Microstructure examination of steel.
To observe the constituents and structure of metals and their alloys by means of an optical microscope.
Metallography can be defined as the investigation of metals and their properties through the use of an optical microscope. Metallography is being used in instances such, we need to compare some metals and their respective properties. This is really important for materials engineering as we need to study about various materials and their behaviors under various circumstances.
Macrostructure and Microstructure
To observe these structures we need to use a high scope of amplification. Specifically these structures are seen at the magnification between 20X to 2000X.
The structures which can be seen under low scope of amplification are called as macrostructures. (between 5X-10X)
This equipment is used to reveal microstructures which can’t be seen to a naked eye. Mainly there are three types of microscopes.
Uses the principal of optical technology.
The optical microscope is used for the measurements regarding the resolutions to nearly the equivalent to light and the maximum possible magnification is approximately 2000 diameters.
Its hard to observe the properties of the materials which doesn’t transfer light.
The observation is done by the use of an electron beam.
The scope of amplification is really high.
There are two main types of electron microscopes.
1. Transmission Electron Microscope.
2. Scanning Electron Microscope.
3.Scanning probe Microscope-
The tool generates a photomicrography map on atomic scale that is a representation of the specimen’s characteristics and surface features.
Really high scale examination possibility. (More than the electron microscaope)
The main aspects of this practical are as follows-
3. Chemical homogeneity.
4. Distribution of stages.
5. Elongated structures framed by plastic distribution.
The test is all about revealing the microstructures of mild steel and figuring out the properties of the particular material. For this test we use mild steel which contains around 1% carbon.
Selecting: A random sample should be selected for the inspection. Basically an average piece of metal is enough. It may not be same or either composition or crystal structure.
Mounting: If we undergo this step we can optimize our result.
Grinding: This is done to remove the surface damage points which occurred which the cutting process.
Polishing: Soon after grinding the specimen is undergone by this process. We use a smooth velvet surface to polish the specimen more efficiently while using the diamond paste.
Etching and the use of alcohol: This helps us to wash off the thin surface layer which was deformed by the previous steps. Doing this step more accurately helps us to observe the micro structures more clearly and precisely. For this we can use –
1. HNO3-2cc-Ethly alcohol
2. 1-5%HNO3 in alcohol
Anyhow the surface of the specimen should remain nutral as it could react with Oxygen.
Materials and Apparatus
1. A specimen (mild steel)
2. Power supply
3. Water supply
4. Diamond lapping paste
5. Nitric acid
6. Ethyl alcohol
7. Light microscope
8. Grinding and polishing machine
12. Piece of clothes
1. Initially a random specimen was chosen which was cut from a mild steel bar.
2. As we could observe some surface cuts and damages in it, the specimen was grinded using the grinding machine. For the grinding process silicon carbide papers were used. The grinding process was carried out for 30 minutes.
3. After that the specimen was polished using a red velvet along with some diamond lapping paste on it. The specimen was polished until it got a mirror-like surface.
4. The piece of mild steel was dipped in a concentrated Nitric acid solution (2%). This process is called as the etching process.
5. For the specimen surface to be neutralized it was then washed by ethyl-alcohol.
6. After that the specimen was left to dry.
7. The cleaned surface of the specimen was kept upwards in the microscope to be observed by us.
8. The grains and the intersections were observed with the use of the microscope.
9. The number of grains in each line were counted and the total number of grains were found.
10. Finally the calculation was done precisely.
Observation and Calculation
The practical microstructure is a test which doesn’t harm the object physically, there for its known as a non-destructive experiment of materials testing. All steps of this practical should be done in room temperature so that it prevent the properties of the specimen to change.
Errors that can occur
When we try to separate a piece of steel for our observations there could be little fractures in the sawing process. If such fracture occurs we won’t be able to observe the specimen properly. During the process of polishing we should add the correct amount of diamond lapping paste to the velvet. Etching process should be done with the correct concentration of HNO3. Using HNO3 more than the required amount would lead the specimen to burn or have defects. Therefor we need pay close attention to the practical and prevent errors from occurring until all steps are done and the results have been figured out.
Improvements/measurements to avoid errors.
We should add a little water to the rotating wheel plates to minimize the friction and the scratches which would happen on the specimen. The concentration of the acids and the alcohols should be measured properly as the specimen could react with Oxygen in air if there’s any acidity left. Ultimately the grains in the lines should be counted twice in order to prevent any calculation error.
Identify each phase present in the observed microstructures.
“A phase may be defined as a homogenous portion of a system that has a uniform physical and chemical characteristics.”(Callister, Jr and G. Rethwisch, 2000). According to the citation above, it implies us that a phase could be either pure solid, liquid or gas. They can be graphically identified by phase diagrams.
Phases are mainly categorized under two aspects-
Systems with single phase diagram (Homogeneous) Eg- Metal
Systems with two or more phase diagrams(Heterogeneous) Eg- Polymers.
Usually when we categorize the microstructure of metals we consider the the number of phases present and their arranging order.
The microstructure alloys depends on:
Alloying element present.
Heat treatment of alloy.
Phase equilibrium- The equilibrium is a system in which more than one phase in existence.
Metastable: -The system where rate/approach to equilibrium is very slow.
The symbols used to mention each phase is given below.
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