Experiment 3

Making Metals Strong

Processing Metals

Objective:

The objective of this lab is to demonstrate the effect of cold-working (strain-hardening) and annealing on the ability of wires of the same metal to support a load.

Review of Scientific Principles:

Because plastic deformation results from the movement of dislocations, metals can be strengthened by preventing this motion. When a metal is deformed, new dislocations are produced. As dislocations are generated and move, the metal can be bent or shaped without cracking. As the number of dislocations in the crystal increases, they will get tangled or pinned and will not be able to move. This will strengthen the metal, making it harder to deform. When this is done at or near room temperature, the process is known as cold-working. When cold-worked metals are annealed (heated gently), new grains form from the cold worked structure and grow until they replace it with new, soft crystals. Steels (alloys of iron with up to 1% carbon) can also be hardened by heating and quenching. At high temperatures (red hot), iron has an FCC structure which can dissolve carbon. At low temperature, the iron changes to BCC which cannot dissolve carbon, so it precipitates as an iron-carbon compound. If quenched, this compound does not have time to form, the carbon is trapped and distorts the BCC crystal structure to create a new, hard and brittle structure called Martensite. If Martensite is gently heated, the carbon can precipitate giving a strong, tough structure.

Applications:

The properties of metals can be altered by processing. Since the properties of a material are dependent upon its structure on the atomic level, altering its structure should alter its properties. Common treatments include cold-working and heat treating.

Time: 50 minutes, part I; 50 minutes, part II; 30 minutes, part III

Materials and Supplies:

Hammer

Bunsen burner and tongs

16 or 18 gauge solid wire of copper (or aluminum)

16 or 18 gauge solid wire of other metals

high carbon steel wire or bobby pins

Pair of 3" C-clamps (or other size if 3" not available)

Wire gauze

General Safety Guidelines:

Procedure (Part I):

  1. Hammer one of the pieces of copper wire until it is about half its original thickness.

  2. Bend it and the other piece of wire back and forth several times. Observe.

  3. Heat the flattened (work hardened) piece of copper in the burner flame until red hot.

  4. Let it cool slowly on the wire gauze.

  5. Label and save for experiment 4.

  6. Repeat procedures 1 - 5 for the other wires.

  7. Label and save the wires for later.

Procedure (Part II):

  1. Obtain 7 samples each of high carbon steel wire (bobby-pins) and other metals.

  2. Bend one of the wires until it breaks. Count and record the number of bends needed to break the wire.

  3. Heat the second and third steel wires in the middle until they are red hot. Let them cool slowly in air.

  4. When the wires are cool, bend one of them back and forth as before. Count and record the number of bends needed to break this heat treated wire. Label and save the other wire for later.

  5. Fill the beaker with cold water.

  6. Heat the fourth and fifth wires in the flame until they are red hot and immediately plunge it into the water in the beaker.

  7. When the wires are cool, bend one of them as before and record the number of bends needed to break it. Label and save the other.

  8. Heat and quench the last two wires as in procedure 6. Heat them again but cool them slowly in air. This process is called tempering. As before, note the properties of the tempered wire. Label and save one.

  9. Repeat steps 1 - 8 for the other metal wires.

  10. Save the extra wires for Experiment 4.

Procedure (Part III):

  1. Using a pair of 3" C-clamps attached to the ends of the wire stretch a section of annealed copper wire by 5% and another by 10%.

  2. Repeat for the other metal wires.

  3. Save these wires also for Experiment 4.

Questions:

1. What is the hammering in Part I, procedure 1 called?

2. In Part I, procedure 2, what did you observe about the ease of bending for each wire? Why were they different?

3. In Part II, procedure 2, how many bends were required to break the wire? Did it break easily? Briefly describe the mechanical properties for this sample.

4. What term describes the heat treating method used in Part II, procedure 3 (heating, slow cooling)?

5. In Part II, procedure 4, how many bends were required to break the wire? Did it break easily? Briefly describe the mechanical properties for this sample.

6. What is cooling the hot metal rapidly as in Part II, procedure 6 called?

7. In Part II, procedure 7, how many bends were required to break the wire? Did it break easily? Briefly describe the mechanical properties for this sample.

8. In Part II, procedure 8, what were the properties of the tempered wire?

Teacher Notes:

Answers to Questions:

1. Cold-working.

2. The hammered wire was harder to bend, but broke more easily. The hammering produced many dislocations which became tangled, inhibiting the sliding of planes of atoms.

3. Answers will vary. The unworked wires should be easier to bend and bend more times before breaking.

4. Annealing.

5. Annealing the wires should soften the metal allowing it to bend more easily and more times before breaking.

6. Quenching.

7. The quenched wires should be harder and bend fewer times before breaking.

8. The tempered wire should bend more times than the quenched wire did before breaking.

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