Department of Mechanical Engineering National University of Singapore
TABLE OF CONTENTS LIST OF ILLUSTRATIONS INTRODUCTION METALLOGRAPHIC PREPARATION OF SPECIMENS THEORY OF WELD STRUCTURE SCOPE PROCEDURE REFERENCES LIST OF ILLUSTRATIONS Figure 1 Figure 2 Fe-Fe3C Phase Diagram Schematic of Weld Structures
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Metallography can be defined as the visual study of the constitution and structure of materials. Metallographic examinations can be broadly classified into two types namely, macroscopic examinations and microscopic examinations. Macroscopic examinations refer to the observations carried out at a magnification of X10 of less. Microscopic examinations, on the other hand, refer to the examination of the structure at a magnification greater than X10. Microscopic examinations, depending on the nature of information to be extracted, can be accomplished using an Optical Microscope (up to X2000) or Scanning Electron Microscope (up to X 50000) or a Transmission Electron Microscope (up to X500000). For most of the routine purposes in optical microscope is used to obtain first hand information on the geometric arrangement of the grains and phases in a material. In order to retain the information visualized using the microscope, microstructural details are often recorded on a 35 mm film or a Polaroid film. The photograph thus obtained, revealing the microstructural details, taken at a magnification of greater than X10 is known as a photomicrograph. Maintaining a record of the microstructural studies in the form of photomicrographs is a common practice employed by research scholars and leading laboratories all over the world. The study of microstructaral details is important due to its correlation with the ensuing mechanical properties of the material. As an example, if material A exhibits a more homogeneous and refined microstructure than material B , it may very well be anticipated that material A will exhibit better room temperature properties when compared to material B. In order to metallographically examine a specimen, it is essential to learn about the various steps that are required to prepare it. The following section briefly describe the various steps involved in the metallographic preparation of the samples. METALLOGRAPHIC PREPARATION OF THE SPECIMENS The basic operation outlining the metallographic preparation of the specimens is as follows: Selection of the Size of the Specimen : The selection of the size of the specimen is dependent on the nature of material and the information to be gathered. Normally, the linear dimensions may vary from 5 mm to 30 mm while the thickness is kept lower than the linear dimensions. Mounting the Specimen : Mounting of the specimen is normally carried out, if the specimen does not permit convenient handling . Plastic mounting is normally carried out by placing the specimen in a plastic or rubber mold face down, filling the mold with mounting grade of plastic and allowing it to dry for a few hours. The plastic mounting is carried out such that the surface to be examined is exposed on one side of the plastic mount. Rough Grinding : Rough grinding is carried out on the emery belt surfacer in order to round off the corners, if necessary and to remove deep scratches from the surface.
Fine Grinding : Fine grinding involves rubbing of the specimen surface against the silicon carbide powders bonded onto specially prepared papers. There are various grit sizes of silicon carbide papers and. the ones normally used are 400 grit, 600 grit and 1000 grit papers. These papers are normally mounted on a flat surface. Grinding involves holding the specimens face downwards on the abrasive paper followed by rubbing in forward and backward directions until the surface is covered with an even pattern of fine scratches. The process is repeated with successively finer grade papers (increase in grit number). With...
References: 1. 2. 3. 4. 5. 6. 7. W.D. Callister, Jr.,in "Material Science and Engineering, An Introduction," (John Wiley And Sons (SEA) Pte Ltd, Singapore, 1994). R.E. Reed - Hill and R. Abbaschian, in "Physical Metallurgy Principles,"(PWS- Kent Publishing Co., Boston, USA, 1992). Metals Handbook, ASM Desk Edition, Eds: H.E. Boyer and T.L. Gall, ASM, Metals Park, OH, USA, Vol. 2, 1985. Metals Handbook: Metallography and Microstructure, Vol. 9, 9th Edition, ASM, Metals Park, OH, USA, 1985. M.N.A. Hawlader, Metallography Laboratory Manual, 1984. D.S. Clark and W.R. Varne, in "Physical Metallurgy for Engineers", (Van Nostrand, 1962). G.L. Kehl, in "The Principles of Metallographic Laboratory Practice", (McGraw-Hill, 1949).
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