Metallography

Metallography

Hussien Amr El Gendy

34-3655

Experiment (A)

To conduct a metallographic examination of a material we first have to go several procedures to be able to observe the material. First of all the material has to be mounted to be able to handle all difficult and different shapes and sizes of materials. Secondly, the material has to be grinded to minimize and eliminate damage on the material’s surface. Then the material is polished to remove scratches done during grinding. Finally the material undergoes etching so we are able to identify where the grain boundaries of the material are. After these procedures the material is ready to be observed.

Table of Contents

1. Introduction……………………………………………… 4
2. Experimental Procedure…………………………… 5

A-Mounting

B-Grinding

C-Polishing

D-Etching

E-Microscope Procedure

1. Results …………………….……………………………….8
2. Discussion…………………………………………………9
3. References………………………………………………10

I.Introduction:

Metallography is the study of structure and properties of metals or alloys to determine various things including: Heat treatment, mechanical processing, material properties and phases present. There are two main types of metallographic observation; Microscopic and Macroscopic. Macroscopic examinations refer to the observations carried out at a magnification of x10 or less which can be done by the naked eye. While Microscopic examinations refer to the examination of the structure at a magnification greater than x10 which require sophisticated microscopes to achieve kind of magnifications such as: Optical Microscope (up to x2000) or Scanning Electron Microscope (up to x 50000). We can predict the general behavior of materials by observing their microstructure. Besides the crystallographic nature of a material, imperfections inside a material have an even greater influence on the mechanical properties, i.e. tensile, fatigue, creep, fracture toughness, impact properties. Some defects such as missing planes of atoms, called dislocations, are responsible for plastic deformation of crystalline solids. Others such as grain boundaries, precipitates, twins and cracks alter stress distribution in a material and the accompanying motion of dislocations. Some defects such as missing atoms and dislocations cannot be observed optically except by their effects, i.e. strain, etch pits, slip lines. Other defects such as grain boundaries, twins, precipitates, can be observed readily in the microscope. We will now discuss the essential procedures so we can observe the material under a microscope.

II. Experimental Procedure:[pic 1]

Equipment:

• Struers LaboPress 1 sample mounting device.
• Struers LaboPol 5 sample Polishing device.
• Zeiss Microscope with digital camera.
• Data acquisition system.

A. Mounting:

Mounting is the first step for preparing a specimen for examination. It is necessary for easier handling of the material.

First the specimen is placed in the Struers LaboPress 1 sample mounting device which molds the specimen using heat (Hot Mounting), and then the appropriate mounting resin is placed. The mounting device reaches a temperature of about 180 degrees Celsius where the where the specimen is left to mold for about 10 to 12 minutes. Once the molding is complete the specimen appears the in this fashion.

[pic 2]

B. Grinding:[pic 3]

Grinding is the second and most vital step in specimen preparation. It is necessary for minimizing and removing surface damage to the specimen.

We use a Struers LaboPol 5 sample polishing device for both grinding and polishing the specimen by only changing the paper used. Firstly a silicon carbide paper with low grit number (100) is mounted onto the device. Turn on Struers LaboPol 5 for 5 min with 150 rpm. The specimen is held face down on the device as the paper is being rotated rapidly and grinding the surface damages until there is an even pattern. This procedure is repeated with finer papers each time (from 100 grit to 1000 grit). Also after each paper change the specimen should be rotated 90 degrees so as to eliminate the scratches done by previous rough grinding. Water is very essential and should be present in all of the grinding processes. Water washes away the removals and dirt from the grinding. It also prevents friction of the specimen with the rough paper so not to damage the structure of the material and prevents it from overheating.

C. Polishing:

Polishing is the third step of specimen preparation and is pivotal for removal of scratches done during grinding procedures.

Polishing is also done on the Struers LaboPol 5 with a difference in the paper discs. Instead of the silicon carbide paper, the polishing discs are covered with soft cloth impregnated with abrasive diamond particles and an oily lubricant.

During the polishing, the specimen should be held firmly in contact with the polishing wheel, undue pressure should be avoided. During polishing, the specimen should be rotated 90 degrees around the wheel to give an even polish.