• 4 bobby pins
• 4 large paperclips
• Heat source – Bunsen burner or propane torch
• Container of water for quenching
• Tongs or pliers
1. Set aside one bobby pin and one paperclip for comparison. (Control)
2. Heat the loop of one bobby pin and a loop of one paperclip red hot and slowly lift out of flame.
Slow cool. (Anneal)
3. Heat 2 of each red hot and quench in ice water. Quick cool. (Quench)
4. Take one of each from step #3 and heat again in the top of the flame for a few seconds and then quench. Do NOT let it get red hot. (Temper)
5. Make a data table for comparison of properties when the heat-treated area is bent.
6. Test the bobby pins one at a time by slowly pulling the ends apart. Record observations.
7. Test the paperclips one at a time by pulling the heat-treated loop open. Record observations.
8. Propose reasons for any differences in results.
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Notes:
• Don ’t heat beyond red hot – too much heat and you start burning out the carbon.
• Bobby pins are high carbon steel and paper clips are low carbon steel.
• This lab is usually done after the iron wire demo so that students have been exposed to the idea of allotropes and solid state phase changes in regards to iron.
• This lab is also usually done after the work-hardening and heat-treating of copper lab.
Typical Results: bobby pin paperclip control
“normal”
“normal” anneal softer, easier to bend, lost the springiness softer, easier to bend quench snaps in two, brittle varies depending on the paperclip, sometimes harder, sometimes more like the control, but is NOT brittle, doesn’t break temper feels much like the control doesn’t feel much different than the other paperclips
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Why steel is affected differently by heat treating:
• Iron is allotropic – BCC below 912° C and FCC above 912° C
• Steel is an interstitial alloy – most other alloys are substitutional
• FCC is more tightly packed than BCC but has larger individual gaps – carbon fits into FCC better than it does BCC
• Carbon can be in different locations and in different forms: o
Between grain boundaries o
In the interstices (gaps within the crystal) o
Bonded with iron as Fe
3
C (cementite) which is a ceramic
• Annealing is like what happens with copper o there is crystal regrowth and dislocations are decreased o the metal is softened o carbon has time to migrate to lowest energy positions
• Quenching causes steel to become very strong but also very brittle o forms an intermediate crystal structure which is metastable (has extra energy stored) – it is body-centered tetragonal o
Carbon does not have time to migrate out to lower energy positions o
Extra “stress” is in the crystal
• Tempering is a secondary heat treatment o
Strength is maintained while improving toughness – removes some of the brittleness o
Carbon has time and energy to migrate to lower energy positions o
Remains body-centered tetragonal o
Called martensite
Bobby pin is high carbon steel – approximately 0.7%. Therefore, the carbon has a large effect on the properties and greater differences are observed in the various heat treatments.
Paper clips are low carbon steel – approximately 0.2%. The strengthening of the steel during quenching is mostly due to the solid state phase change from FCC to BCT. Therefore tempering doesn’t alte r the properties really – there isn’t enough carbon to cause brittleness during quenching.
Copyright © 2010, ASM International. All Rights Reserved.

Copyright © 2010, ASM International. All Rights Reserved.