Thermal Processing of Bobby Pins Lesson

Thermal Processing of Bobby Pins at a glance

Background

Metals can be both strong and flexible, depending on how they are treated. In this lesson, students will explore thermal processing, a technique that alters the properties of metals through heating and cooling. The process begins by heating a metal, like a bobby pin, to a red-hot temperature, allowing the metal's atoms to move more freely and change its internal structure.

If the metal is allowed to cool slowly after heating, the atoms arrange themselves more orderly, making the metal softer and easier to bend. This process, called annealing, is used to make metals more malleable, ideal for shaping items like jewelry or car parts. Conversely, if the metal is quickly cooled by quenching it in water while still hot, the atoms are "frozen" in a disordered state. This results in a hard but brittle metal, suitable for tools or parts that need to be tough and wear-resistant.

Students will experiment with heating and cooling bobby pins, comparing their flexibility before and after each process. By the lesson's end, they'll understand how thermal processing can make metals either soft and flexible or hard and brittle, highlighting the importance of these changes in engineering.

Lesson Objective

This lesson demonstrates how thermal processing, such as annealing and quenching, affects the mechanical properties of steel bobby pins. Students will observe differences in strength and ductility by testing untreated, annealed, and quenched bobby pins.

Experiment Description

Students will heat and cool three steel bobby pins using different methods: one untreated (control), one annealed (slowly cooled), and one quenched (rapidly cooled). They will then test the pins' deflection and strength by loading them with pennies and measuring their response.

Materials List

3 steel bobby pins
1 cup with twine
1 C-clamp
Ruler
Bunsen burner
Pliers or tongs
~300 pennies
1 cup filled with cold water

Safety Precautions

Always wear safety glasses when handling the Bunsen burner, and use pliers or tongs to avoid burns. Ensure the bobby pins have cooled before handling them to prevent burns.

The Experiment

1. Set aside one pin to be used as the “control.” The control sample will not receive any heat treatment, and will be tested “as received.”

2. Heat a second pin using the Bunsen burner. The entire pin should be heated until the bobby pin glows red-hot. (When heating a pin, it is best to use pliers to grip the “open end” of the pin so that it separates the two sides of the pin. This allows the pin to heat much faster.) Keep the bobby pin in the flame for 20 - 25 seconds after it starts glowing red.

3. After the bobby pin has been removed from the flame and returns to a gray color, set the bobby pin on a paper towel and allow it to continue to cool for several minutes. This bobby pin has been “annealed.”

4. While the second bobby pin is cooling, heat another bobby pin using the Bunsen burner. Place the looped end in the flame, heating the loop and about 1/3 of the pin. The pin should again be kept in the flame until glowing hot for 20 - 25 seconds.

5. Remove the pin from the flame and immediately plunge it into the cup of cold water.

6. Set the bobby pin on a paper towel and allow it to dry completely. This bobby pin has been “quenched.”

7. Measure and record the width (mm) and height (mm) of the “smooth” side of the control bobby pin.

8. Punch a hole in each side of the cup and attach the twine as shown in Figure 1. Set up the control bobby pin as shown and be sure that the cup and string are hanging from the end of the bobby pin.