METALLOGRAPHY LABORATORY EXPERIMENT MANUAL

International ProgramMetallurgy and Material Department

2 nd Sementer 2008/2009

Department of Metallurgy and MaterialsEngineering Faculty

University of Indonesia

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.

I. 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 ( -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 %.

Cementite (Fe3C): a compound of carbon and iron, having a chemical formula o Fe3Cand a carbon content 6.7%.

Pearlite ( + 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) Deposited Metal Zone Weld Metal Zoneb) Fusion Zone -c) Grain Growth Zoned) Grain Refinement Zone The Heat Affected Zonee) Transition Zonef) Unaffected Zone

Figure2. Schematic diagram showing six microstructural zones in the welded steel.

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

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.

II. FERROUS AND NON FERROUS MICROSTRUCTURES

The property of materials totally depends on their microstrectures which consists of many phases. We have studied about their characteristics in our last Subjects of “ Microstructure Analysis Technics of Materials”. Each phases has their own properties and their mechanism to obtain. The following is some examples of ferrous and non ferrous microstructures:

Fig. 3 Aluminum-killed 1008 steel, normalized after 60% cold reduction to a final thickness of 0.8 mm (0.03 in.). The ferritic structure contains fine pearlite (dark areas) at the grain boundaries. 4% nital. 1000×

Fig. 4 High-strength low-alloy steel (0.2% C), hot rolled. The structure is ferrite and pearlite. 4% picral, then 2% nital. 200×

Fig. 5 0.20% C steel, water quenched. The structure is lath martensite. 8% Na2S2O5. 500×. (R.L. Perry)

Fig. 6 10B35 steel, austenitized 1 h at 850 °C (1560 °F), quenched in still water, and tempered 1 h at 175 °C (350 °F). Structure is ferrite (small white areas) and lower bainite

III. THEORY OF HARDENABILITY OF STEEL.

What is hardenability? The primary objective of austenitizing, quenching and tempering a steel part is to produce a hard martensitic microstructure in the material. The ease of accomplishing this task is a measure of the steel alloy’s hardenability. The higher the hardenability the easier it is to produce a martensitic microstructure throughout the part. Alloying elements are added to iron to influence the hardenability of steel. The most significant being: C, Mn, Si, Ni, Cr and Mo. Additions of V, W and Ti are also useful depending on the application. If the steel alloy has a very high hardenability it is possible the produce nearly 100% martensite in large parts.

Measurement of hardenability is accomplished experimentally through the use of an end-quench apparatus, illustrated in Figures 1 and 2, that produces a severe quench at one end of an austenitized steel bar. The linear heat transfer through the length of the bar provides an infinite number of continuous cooling paths that can be correlated to the kinetics of the decomposition of austenite. The quenched end is nearly 100% martensite. At some distance from the quenched-end, for HSLA steels, the microstructure becomes pearlite and ferrite. In between, mixtures of martensite, bainite, ferrite and pearlite1 are possible depending on the distance from the quenched end and the specific alloy being evaluated.

1 Pearlite is not a phase. It is a mixture of ferrite and cementite (Fe3C).

SCOPE

In accordance with the subject matter covered in the present manual, the scope of this laboratory exercise will be two fold:

1. To obtain experience in the metallographic preparation of metallic specimens;2. To observe the various microstructures of various metal ferrous, non ferros , and

of welded mild steel joint;3. To observe the effects of cooling rate to the amount of martensite and to the

hardness of a steel bar.

PROCEDUREExperiment 1

In this experiment, students will assigned to do a sample preparation, start from cutting and mounting the sample. Each student will have one specific microstructure sample of their own, and the other 3 sample ferrous and nonferrous which have been prepared by the instructure for a whole group member.. Besides the 3 samples, each student have one sample to be heat treating to be prepared and observed, and one macrostructure sample. All the samples have to be obreved with optical microscope, and to be analyzed the microstructure. Sample Preparation have the steps 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.

2. 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.

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

4. 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.

5. Wash and dry.

6. 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.

7. Wash and dry the specimen using a specimen drier.

8. The specimen is now ready for observation.

9. Observe the specimen under a optical microscope.

10. 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.

11. 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.

The photograph of the sample have to be made in micro( more than 100X) magnification and macro scale(less than 100X) magnification .

Experiment 2

In this laboratory experiment will consist of Photomicrograph making and a discussion of some of the microstructures features present in steeel and nonferrous alloys. How the microstructure is modified by thermal processing (heat treatment) is part of this experience. An optical Olympus microscope will be used to present alloy microstructures. The alloys selected are presented in Table I.

Table I. Summary of specimens prepared for viewing from the Metallurgy Department’s metallography sample collection. The listing is in the order of presentation. Composition is

weight percent unless otherwise specified.

Sample Description1234

Heat Treatment SampleMacrostructureMacrostructure

A photomicrograph of each alloy listed in Table I have to be made by students and they have to make note of phases and microstructure features identified during the lecture. This document with student notations is the laboratory report. Turn this document in at the end of the laboratory class.

As the instructor projects each specimen, (1) sketch the basic microstructural features and label these features as they are identified, or (2) identify features on the photomicrographs supplied.

Experiment 3

The recommended austenitizing temperature for medium steel is between 845-950°C.2 The student is expected to research other aspects of the heat treatment process such as: quenching, tempering and the role of alloying elements on hardenability. A standard Jominy end-quench specimen will be heated to the suggested temperature and placed in the Jominy end-quench test apparatus. After the quenching and cooling process is completed, a flat is ground on the longitudinal axis of the Jominy bar. A series of HRC hardness measurement will be made at approximately 1/16-inch intervals along the length of the bar. This data can be compared to published and computed data.

2 ASM Metals Handbook, 8th edition, Volume 4, Heat Treating, American Society for Metals, Metals park, Ohio 44073, p.29.

Figure 7. Schematic of Jominy end-quench facility. After Smith.3

Formal Pre experiment ReportsWrite the report for readers with a broader background then what is expected for memorandum reports, including:

Team Information Team Information transmittal will contain details of team member. Who in the team was in charge to some kinds of materials observed.

Summary Be concise, 100 words or less, present primary objectives, how objectives were accomplished and the most important conclusions.

IntroductionBackground information developed, usually from the literature research, to acquaint reader with the problem and the purpose for carrying out the work (objective of the experiment work).

Experimental ProcedureDiscuss the equipment used and any relevant details. For example, calibration of the equipment.

Final Report

Pre experiment Report will be attached together with Final Report. Final report is consist of:

Experimental Results

Present the results in the appropriate from. For example, present the data as computations, as tabulated data or in figures (photographs or graphs).

Discussion of ResultsThe main point of the discussion is to determine if your experimental measurements have any correlation to measurements made by others. Are the results valid? Why? Were any problems encountered with the experiment design?

Conclusions

3 William F. Smith: Foundation of Materials Science and Engineering, second edition, McGraw-Hill, New Your, 1993, p. 457.

Prepare very concise statements about the experiment. Possibilities are; what was learned, quality of the engineering data, problems with the experimental study and what improvements could be made if the experiment were repeated.

ReferencesList reference in the order they are used in to report. Identify in the report where references are used.

Appendices (Attachements)Use an appendix for any computations or lengthy discussions that is important to the knowledge base, but would detract from the main flow of ideas in the report.

TablesTabulated Data FiguresPhotographs, graphs or other artwork

A set of question have to be answered in the Final report

Reference ExamplesPlace the references at the end of the written report. There are three types of references. There are several accepted formats. The goal is to assure the reader can find the reference in the library.

Technical JournalsAuthor: Journal, year, ser., vol., pp.Cahn, J.W.: Trans TMS-AIME, 1968, vol. 242, p. 166.

BooksAuthor: Book, edition, vol., page, Publisher, Place, Date.Barrett, C.S.: Structure of Metals, p.25, McGraw-Hill, New York, 1954.

Private CommunicationsName: Date, Place, Address.Kellar, J.: January, 2002, South Dakota School of Mines and Technology, 501 E. St. Joseph St., Rapid City, SD, 57701-3995.

Standard Numbering Conventions for Appendices, Tables and FiguresAppendix A, B, C, D --- (Capital letters)

Table I, II, III, IV --- (Roman numerals)

Figures 1, 2, 3,4 --- (Numbers)

Appendix ExampleThe appendix title must stand-alone. This means reference to the main report is not needed to understand what the author is demonstrating. Consider the following.

Appendix A: Derivation of the equation for the theoretical shear strength of a crystalline solid. This model assumes deformation involves entire planes of atoms sliding over each other.

The above is excessively complicated and therefore could distract from the flow of ideas. This appendix could be addressed from the main report as follows.

The theoretical strength of a crystal, assuming the deformation process involves the sliding

of entire planes of atoms over one another, is shown to be This result is

developed in Appendix A.

Table ExampleLike the appendix title, the table caption must also stand-alone. Table captions are always located above the table body.

Table 1. Summary of ASTM E112 grain size measurement obtain from a hot-rolled steel rod. Results from ten independent measurements.

Longitudinal Specimen Transverse Specimen

n Grain Size n Grain Size ASTM E112 Micron-meters ASTM E112 Micron-metersAverage 9.93 10.2 10.14 9.5StdDev 0.15 0.5 0.09 0.3Max 10.11 10.8 10.29 9.8Min 9.78 9.6 10.07 9

Figure ExampleFigure captions must also stand-alone. Figure captions are always located below the figure.

Figure 1. Hot rolled steel rod. The left figure is from the round (longitudinal cross-section) and the right figure is from the transverse cross-section. The arrow in the transverse specimen designates the longitudinal axis of the rod. The dark regions in the photomicrograph are pearlite. The white magnification bars are 50 micron-meters long on the photomicrograph.

Each group (consist of at least 2 Student) will grind,mounting, polish and etchingList of Materials;

1. Carburized Steel; Medium Carbon Steel, Mild Steel2. Welded Steel3. Plating Steel4. Aluminium Alloy;AC4B (Al-Si)5. Cu-Zn (Brass Alloy)6. Al-Mg-Si