Steel It? 
The variety of properties of steel

Background:

            Steel is made of 2 main components, iron (usually over 95%) and carbon (usually less than 5%).  Not only will different amounts of carbon and iron will produce different kinds of steel, but heating the steel in different ways will produce various properties.  These different properties result from different arrangements of the Fe and C atoms. 

Pure iron (alpha or ferrite) is in an arrangement called body centered cubic (BCC).  There are 6 Fe atoms on corners of a cube and one Fe atom in the middle (see figure 1).  If you heat the iron until it is red, the arrangement of Fe atoms changes into face centered cubic (FCC) or austenite.  There are 6 atoms of Fe on the corners of the cube, and 6 Fe atoms on each of the 6 sides (see figure 2).  Which do you think is more dense?  The FCC because there are more Fe atoms per given space. 

 

FIGURE 1:  Body Centered Cubic (6 Fe atoms on the sides of a cube and one Fe atom in the middle)	FIGURE 2:  Face Centered Cubic (FCC)  (Note 6 Fe atoms on the corners & 6 Fe atoms on each face)

 

Since carbon atoms are smaller than Fe atoms, the C atoms will need to fit into open spaces in these cubes.  It appears like the less dense BCC has more open space because it less dense.  However, the shape of the open spaces in the crowded looking FCC are octahedral and can more easily alloy carbon atoms inside.  The FCC can dissolve up to 1.9% carbon atoms, while the BCC can dissolve less than 0.01% carbon, a huge difference. 

There are 3 heating techniques that will be performed. 

 

I.  Annealing:  This involves heating the steel (a mixture of Fe & C atoms) until red and then slowly cooling.  This allows precipitation of excess carbon.  Annealed steel is soft and workable.

 

II.  Hardening steel:  This involves heating to red heat to dissolve almost 2% carbon, and suddenly cooling it in water.  This forces more carbon to dissolve in the BCC structure.  The resulting shape is distorted and is called martensite.  Hardening steel is hard and brittle. 

 

III.  Tempering steel:  This involves starting with hardening steel and heating until a blue color appears.  The result is that the distorted shape becomes somewhat regular.  Tempering steel is hard and springy. 

           

We will use bobby pins which are a cheap, easy source of steel.  You will try all 3 different techniques.  Bobby pins are made of steel with a thin plastic coating, which when heated will burn.  You might smell the burned plastic (Blah!), but that will not affect the results. 

 

Procedure: 

1.  Spring steel.  Start by taking a bobby pin and manipulating it for a few minutes to discover the properties of spring steel.  Record in detail on the data table as many properties as you can.  Consider these questions to guide you:

Is it springy or brittle (breaks easily)? 

Can it go back to its original shape (more or less) or any new shape?

If you bend it repeatedly (3-6 times) does it break?

 

2.  Annealing.  Take a new bobby pin and heat the folded end with a Bunsen burner until the end is red-hot.  Let it cool briefly.  Now try to open it straight (use tweezers).  If it can not open, re-heat, then straighten it out using tweezers.  Allow the pin to cool.  Once it is cool, squeeze the two ends together gently, and notice how it is different. 

           

3.  Using the same pin, heat the entire length of the bobby pin until it is red-hot.  Slowly lift the pin up out of the flame very slowly.  Allow it to cool. Observe the properties of the wire now.  Try to bend it into a hook.  Put some string around the hook and pull on it.  Does annealing support much weight?

 

4.  Hardening.  Get a 400 mL beaker & fill with cold water.  Use a new pin and heat the entire hook to red-hot, then quench (cool rapidly) it by putting it quickly from the fire into the cold water.  Note all of the different properties. 

 

5.  Anneal another bobby pin just like you did in step #3.  Shape it into a hook and then perform the hardening procedure.  Be very gentle not to break it.  Can annealed steel turn into hardened steel?  

 

6.  Tempering.  Slowly lower the hardened hook into a flame, but do not let it get red hot.  You might see a blue oxide coating (about 3 minute) over the entire surface of the pin.  Remove the pin from the flame and let it cool slowly on the lab table.  Analyze the properties of the tempered steel. 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Data Table: 

Changing the Properties of Steel
Type of Steel	Properties (Describe in detail)
Spring Steel (no heating)	Springy or brittle?       Straighten?       Original Shape       New Shapes?     Repeated bending?     Any Other properties?   Properties good for bobby pin? Explain.
Annealing
Hardening	New pin           Annealed steel first:  Did it turn into Hardened steel?
Tempering

 

 

 

Questions:

1)  Make a chart of the variety of types of steel & how each is made.  Identify each as FCC or BCC or unknown.  

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

2)  Use your textbook to determine if there are any other crystalline shapes other than FCC or BCC.  Explain how they are different.