Kasırga,
MATERIAL TESTING : MACRO/MICRO
HÜSEYİN KASIRGA 030040323 GROUP
B
Instructor: Prof. Dr. Ahmet ARAN
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Kasırga, TABLE OF CONTENTS
I.
Introduction 1. Definitions a. Metallography b. Microscopy c. Macroscopy
II.
Experimental Procedure 1. Specimen Preparation a. Grinding b. Polishing c. Etching 2. Microscopical Examination a. Objective b. Experiment 3. Macroscopical Examination a. Objective b. Experiment i. Sulphur Printing ii. Flow Lines iii. Welded Sections
III.
Data Analysis 1. For Microscopical Examination 2. For Macroscopical Examination
IV.
Results & Discussion Of Results 1. Results of Microscopical Examination 2. Results of Macroscopical Examination
V.
Conclusion
VI.
References
VII.
Tables and Figures
VIII. Appendix
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I. INTRODUCTION 1. DEFINITIONS a. Metallography: Metallography is the science and art of preparing a metal surface for analysis by grinding, polishing, and etching to reveal microstructural constituents. After preparation, the sample can easily be analyzed using optical or electron microscopy. A skilled technician is able to identify alloys and predict material properties, as well as processing conditions by metallography alone. There are two examination methods in Metallography: 1. Microscopy 2. Macroscopy b. Microscopy: In microscopy, the examination of the prepared specimen is done with the optical microscopes applying magnifications from 10x to 2000x. This examination provides information regarding the morphology and distribution of constituent phases as well as the nature and pattern of crystal imperfections. c. Macroscopy: In this method the examination of the material (generally metals and alloys) is done by the unaided eye or a low-power microscope with a magnification up to 10x. This examination method provides information about the nature of inhomogenities, flow lines, segregations, and fabricating defects that cannot be examined by microscopy. However, these data represents the characteristics only at a particular section of the material.
II. EXPERIMENTAL PROCEDURE 1. SPECIMEN PREPARATION The purpose of specimen preparation is to prepare the material piece for analysis by minimizing mechanical surface damages to obtain reproducible results. Preparation of a metallographic specimen generally requires following major operations: 1. Grinding 2. Polishing 3. Etching a. Grinding: The purpose of grinding is to minimizing the mechanical damages on specimen’s surface by lessening the depth of deformed metal to the point where the last vestiges of damage can be removed by series of polishing steps. The scratch depth and the depth of cold worked metal underneath the scratches decrease with decreasing particle size of abrasive. Grinding is done using a machine carrying rotating discs covered with silicon carbide paper and water. There are a number of grades of paper, with 180, 240, 320, 400, 600, 1000, 1200,
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grains of silicon carbide per square inch in the order from coarser to finer. Most grinding of metallographic specimen is performed by manually holding the specimen with its surface against that grinding material covering discs. To establish and maintain a flat surface over the entire area being ground, equal pressure must be applied on both sides of the specimen avoiding any rocking motion that will produce a convex surface. Success in grinding depends in part on the pressure applied to the specimen. A very light pressure removes insufficient metal. Somewhat heavier pressure produce polishing, while still heavier pressure brings about the desired grinding action. Very heavy pressure results in nonuniform scratch size, deep gouges, and embedded abrasive particles. Generally, a medium to moderately heavy pressure applied firmly gives the best results. To avoid from excessive warm of the specimen water is used as cooler. The grade of the silicon carbide paper to begin the grinding operation is the coarsest paper, 180 grade. Always light pressure is used, applied at the centre of the specimen and continued grinding until all the blemishes have been removed, the sample surface is flat, and all the scratches are in a single orientation. Then the sample is washed in water and moved to the next finer grades, orienting the scratches from the previous grade normal (90˚) to the rotation direction. This makes it easy to see when the coarser scratches have all been removed. After the final grinding operation on 1000 paper, the sample is washed in water and dried before moving to the polishing operation. b. Polishing: Polishing is the final step in production a surface that is flat, scratch free, and mirror like in appearance. Such a surface is necessary for subsequent accurate metallographic interpretation, both qualitative and quantitative. The polishers consist of rotating discs covered with soft cloth (broadcloth) impregnated with diamond particles and an oily lubricant. Typically, a sample is polished with slurry of alumina (Aluminum oxide is a chemical compound of aluminum and oxygen with the chemical formula Al 2O3. It is also commonly referred to as alumina in the materials science communities.), silica, or diamond as polishing agent to produce a scratch free mirror surface. In polishing operation, alumina is used to prevent excessive warm of specimen instead of water in grinding operation. During polishing the specimen should be rotated or moved around the wheel so as to give an even polish, but excessive pressure should be avoided. After polishing the specimen should be thoroughly cleaned and dried without any contact of hands directly with the polished surface of specimen. c. Etching: The purpose of etching is two-fold. Grinding and polishing operations produce a highly deformed, thin layer on the surface which is removed chemically during etching. Secondly, after polishing, the microstructural constituents of the sample are revealed by using a suitable chemical or electrolytic etchant .The etchant attacks the surface with preference for those sites with the highest energy, leading to surface relief which allows different crystal orientations, grain boundaries, precipitates, phases and defects to be distinguished in reflected light microscopy. There are many tried and tested etchants available, but the ideal etchant is dependent on sample composition, and the microstructural feature(s) of interest. As a quide following etchants are commonly used:
Kasırga, - Nital (ethyl alcohol + 2% HNO3)
iron and steel
- Alcholic Ferric Chloride
copper alloys
- Mixed Acids
aluminum alloys
- Dilute HCl
zinc alloys
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Etching times varies according to specimen, but a general procedure is to observe the surface during the operation and to end the operation when evidence of the grains first appears. Further etching then follows to strengthen up the details as required for a good contrast in microscopical examination. After etching, the specimen should be thoroughly washed and dried with acetone or alcohol.
Figure 1: Machine used for grinding and polishing
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2. MICROSCOPICAL EXAMINATION After the specimen prepared for examination, the microstructural study of the material can begin. With this examination information can be provided about the distribution and morphology of the phases and if their properties are known, a quantitative analysis of the micrographs provides information about the bulk properties of the specimen. Microstructural examination can provide quantitative information about the following parameters: Specimen’s grain size Interfacial area per unit volume Dimensions of constituent phases Amount and distribution of phases a. Objective: To examine the microstructural characteristics of prepared metal specimen employing magnifications with an optical microscope. b. Experiment: First, the specimen is prepared as said in the section of specimen preparation. That is the specimen grinded, polished and finally etched. Then with the aid of microscope successively magnifications are used to resolve the fine details and obtain a good image with a good contrast. In focusing, the stage is gradually moved towards the objective and when the image appears, focusing is completed with the fine adjustment. However, something important that must be considered is that the prepared surface represents a two-dimensional picture whereas the structure of specimen exists in three dimensions.
3. MACROSCOPICAL EXAMINATION In this method the examination of the material (generally metals and alloys) is done by the unaided eye or a low-power microscope with a magnification up to 10x. This examination method provides information about the nature of inhomogenities, flow lines, segregations, and fabricating defects that cannot be examined by microscopy. However, these data represents the characteristics only at a particular section of the material. a. Objective: To examine the nature of inhomogenities, flow lines and welded sections of a metal by unaided eye (or with a low-power microscope). b. Experiment: i. Sulphur Printing ii. Flow Lines iii. Welded Sections
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i. Sulphur Printing: The purpose of this experiment is to examine the impurities that may exist in steel products. The importance of these impurities is because of their amount and distribution in the steel. These impurities degrade steel’s mechanical properties especially it’s strengthen. Sulphur existence is one of these impurities and makes the steel brittle. Sulphur printing is a common method to detect and record the distribution of Sulphur in steel. To prepare the surface of rail material to the experiment it must be cleaned from the foreign matters on the surface by grinding and polishing. Because it is a macroscopic examination with unaided eye, etching is not required. Preparing the surface, a special photo paper, photographic bromide paper, is soaked in a 2% aqueous solution of sulphuric acid for approximately 3-4 min. After the paper is removed from the solution and allowed to drain from excess solution, prepared surface of the specimen is pressed on the paper with a pressure for 1-2 min to let the chemical reactions between the surface and acid solution proceed. Then it removed and the paper is placed into a photographic fixing solution for about 15 min. to fix it permanently. When fixation completed the paper is washed in running water and dried. Presence of darkly colored areas on paper indicates the presence of sulphur on the material and distribution of these areas provides information about the mechanical properties of rail material. ii. Flow Lines Flow lines are the natural consequences of applied mechanical working on material. Flow lines provide information about the defects of material and excessive amount of inclusions and segregated areas as well as the direction of metal flow during deformation. In this method, because the elongated inclusions of impurities, such as oxides and other heterogeneous areas are selectively attacked by etching reagent, flow lines are made visible. iii. Welded Sections: The purpose of welded section experiment is to find the locations of weld sections of a welded material. After the surface of the specimen cleared from foreign matters and deformations by grinding and polishing, prepared surface is etched with nital for 10-20 sec. This operation is repeated several times until some relief of the macrostructure is produced. Finally the specimen is rinsed in alcohol and dried.
III. DATA ANALYSİS 1. MICROSCOPICAL EXAMINATION
d = the average grain diameter nL = the number of grain boundaries intersected per unit length of a test line M = magnification C = constant (generally C=1,5)
d = C/nL .M
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IV. RESULTS & DISCUSSION OF RESULTS 1. RESULTS of MICROSCOPICAL EXAMINATION 1. The Sketch Of Microstructure:
Figure 2. Micrograph of the Microstructure of specimen. Dark colored areas indicates perlits (α + Fe3C ) and other white areas ferrits (α). 2. Carbon Content of the Steel: Containing the compound of Carbon (C), the amount of the dark areas, perlits (α + Fe3C ), per unit area can give the Carbon content of the steel. However, the microstructure of the specimen in figure 2 is not clear enough to calculate this content, because the grain boundaries are not distinct enough. The reason of this situation is inadequate time of the operation for etchant to attack and resolve details! 3. Average Grain Diameter: Average grain diameter of a microstructure can be calculated with “Linear Intercept Method”. In this method, the grains intercepted by a theoretical line on the specimen surface are counted. The average grain size is indicated by the inverse of the number of grain boundaries intersected per unit length of the test line, corrected for the magnification of micrograph. Then the average grain diameter is given by following formula:
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d = the average grain diameter nL = the number of grain boundaries intersected per unit length of a test line M = magnification C = constant (generally C=1,5)
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d = C/nL .M
However, because of the unclear micrograph above, the average grain diameter cannot be calculated for this figure (Figure 2.). 2. RESULTS of MACROSCOPICAL EXAMINATION a. Sulphur Printing 1. Sulphur Inclusions in Rail Material: As explained above in the sulphur printing section, a consequence of the chemical reaction between the sulphuric acid on the photographic bromide paper and the sulphide regions of the specimen’s surface, darkly colored stains and areas exist on the paper. That is, dark colored points indicate sulphur inclusions of the rail material. 2. Distribution of Sulphur Inclusions:
Figure 3. Sulphur Inclusions
As it is seen in the figure 3, sulphur inclusions are not distributed homogeneously. They are distributed as segregated points in the material. This distribution affects mechanical properties of the material, especially its strength. This inhomogenic distribution of sulphur makes the material brittle where the sulphur density is higher. b. Flow Lines
3. Existence of Flow Lines: Flow lines appear as a consequence of applied mechanical working on materials. For example, when a plastic formation, for instance rolling is applied to the material, orientations in the rolling direction are started in the microstructure of the material due to the movement of dislocations. Flow lines do not exist in casted materials, because in casting, the material in liquid form is poured into a mold where it is allowed to solidify by cooling. The liquid and homogeny form of the material prevents any lines inside the product while it cools slowly. 4. Flow Lines and Mechanical Properties: Flow lines are the kind of discontinuousness in the constitution of the metals that affects the mechanical properties of the metal. A pressure applied on the metal is transmitted on the flow lines through the metal and according to the distribution and direction of the flow
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lines, some stress stacks appears in particular sections of metal where have the risk of brittle fracture. (See the figure 4.)
Figure 4. Flow lines
c. Welded Sections: 5. Welded Section
6. Welded Section and Mechanical Properties By examining the welded-sections it is found that the the material is not manufactured as a whole. The elliptical projection of the material is welded. Because any distinction in appearance cannot be seen with unaided eye without any polishing and etching operation, it can be said the weld quality is very good. The reason of this is the similarity of two materials.
Figure 5. Welded Section of Specimen There are some relations between the weld section and the mechanical properties of the material. Welding causes a heat treatment process of which effect decreases according to the distance from the welded section. Due to the high temperature, some grain growths exist in and near the welded section. The growth of the grains means decline of the grain boundaries that affect the hardness and strength of the material in a negative way. Because the grain boundaries are amorphous, they prevent from the dislocations. Therefore a decrease in the amount of the grain boundaries results in decreases in the hardness and strength of the material. V. CONCLUSION In conclusion, the constitutional characteristics of a metal or an alloy can be related to its physical and mechanical properties with the aid of methods contained in metallography.
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VI. REFERENCES [1] University of Cambridge, retrieved from http://www.msm.cam.ac.uk, September 28, 2006. [2] D.R. Askeland, “The Science and Engineering Materials”, P.P Phule Thomson Pub., 5th edition 2006. [3] Wikipedia, the free encyclopedia, retrieved from http://en.wikipedia.org, September 28, 2006. [4] Arnes Corporation, retrieved from http://www2.arnes.si, September 29, 2006. [5] Aran, A., “Malzeme Bilgisi Ders Notları”, Makina Fakültesi, İTÜ. [6] “Material Testing Laboratory Manual”, ITU, 2006. [7] Aran, A., “Manufacturing Prop. of Materials”, ITU, 2006. [8] Materials Evaluation and Engineering, inc, retrieved from http://www.mee-inc.com, September 30, 2006.
