Does the annealing process essentially take you back to the
original grain structure
In a word, no it does not take it back to some original structure.
When you heat basic carbon steel (iron-carbon alloy) to between
727-912 C or 1341-1673 F It changes its crystal structure to become
austenite a face centered cubic or FCC crystal. This pretty much
obliterates the previous crystal structure of the steel. This is a
radical change in structure from the body centered crystal BCC
structure of lower temperature steel. You must heat to above the
austenite transformation temperature to harden the steel. One way to
test this is that austenite is not ferro magnetic so steel that is
hot enough to harden is no longer attracted to a magnet. Once you go
through this austenite phase transformation you can cool it in such
a way as to cause it to transform to martensite. This is a fine
needle shaped grain structure that is the hardest form of steel that
is also quite brittle, for a micrograph of the martensite grain
structure go to
File:Steel 035 water quenched.png - Wikipedia.
In basic carbon steel you need to cool or “quench” the steel at a
rate in the neighborhood of 1000C per minute to freeze the steel in
the martensite crystal structure. This almost has to be done in water
as other quenching media are too slow.
If you allow the steel to cool at a slower rate then it will
transform to a mixture of one or more of these other structures
pearlite, bainite, ferrite (pure iron), or cementite (iron
carbide).Pearlite is a lamellar (layered) structure of 88% ferrite
and 12% cementite See
for a micrograph of pearlite, the pearlite is the striped crystals.
Bainite is a lamellar structure as well but the layers are too thin
to be seen with a light microscope and can only be resolved with an
electron microscope. The proportions of these crystals in the
quenched steel will vary depending on cooling rate and alloy
composition.
If you succeed in cooling the steel fast enough to make martensite
you need to temper it. By heating to a specific temperature some of
the martensite transforms to a mixture of bainite, pearlite, ferrite
and cementite. This makes the steel softer tougher and increases the
grain size. The temperature you need to heat to and time to be held
at that temperature is going to depend on the use of the item and the
alloy composition of the steel. There are lots of references to
approximate temperatures for various uses in many of the jewelry
texts so I will not go into them here. But often these texts do not
cover time at temperature. For the best results holding the tool at
the tempering temperature for an hour or so will make for a much
longer lived tool, it takes time for the carbon to diffuse in the
steel matrix. Just a quick trip to the desired temperature will
provide only a partial transformation of the martensite to the other
forms of steel crystal structure and can mean that the tool will be
more brittle than desired.
Ok so for plain carbon steel you need that really fast cooling rate
to capture the martensite structure. When other elements are added to
the steel for various property enhancements the speed of
transformation changes and typically it becomes slower as other
elements are added to the steel alloy. This is where we get oil and
air hardening tool steels. The alloy additions have slowed the
transformation of martensite to other structures to the point that a
slower quench sometimes even air cooling will still capture the
martensitic structure. The mechanics get much more complicated when
talking about alloy steels and I will not go into them but the thing
you need to know is that martensite is a highly stressed structure
and in some of these alloy steels using too fast a cooling method
will stress them to the point of self destruction. Many steels in
their martensitic state will fracture if you drop them some will
fracture if you allow them to sit around at room temperature before
tempering them so you allow them to cool to 150 F then take them
right into the tempering oven. The upshot is that if you don’t know
what kind of steel you are working with you can easily ruin it if the
wrong cooling rate or tempering process is used. There are literally
hundreds of alloy steels in regular use by manufacturers 40 or 50
years ago you could count on finding scrap steel that had fairly
simple characteristics that you could easily repurpose but modern
manufactured goods are much more likely to have more complex steels
in them. Known quality alloy tool steel is easily purchased in a wide
variety of shapes and sizes. why put a bunch of work into steel that
you have no idea of what its composition is or its suitability for
your use.
Hope this helps and doesn’t confuse things too much,
Jim
James Binnion
James Binnion Metal Arts
