ENSC 330
Lab 2
Due Date: March 1, 2004
Instructor: John Jones jones@sfu.ca
Lab Support: Carl Chun chunq@sfu.ca
NOTE: Read the Safety and Procedure sections completely before starting the lab. Lead is a poison, wear gloves or use forceps when handling, and do not breathe fumes.
N.B. Report any problems or breakage. You will not be penalized for problems, even if you helped to create them. We want only to fix problems, not to fix blame.
Objectives
To make 4 different alloys of lead and tin (5, 20, 60 and 90% by weight tin).
To melt samples of 100% lead and 100% tin.
To determine a rough phase diagram of the Pb-Sn alloy system.
To observe and record the microstructure of each alloy.
To report on the relative electrical and mechanical properties of these alloys.
2.0 Materials and equipment
Ring stand with:
ring and clamp,
wire triangle,
two-jaw clamp to hold thermocouple.
K-type thermocouple
Propane torch with flint lighter/lighter
Crucible
Dish, aluminum, weighing
Spatula, stainless, large
Spatula, stainless, small
Crucible tongs
Ohaus balance
Forceps, metal, coarse
Tin, bulk stock cut up
Lead, chunks of cut up “pencil lead”
Plastic beaker
Metallurgical microscope, Leitz or equivalent
Stereo microscope, B&L or equivalent
Paper towel
3.0 Safety
1. Read the Material Safety Data Sheets (MSDS) provided for lead and tin, before starting work.
2. Note position of nearest fire extinguisher.
3. Note position of nearest telephone. It has emergency numbers marked on it.
4. Turn on fume hood fan and light switches (switches on either side of fume hood glass). Check that Magnehelic gauge indicates acceptable flow rate/pressure differential (see sign on fume hood for acceptable reading).
5. All team members must wear goggles or face shields at all times.
6. Keep fume hood glass about half way down and keep the glass between your face and the experiment.
7. Remember that crucibles and samples are hot. Carefully use tongs to handle crucibles. When pouring molten material, grip edge of crucible firmly and carefully with point of tongs. Trying to cradle crucible does not work.
8. Apply only enough heat to the crucible to liquefy the sample. DO NOT continue to heat indefinitely after melting is complete; you may be able to turn down torch and still maintain melt.
9. Melt should not become red hot. If it does, it is probably covered in excess dross. In any event, do not plunge thermocouple into it until it has taken on the liquid silver appearance of mercury. Otherwise the thermocouple may break.
10. Turn off torch firmly but gently -- gas control is by way of a delicate needle valve which is easily damaged by overtightening.
11. LIGHT AND USE TORCH ONLY WHEN ACTUALLY HEATING SAMPLES – DO NOT LEAVE IT ON OTHERWISE – THIS IS WASTEFUL AND DANGEROUS
4.0 Procedure
N.B. Start your work with the high % tin alloys. These melt at lower temperatures and will give you a feel for temperature, melt time, use of thermocouple, etc.
N.B. Please read each numbered section of this procedure completely before starting to perform the operations called for by that section.
N.B. You should not have to adjust the position of the crucible – just slide whole torch assembly to one side for lighting or to remove it as a heat source.
1. Use an aluminum weighing dish and weigh out the correct proportion of lead and tin. Record the weights of your metals. Use larger pieces of material to get close to the desired weights and then use progressively smaller pieces to reach the actual final weight. Your sample mixtures need only be within ~ +/- 1% of the stated sample proportions, but be sure to record exactly what the final figures are.
Use the coarse metal tweezers and/or spatulas to handle the tin and lead.
A total weight of about 50 grams for each sample should suffice --this will provide the needed depth-of-melt of approximately 5-10 mm. The 90% tin mixture will require about 40-45 grams of tin, and the 5% tin mixture will require about 45-50 grams of lead.
Be sure that the Ohaus balance is at zero with nothing on either pan. Your sample goes on the right pan of the Ohaus balance. Move the black “micrometer” scale weight to centre the red indicator on “0" and read the weight. Do not touch the knurled brass nut on the rod above the “micrometer” scale if the scale reads “zero” with no weight on either pan. This knob is used only for zeroing an empty balance. Decide if weight of weighing boat should be taken into account.
2. Mix the weighed samples in a small crucible. Place the crucible in the ring stand triangle.
3. Make sure the thermocouple is raised well away from the propane torch so that the torch flame will not affect it. Move the torch to a convenient spot for lighting and adjustment of the controls. Light it with the flint striker/lighter and adjust the flame so that its flared blue cones extend about 1. -1.5 cm beyond the end of the torch. Adjust the position of the torch so that the end of the highest flame is just about 3- 5 cm. from the bottom of the crucible. It does not have to be pointing directly up at the crucible; pointing at the bottom of the crucible from below and to one side is fine. DO NOT TURN UP THE FLAME OR MOVE THE TORCH CLOSER IN ORDER TO TRY TO SPEED THINGS UP. Ensure that the flame heats the crucible evenly.
4. While waiting for the melting to occur, fill the plastic beaker at least 3/4 full of cold water.
5. Melt should take about 5-10 minutes, depending on mix and flame intensity. Your sample will melt a little faster if you have small pieces on the bottom to melt first -- once there is a liquid present, heat transfer to the other pieces will be a bit quicker.
As material melts, stir the melt carefully with the stainless steel spatula. Do not splash melt out of crucible or hit thermocouple, which should be clamped with the tip well out of the crucible at this point.
Use the small spatula to remove any excessive dross or scum (the granular junk floating on the surface), and place this in a weighing dish to cool, then in the designated waste container. Lab staff will dispose of this waste correctly later. Most dross will be dark coloured and may even look like lead that refuses to melt. If you see a lot of coloured dross (yellow, white or red), turn heat down slightly.
Failed samples may also be placed in this waste container.
6. When material has melted, is reasonably free of dross and has a silvery appearance, carefully turn propane torch flame down, but not off, and gently lower the thermocouple into the melt, while watching the temperature reading to ensure that thermocouple does not go off scale (remove it if it does and cool sample). Clamp thermocouple gently but firmly so that it is suspended in the middle of the crucible and does not touch the crucible bottom. About 2-3 mm off bottom of crucible is good.
7. Record liquid temperature. Turn off the propane torch carefully to avoid spilling the crucible. Be sure valve is fully and firmly off, but don’t overtighten. Begin the rest of the temperature measurements and observations. Cooling time will be about 1-2 minutes; if cooling happens too quickly, try turning the torch flame down but not off. It is suggested that temperature readings and observations of the melt be made about every 10 seconds. MAKE YOUR DATA CHART UP AHEAD OF TIME, LEAVING ROOM FOR LOTS OF TEMPERATURE/TIME MEASUREMENTS, AND SOME ROOM FOR COMMENTS ON THE VISCOSITY OF THE MELT. Stir gently with the small spatula during cooling. Describe viscosity in terms of butter, margarine, putty, plasticine, glue, syrup, etc., whatever comes to mind.
8. Remelt sample and record temperatures and observations, both on the way “up” and during cooling again. Remelt time may differ from the original melt time, because you are now melting the alloy, not its components.
9. Remelt sample one last time (recording data on the way “up”) and then lift the thermocouple well out of crucible. Grasping the edge of the crucible firmly with the tongs, VERY CAREFULLY pour half of the melt into the water in the plastic beaker and the other half into one of the aluminum weighing dishes – have this sitting on the square of wire screening to speed the cooling process and to prevent damage to the counter top.
10. When the samples have cooled, identify them (i.e. by marking them) to avoid confusion later.
11. Repeat procedure for other tin/lead proportions and for the 100% lead and 100% tin samples.
12. Using the microscopes, observe any crystal structure on the surface of the alloy specimens.
13. Carefully scrape crucible clean with spatula when finished and deposit scrapings in the designated waste container mentioned above. The crucible won’t look like new, but it should not have a thick film of metal in it.
14. Turn off light and fan in fume hood. MAKE SURE TORCH IS TURNED OFF.
15. Plot temp/time observations and comment (4 data sets plotted on one graph for each sample).
16. Save all samples and return to Gary in an envelope with your group name on it..