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ME2151 Lab2 Metallography

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ME2151 Lab2 Metallography
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    ME2151-2 METALLOGRAPHY 2014/2015 Department of Mechanical Engineering  National University of Singapore  CONTENTS TABLE OF CONTENTS (i) LIST OF ILLUSTRATIONS (i) INTRODUCTION 1 METALLOGRAPHIC PREPARATION OF SPECIMENS 1 THEORY OF WELD STRUCTURE 2 SCOPE 5 PROCEDURE 5 REFERENCES 6 LIST OF ILLUSTRATIONS Figure 1 Fe-Fe 3 C Phase Diagram 3 Figure 2 Schematic of Weld Structures 4  INTRODUCTION 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 each change of paper, the specimen should be turned through 90 °  to facilitate the observation of the disappearance of the previous scratch marks. In addition, at every new stage the specimen and equipment should be washed of grit and dirt from the preceding grinding. Rough Polishing: This stage involves the polishing of the specimen surface on a rotating wheel using alumina or diamond abrasive with a particle size of about 5 microns. Polishing aids include diamond  particle suspension or alumina powder suspension. In the polishing stage, the specimen is moved around the wheel in the direction opposite to the wheel itself. This ensures a uniform polishing action. Fine Polishing: This stage involves the removal of very fine scratches and the thin distorted layer remaining from the rough polishing stages. Fine polishing is usually carried on a polishing wheel using fine alumina particles with an average size of less than 1 micron (normally 0.5 micron size is used). Fine polishing, if properly carried out, yields a scratch free surface ready for etching. Etching: Etching is carried out on the properly dried specimen obtained from fine polishing step. Etching involves chemically treating the specimen surface using a mild acidic or alkaline solution. The etchant differentially attacks various microstructural features as a result of their different chemical affinities. This differential attack leads to a non-similar reflection of light into the objective lens leading to the generation of contrast between the various microstructural features. After etching is successfully carried out, the specimen can be taken to the optical microscope for microstructural examination. THEORY OF WELD STRUCTURE In the present laboratory session, we will be examining the microstructure of fusion welded steel. In order to gain better understanding of the various features we will see in the welded area, it is essential to know about Fe - Fe 3 C metastable phase diagram and the various microstructural zones that are normally formed following welding operation. Figure 1 shows a typical Fe - Fe 3 C metastable phase diagram. The various phases in this phase diagram which may be of interest to us can be defined as follows: Ferrite ( ! -iron):a solid solution of carbon in iron having a maximum carbon content of about 0.022%. Austenite ( -iron):a solid solution of carbon in iron formed at high temperatures containing a maximum carbon content of about 2.11 %.

15026_mp_1_en

Jul 23, 2017
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