Making Metals Strong
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.
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:
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)
General Safety Guidelines:
General Safety Guidelines:
Procedure (Part I):
Procedure (Part II):
Procedure (Part III):
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?
Answers to Questions:
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.
5. Annealing the wires should soften the metal allowing it to bend more easily and more times before breaking.
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.