- Education and Science
Metallography and Analytical technique
In the land of Mechanical and Metallurgical Engineering, it is hard for any student to graduate from the department without offering a topic called Metallography and Analytical technique. I decided to write on this topic because that was one of the works I performed in National Metallurgical training Institute, during the period of my industrial training. I worked with metallurgical Engineers for the period of six months until I finished my training with them. What is metallography? Metallography is the study of the topography or the micro-structure of material by passing the material through various stages and finally viewed under electron microscope. The stages that take place (analysis) in metallography are cutting, filing, grinding, polishing, etching and viewing. All these analysis are carried out one after the other.
This is the first stage and it is carried out in the foundry workshop. The workshop is where the portion of the metal to undergo metallography is cut before proceeding to metallography laboratory. I will like to sound a warning at this point; do not enter the foundry workshop without taking note of the safety precautions. This is to avoid injury when working. The portion of the metal that is cut out is called sample. This is the specimen to work with in order to obtain the micro-structure of the specimen at the end. The specimen I used during the experiment is mild steel. A long mild steal is placed between the vice and then held tight before it is cut out with hard saw. The cut portion of the metal is to be levelled during the cutting process to ensure good workable surface. The pressure exerted during the cutting stage must be minimal so that heat will not be generated, which may affect the final result.
It is the second analysis that comes after cutting. This is done to smoothen the surface of the sample. Filing stage is carried out with the use of metal file which removes some metal particles by rubbing the file against the surface of the sample until the desired result is achieve. Make sure you use the surface of the file that is less rough so that you do not make the surface of the sample rougher. Once you see that the surfacing you are filling is smooth, stop filing and proceed to the next analytical stage. You can call your instructor or any metallurgist around to help you when you get confused at this stage.
I discovered that this stage was the most difficult when I was working in metallography laboratory. You may ask how? The reason is because this analysis involves grinding from one emery paper to the other. I grinded until my hand started to pain me. I left the sample in the laboratory to come back the next morning and continued from where I stopped. Grinding in this context means rubbing the sample surface against emery cloth of various sizes. Note that emery paper is the same with emery cloth. You have to grind until no strips is seen on the surface of the sample. Note that if this standard is not maintained, the strips will affect the final result of this experiment when viewed through electron microscope. After grinding on the surface of an emery paper, ensure that you clean the surface of the sample with cotton wool before transferring to the next emery paper which is of smoother surface. Why you should clean the surface is to avoid transfer of mesh from one surface to the other. Any mesh transferred can alter the smoothness of the surface of the sample. During grinding stage, little or no pressure is applied. The sizes of emery papers used during grinding of mild steel are P60, p100, p240, p320, p600, p800 and p1200. The given sequence is from the rough to the smoothest surface. It means that p60 is the roughest of the emery paper sizes and p1200 is the smoothest. The grinded surface of the sample is cleaned before going to the next stage, which is polishing.
This comes after the surface of the metal sample has been grinded using emery paper of various sizes. Polishing is conducted using gamma-Alumina powder. The powder used in polishing the surface of mild steel is usually of two types. One of the powders used in mild steel polishing is rough and the other smooth. What make them rough or smooth are the grain sizes. Rough gamma-Alumina powder has larger grains and the smooth has smaller grain sizes. The rough is used on the grinded surface before the smooth. Polishing is carried out by rubbing the surface of the grinded surface against Gamma-Alumina powder. The polishing is continued until mirror image of the metallurgist carrying out the experiment can be seen with the polish surface. Some students do not achieve desired result in metallography because they become tired at a point and do not obtain the desired mirror-like surface needed. You must obtain mirror-like surface, if not, you are not done with polishing.
It is treating the surface of polished sample with chemical called etchant. During the time I was conducting the analytical process, I made use of an etchant called Nital. This etchant is popularly used on mild steel. The etching time varies, depending on the metal you are working with. When you etch, make sure that you wear hand gloves so that you will not be attacked by chemical. Note that etching is done in a place called fume chamber. Other etchants that can be used in etching are Picral, Vilella reagent, Marble reagent and Keller reagent.
It is the last stage in the analysis process of metallography. This stage involves viewing of the etched surface. The etched sample is properly suited under the electron microscope and then viewed. The sample is continuously adjusted until the microscope is well focussed. The viewing is carried out under microscope of various magnifications. When I viewed with mild steel, the result observed is martensite. Under the electron microscope, it has needle-like shape. The observation also shows that the martensite has more of white patches. The white patches are indication that mild steel has less percentage of carbon and exhibits soft property.
In conclusion, this topic has gone deep on metallography and analytical technique. The sequence of operations that went on during this practical was explained by me. I am glad that you understood everything on this page.
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