This demonstration shows how heating and cooling rates affect the properties of steel. By comparing annealed and quenched bobby pins with an untreated one, students can observe changes in hardness, flexibility, and brittleness that result from different heat treatments.

1. Keep one bobby pin as the control (untreated).
2. Hold a second bobby pin with pliers and heat it in a Bunsen burner flame until it glows red-hot.
3. Remove it from the flame and allow it to cool slowly in air on a paper towel or fireproof surface, this is annealing.
4. Hold a third bobby pin with pliers and heat it in the flame until it glows red-hot.
5. Immediately plunge the hot pin into a cup of cold water to cool it rapidly, this is quenching.
6. After cooling, test each bobby pin by gently bending it and compare their properties.

Heat Treatment of Steel - FlinnScientific:

Thermal Processing of Bobby Pins: Interactive Science - Ceramic and Glass Industry Foundation:

• Use oil instead of water for quenching and compare results.
• Reheat the quenched pin to moderate temperature and allow it to cool slowly to demonstrate tempering, which increases toughness.
• Try using other small steel items such as paper clips or sewing needles.

• Wear heat-resistant gloves and safety goggles.
• Handle hot metal only with pliers or tongs.
• Keep flammable materials away from the flame.
• Allow all metal to cool fully before handling.

• How does the cooling rate affect the hardness and flexibility of steel?
• Why does quenching make the steel hard but brittle?
• What happens at the atomic level during annealing compared to quenching?
• How does tempering modify the properties of quenched steel?