field to another. This means that in engineering practice, the distinction between the terms "phase" and "phase field" is seldom made, and all materials having the same name are referred to as the same phase.
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Cooling curve for pure iron. As iron is cooled slowly from the liquid phase.
The iron-carbon diagram.
Iron-Iron Carbide Phase Transformation on the T-T-T Diagram. The T-T-T curves correspond to the start and finish of transformation, which extend into the range of temperature where austenite transforms to pearlite. Above 550°C, austenite transforms completely to pearlite. Below 550°C, both pearlite and bainite are formed and below 450°C, only bainite is formed. The horizontal line C-D that runs between the two curves marks the beginning and end of isothermal transformations. The dashed line that runs parallel to the solid line curves represents the time to transform half the austenite to pearlite. Below we have listed some simple examples as an exercise at other temperatures that result in different phase transformation and hence different microstructures.
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5.0) CONCLUSION:
Microstructural analysis is extremely important in today's world where metals, ceramics, polymers, and composites are used to improve our everyday lives by insuring safety and reliability in the products that we use. In order, to correctly analyze these microstructures, proper specimen preparation is required to eliminate preparation induced microstructural artifacts. By knowing the physical properties of the material such as hardness, recrystallization temperature, fracture toughness; an appropriate specimen preparation procedure can be developed. In general, minimizing the damage early in the microstructural preparation stage by choosing the correct abrasive, lubricant, and equipment parameters, the steps required to highlight the representative materials microstructure obtained by following the basic preparation guidelines outlined in this paper. The optical microscope is a very useful tool for the observation of materials and can be used to gain valuable information about a large variety of specimens. Some knowledge of the material and the information that is required is essential to determine the best techniques to employ when preparing and examining specimens. Sample preparation is a critical part of microscopy, as this determines the quality of the images produced. Many techniques, when correctly applied to a specimen, can enhance the information present.
6.0) REFERENCE
1) Lab sheet: Introduction to sample preparation and o ptical microscope for metallographic observation 2) Materials science and engineering an introduction, William D. Callister. Jr. 6 Edition Wiley & Sons 3) Pace Technologies Educational Web page at http://www.metallographic.com
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