Heat Treating Metals

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Heat Treating Metals
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Working with Copper Wire
How many turns were you able to make with the
copper wire before heating?
Why do you think the wire got more difficult to
twist with each turn?
Explanation As copper is twisted and turned,
small dislocations in the arrangement of atoms
begin to block the movement of atoms. The
dislocations get bound up with each other like
cars bunching up on the freeway. The metal atoms
become more difficult to move and the metal gets
tougher, more brittle, even to the point of
breaking.
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Dislocations in Metal Crystals
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More Dislocations
Images show grain boundaries in crystal lattice
www.seas.harvard.edu/.../crystal/Crystal3D.png
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Work Hardening

• Hardening of a metal (while cold) by bending,
  twisting, rolling, hammering, etc. to strengthen
  the material.

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www.thefabricator.com/.../Photos/1165/Fig5.jpg
In this example, work hardening of the piece has
caused cracks on the edges.
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Heating the Copper - Results

• How many turns were you able to make with the
  copper that was heated and cooled quickly with
  water (quickly quenched)?
• How many turns were you able to make with the
  copper that was heated and cooled slowly
  (annealed)?
• What does this tell you about these processes and
  copper?

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Heating and Cooling Copper

• Explanation
• Heating copper allows the atoms to rearrange
  themselves and remove some of the dislocations
  which were blocking the movement of atoms. As a
  result, atoms move easier in the metal . The
  metal as a whole is more workable, more ductile
  (stretchable) and malleable (deformable).

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Quenching

• Heating, followed by quick cooling of metal to
  harden metal dislocations are locked into
  place.

www.silverbuckles.com/Images/Studio/quench1.jpg
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Annealing

• Strong heat followed by slow cooling atoms have
  time to rearrange, reduce dislocations and
  increase ductility and strength.

Annealing oven
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Moral to the story of Copper!

• If you want to reduce brittleness in copper
  heat it!
• Does it matter if you quick quench it, or slow
  cool it? Does one or the other produce a more
  workable piece of copper?

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The rest of the Story!

• What about iron or the bobby pin (steel)?
• How does work hardening iron compare with work
  hardening copper?
• How does work hardening a bobby pin compare with
  copper?

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Annealing Iron and the Bobby Pin

• How does annealing iron compare with annealing
  copper? Does annealing have the same effect or
  something different? Why?
• How does annealing the bobby pin compare with
  annealing copper or iron? Same effect or
  different? Why?

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Heating / Quenching / Reheat at lower Temperature

• How does this process affect the strength of
  iron? Is it more or less ductile than just
  quenching or annealing (slow cooling)?
• How does this process affect the strength of the
  bobby pin (steel)? More or less ductile than
  quenching or annealing?

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Tempering

• The process of heating, then quenching, then
  reheating at lower temperature and cooling slowly
  creates a stronger, tougher steel that is less
  brittle, longer lasting.

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i.ehow.com/.../2094362/knifetemper-main_Full.jpg
www.upaa.org/winners_mic/2006_02/sr1.jpg
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Why does Steel or Iron need to be Tempered?

• Remember that iron goes through a crystal phase
  change at a high temperature (912o C). It
  changes from BCC to FCC. This causes multiple
  forms of crystals in iron as it is heated and
  cooled. Many dislocations are formed and the
  heating, cooling, heating and slow cooling of the
  process of tempering works out the kinks caused
  by the dislocations and produces a tougher, less
  brittle material.

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Senior Project Idea

• So if youre really into this sort of stuff, a
  project involving a hot furnace, some iron, a
  bucket of water and a big heavy hammer might
  allow you to learn about how to create iron with
  a variety of properties brittle, soft, tough
  etc.
• Basically, this is the science behind what a
  farrier or blacksmith does while fabricating
  horse shoes for a horse.

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In Conclusion

• How does work hardening change the properties of
  iron?
• How does heating and quick quenching change the
  properties of iron?
• How does annealing change the properties of iron?
• How does tempering change the properties of iron?

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In Conclusion (cont.)

• How does work hardening change the properties of
  the bobby pin (steel)?
• How does heating and quick quenching change the
  properties of the bobby pin?
• How does annealing change the properties of the
  bobby pin?
• How does tempering change the properties of the
  bobby pin?