Experiment #1Chemical attack applied to metallographic specimens

Objective

Identify the phases and microstructures of metallic specimens in the metallographic microscope.

Materials

Chemical reactant. Ethanol. Metallographic specimens of ferrous and nonferrous materials, the specimens should have.

shinning superficies (a mirror finished ).

Equipment

Fan (to inject warm air).

Work to do

Work in the specimens superficies. Identify the microstructure of the metallographic specimens.

Technic

When the specimen superficies has a mirror finish, the superficies should be cleaned with ethanol, and then is drying with the hot air injected by the fan. When the superficies have been dried we should applied the chemical reactant, we need to know which kind of chemical reactant we are going to use, because each material have a specific cleaning compound, but sometimes just some chemicals are used for different materials.With the chemical reactant working in the specimen surface, we should wait a few minutes after remove the chemical compound.Then, this compound is removed with cotton, the specimen is washed with alcohol and then is dried with a hot air flow.After this procedure the final step is see the microstructure using the metallographic microscope.

Results:

In the figure 1 we can see the finished in a) and the microstructure in b) for the ferrous specimen (1018 steel). Looking the microstructure we can notes the proeutectoid ferrite and perlite. Then, in the figure 2 we can see de same, the finished in a) and microstructure in b), but now is for the nonferrous specimen (Cu-Sn). Looking the microstructure we can notes that betha phases are the dark parts and alfa phases are the white parts.

a) b) Figure 1. Finished and microstructure, in a) and b) respectively.

a) b) Figure 2. Finished and microstructure, in a) and b) respectively.

Polished technic

We used a different polished technic, taking care of two things:-The specimen should maintain a low temperature similar to the environmental temperature.-The sandpaper should resist the operating conditions.

The technic consist in use a lathe, the steps followed are:-The specimen should maintain a low temperature similar to the environmental temperature.-The sandpaper should resist the operating conditions.

The technic consist in use a lathe, the steps followed are:

1. The lathe is configured to operate at 100 rpm.2. Set the specimen at the pegbox.3. Cool the specimen with a constant flux of water.4. Do a facing cut process.5. The lathe is configured again to operate at 400 rpm.6. Do a facing polish process with the first sandpaper. (Each process should take twenty

seconds).7. Do a facing polish process with the second sandpaper. 8. Do a facing polish process with the last sandpaper. 9. Do a facing polish process with alumina in the turn table.

The whole technic of above is illustrate in the figure 3, where in a) we can see the lathe holding the specimen, in b) and c) shows the lathe working and freshing the specimen with water, obviously changing the sandpaper, and in d) shows the specimen doing to it the facing polish with alumina in the turn table.

a) b)

c) d)

Fugure 3. Whole polish technic.

Experiment #2Thermic treatments

Objective

Learn the methods and procedures of the different thermic treatments used to modify the original properties of the material.

Materials

Metallographic specimen: (steel, brass, cooper, aluminum alloys). Liquid solution of ammonium persulfate. Liquid solution of fluorhydric acid. Mix of Nitric acid and alcohol. Cooling medium: water, salt, oil and sand.

Equipment

Rockwell hardness tester. Vickers hardness tester. Furnace.

Work to do

Get the hardness of the specific material used in this practice after and before the thermic treatment.Classify and identify the phases and microstructures presents in the metallographic specimens after and before the thermal treatment.

Technic

First we should get the hardness of the material; it will be the initial hardness of our initial material.Under microscope we should determine the microstructure of the material after and before the treatment.Consulting the transformation graph and the phase diagrams, the temperature and the time of the treatment are determined. After the thermal treatment the superficies is polished again.The phases and the microstructure are determined again.The new hardness is obtained.

Our thermical treatment for the ferrous specimenTempered with oil like cooling media

Application (Truck springs)

As we know, the tempered process will increase the material hardness. But a high hardness is translated in a high fragility. Exist applications in which the fragility is not an important property of service, for example, a shave knife shall be hard in order to preserve the edge sharp and the high fragility of this knife is not considered because this kind of knives are never used under high forces. But, also exist situations in which the fragility is very important, in our application, a soft steel is not desirable because it has a “high” ductility, if we want to make truck springs this property give us deformables springs which give like a result an useless suspension system.

But if we do a tempered treatment with a fast cooling process, the steel will present martensite in its microstructure. Now the soft steel increase its hardness, but increase the fragility. Fragile steel is not desirable because it will break under the application of an intense force, give like a result a useless suspension system.

Is clear. A soft material will fail, but a hard steel will fail. Now the question is, how obtain the specific steel for this use?The answer is in the steel, in the alloy carbon contain, but the important part of the answer is in the thermical treatment, specifically in the cooling medium. If we cool the specimen in water is predictable obtain martensite (obtaining a hard material), but if we cool with a different medium, the hardness will be different. In our case, the oil cooling process will increase the hardness in a desirable amount.So, the truck spring is show in the figure 4. Figure 4. Truck Spring.

So based on the figure 5, shows the stated and knowing material composition. Observing the iron carbon diagram, the treatment temperature is defined, this temperature is 900°C in a enough time to get a temperature that is defined inside the austenite zone.As we can see in the fugure 7a, the heating was made in the electrical furnace in a nonprotected atmosphere.Is important wait until the furnace indicates that the temperature was achieved.Now, with the specimen at the increased austenizing temperature, the next step is put the specimen in the cooling medium as we can see in the figure 7b, for this case this medium is oil.After some hours the specimen has the environment temperature and is ready for the cleaning process.When the hot specimen is cooled in oil, an important layer of carbon and other compounds is formed on the superficies of the specimen as we see in the figure 7c, if we took the Rockwell hardness with this dirty specimen the Rockwell hardness will be an incorrect lecture, because the layer will absorb part of the force applied in the hardness test.

Figure 6. Phase diagram of the steel.

a) b)

c)Figure 7. Specimen after the Thermical Treatment.

The figure 8 shows the microstructure after the chemical attack where we can see the much finer ferrite structures with increased perlite.

Figure 8.

Increasing the amount of fine pearlite and reducing the size of the visible grains, we have a greater hardness but not so high as in cases where the appearance of a high amount of martensite occurs.Now compared with ferrite structure and fine pearlite we could find similar phases as it shows in the figure 9.

Figure 9.

Now, the hardness obtained before and after treatment are show in the next table:

Hardness test taken: Rockwell B hardnessBefore the treatment: 63-65

After treatment: 64-68.5One day before the treatment: 67-68.5

As we see the hardness of the material increases but in a lightly way. It is a expected result which corroborates our assumptions.

Justificacion de resultados en español:Al haber hecho un proceso de templado lo esperado es la obtención de martencita en la microestructura lo cual como se mostro anteriormente no fue del todo cierto, si no que se obtuvo perlita fina, esto pudo ser debido a una descarburización en el material o bien a que la velocidad de enfriamiento no fue la contemplada.Del libro de Avner “Metalurgía Física” se encontró la siguiente tabla:

Dicha tabla sugiere que el enfriamiento del acero en aceite lento oscila entre 36 y 32 grados Farenheit por segundo es decir, aproximadamente entre 20 y 18 grados centígrados por segundo.Ahora al observar un diagrama TTT para el acero

Se denota que durante los 10 primeros segundos del enfriamiento se reducen 200 grados lo que coincide con el inicio de la transformación de la Perlita gruesa para instantes después terminar como perlita fina

Our thermical treatment for the nonferrous specimenAging with air like cooling media

Application (Industrial valves, using environmental temperature fluid)

The bronzes are highly used in the field of engineering, in our case our starting material was perfect for applications such as bearings however the hardness of this material was low, it means that its ductility was high, the explanation of why a brass bearing must be soft is based on the fact that a hard material will penetrate a soft material first, thus a hard material wears a soft material first, in all bearings is desirable to replace a worn bearing before a worn shaft.As seen the original properties have their advantages but, what happens if we make a bronze threaded?

A ductile material will give us a threaded that will deforms in presence of high stress, this deformation will cause a fail in the threads. This is totally undesirable, for this reason this aging treatment was done, our objective was increase the bronze hardness. So, figure 10 shows the application of a industrial valve.

Figure 10.

Based on the diagram in the figure 11 and taking the correct data for this composition, the team choose a aging treatment to increase the bronze hardness.

This treatment should be made at 300°C for this case, we made the treatment al 350°C to ensure the phase change.

After the cooling process, the specimen is cleaned; the new microstructure is now defined in the figure 12. We can see a solid solution of solid betha, solid alpha and the most important phase for this case, the precipitate.

Figure 11.

Finally we do the hardness test, having the next results show in the table:

Rockwell hardness test Hardness ranges Rockwell B obtainedBefore the treatment: 15-17

One our before the treatment: 22-25Four hours before the treatment: 25

At nex day: 27

As we can see the aging process is a successful way to get an increasing in the hardness, with these results we confirm our initial assumptions.Is important remember that the hardness will decrease with the time, in this case we only see a increase in the hardness because the time between lectures was relatively short, for future hardness test the values will be different and smaller than the initial and medium data.

Justificacion de resultados en español:Como se mostro anteriormente el tratamiento propuesto fue el de endurecimiento por envejecimiento, para lo cual es necesario que el diagrama de fases cumpla con lo siguiente condición:

Y se puede denotar que para la composición planteada en el material no ferroso el diagrama cumple con el limite de solubilidad y a la temperatura planteada, lo que nos da la certeza que el tratamiento fue bien realizado.Ahora bien al comparar la tabla de mediciones de dureza obtenida con un diagrama típico de endurecimiento por envejecimiento:

Se puede denotar que el diagrama sugiere que después de cierto tiempo la dureza va aumentando hasta llegar a un valor máximo esto debido a que la fase inestable se va estabilizando conforme pasa el tiempo, para después ir reduciendo gradualmente hasta llegar a un equilibrio, tal y como sucede en la tabla presentada.