Introduction to sample preparation and optical microscope for metallographic observation.

Introduction

Title

Introduction to sample preparation and optical microscope for metallographic observation.

Objectives

Upon completion of this experiment, students should able to:

·         To understand preparation procedure of metallographic sample.

·         To understand the concept of etching and the purpose of this process.

·         To understand basic function of optical microscope and its operation.

·         To use and perform analysis from the microstructure’s observed.

Apparatus

Abrasive cutter machine, Specimen cut off machine, Auto mounting press Machine, Grinding Machine, Silicon carbide paper, Polish machine, Racine, Hardener, Phenol Powder, Polishing Powder, p Glove, and Goggle.

Background

Materials engineers 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.

Sample prepartion

In the study of matellic materials it is often to analyze the phases exist and grain size in the structure.The structure of metal cannot be see through naked eyes but it can be seen in microscope.Characteristics of the metal such as grain structure,effect of heat treatment and carbon content of steel can be determined by studying the micrograph.For this experiment ,the metal used in the metallurgical examination must be prepared and polished carefully before good image of the microstructure can be seen.This is very important to ensure that surface is smooth and to avoid a confusion when we obseve the sample.Saveral method are required:

1)      Cutting

-This process is to cutting the sample to the size that we need  and make it easier to hold or molded.

   

2)      Molding

                                         

i) hot mounting                                                            ii) cold mounting

-Molding process is make the sample easy to hold.This process have 2 type, cold mounting and hot mounting.Basically we will use hot mounting to prepare the sample compare with cold mounting because it only take 10 minutes to be provided.

3)         Grinding  

-This process is use remove the rough surface and  improve the surface to shine.

      4)      Polishing

- Polishing is carried out on cloth covered rotating wheels. During the polishing,the specimen    should be held firmly in contact with the polishing wheel and th e specimen should be rotated around the wheel to give an even polish.

     5)    Ecthing

            

- Etching is done to bring out the structure of the polished specimen.  It is usually performedby subjecting the polished surface to the chemical action of an appropriate reagent. 

Introduction to Optical Microscope.

            Because of its ability to study objects with highly polished like metals, a metallurgical microscope ids different from other microscopes. The many metallurgical microscope will allow them ti explore different fields and broaden their knowledge with just one tool. The study of metals and alloys and more specifically metallographic, the microscopic examination of metals and alloys, a metallurgical microscope, especially a high end one, is generally equipped to provide great help in other fields of materials science as well. After a mirror-like metal surface has been prepared, now the structure can be observed under optical microscope for analysis. Before interpreting of the structure that you observe, it is important to understand some of basic operations of optical microscope in metallurgical study. The optical microscope magnifies an image by sending a beam of light through the object as seen in the schematic diagram. The condenser lens focuses the light on the sample and the objective lenses (10X, 40X,…, 2000X) magnifies the beam, which contains the image, to the projector lens so the image can be viewed by the observer. In interpreting the microstructure, it is helpful to consult with the phase diagram and to have some knowledge of the composition and thermal history of the specimen. If both resources are used, phase can be often identified and the sequence in which they formed can be traced.