TRANSFORMATIONS AND STRUCTURES OF THE STEELS
It was “Ali Baba” who is quoted as saying, “Those who do not know how to
take the Philistine, better hadn’t!” or words to that effect.
Now through these chapters we have attempted to discuss in an entirely
non-technical manner the subjects presented. On this account we were
compelled to forego discussion of many things which are highly important
and interesting but which are more or less difficult of explanation
without the use of scientific terms and theories. One such has been the
“mechanism” of the hardening of steel and its opposite, its softening by
annealing. For those who may desire to get a glimpse into this
“wonderland” it is hardly fair to refrain from brief discussion of the
subject just because it is technical and difficult and so may prove to
be tedious to some who have little reason to be interested.
It seems desirable, therefore, to impose this more technical chapter or
two that the subject of the real metallurgy of iron and the steels may
at least be “hinted at.” We say “hinted at” advisedly for it is a long,
long story, and, even now, after a great many years of serious study no
one has yet read it to the end. We are not saying this in a discouraging
way, however, for there seems little reason to doubt that the multitude
of facts which have been disclosed through the tireless experiments and
the study of hundreds of investigators have put us well on our way to
the solution of this one of Nature’s great problems.
To those, however, who are not interested in the known details of “how”
and “why” hardening and softening of steel is possible and why hardening
of pure iron and mild steels does not and cannot take place, we must say
as would “Ali Baba”:—“Those who do not care to study it better hadn’t.”
Anyway, the study of this rather intricate subject is conducive of
“headaches,” and perhaps it is not extremely important when viewed from
the non-technical standpoint of these articles.
We have several times referred to the debt which civilization owes to
iron and steel structural materials, machinery and tools and
particularly to those tools which have hardened cutting edges. Almost
every one knows that hardened cutting edges are imparted to tools by
sudden cooling in water or oil from a good red heat. Probably most of
us, too, know that the blacksmith can again soften such tools by
reheating to the same red heat and allowing them to cool slowly. This he
calls annealing. In this softened or annealed condition a piece can
readily be sawed or filed, while in its hardened state a saw or file
produces no result upon it.
Now what are the facts, meaning and the cause of this dual life of the
alloy, steel, without which we would be so greatly handicapped.
To be better prepared to understand the answer, let us consider three or
four accompanying and closely allied phenomena which close observation
of the habits of steel has disclosed.
The “Point of Recalescence”
If we drill a hole in a small piece of carbon tool steel which we are
about to put into the heating furnace, and if into this hole we insert
the bare tip of an electric pyrometer, this heat-measuring instrument
will indicate at all times the rising temperature of the piece of steel
as it heats in the red-hot muffle or chamber of the furnace.
As we watch the piece grow red, the pyrometer registers 900°, 1000°,
1100°, 1200° F.,—gradually and uniformly indicating higher and higher
temperatures.
[Illustration:
APPARATUS FOR DETERMINING THE CRITICAL POINTS OF STEEL
]
But lo! Something must be wrong! The pyrometer needle does not now move
forward but is standing still. Though we know that in that hot furnace
the piece must be absorbing heat at the same rate as before, yet the
pyrometer needle does not budge!
But, as our wonderment grows and we are still undecided as to the
meaning, the needle again begins to advance and continues again
regularly and uniformly to higher and higher temperatures as though it
had never taken the vacation.
With the piece now at a white heat, we have proceeded far enough with
the heating.
Turning off the electric current from the furnace and allowing it to
cool we again watch the pyrometer needle as the temperature of the piece
in the cooling furnace gradually falls. Lower, lower, lower swings the
needle, always at a rate approximately uniform.
But again it suddenly stops and remains immovable, or perhaps even rises
slightly, for a period of several seconds, after which it resumes its
uniformly-timed downward course as though nothing had happened.
Yes, these pauses of the needle occurred at very nearly the same marking
on the pyrometer dial, but not at exactly the same ones. Going up it was
at 1350° F., and on the downward way it was at 1250° F. And you are
correct in surmising that these two points are closely related. They are
parts of the same, if we may so speak, and, in reality they represent
one point which is located about halfway between them, the divergence
resulting from what is known as “hysteresis” or “lag,” which means, of
course, a “being-behind-hand” or tardiness.
For the present we may say that all carbon steels have this “critical”
range as shown by such pauses of the pyrometer needle during heating or
cooling of the steel.
Now as the piece is most certainly continuing to absorb heat in the
furnace as it grows hotter and is losing it uniformly to the air as the
furnace cools, we have no alternative but to judge that the pause of the
needle on its upward way was caused by some internal affair of the piece
of steel itself, for which, at just that stage of its journey, it
required and used for its purpose (which was other than making itself
hotter), the heat furnished it by the furnace; and, that on the downward
journey, at just that same point, it gave out again that same heat. It
must have been the setting free of this imprisoned heat, if we may so
term it, which kept the piece for those few moments from cooling at the
usual rate. Indeed, had we conducted our experiment in a rather dark
room and observed the piece closely we would have noticed that during
the pause of the pyrometer the piece of steel did brighten or glow
somewhat, showing that it had extra heat from some hidden source.
Because of this “self-heating” of the steel as shown by the pyrometer
and the brightening, the temperature at which the phenomenon occurs has
been named the “point of recalescence,” which means the point at which
it spontaneously becomes hotter.
Loss of Magnetism
Now another curious thing took place had we but noticed it.
We all know that iron and steel are our most magnetic materials. From
childhood we have seen pins, needles, steel pens, and various other
steel or iron objects jump to a magnet held near them.
What, now, when we find that our piece of steel in the furnace when at a
red or higher heat is entirely unresponsive or dead to the attraction of
a strong magnet?
Strange! Do you suppose that our magnet has lost its power?
Let us see.
Suppose that every minute or so, while watching the pyrometer needle go
slowly down again after turning off the heat, we put the magnet to the
steel.
Continually lower comes the temperature of the piece—1500°, 1400°,
1375°, 1350°, 1325°, 1300°, 1275° F.,—and lo! the piece jumps, and from
this all the way down to cold it responds to the attraction of our
magnet. Just to make sure that we are not “seeing things,” we again
start our furnace, and, as the steel heats, we test it with the magnet.
So far there is no doubt about its being magnetic!
At 900° F., the pieces begins to show dark red, at 1000°, 1100°, 1150°,
1200°, 1250°, stronger and stronger red. At all of these temperatures
the steel is attracted. So it is at 1275° and at 1300° F.
But just as we are thinking that we must have been mistaken before, we
find that again the steel is suddenly “dead” to the pull of the magnet!
And at what temperature? The pyrometer indicates 1320° F. But was not
this the same or very nearly the same reading at which the pyrometer
needle paused on the way up, and do you not remember that it was only a
little below 1250° F. that it paused on the way down, and the
disagreement of the two temperatures we ascribed to “lag”?
No, we made no mistake. Steel loses all of its magnetic properties at
the “critical range” and has none above it.
Dilatation and Conductivity
Certain other great changes, too, occur here.
We know that most materials expand uniformly upon heating and contract
as they cool. Steel is no exception, but at the critical range on
heating it becomes fickle and for a short space contracts instead of
continuing its uniform expansion. Conversely, during cooling, it ceases
its uniform contraction and suddenly dilates or expands for a short
period when it reaches the critical range, after which aberration it
again resumes its old habit of uniform contraction as the temperature
falls.
Just so with its electrical conductivity. At the critical range the
electrical conductivity suddenly decreases abnormally as the piece gets
hotter and as abnormally increases as the steel cools through the
critical range on the return trip.
There are certain other happenings at or near this particular
temperature but we will not consider them here.
Manifestly all of this has a deep meaning.
Recalescence Indicates the Hardening Point
You remember that we said the divergence between the going up and the
coming down pauses of the pyrometer needle was due to tardiness or lag?
Among humans habitual tardiness is not considered a desirable trait, but
it is undoubtedly through this very lag or tardiness that steel becomes
so serviceable to us.
[Illustration:
HEATING AND COOLING CURVES OF STEEL WITH
.9 PER CENT OF CARBON
]
This lag is peculiar in that it grows less the more slowly we heat or
cool the steel, and, if the heating or cooling is done slowly enough,
the lag disappears almost entirely, i.e., the pause of the pyrometer
needle occurs at the same temperature on the upward as on the downward
way. Conversely, the disagreement or split grows or widens the faster
the temperature is raised or lowered.
Here is the vital point.
By extremely sudden cooling, such as quenching in water, the lag becomes
so great that it never catches up at all and any structure with its
consequent properties which was brought about in the steel by the higher
temperature is thus frozen or fixed and made to “persist” after the
steel has become cold.
It is just at this point, the “point of recalescence,” that steel
changes from its soft and malleable, to its extremely hard and brittle
condition. If it is quenched from temperatures above this point, it is
extremely hard, if from temperatures below it, even those only a little
below, it is soft and ductile. It is from just a little above this
point, then, usually between 1350° F., and 1500° F., that the blacksmith
hardens his tools by plunging them into cold water.
[Illustration:
HEATING AND COOLING CURVES OF STEEL WITH .46 PER CENT OF CARBON
]
Steels of Other Composition
Now it should be noted particularly that the specimen with which we have
been experimenting is a tool steel of .90% carbon or thereabouts. This
is important, for, while all of the carbon steels show this same
critical temperature, at which occurs the point of recalescence, those
containing from .45% to about .85% carbon have another point somewhat
higher on the temperature scale, and steels which contain from .10% to
.45% of carbon have two others, or three points in all. Further, steels
having less than .10% of carbon and iron with no carbon at all have the
two upper points but no point at 1290° F. This lower one has
disappeared.
All of this means that if instead of a piece of .90% carbon steel we had
used one having .60% of carbon, say, we would have found two different
critical ranges or points at which the pyrometer paused, the one at
1290° F., and another when we got to 1360° F. Had the steel been one
containing .30% carbon we would have discovered pauses at three
different points, viz., at 1290° F., at 1395° F., and at 1480° F. With
very low carbon steel or with wrought iron, the pyrometer would have
registered two pauses, one at about 1395° F., and the other at 1650° F.
When records are carefully kept of the time which is required for the
temperature to rise or lower over each and every twenty-five degree
period, say, on the upward and downward way, and these are “plotted,”
what are called “heating” and “cooling” curves can be drawn through the
stars and dots so set down and these form a record of the behavior of
the pyrometer needle at each temperature along the scale. Two
illustrations of such curves are shown.[9]
Footnote 9:
Special apparatus is now obtainable for determination of critical
points, heating and cooling curves.
Now if on properly spaced, dotted, vertical lines, which we will let
represent these various alloys, we mark points number three, two and one
as shown by our “cooling” curves, calling the topmost point three, it is
readily seen that the points are related. The lines and the alloys which
they represent are,
(a) The wrought iron,
(b) .15% carbon steel,
(c) .30% carbon steel,
(d) .45% carbon steel,
(e) .60% carbon steel,
(f) .75% carbon steel, and
(g) the steel with .90% carbon.
[Illustration:
CRITICAL POINT DIAGRAM OF PURE IRON AND THE STEELS
]
Of course many more cooling curves, especially of steels with other
percentages of carbon would be desirable, but we have enough that we are
safe in sketching the horizontal and oblique lines, Ar_{1}, Ar_{2},
Ar_{3}, Ar_{3·2} and Ar_{3·2·1}, through the points which we have
arranged.
For convenience, metallurgists everywhere mark these points Ar_{1},
Ar_{2} and Ar_{3}, the first being the lower, and Ar_{3} the upper one.
Ar_{cm} represents an upper point found in steels having more than .9%
of carbon. The letter “r” is derived from the French word,
“refroidissement,” meaning “cooling.” The corresponding points disclosed
during heating are marked Ac_{1}, Ac_{2}, Ac_{3·2·1}, etc., from the
word, “chauffage” meaning “heating.” The “A” apparently “just happened.”
Before the upper critical points were known it had been used by
Tschernoff to designate the temperature at which steels harden.
But it must not be supposed that the skeleton which we have constructed
can be fully accepted as true until it has been checked and rechecked
hundreds and hundreds of times by other investigators. A great many have
worked upon these critical points and upon the “freezing-point” curves
of the various alloys. It was of course impossible for all of them to
make their determinations in just the same manner and with exactly the
same materials. Examination of their work and consideration of the
results which they obtained show some discrepancies as might be
expected, largely probably because of difference in purity of the
materials tested, every impurity such as manganese, nickel, silicon,
sulphur, phosphorus, etc., modifying more or less the results obtained.
As we discovered, speed of heating and cooling also modify the results.
When we consider the difficulties which attend the making of
determinations on metals and alloys at high temperatures, the wonder is
that there is such close agreement. From these standpoints the
differences which exist in the published results seem quite small.
It was but twenty years ago that the first outline was drawn and the
whole “fusibility” or “equilibrium diagram” of the iron-carbon alloys
given in the next chapter has practically been developed within this
time. But over this period of twenty or so years the points upon which
these lines Ar_{1}, Ar_{2}, Ar_{3}, Ar_{3·2} and Ar_{3·2·1} are based
have been checked many times and they are now well substantiated. These
lines form but a small part of the complete “iron-carbon diagram.”
The Meaning of the Points
You remember that wrought iron and steels having less than .10% carbon
showed no point Ar_{1}, and that in all other steels this point becomes
stronger as they contain higher and higher carbon. There is little doubt
that the point Ar_{1} exists or results from and because of the carbon
of the alloy. In wrought iron there is no carbon, hence there is no
point Ar_{1}. If the extremely low carbon steels have an Ar_{1} it is so
weak that it cannot be detected.
Had we tested the .45% carbon steel for magnetic properties we would
have found that it lost magnetism at about 1395° F., instead of at 1290°
F., at which temperature the .90% steel became non-magnetic. The point
Ar_{2}, then, shows the temperature at which loss or gain of magnetism
occurs. The electrical conductivity change comes at neither of these
points, Ar_{1}, nor Ar_{2}, but at Ar_{3}.
However, with increase of carbon the line Ar_{3}, which was drawn
through the points, Ar_{3}, rapidly descends. At about .45% or .50%
carbon content, this line Ar_{3}, representing the changes in
conductivity, joins line Ar_{2}. Hence in steels having .45% carbon or
more, there is a common point, or one which in reality is made up of
both points. At this common point the phenomena peculiar to each of the
points occur.
This common line, now called Ar_{3·2}, itself lowers with further
increase of carbon until, in steels of around .90% carbon, there is but
the single point Ar_{3·2·1}, and the phenomena corresponding to all
three of the points occur at this one point at 1290° F., as we found in
our experiments.
As points Ar_{2} and Ar_{3} occur in carbonless iron, they cannot result
in any way from carbon but must have to do with the iron itself. From
their experiences with other materials, chemists and physicists are well
acquainted with such evolutions of heat as occur at Ar_{2} and at
Ar_{3}. These heat absorptions and evolutions, with the sudden
dilatation, gain in conductivity, etc., indicate that some internal
change or reorganization takes place in the iron itself.
Such changes seem to indicate what are known as “allotropic”
modifications. More familiar examples of allotropic forms of materials
may be mentioned. Phosphorus, for example, may exist, either as the
“yellow” variety which is poisonous and so inflammable that it must be
kept constantly under water, or as the “red” variety which is
non-poisonous and non-inflammable. Too, there is carbon, which may exist
in any one of several forms such as amorphous carbon (soot), graphite,
and the diamond. It is believed that iron, itself, exists in three
allotropic states. These have been named “alpha,” “beta” and “gamma”
iron. We do not need to go into this part of the great subject except to
state that at ordinary temperatures and up to Ar_{2}, we have alpha
iron, between Ar_{2} and Ar_{3}, beta iron, and above Ar_{3}, gamma
iron. Both beta and gamma iron are non-magnetic, while alpha iron is
strongly magnetic. In cooling through Ar_{3}, i.e., from gamma to beta
iron, some rearrangement of its molecules produces the dilatation or
expansion and the change in conductivity which was noted above.
From the fact that by chemical analysis any certain steel must have the
same composition in its hardened that it has in its unhardened
condition, it will readily be seen how futile it would be to expect
chemical analysis to give us complete information regarding it. Too,
tensile strength and the other usual physical tests can hardly tell us
all that we wish to know. Microscopic analysis or metallography,
however, shows us internal structure of properly prepared pieces of
either the hardened or unhardened alloy that we may see the actual
condition or grouping of the constituents. The view points given by all
three of these methods, chemical, physical and metallographical, are, of
course, much better than any one or two alone.
The Structures of Quenched and Unquenched Steel
We saw that the lag or tardiness is greater the more rapid the cooling.
Along with this very great lag which is brought about by very rapid
cooling comes increasing slowness, i.e., less ability to catch up, as
the temperature is lowered. Hence quenching produces such a wide lag and
so slows the changes which should take place that they do not take place
at all, i.e., the structure which the piece had at the higher
temperatures cannot change but is set or fastened by the quickness of
the cooling.
Though no degree of suddenness is sufficient to set completely the
structure existing at very high temperatures, for our present purposes
we can say that by quenching in cold water we can freeze or fix any
structure. Then after we have quenched a piece of steel, it will have
when cold, the structure which corresponded with or resulted from the
temperature which it had at the moment before the quenching.
If so, the microscope should give us aid.
[Illustration:
ILLUMINATION OF THE SAMPLE UNDER THE MICROSCOPE
]
By breaking off pieces of a quenched piece and very carefully and slowly
grinding and polishing without heating a surface which was an interior
part we find after etching that we can actually see the kind of
structure which corresponded with the temperature from which the piece
was quenched.
Photomicrograph No. 80 shows the appearance of a piece of hardened
carbon steel. Note the needle-like structure under the microscope at
magnification of 400 diameters. This structure is characteristic.
The constituent, having this needle-like appearance has been named
“martensite” in memory of a distinguished European metallurgist, A.
Martens. It is supposed to be “beta” iron, much the hardest allotropic
variety of iron, and to hold in solution the carbon of the alloy, either
as carbon alone or as the extremely hard chemical compound, iron
carbide, Fe_{3}C.
Martensite, then, is the extremely hard structure, necessarily
containing considerable carbon or iron carbide in solution which gives
to our carbon tool steels their hardness and great usefulness.
Unhardened steels never look like this. Their appearance is shown in
photomicrographs Nos. 3b, 5, 22 and 24a.
[Illustration:
No. 80. MARTENSITE, THE CONSTITUENT OF HARDENED STEEL
(_Magnification 400 Diameters_)
]
In unhardened steels having less than .90% of carbon we find two
constituents.
“Ferrite” is the name which has been given to one, the soft and ductile
constituent, pure iron. With ordinary etching the ferrite usually shows
as light-colored or white grains bounded by black lines, which, if the
patch is large enough, give a fish-net appearance. It is soft and
ductile like copper, for pure iron and pure copper are not so greatly
different in malleability and ductility as one might suppose.
The darker and more or less triangular patches at the corners of the
ferrite grains are “pearlite,” a name originating because of their
“pearly” appearance under the microscope. How this pearly appearance
comes about will be readily understood from photomicrograph No. 23e
which was taken at a magnification of 400 diameters. It is seen that it
results from alternate black and white layers.
Again we must give up the idea of any finality in the things we learn or
think we have learned. We just learned, for instance, that ferrite
usually was light or white in color. Well, in pearlite, as shown in
photomicrograph No. 23e, every other plate is of ferrite but they are
not the white but the black ones.
[Illustration:
No. 23e. PEARLITE AT MAGNIFICATION OF 400 DIAMETERS
]
You may not have understood before that color as shown under the
metallographic microscope depends not so much upon actual color of the
material itself as upon its ability to reflect light. For metallographic
observations it is necessary to have very strong illumination. Usually
the powerful beam from an electric arc is concentrated by means of
condensing lenses upon a thin disc of glass called an oblique reflector
which directs the beam upon the polished and etched specimen beneath the
objective of the microscope. Often a prism is used. The rays of light
returning from this highly illuminated “field” under observation return
up through the tube and eye piece of the microscope and can be focused
upon a small screen convenient for observation or upon the ground glass
of the attached camera by means of which the pictures are taken. Unless
the surface of the specimen being examined is perfectly plain and level,
not all of the vertical rays thrown down upon it will be reflected back
up through the tube and eye piece. Those portions of the field which are
absolutely at right angles to the vertical rays appear at the eye piece
or upon the screen as white or light-colored portions, while those
which, during the polishing or etching have been dug or eaten away
reflect the light imperfectly or in directions other than up the tube of
the microscope, wherefore such portions show as darker or black
sections.
The pearlite, then, is made up of little plates of soft ferrite
alternating with others of a very much harder constituent. The harder
plates are much less affected during polishing and etching than are
those of the softer ferrite, hence they stand out in relief and reflect
abundant rays of light, whereas the “dug-out” ferrite plates reflect the
light imperfectly or not at all and therefore appear as dark lines.
These white, hard plates of the pearlite contain all of the carbon of
the low carbon alloys. They are this other constituent, “cementite,” so
named because it was first discovered in steel made by the “cementation”
process. It is a very hard and brittle substance, hard enough to scratch
glass. It is the chemical compound (Fe_{3}C), unvarying in composition
as chemical compounds always are. It consists of just three atoms of
iron (93.4% by weight) and one of carbon (6.6%).
Pearlite, therefore, is a sort of mechanical mixture of two separate
constituents, ferrite or pure iron, and this chemical compound, carbide
of iron, which is called cementite. Pearlite is common to all unhardened
steels whether of low, medium or high carbon content and may be
considered characteristic.
That we may understand clearly the structures of the annealed steels,
let us start with pure iron and gradually change it into higher and
higher carbon steels by gradual addition of carbon. Pages 328 and 329
show such a series.
Photomicrograph No. 99b is open-hearth iron which is entirely made up of
free ferrite. In No. 3b there is considerable pearlite, here appearing
black, though the sample of steel yet contains but .10% of carbon. In
No. 5, which is of a steel containing .30% of carbon, we have more
pearlite and in No. 22c with .50% carbon we have yet more. Manifestly at
this rate the comparative pearlite areas are growing so that there will
soon be room for no ferrite at all. In No. 23g this has occurred. This,
the photomicrograph of a steel containing .86% of carbon is one of the
steels in which we found that the point of recalescence, loss of
magnetism, decrease in electrical conductivity and rate of expansion
take place all at the one point.
[Illustration:
No. 99b. CARBONLESS IRON
]
[Illustration:
No. 3b. STEEL WITH .1 PER CENT CARBON
]
[Illustration:
No. 5. STEEL WITH .3 PER CENT CARBON
]
[Illustration:
No. 22c. STEEL WITH .5 PER CENT CARBON
]
(_Magnification 60 Diameters._)
Now as we go still farther on up in percentage of carbon content, i.e.
(beyond .86%, we have a white constituent beginning to appear as cell
walls around the grains of the pearlite and this increases with increase
of carbon until, with alloys having carbon around 3%, we have a
proportionately small amount of pearlite while the white areas have so
increased that it appears that the more or less round patches of
pearlite float in a lake of white. This white which appears first as
cell walls, and later in greater and greater quantity is free
cementite.)
[Illustration:
No. 23g. STEEL WITH .9 PER CENT CARBON
]
[Illustration:
No. 24a. STEEL WITH 1.25 PER CENT CARBON
]
[Illustration:
No. 36b. STEEL WITH 2 PER CENT CARBON
]
[Illustration:
No. 109. WHITE CAST IRON WITH 3 PER CENT CARBON
]
(_Magnification 60 Diameters._)
Such are illustrated in photomicrographs Nos. 24a, 36b and 109 which
contain 1.25%, 1.98% and 3.00% of carbon respectively. While steels with
the typical white, free ferrite areas are so soft that a needle-point
will plow furrows across them, those with over 1.25% of carbon have such
excess of free cementite that they are very hard to scratch and too
brittle to use except for special purposes.
[Illustration:
AUSTENITE (WHITE) AND MARTENSITE (DARK) MAGNIFIED 1,000 TIMES THEIR
ACTUAL SIZE
]
So during ordinary cooling from the molten alloy or the slower cooling
of the steel during the annealing process, the martensitic structure
breaks down at the recalescent temperature into pearlite and ferrite
(soft iron) if the carbon content of the steel is lower than about .90%,
or pearlite and the other and very hard constituent, “cementite,” if the
steel has more than .90% of carbon. If the carbon content happens to be
just .90%, or thereabouts, there is exactly sufficient pearlite to make
up the total area of the field shown under the microscope.
Another constituent which is of great interest scientifically, though
not at all commercially, is “austenite.” By quenching _very_ high carbon
steels from a _very_ high temperature _very_ suddenly and completely, we
can fasten the “austenite” structure, which exists only at temperatures
higher than martensite, i.e., austenite is our gamma iron with the
carbon of the alloy in solid solution, perhaps as iron carbide, while
martensite is thought to be the beta iron solid solution, perhaps with
some gamma iron mixed with it.
While ordinary quenching fastens structures pretty well, it is not
usually quick enough to prevent the austenite from sliding along down
into martensite. However, carbon discourages such slipping, so, with
high carbon to act as a brake, we can fasten some of it by chilling very
suddenly and completely from a very high temperature. Steels with 1.5%
of carbon and temperatures of 2000° F., or over, are usually necessary
to accomplish it.
However, austenite, after we get it, is not as hard as martensite and we
have little use for it commercially. As was stated before, martensite is
the useful and proper structure for carbon steel tools.
[Illustration:
No. 73. ANNEALED STEEL HAS FINE GRAIN
(_Magnification 70 Diameters_)
]
Tempering or Drawing
“Tempering” is done to relieve the intense brittleness of steel after
quenching to martensite. While we dislike to sacrifice any of the
hardness, it pays to temper or “toughen” the steel, as the toolmaker
calls it, by reheating it to somewhere between 400° and 570° F.
The higher the temperature, the freer and quicker is the change from one
structure to another, as, for instance, the austenite to martensite. At
the low drawing temperatures the changes from martensite to the
pearlitic structure may be said to just creep along. A second quenching
then fastens it at the new structure which gives a trifle less hard but
a tougher steel. As you would guess, the microscope shows on these what
we may term a “transition” or “breaking-down” appearance and structures
not at all definite. These, of course, give to the steels the various
degrees of hardness and brittleness and other qualities which are so
desirable from the practical standpoint. The production of these fine
shades of temper by the practical tool maker or blacksmith may almost be
considered a fine art.
How and Why Do the Steels Harden?
Now, from all of these facts, what, shall we say, is the cause of the
hardening of steel?
The explanation most generally accepted seems to be, that, of the three
allotropic forms of iron the gamma and beta varieties are very much
harder than alpha iron, which is the one which we have in annealed steel
at ordinary temperatures, and, of the two, the beta is harder than the
gamma variety. It is thought also, that carbon, perhaps as carbide of
iron, is held in solution in gamma or beta iron after the quenching, and
increases the hardness proportionately with increase of the carbon
content of the steel. While this carbon or iron-carbide solid solution
may and probably does itself confer additional hardness to the steel,
its main function is to retard or slow down the change from gamma into
beta and alpha iron, which change in carbonless iron and low carbon
steels is so insistent and extremely rapid that not even the most severe
quenching, as in ice-water or liquid air, can prevent or stop it. Not
only do we fail to get austenite, which is gamma iron, until we get 1%
or more of carbon present and quench from very high to very low
temperatures, but we cannot even stop the transition at beta iron, the
next lower allotropic variety until we have at least .30% of carbon,
and, for serviceable hardening fully .60% of carbon in the steel.
Fortunately for us, this beta form of iron is the one we want, for it is
harder and more useful than the gamma form.
Our most serviceable constituent, martensite, then, is a solid solution
of carbon or iron carbide in beta iron. It is magnetic but this probably
results from its containing some alpha iron through incomplete stoppage
of the change by the quenching.
As has been stated, “tempering,” which means careful reheating to 400°
F., 500° F., or 600° F., allows the slight “slipping” of enough of the
beta solution, always eager at temperatures below the point of
recalescence to return to alpha condition, to relieve the excessive
brittleness of the hardened steel.
[Illustration:
No. 72. THE “SORBITE” GRAIN IS PRODUCED BY COOLING IN A BLAST OF AIR
AFTER ANNEALING. IT GIVES GOOD WEARING PROPERTIES
(_Magnification 70 Diameters_)
]
Annealing is the complete release of beta iron and the “trapped” carbon
which allows of their return to the normal condition of pearlite with
alpha iron. To accomplish this, the hardened steel has to be heated
above its point of recalescence and cooled more or less slowly.
Different speeds of cooling give different grain size, structures and
physical properties.
This explanation of hardening, which is known as the “allotropic”
theory, is not universally accepted, conclusive evidence being lacking
at more than one point.
It must be stated also that some who hold the “allotropic theory” of
hardening doubt the existence of beta iron. These contend that the
so-called beta iron and martensite are only decomposition or transition
forms of gamma iron and austenite.
Two or three other theories have been more or less strongly advocated
but these also suffer from lack of evidence. The one which perhaps ranks
next in number of advocates is the “carbon theory.” Its supporters
contend that by quenching, alpha iron is made to hold carbon or iron
carbide in solid solution and that it is this solution of carbon or
carbide which gives to the steel hardness in proportion to its carbon
content. Others hold that the great density or strain under which
quenched steel exists accounts for the hardness.
Like many other great problems of the universe this one is not yet
conclusively or satisfactorily solved, so reluctantly does Nature yield
her secrets. But while it may not have explained all of the “whys” and
“wherefores,” the work which investigators have done has brought about
great improvement in methods of manufacture and quality of the alloys
which are making our civilization greater and this Age more wonderful.