Materials Polishing Manual. By Thomas Perry Daniel Webster College Version 1 August 10, 2007

Transcription

1 Materials Polishing Manual By Thomas Perry Daniel Webster College Version 1 August 10, 2007

2 Materials Polishing Manual This manual describes how to use the equipment at Daniel Webster College to create specimens of material samples for study under a metallographic microscope. The general procedure is: 1. Create specimens in the Pneumet press. 2. Mount the specimens in the polishing retainer. 3. Polish specimens: a. Rough Grind specimen flat and expose material surface. b. Fine grind to remove scratches from rough grind. c. Polish specimen using alumina powder suspension. 4. Etch specimen to expose grain boundaries. Figure 2 Pneumet Press Figure 1 Grinding/Polishing Table setup

3 Creating the specimen For this operation, we will be placing the sample in the cylinder at the top of the Pneumet press pictured at right, and then covering it in a thermoset powder, which, when heated will encase the sample so it looks like the sample in Figure 4 Finished sample. Figure 4 Finished sample Figure 3 Buehler Pnuemet Press 1. Place the sample in the Pneumet I Press a. Make sure the air hose is connected to the hose in the rear of the cabinet. b. Unscrew and remove the Tetramet Closure from the top of the cylinder (see Figure 5 Tetramet Closure) c. Turn the Pressure Regulator knob to the right until the air pressure reaches 80 ksi and then move the Ram Control lever to the right (Up) and wait for the ram piston to move to the top. d. Place the material sample on top of the piston so that the surface you want to view is facing down at the piston surface (see Figure 6 Sample placed on ram piston). Figure 5 Tetramet Closure Figure 6 Sample placed on ram piston

4 e. Move the Ram Control lever to Down and wait for the piston to move to its lowest position. f. Pour enough black crystal phenolic powder into the cylinder to completely encase the sample plus some extra (typically ~20 ml). g. Screw the Tetramet Closure to the top of the cylinder, making sure to not overtighten it. 2. Heat the specimen to cure it a. Move the Ram Control lever to the right ( Up ). Place the Automatic Heater over the cylinder and turn it on. Figure 7 Pouring Phenolic powder into cylinder b. Set the timer to 7 minutes. c. It is a good idea to degas the sample. To do this, when the air pressure reaches 80 ksi, move the Ram Control lever to Down. When the air pressure reaches 0, push the lever back to Up. Repeat this process one more time approximately 30 seconds later. 3. Cool the specimen a. When the timer reaches 0, turn off the Automatic Heater and place it on the post on the back of the machine. Be very careful as the ring will be very hot. Figure 8 Curing specimen b. Place the Cooler over the cylinder. c. Set the timer for 5 minutes. Do NOT lower the ram until the specimen has cooled for at least 5 minutes with the Cooler over the piston. Figure 9 Cooling the specimen

5 4. Remove the specimen Notes: a. When the timer reaches 0, turn the Ram Control lever to the Down position and wait for the piston to reach the bottom of the cylinder. b. Unscrew the Tetramet Closure and let it sit there. It will likely not want to come out of the cylinder. DO NOT force the Tetramet Closure out of the cylinder. c. Turn the Ram Control lever to the Up position and wait for the piston to push the new specimen and the Tetramet Closure to the top and out and then remove the specimen. If the specimen remains attached to the Tetramet Closure, just hit it softly on the side with a screwdriver or similar implement to knock it off. Be careful as the specimen and all the metal parts are very hot. d. Using the belt sander, grind two small flat surfaces opposite each other onto the sides of the specimen to ease manipulation of the specimen with tongs later in the process. 1. It is very important to maintain the appropriate pressure throughout the whole process. 2. The heater works very quickly, there is no need for preheating. Figure 10 Removing the specimen 3. Make sure to thoroughly cool the specimen before removing it, or the specimen will likely be damaged. 4. Do not allow the air pressure to go over 110 ksi or the unit may be damaged. 5. Do not supply the unit with over 125 ksi of air pressure or the unit may be damaged. 6. Note the symbols on the pressure gauge. The cylinder is 1.5 diameter and pressure is at 4.2 ksi, so the symbol applies when reading the gauge.

6 Mounting the Specimen in the Retainer In this step we mount the specimens in the Specimen Holder (see Figure 11 Loading the Specimen Holder.) that holds the specimens against the wheel during polishing. It is recommended to polish a minimum of 3 specimens at a time. 1. Make sure the set screws in the Specimen Holder are loosened. 2. Place the specimens into the holes in the Specimen Holder. Make sure the samples to be studied are facing down, and that the pins of the holder are facing up. The specimens should be sticking out of the bottom of the holder by about 3/8 of an inch. 3. Place the Loading Fixture Base on the Swivel Base. 4. Place the Specimen Holder on Loading Fixture Base, taking care to align it with the locating pin. 5. Push the specimens down so they are in contact with the Loading Fixture Base. The samples should be facing the Loading Fixture Base. 6. Place the Pressure Plate down onto the assembly and tighten the Loading Fixture Knob as tight as it will go. 7. Tighten each of the set screws around the outside of the Specimen Holder. 8. Loosen the Loading Fixture Knob and remove the Specimen Holder from the assembly. You are now ready to grind and polish the specimens. Figure 11 Loading the Specimen Holder.

7 Grinding and Polishing the Specimens In this multi-step stage of the process you will grind the faces of the specimens flat and perpendicular, and then polish the surfaces of the samples smooth. For all steps, we will use the Automet attachment to hold the specimens to the grinding/polishing wheel and apply pressure. First, you will rough grind and then fine grind the samples using several grades of adhesive backed sandpaper on a bronze polishing wheel. To avoid burning the samples, always apply running water to the wheel while grinding. Then we will place an adhesive backed polishing cloth on another wheel and apply a 0.3 micron alumina slurry to the wheel to polish the samples smooth. For grinding, we will use the following sand paper grades in order (see Table 1 Sandpaper grades for grinding.). Figure 12 Sandpaper Disks and Polishing Cloth Disk shows the various grades of adhesive backed sandpaper disks and a polishing cloth. Table 1 Sandpaper grades for grinding. Operation Rough Grind Fine Grind Fine Grind Fine Grind Grit Polishing 0.3µ Alumina Slurry on Polishing cloth Figure 12 Sandpaper Disks and Polishing Cloth Disk

8 Figure 13 The Grinding/Polishing Table

9 1. Rough and Fine Grinding a. Wash and dry the bronze polishing wheel and place in polishing/grinding unit. b. Remove the backing from a 180 grit sandpaper disk and apply the disk to the polishing wheel, making sure to not trap any bubbles underneath the paper. c. Turn the water spigot over the left side of the polishing wheel and wet the wheel. d. Place the Specimen Holder on the polishing wheel, aligning the pins with the detent in the ring on the polishing/grinding unit. Figure 14 Wet wheel before placing Specimen Holder e. Before mounting the Automet on the polishing wheel shaft, turn the Automet Loading Handle counterclockwise until it catches in the safety. Then pull out the safety and continue turning counter clockwise about 180 degrees more. Figure 15 Place Specimen Holder on Wheel Figure 16 Adjust Loading Handle before mounting Automet on polishing wheel

10 f. Place the Automet over the shaft on the polishing wheel with the Automet turned about 90 degrees counterclockwise from where you placed the Specimen holder. Make sure the Automet is fully seated on the shaft, as it can sometimes get caught up a little. Attach the Mounting Cap to secure the Automet. Figure 17 Mounting the Automet Figure 18 Center shaft over Specimen Holder g. Turn the Automet over the Specimen Holder until it catches in the détente in the grinding/polishing unit ring, taking care to not damage either part. h. Making sure that the shaft is over the center of the Specimen Holder, turn the Loading Handle clockwise until it catches in the safety lock. Then pull out the safety lock and turn it further about 90 degrees clockwise. DO NOT put any pressure on the specimen holder before you turn on the grinder/polisher motor. Figure 19 Turn Loading Handle until it reaches the Safety catch. Then lift the Safety Catch knob and turn the Loading Handle 90 degrees further clockwise. Figure 20 Make sure shaft is inserted in hole in center of Specimen Holder.

11 i. Turn on the water so it is flowing onto the polishing wheel and turn on the polishing wheel motor to about 500 rpm (the end of the green range on the rpm indicator). Figure 21 Motor Control Panel j. Making sure enough water is flowing to keep the paper wet, turn the Loading Handle on the Automet clockwise until the indicator is pointing at about the second line on the top and let the system run for about 2 minutes. Figure 22 Grinding the Specimens k. After grinding for about 2 minutes, unload the pressure by turning the Automet Loading Handle counterclockwise until the safety catches. Then turn off the motor. Then, pull the safety catch on the Automet Loading Handle and turn the handle about 180 degrees more counterclockwise to disengage it from the Specimen Holder. l. Turn the Automet away from the Specimen Holder, remove the Mounting Cap from the Automet, and remove the Automet from the polishing wheel.

12 m. Pull the sandpaper off the polishing wheel and thoroughly wash both the Polishing wheel and the Specimen Holder to make sure no particles are left over from the grinding process. If you fail to wash them thoroughly, specks from the previous process may grind scratches during the next grinding/polishing process. Figure 23 Wash the specimens and polishing wheel after each step n. Repeat this process for each of the other Sanding paper grits. If the scratching on the specimens does not look even, you may want to do a second pass with another piece of the same paper before you move on to the next step. It is very important to clean off both the wheel and the Specimen Holder after each step in the process. Particles left behind can leave scratches in the samples that won t be removed by the next step. Even after using the 600 grit paper, you will still be able to see scratches in the surface of the sample. Move on to the polishing step anyway, as it is quite adept at removing the scratches. But, if after polishing, you still see too many scratches, you ll have to start from the beginning and do it all over again.

13 2. Polishing In this step, we will use alumina slurry to polish the samples to a mirror finish. When you are done, you should not see any scratching or pitting in the surface of the sample. The process of polishing is the same as above except that, rather than applying a piece of sandpaper to the polishing wheel, you should switch to the other polishing wheel (the white one) and apply a polishing cloth disk instead. Then, starting at step i: i. If the alumina slurry bottle (see Figure 24 Alumina Slurry Bottle.) is empty or low, pour a couple of spoonfuls of alumina powder into the bottle and fill it, cap it, and shake vigorously. It should have the consistency of skim milk when properly mixed. j. DO NOT turn on the water spigot, as this will dilute the action. Squirt the alumina slurry a couple of times onto the polishing wheel and turn on the motor to 500 rpm (the end of the green range on the rpm indicator) and continue to squirt the slurry onto the wheel making sure the wheel stays wet. Not too much, but enough to make sure the slurry is acting on the specimens. k. Run the wheel for 2 or more minutes, occasionally squirting more slurry onto the wheel as it dries up. l. After two minutes, unload the Loading Handle as above and Figure 24 Alumina Slurry Bottle. turn off the motor. Then remove the Automet and wash the specimens off. They should now have a mirror finish, and you should not see any scratches left over from the grinding process. Figure 25 Polishing the specimens

14 3. Etching Once you ve polished the specimens, you will need to etch each specimen in order to see the grains in the surface. This is done with a 98 ml ethyl alcohol and 2 ml Nitric Acid solution applied to the face of the samples for 45 seconds. Be sure to do this under an approved hood in the Chemistry lab. After 45 seconds, thoroughly wash the specimen with distilled water to stop etching action. Then wash it again with alcohol and use a heat gun to dry the sample. DO NOT touch the polished/etched specimen, as this will alter the metal surface condition. Always use a new beaker of 2% Nitric Acid solution for each specimen. Never reuse the 2% Nitric Acid solution. Etching Process 1. Make sure the you have all the correct materials under the hood before you start: a. Bottle of 2% Nitric Acid in Ethyl Alcohol solution. Nitric Acid is extremely corrosive. Do NOT under any circumstances allow the solution to touch your skin, or try to create the solution yourself. b. Squirt Bottle of distilled water. c. 3 Petri dishes. d ml beaker e. Tongs for handling the specimen f. Safety gloves g. Heat gun/hair dryer h. Squirt bottle of alcohol (not for drinking). i. Large beaker for waste liquids. 2. Put on the safety gloves and turn on the hood fan. 3. Pour 40 ml of the 2% Nitric Acid solution into the 50 ml beaker. Never reuse the 2% Nitric Acid solution or etching will not work. 4. Place the specimen sample side up into the solution in the beaker. 5. Allow the solution to act on the specimen for 45 seconds. 6. When the time is up, remove the specimen, place it in the first Petri dish and immediately wash it with distilled water from the squirt bottle to stop the reaction. Repeat this process 3 times.

15 7. After washing with distilled water, wash it off again with the alcohol in the squirt bottle and dry the specimen off with the heat gun. DO NOT touch the polished/etched specimen, as this will alter the metal surface condition. 8. Dispose of all materials in beaker and Petri dishes in the non-hazardous materials waste container. Never reuse the 2% Nitric Acid solution or distilled water or alcohol. Your specimen should look a little oxidized after etching. This is fine. You may want to look at the specimens under the microscope before and after etching. When using the metallographic microscope, please be sure to turn off the light in the microscope when you are done and replace the dust cover.