Chemistry Comes Alive! C C Alive! Table of Contents Index Textbooks

Heat Treatment of a Metal Bobby Pin

A bobby pin is heated until it glows red hot and then slowly cooled. It loses its springiness and becomes very flexible. A simulation illustrates this process at the atomic level. When the bobby pin is reheated and then rapidly cooled, it becomes very brittle. A simulation illustrates this process at the atomic level. When a hardened bobby pin is slowly heated until a blue coating forms on its surface, it once again becomes springy. This process is also illustrated with an atomic-level simulation.

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Keywords

crystal lattice, solid state structure, practical applications, metals, solids and liquids, metals, bonding, real life/environment


Multimedia

_Play movie (51 seconds, 3.1 MB)

      
When a piece of metal is heated and then cooled, the physical properties of the metal are determined by the rates of heating and cooling.

A bobby pin is straightened and placed in a burner flame until it glows red hot. The bobby pin is then allowed to cool very slowly. This treatment process is called annealing. After this treatment, the bobby pin is very flexible, and is no longer "springy."

_Play movie (17 seconds, 0.9 MB)

  
At the atomic level, as the bobby pin is heated, the metal atoms move more freely, and, during the gradual cooling, the atoms slow down and settle back into a very orderly array with few defects.

_Play movie (21 seconds, 1.4 MB)

    
An annealed bobby pin is again heated until it glows red hot. This time, instead of being allowed to cool gradually, the red hot bobby pin is cooled quickly in cold water. This process is called hardening. The hardened bobby pin is brittle.

_Play movie (11 seconds, 0.6 MB)

  
At the atomic level, as the metal is heated the atoms move more freely, but the abrupt cooling process leaves many defects in the crystal structure.

_Play movie (15 seconds, 1 MB)

    
In the tempering process, a hardened bobby pin is gently heated until a peacock blue oxide coating forms on its surface. The bobby pin cools slowly. The tempered bobby pin is springy.

_Play movie (8 seconds, 0.5 MB)

  
The tempered crystal structure has fewer defects than the hardened structure, but more defects than the annealed structure.


Discussion

The movies presented above present voice-over combined with sound effects. The following matrix allows access to alternative versions of the movies with or without the sound effects.

voice only
voice and effects

Annealing

bobby pin

AV
N/A

simulated with BB board

BV
BVE

Hardening

bobby pin

CV
CVE

simulated with BB board

DV
DVE

Tempering

bobby pin

EV
N/A

simulated with BB board

FV
FVE

See Simulation of Dislocations in Metals for models illustrating the formation and transmission of defects in a crystal lattice.

Information on the BB board used in the atomic level simulations can be obtained from the Institute for Chemical Education, Department of Chemistry, University of Wisconsin - Madison.

Additional still images for this topic

Demonstration Notes: Warnings, Safety Information, etc.


Exam and Quiz Questions

1. Describe and explain the structural differences between annealed, hardened and tempered metals.

2. How does the rate of cooling affect the number of defects? Explain.

3. How does the number of crystal defects affect the hardness of the metal? Explain.

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