AN EXPERIMENT PERFORMED WITH A PIECE OF PEARLITIC STEEL.
However, let us not trouble ourselves with too many definitions at one
time, but instead amuse ourselves for a while by running through a
little experiment with a piece of carbon tool steel similar to that
which we have just been discussing. For our investigation we will also
need a special kind of thermometer for measuring high temperatures.
Such an instrument is known as a “pyrometer”. Now we will drill a
little hole in the test piece of carbon steel and after inserting the
“couple” of the pyrometer into it, place the same in the electric
furnace.
As the current is turned on, the test piece begins to grow warm and
then hotter and hotter, gradually up through a range of temperatures
which are continually recorded by the needle of the pyrometer. 800,
900, 1000, 1200 degrees Fahrenheit are uniformly reached, and the
temperature of our test piece continues to rise, as the absorption of
heat progresses. Suddenly, however, the test piece assumes a bright
glow and the needle of the pyrometer ceases to advance, and we note
that it is pausing at about 1350 degrees Fahrenheit. Then after its
pause, the advance is again resumed until the piece has become almost
ready to melt. By plotting the uniform periods of time at which we read
the different temperatures recorded by the needle of the pyrometer,
against the temperatures as read, we would have a picture of our
phenomenon something as follows:
[Illustration: Graph showing the course of the temperature curve as a
function of the heating time of the metal sample.]
Now let us begin to let our test piece cool off gradually. The
temperature of the furnace is lowered and the uniform range of cooling
temperatures is recorded by the ever sensitive needle of the pyrometer.
Suddenly as before, the test piece assumes the brilliant glow noted
previously, and again the needle comes to rest, but this time we note
that the recorded temperature is about 1250 degrees Fahrenheit instead
of 1350 degrees Fahrenheit as before. Evidently there has been a
certain tardiness or “lag” which has caused the phenomenon to take
place a little too high going up and a little too low coming down, and
in fact the metallurgists tell us that such is exactly the case, and
that the real point in which we are interested lies just half way
between the two points indicated, as we shall presently see. If we
again represent the results of our latest experiment graphically, we
would have a picture something as Fig. 2.
[Illustration: Graph of the cooling curve of the metal sample
over time]
Now placing the second curve so obtained on the first, we are able to
study the following interesting relationship. Fig. 3.
[Illustration: Graph combining the heating- and cooling-curves from
before and demonstrating the critical range]
It is natural to suspect that both of the parallel sections of our
curves have something to do with the same thing, and for convenience
since we noticed that mysterious glow of the test piece just as the
needle came to rest, we might call the particular point which lies just
half way between the temperatures under discussion, the point of glow,
or as the metallurgists call it, the “point of recalescence” and the
range between these two temperatures the “critical range”.
I suppose it would be difficult to explain this phenomenon of the test
piece unless we imagine that as the critical range is reached some
internal reaction of the steel causes it to spontaneously take on heat
at the same temperature in the first place and give off the stored heat
at the same temperature as the piece was being cooled down, and this
heat caused it to glow as was noticed. Now if we were to experiment
further with our piece while at the critical range, we would find
certain other remarkable changes, one of the most noticeable of which
is the loss of magnetism at and above the critical range.
Irons and steels are usually the most magnetic materials, but the
attraction of the magnet is completely lost at or above the critical
range.
We can easily satisfy ourselves in this respect by noting the
attraction of a simple horse shoe magnet when our piece of test steel
is brought into its magnetic field. As the pyrometer needle passes on
up through the range of temperatures noted above, the magnetic
attraction is perfectly evident when suddenly the recalescence point is
reached, the spell is broken and the magnet and the test piece fall
apart. But let us just consider this phenomenon a moment. We are told
by the physicists that magnetism is induced in a piece of iron or steel
by a “rearrangement of the internal molecular structure, in which the
positive ions face one direction and the negative ions in the opposite
direction”. Therefore, if magnetism suddenly ceases to exist it would
seem as if something had happened to the “internal molecular structure”
of the test piece. Thus when the recalescence point is reached we may
conclude that something more than a mere absorption of heat units has
taken place. In fact we may really believe that an actual internal
molecular revolution has occurred and that some of the natural laws
which formerly had governed all of these little molecules which go to
make up the whole piece of steel, have been overthrown and that the
molecules are more or less free to set up a new form of government for
themselves, and that, therefore, when a piece of steel is brought to
the recalescence point it is really in a very sensitive condition. In
fact, if we should care to investigate further we should find that
certain other great changes take place at this critical point, such,
for instance, as partial failure of the test piece to conduct an
electric current, which formerly, of course, it did with great ease.
Also when the critical range is reached, a peculiar contraction of size
interrupts the gradual expansion which had been developing as the test
piece absorbed heat units, and therefore these several observations
give us reason to believe that our conclusions as noted above must be
more or less correct.
Now if all steels acted exactly like the little test piece which we
have been observing above as they were placed in the hardening furnace,
it would not take us very much longer to finish our preliminary
investigations. You remember the piece of steel which we have been
investigating was a piece of simple carbon tool steel, containing about
0.90% carbon. But all steels do not contain just this same percentage
of carbon, and may also contain various elements other than carbon, all
of which produce many and varied results during the process of heating,
treating and hardening.
In order to better visualize the investigation which we are making, let
us picture graphically each step which we take. If therefore, we let
the vertical lines represent the different carbon contents which steel
might have, and the horizontal lines the different degrees of
temperatures through which we might desire to heat the steel under
discussion and then plotted the phenomenon described above we would
have a picture something as follows:
[Illustration: Graph showing the point of recalescence]
Now all that picture means is that as we heated up a piece of simple
carbon tool steel containing O.9% C, we discovered a certain very
noticeable reaction which occurred just about half way between 1250
degrees and 1350 degrees Fahrenheit, which we decided to call the point
of recalescence, and then on further heating of the piece no other such
phenomenon was noticed.
Now let us go through the same experiment with a piece of steel
containing .45% C. Yes, just as before, as the temperature 1250 degrees
Fahrenheit is reached we note all the strange symptoms which are
characteristic of the point of recalescence and then, just as we are
about to decide that it is hardly necessary to go further we notice
that the pyrometer needle has again come to rest, but that this time it
is registering 1390 degrees Fahrenheit. Therefore, it would seem as if
this piece had two critical ranges instead of one and we are now quite
ready to again proceed with our heating to see if anything else occurs.
However, as nothing does happen we turn to our picture and plot the two
points just observed, together with the one point found on our first
investigation, and the drawing then looks something as follows:
[Illustration: Graph additional shows the recalescence of a
second sample containing a different rate of carbon]
Now let us take a piece of carbon steel as before, but this time
containing .15% carbon, and again proceed with our observations. Again
the needle of the pyrometer records the point of recalescence and also
the point designating the second range of critical temperature, but
this time strange to say, as the test piece continues to absorb heat, a
third critical range is registered, all of which when added to our
former picture gives a result something as follows:
[Illustration: Graph showing different behavior of
samples containing different rates of carbon]
By repeating the operations as outlined above, with pieces of steel
containing various percentages of carbon from zero to 1.25% and by
plotting the different critical temperatures so obtained, we finally
obtain a chart which graphically expresses the critical ranges of iron
and steels due to the variation of the carbon content. With very low
carbon steel it is interesting to note that the first critical point
would not occur until 1395 degrees Fahrenheit was reached.
Metallurgists have long designated the lines so obtained by letters,
“r”, standing for, “refroidissement”, which is the French word meaning
“cooling”, the suffixes 1-2-3 simply standing for the lines in the
order drawn.
From the completed chart it is further evident that our first piece
containing 0.9% carbon in one way is the most interesting of all since
it is the only case where only one point of critical temperature
occurs.
It will be noticed from the chart that steels containing less than .10%
carbon have no point Ar1 and it is therefore undoubtedly due to the
carbon content that this, the point of recalescence, occurs. From tests
which we made with the magnet we would also find that the temperatures
at which loss of magnetism occurs are those designated by the line Ar2,
whereas the loss of ability to conduct an electric current occurs at
the point designated Ar3. In steels containing .45% carbon to .75%
carbon loss of magnetism and loss of ability to conduct an electric
current occur at the same points designated on our chart by the line
Ar3-2; whereas in the steel containing .90% carbon—all these changes
take place at the same time.
Now, as we concluded before, it is evident that some internal change
must have taken place in the steel itself, and as we know that the
chemical content does not vary, it is further evident that the change
must be of a physical nature, or as in the language of the
Metallurgist, an “allotropic change”. Therefore, another conclusion
which we can draw at this point is that a very much more thorough
investigation is required for the proper handling of steel at high
temperatures than a mere knowledge of the chemical analysis of the
same.
There is one very fortunate circumstance connected with the passing
from one of these allotropic changes to another, and that is that the
effecting of one of these changes takes =time=. It does not take a very
long time, however, for in some instances the change is affected in a
very small fraction of a second, while rarely more than one or two
seconds are required. The higher the temperature the quicker the
change.
Would it not be interesting if we had been so constructed as outlined
in the beginning of this little volume; that we could have withstood
the high temperatures in which some of these very interesting changes
occur, because we could then handle the steel, examine it and
experiment with it at our leisure. However, such not being the case, we
will have to derive some other means for “catching” the steel while it
is in one of these interesting conditions, and then bringing it in its
entrapped condition down to room temperature. How shall we do it? Well,
we remember that we said it took =time= to effect the changes under
discussion and furthermore we remember that the changes can only take
place when the steel is within the proper critical range. Therefore, if
we could do something to lower the temperature of a piece of steel
while in one of the critical ranges before the steel had time to effect
the usual allotropic change of form, we might be able to catch a piece
of steel while in one of these unusual conditions, before it had really
had time to get back to normal.
Therefore, let us place a piece of .9% carbon tool steel in the heating
furnace and bring it up to and beyond the point of recalescence. Now,
grasping the piece firmly in a pair of tongs with all possible speed we
plunge it into a nearby pail of ice water, keeping the steel
constantly in motion. Almost instantly the steel becomes black and
within a few seconds is actually brought down to room temperature.
Now let us take the steel out and examine it. The act of tapping it on
the anvil in order to knock off the surplus water gives us a hint that
our test piece has undergone some sort of a change. For now it rings
with a bell-like clearness and gives the hammer with which we strike it
a quick snapping rebound which in itself indicates great hardness.
Next, we test the piece with a hardened steel file with which we could
easily have made a deep ridge before we attempted the heating operation
and to our surprise the file has as little effect as if it had been
made of wood. And to our surprise on closer examination, we actually
find that our test piece has scratched the file—surely it must be very
hard. We are convinced that some marked change must have taken place.
What can it be? Why it must be that due to the rapid cooling in the
pail of ice water we brought the temperature of the test piece down
below the critical range =before= the abnormal condition at which it
existed while at and above the critical range had found =time= to
change back to its former condition. And we remember that if one of
these allotropic changes is going to take place at all, nature says it
=must= do so while the steel is within the critical range and therefore
having forced the steel through that critical range which separates one
allotropic condition from another, before it had found =time= to effect
its desired change, we managed to entrap the abnormal condition so that
we could see it and feel it and get familiar with it at room
temperature.
If we so desire we can now make other hardness tests on our piece of
steel at our leisure. For these scientists have invented several
machines. One of the most common is called the scleroscope in which a
hardened steel ball is allowed to drop from a given height on to the
piece of steel to be tested. Then the rebound of the ball is carefully
noted. The higher the rebound, the harder the piece. That is natural
isn’t it? We know that if the ball were allowed to drop on butter, it
wouldn’t rebound at all, because the butter is so soft. A piece of wood
would possibly record a very tiny rebound, while a piece of hardened
tool steel would effect a very material action of the scleroscope ball,
thus indicating extreme hardness.
Now let us take our test piece to the grind stone and grind it down to
the shape of a cutting tool. It is necessary to resort to the grind
stone, in order to get the desired shape, because of course, our test
piece is far too hard to cut with any other metal. After having
produced a tool of the desired shape and size, let us fasten the same
securely into the carriage of a lathe, and then upon applying the
cutting edge to a revolving piece of cast iron, or soft steel, or even
to a piece of the very same grade of steel out of which the tool was
made, only while it is still in the softened or annealed condition, we
find that it is capable of easily and quickly cutting out a good sized
ribbon of chips from the metal which is to be machined.
However, we are soon confronted with a new difficulty, for as the cut
progresses, our tool runs into a rough spot which causes it to tremble
and chatter and then suddenly our tool cracks in two in the middle and
is at once completely ruined.
It is evident that as we are able to increase the desirable element of
hardness in a piece of tool steel, we also automatically increase the
undesirable element of brittleness, and therefore some new method must
be devised which will allow a sufficient degree of hardness to allow
the tool to cut other metals and at the same time not cause so much
brittleness that it will crack in two at the first rough spot which it
encounters.
One method of assisting the toughening of a piece of hardened tool
steel is accomplished by the process of “drawing”. This simply means
heating the piece of hardened tool steel up to some fairly warm
temperature, which of course must be kept well below the critical range
(at which the steel would jump at the chance to quickly change back
into one of its softer allotropic forms) and then keeping the steel at
this drawing temperature for a while until the unusual strains and
stress caused by the rapid cooling have had an opportunity to have
become somewhat relieved. Therefore, the process of “drawing” is quite
as important as is the first act of hardening itself, and great care
must be exercised in undertaking the same.