September 3, 2017 | Author: Faizan Abid Naqvi | Category: Welding, Iron, Abrasive, Microscopy, Steel
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Department of Mechanical Engineering National University of Singapore


















Figure 1

Fe-Fe3C Phase Diagram


Figure 2

Schematic of Weld Structures



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 - Fe3C metastable phase diagram and the various microstructural zones that are normally formed following welding operation. Figure 1 shows a typical Fe - Fe3C metastable phase diagram. The various phases in this phase diagram which may be of interest to us can be defined as follows: Ferrite ( α-iron):a solid solution of carbon in iron having a maximum carbon content of about 0.022%. Austenite ( γ-iron):a solid solution of carbon in iron formed at high temperatures containing a maximum carbon content of about 2.11 %.

Cementite (Fe3C): a compound of carbon and iron, having a chemical formula o Fe3Cand a carbon content 6.7%. Pearlite (α + Fe3C): a two phase mixture that exists with an average carbon content of 0.77%. Pearlite has a lamellar structure.

Figure 1. The Fe - Fe3C phase diagram. (adapted from Metals Handbook: Metallography, Structures and Phase Diagrams, Vol.8, 8th edition, ASM Handbook Committee, T. Lyman, Editor, ASM, 1973, p.275). After gaining a basic insight into the Fe - Fe3C phase diagram, we will in the following section try to gain knowledge regarding fusion welding and the microstructural zones that are formed as a result of fusion welding process. A fusion weld is normally produced by the electric arc welding process. There are basically six zones that can be discerned in the welded area and the parent metal adjacent to it. These zones are created since the various regions of the parent metal and the weld itself are subjected to different degrees of heat treatment during the welding process. The six basic zones are: a) b) c) d) e) f)

Deposited Metal Zone Fusion Zone Grain Growth Zone Grain Refinement Zone Transition Zone Unaffected Zone

Weld Metal Zone The Heat Affected Zone

These six zones are distributed in the welded specimen as shown in Figure 2 for hot rolled metal.


Figure2. Schematic diagram showing six microstructural zones in the welded steel. The Deposited Metal Zone is the portion where the filler metal was deposited. In the Fusion Zone, the parent metal was heated to the melting point and subsequently cooled. These two zones are generally indistinguishable from each other and make for the Weld Metal Zone. The Weld metal zone is generally characterized by the presence of two important microstructural features i.e. Columnar grains and Widmanstatten structures. As steel is a good conductor of heat, the weld is subjected to very rapid cooling from its molten state and the result is essentially a chilled casting having the associated columnar grains. On the other hand, a Widmanstatten structure appears as a result of large austenite grains being put through a moderately fast cooling rate. The Grain Growth Zone exhibits the effect of high temperature treatment at temperatures somewhat less than its melting point. The high temperature exposure leads to a significant growth of austenite grains. On cooling to room temperature, this effect is retained as a region of coarse ferrite grains and Widmanstatten ferrite and pearlite. The demarcation between grain growth zone and weld metal zone is generally more distinct than between the other zones.

In the Grain Refinement Zone , the parent metal is heated into the temperature range corresponding to the austenite phase in the Fe - Fe3C phase diagram. Because of the relatively lower temperatures, the austenite grains began to nucleate at many points to form smaller austenite grains which on cooling will result in fine ferrite and pearlite grains. In the Transition Zone, the parent metal during welding is heated to the region where ferrite and austenite coexist. In other words, mostly only pearlite grains have transformed to small

austenite grains. On cooling very fine pearlite grains with ragged looking boundaries are formed among the mostly untransformed original ferrite grains. The Unaffected Zone represents the region of the parent metal that was not heated beyond the eutectoid temperature (727 °C) and there is therefore no observable structural change. SCOPE In accordance with the subject matter covered in the present manual, the scope of this laboratory exercise will be two fold: 1. 2.

To obtain experience in the metallographic preparation of metallic specimens, and To observe the various microstructures in a welded mild steel joint. PROCEDURE

Mild steel plates having carbon content of about 0.15% have been welded together by an electric arc welding process using a general purpose electrode. The welded plates were then cut and portions of it mounted in thermosetting plastic mounts. You have been supplied with one mount in which one section of the plate was encapsulated. This contains the weld with the heat affected zone. Proceed with the metallographic preparation as follows: 1.

Grind the specimen using the coarser grade silicon carbide paper laid on the hand grinder. Hold the specimen face downwards on the silicon carbide paper and rub forwards and backwards with as even a pressure as possible to obtain an even pattern of finer scratches.


Wash the specimen and repeat step 1 using the finer grade silicon carbide paper. The specimen should be turned through 90° to facilitate removal of the previous scratch marks.


Wash the specimen when only fine parallel scratch marks are obtained.


Polish the specimen using a cloth-covered rotating disc with Alumina (fine abrasive aluminum oxide powder, suspended in water ) as the polishing agent until a flat and scratch-free mirror -like finish is obtained.


Wash and dry.


Etch the surface of the specimen with 2% Nital (2 parts of concentrated nitric acid and 98 parts of ethyl alcohol by volume ) for 15 to 25 seconds.


Wash and dry the specimen using a specimen drier.


The specimen is now ready for observation.


Observe the specimen under a optical microscope.


Ask the experiment supervisor to have a look and if he is satisfied that your preparation has produced clearly observable microstructures then proceed to step I 1. If not, repolish and re-etch the specimen until the microstructures are observable.


Sketch the general microstructural arrangement of the various distinguishable zones stating the magnification used. Your sketches are to be of high quality. Label all important features neatly.


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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