Project: Heat Treat a Piece of Metal

You might have heard about some metal being heat treated.  This means the way they behave has been changed by how they were heated and cooled.  Heat treating steel is done by heating it up and cooling it down at different speeds.  In this tutorial, we will show you how to heat treat a piece of spring steel.

 

Steel is a metal which is a combination of the elements iron and carbon. Not all steels can be heat treated.  The steel has to have enough carbon in it before the chemistry that changes its properties can work its magic.  A steel paper clip does not have enough carbon in it to be heat treatable.   A steel hair pin is in fact a spring, and it can be heat treated.  Springs have to move and snap back to their original shape.  You can heat treat a hair pin (or any other spring you can get your hands on).   You just need to be able to heat it up to a nice glowing orange.

By the way, a really nice science fair project might be to experiment with different metals and find out about how they respond to the heat treating methods we are going to try out.

There are three ways you can change the properties of a metal by heat treating it: 1) Make it softer, 2) Make it harder, 3) Make it springy (hard but not brittle).

Here is how to heat treat a hair pin those those three ways.

Warnings

  • This project requires adult supervision
  • Burn hazard
  • Fire hazard
  • Flying material (eye injury) hazard
  • Be sure to read and follow the torch instruction manual
  • Paint or adhesives may emit toxic fumes.  Use unpainted steel and do this project outside.  If there is smoke or a funny smell, don’t breathe it.

Tools & Materials

  • At least 3 small springs (hair pins work nicely)
  • A butane torch (or Bunsen burner)
  • Coffee cup
  • Water
  • Pliers
  • Safety Glasses

To Make Steel Softer (Annealing)

To change the hair pin from a spring to a piece of metal that can be easily bent and formed, you want to anneal it.  Annealing is heating the steel up until it is glowing nicely, and then cooling it as slowly as  you can. Here is what to do:

Step 1 – Heat the Steel

Begin by seeing how springy the hair pin is.  Flex it open and let it snap back.   Now light the torch and hold the hair pin (with pliers) until it glows orange.

Step 2 – Cool the Steel Slowly

The key here is to let the pin cool over about a minute .  The steel should still be glowing from Step1.  Cool the metal by slowly moving the pin farther and farther away from the flame.   When it stops glowing, keep moving it slowly away from the flame.  When the metal is cool to the touch, it is fully annealed.

Step 3 – Test it

Now open the pin up and you will see that it is not much of a spring anymore.  You can bend it easily and it will only snap back weakly. Keep this pin and you can make it springy again.


To Make Steel Harder (Quenching)

A hard steel is one that resists scratching or denting.  Hardened steel can be used to cut other metals.  Drill bits and taps are examples of hardened steel.  To do this part, start with a new pin or use the one you used in “To Make Steel Soft.”  We are going to break the pin at the end of this part to see how easy it is to break.

Step 1 – Heat the Steel

Holding the pin in the pliers, heat it until it glows orange.

Step 2 – Quench

While it is still glowing, drop the pin into a coffee cup of room temperature water.  The pin should make a “pssst” sound and you will see little black flakes come off the sides.  The black flakes are carbon that has come out of the steel.

Step 3 – Test It

Take the hair pin out of the water.  It should be cool after only a couple of seconds.  Carefully try to pull it open and it will snap with only the slightest force.   This steel is very hard but this also means it is very brittle.  If we want to make something useful out of really hard steel we will need to temper it.


To Make Steel “Springy” (Tempering)

A good knife blade will have the same properties as a good spring.  It will be a mixture between hardness and toughness.

Hair pins are already in this state when you get them in the store.  When they make springs, they form them while they are soft (annealed) .  Then they quench and temper the steel.  Tempering takes away some of the brittleness but leaves some hardness.

Step 1 – Anneal the Pin

You can start with the pin you used in “To Make Steel Soft” or you can anneal another one.  Do the same slow cooling you did  before.

Step 2 – Shape the Pin

While the pin is annealed, you  can bend it into whatever shape you like. without breaking.

Step 3 – Quench the Pin

Heat the pin and drop it into the water like you did in “To Make Steel Harder.”  When it is cool, don’t try to bend it or it will break.

Step 4 – Clean the Pin

Use some fine sandpaper to clean off any carbon residue that is on the pin.  You need to be able to see the color of the steel clearly.  Just be careful not to break the steel, it is very hard and brittle!

Step 5 – Temper the Pin

Tempering helps take some of the brittleness out of the pin.  When done correctly it will make the pin as springy as it was when it was new.  To temper the pin you want to heat it slowly and carefully until it just turns a beautiful deep blue color.    Hold the pin an inch or two away from the butane flame and heat it slowly until it begins to change color (nowhere near glowing temperature!).  It will first turn light brown, then a blue color.  When the blue becomes a very deep, rich color stop heating and let it cool in still air.  If you heat it too much it will go from blue to dark brown (too far!).

The color in the photo below looks lighter than it really is.  The blue should be a royal blue.  As soon as it turns blue, you are done!

Step 6 – Test It

The pin should be a spring again!

 

You can try these same techniques with a paper clip or copper wire, but you expect different results.

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