Heat Treatment of Iron



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Heat Treatment of Iron

© 2009 by David A. Katz. All rights reserved,

Department of Chemistry

Pima Community College

The purpose of this experiment is to investigate the effects of heat-treating on the properties of iron.


Background


Iron (element 26, atomic weight 55.845) is one of the most common metals encountered in everyday life. It has been used for over 2000 years to make tools, cooking utensils, weapons, and machinery. Although iron has a bright silvery surface when polished, it is quickly oxidized in moist air to form rust, a red-brown mixture of FeO and Fe2O3 usually written as Fe3O4. Iron, itself, has a hardness of 4 on the Mohs hardness scale (talc has a hardness of 1, copper has a hardness of 3, and diamond has a hardness of 10), but is made harder by alloying it with carbon, silicon or other metals to form carbon steel, nickel stainless steel, or other specialty steels. (Steels have a hardness of about 7). Iron is attracted to a magnet and also has the property of being ferromagnetic, a result of its crystal structure being aligned in one direction.

Depending on how the iron is processed, it can take various forms. Pig iron, with about 4-5% carbon and some other metallic impurities is usually used for cast articles such as stoves, cookware, pipes, lampposts and rails. Iron can be made hard and brittle by heating to a high temperature and then quenching it quickly in water. In this form, the iron is more amorphous than crystalline. Hardened iron is useful for making knives and swords. If the iron is heated and allowed to cool slowly, a process called annealing, then the crystal structure is organized and the iron is softer and easier to bend as atoms can slide over one another. Tempering is the process of heating the hardened iron enough to allow some crystals to reform, but still retains some of the amorphous structure. Tempered iron has more spring than the hardened iron.


Materials Needed four bobby pins,


150 - 300 mL of cold water in a beaker

magnet


Bunsen burner

Matches or striker

Crucible tongs
PRELAB ASSIGNMENT:


  1. Thoroughly read the background paragraph and the procedure. Highlight any questions you have.

  2. Copy the observations chart from the teacher page into your lab notebook. Leave plenty of space – feel free to use two pages. Fill in the vocabulary and procedure columns.



Procedure: Record observations throughout the lab.

1. Examine one of the bobby pins. Try bending it. How easily does it bend? Is it springy (does it return to its original shape after bending)? Is the bobby pin attracted to a magnet?

2. Light the Bunsen burner.

3. Each lab partner should grasp one side of the open end of the bobby pin with tongs. Hold the bent end in the hottest part of the Bunsen burner flame. As the pin heats up and glows red, pull it as straight as it will go and remove it from the flame. Repeat this with the other two bobby pins.

4. Holding a straightened pin in the middle with tongs, heat the entire wire to a glowing red-hot temperature. Let it cool slowly. This process is called annealing. Repeat this with the other two bobby pins. When one of the two bobby pins is red hot, hold the magnet near it. Is it attracted to the magnet?

5. When the annealed wires have cooled, bend them into hooks. Do the wires bend more easily now after they have been heated? Are they as springy as they were before? Are they attracted to a magnet?


6. Place the beaker of water next to the Bunsen burner. Holding one of the hooks with tongs, heat it to a glowing red-hot temperature in the flame of the Bunsen burner. Quickly cool by dropping the red hot hook into the beaker of cold water. Repeat this with the other hook. This will form hardened iron. Look at the bottom of the beaker for evidence that the bobby pin may have lost some particles from its crystal structure.

7. Remove one of the hardened iron hooks from the water. Try bending it. Does it bend as easily as before? Are the hardened hooks attracted to a magnet? Write your observations on the data page.

8. Remove the other hook from the water. Grasp it with the tongs and hold it way above the Bunsen burner flame. Slowly lower it toward the top part of the flame until an iridescent blue coating forms on the hook. It must NOT be heated to a glowing red heat, so keep it out of the hottest region of the flame. Allow the hook to cool slowly. This forms tempered iron.

Try bending this hook. It is brittle or springy? Is it attracted to a magnet? Write your observations on the data page.

OBSERVATIONS CHART: COPY THIS CHART INTO YOUR LAB NOTEBOOK. LEAVE PLENTY OF SPACE – YOU CAN USE TWO PAGES. FILL IN THE VOCABULARY AND PROCEDURE COLUMNS. LOOK UP THE VOCABULARY AS IT PERTAINS TO METALLURGY. THE FIRST ROW IS DONE FOR YOU AS AN EXAMPLE.




STEPS

VOCABULARY (define)

PROCEDURE SUMMARY

OBSERVATIONS

MODEL/EXPLANATION

1

spring steel: very strong alloys of iron and carbon. Their main feature is the ability to resist deformation, and return to shape. 







2-3













4-5

annealing:











6-7


quenching

hardened iron










8

tempered steel:












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