COMPARISON BETWEEN CONDITIONS WHICH EXIST IN THE IRON AND STEEL FAMILY
TO THOSE WHICH EXIST WITH MORE FAMILIAR ELEMENTS.
One of the first physical changes which we would discover would be that
when we desired to “freeze” a “crucible” pailful of our iron water, we
could do so much more easily if the same were in its absolutely pure
state than we could if it were mixed with some other element, such as
carbon. Of course, we have long known that this is the case with water
and salt, and just as it becomes harder and harder to freeze water with
greater and greater percentages of salt mixed with it, so the freezing
of iron with greater and greater percentages of carbon mixed with it,
would also occur at lower and lower temperatures.
If we started to add salt to a pail of water we, of course, would have
different degrees of brine. Just so with the addition of carbon to a
crucible of pure iron, we would likewise have different degrees of the
resulting mixture. In adding the salt to the pailful of water, we would
arrive at a point where the water had absorbed all of the salt which it
was capable of holding at room temperature. If we had added a little
less salt we would have had free water in excess of salt, and if we had
added a little more salt it would have been impossible for the water to
have dissolved it, and we would, therefore, have had salt in excess of
water.
For convenience we will call the mixture above mentioned, at which the
water had become thoroughly saturated with the salt, “cementite”,
because this is the name which our friends, the metallurgists, have
given to a similar mixture of iron and carbon. They call the water,
“ferrite”; the salt, “carbide” and the resulting mixture of brine,
“cementite”. This mixture of iron and carbon always exists in exactly
the same ratio, namely, 93.4% iron and 6.6% carbon, and is expressed
chemically by the symbol Fe3C, which means, in other words, that three
“atoms” of iron have united with one “_atom_” of carbon to form the
“chemical compound”, “iron carbide”, which the metallurgists, as above
mentioned, desire to term “Cementite”.
Now let us go back to the brine solution with which we are already
familiar, and suppose that we added a little more salt than the water
could absorb, and which therefore would exist in a “solid solution”,
and then bring this “mechanical mixture” to such a low temperature that
it would actually “freeze”. For convenience, and in order to agree with
the metallurgists again, let us call the resulting structure
“pearlite”. That is the name which they have given to a corresponding
“mechanical mixture” of cementite and ferrite.
This new constituent “pearlite” contains approximately O.9% carbon and
consists of inter-stratified layers or bands of ferrite and cementite.
It is regarded as a separate and distinct constituent of steel, and
takes its name from the fact that it has a mother of pearl-like
appearance under the microscope. It always occurs at a definite range
of temperature and always contains the above mentioned definite
percentage of carbon.
From the above it may be suspected that a steel containing O.9% carbon,
consisting entirely of pearlite, forms rather a special and particular
class of steels, which the metallurgists have decided to dignify with
the title “Eutectoid Steels”. Having done this much to properly impress
the unsuspecting probers of their secrets, they decided to call steels
containing less than this Eutectoid ratio of carbon (0.9% C)
“Hypo-eutectoid Steels”. These steels, of course, contain certain
definite amounts of pearlite with other amounts of free or excess
ferrite. Likewise, if the carbon content is greater than O.9% there
will be an excess of cementite over the ferrite and we will then have a
structure of pearlite plus free cementite. And these steels are spoken
of as “hyper-eutectoid” steels.
[Illustration: Hypo-eutectoid Steel. Carbon .11%. Structure:
Light—Ferrite; Dark—Pearlite. Mag. 500x]
[Illustration: Hypo-eutectoid Steel. Carbon .37%. Structure:
Light—Ferrite; Dark—Pearlite. Mag. 500x]
[Illustration: Eutectoid Steel. Carbon .90%. Structure: Fine uniform
Pearlitic condition. Mag. 500x]
[Illustration: Hyper-eutectoid Steel. Carbon 1.20%. Structure:
Dark—Pearlitic; White boundaries—Cementite. Mag. 500x]