Microstructure Examination of Steel

To observe the constituents and the structure of a piece of steel with the support of an optical microscope.

Introduction

Metallography is the study of the detailed microstructure of surfaces of metals using a light microscope or an optical microscope. Some of the structures are visible to our naked eye and these are named as macrostructures whereas the structures that are not visible to our naked eye and that have to be magnified to be observed are called as microstructures. Microscopy is used for various purposes and one of its very important uses is to identify and learn about defects in metals which is exactly the thing we have done in this lab experiment. We magnify these objects to examine their microstructures and conduct studies related to them. For this experiment, an optical microscope is sufficient since we do not need to study details down to atomic resolution.

Metals gain different properties due to imperfections in their crystal level structure, Microscopy can be used to obtain information regarding the composition of a metal, its properties and many other details related to it. In this experiment we use an optical microscope to study the grains and the grain boundaries of steel which by the way is a metal. The grain boundary is a transition region in which some atoms are not exactly aligned with either grain. Microscopy can give information mainly about:

1. Grain size.
2. Phases present.
3. Chemical homogeneity.
4. Phases present.
5. Elongated structures formed by plastic deformation.

Theory

Since we need a comprehensible view of the microstructure, the specimen should be handled with absolute care while being prepared. For this purpose, there are certain steps that should be followed while preparing the specimen for observation. To begin with, the surface of the specimen is thoroughly grinded using a grinder with an abrasive paper. After this, since the specimen is rough, it is again polished to get a mirror like surface, for this purpose, the specimen is kept over a red velvet cloth with some diamond lapping paste applied over it. After these steps are done, the specimen is dipped in a solution of Nitric acid of 2% to 5% concentration, in fact, the maximum concentration of Nitric acid that can be used is 50%. It is dipped in acid to remove the tiny bits of impurities that would be sticking over the specimen after grinding and polishing it. In addition to that, this brings the grains to the surface which in turn makes our job of observing it much easier. Since the specimen is now acidic and it can react with air and form other impurities, it is then dipped in some ethyl alcohol to neutralize it. Before it can be observed, the specimen has to be left to dry. Finally, an optical microscope is used to observe the grains. Materials and Apparatus:

1. Piece of steel.
2. Grinding and polishing machine.
3. Red velvet cloth.
4. Diamond lapping paste.
5. Optical microscope with magnification 50x to 1000x.
6. Nitric Acid (2% concentration).
7. Abrasive paper.
8. Ethyl Alcohol 99.9%.

To begin with, a piece of steel specimen of around 4cm was already prepared proceeding to the initiation of the experiment.

Then, this specimen was grinded well using the grinder, it was actually kept over an abrasive paper. While grinding, the speed was controlled using a knob. Moreover, water was added to keep the specimen from getting warm while also acting as a lubricant. This was done for around 20 minutes after which the specimen was well grinded.

Next, the specimen was polished using a red velvet cloth which was in fact wrapped around a part in the grinder itself. Additionally, Diamond lapping paste was applied on the red velvet cloth to increase the efficiency of the polisher. Once again, the speed was controlled by the knob in the grinder. This was continued until a mirror like surface was obtained.

Subsequently, the specimen was etched in a solution of Nitric acid of 2% concentration.

Afterwards, the specimen was dipped in Ethyl alcohol 99.9%.

The specimen was then left to dry for a few minutes.

Finally, the specimen was kept under the microscope and viewed, the required photomicrograph was then obtained after which the calculations were done.

After completing all the steps mentioned above, we were able to observe the view of the grains and the grain boundaries as we expected. In fact, the grain boundaries were not as thick as we expected. The grain boundaries were irregular and the grain areas were scattered.

Talking about the phases present in the microstructure, Phases are regions in atomic level which are homogenous and separated from other similar regions by surfaces and have homogenous properties exclusive to them. In a particular phase, the properties, composition and the structure are indistinguishable. Phase change can occur as a result of increase in temperature (but it depends on some other factors too).

Let’s consider the phases present in the microstructure of steel which is indeed iron with 2% carbon. The main phases present in steel are as follows.

• Phase
• Austenite
• Allotriomorphic Ferrite
• Idiomorphic Ferrite
• Pearlite
• Widmanstätten Ferrite
• Upper Bainite
• Lower Bainite
• Acicular Ferrite
• Martensite
• Cementite

Austenite is one of the phases in steel and it has an FCC (Face-centered cubic) structure. This exists at around 1150°C. When the temperature decreases, a phase change occurs. Austenite transforms into allotriomorphic and idiomorphic ferrite. Ferrite is stable at room temperature. Meanwhile, cementite which is another phase is also formed. Cementite has properties which are drastically different to the properties that ferrite possesses. Pearlite is a result of the combination of the above formed ferrite and cementite. Martensite occurs when red hot steel is quickly cooled to a very low temperature. When austenite is cooled to an even lower temperature, Bainite is formed , they are strong but ductile.

Importance of metallography

Metallography is the study of the microstructure of metals. Today, an enormous number of metals have been discovered and these are being used in our day to day life. The use of alloys and metals take an important place in engineering applications. Therefore, engineers have this task of selecting the best metals for whatever they are modeling. They have to test whether the metals have been processed precisely. Most of these metals are opaque; these materials are observed with reflected light using microscopes of high magnifications. This is the principal use of metallography. Metallography helps a metallographer identify the properties and its composition. It can be used to identify its colors, grain size and its phases too. It can help one track the processes it had gone through until that particular point. Furthermore, the metal can be checked for the mixture of alloys too. Thus, metallography is of utmost importance in producing the best out of the lot we possess while improving the quality of products that already exist.

Errors and Precautions

• When selecting the specimen for the experiment, special precautions should be taken.
• Grinding should be done for at least half an hour and it should be done thoroughly, failing to do so will leave impurities thus resulting in a less accurate observation.
• During grinding, a coolant should be sprinkled on the abrasive paper to prevent the specimen from getting warm and to keep excess shavings from building up.
• When polishing the specimen after grinding, diamond lapping paste should be used to increase the effectiveness.
• The concentration of Nitric acid used should not exceed the concentration mentioned or else it would form a layer of oxide.
• The etching process should not be carried out for more than the specified time as it would result in the deformation of the grain layer.
• After etching, the specimen should be washed with ethyl alcohol or else it would be acidic and corrosion can occur.
• After washing with ethyl alcohol, the specimen should definitely be washed with distilled water and it should be left to dry so that we are sure that we are observing an actual defect in the grain and not one that has arisen because of water particles.
• Should take extra care when drawing lines to measure the grain size because a difference in length would decrease the accuracy of the calculation.

Conclusion

This experiment granted me the opportunity to widen my knowledge about the microstructure of metals and how it can be used to identify their properties. In addition to that, this experiment allowed me to observe grains, grains boundaries and learn about them. Moreover, I could learn about phases and specifically, the phases present in steel. I think this experiment has been very useful since it’s going to be very important to be able to select the best materials for any process in our future engineering careers.