Scientific American Supplement, No. 344, August 5, 1882
by
Various
Part 3 out of 3
CRYSTALLIZATION AND ITS EFFECTS UPON IRON.
By N.B. WOOD, Member of the Civil Engineers' Club, of Cleveland.
[Footnote: Read January 10th. 1882.]
The question has been asked, "What is the chemically scientific
definition of crystallization?" Now as the study of crystallization and
its effect upon matter, physically as well as chemically, will be of
interest, considering the subject matter for discussion, I shall not
only endeavor to answer the question, as I understand it, but try to
treat it somewhat technologically.
Having this object in view, I have prepared or brought about the
conditions necessary to the formation of a few crystals of various
chemical substances, which for various reasons, such as lack of time and
bad weather, are not as perfect as could be desired, but will perhaps
subserve the purpose for which they were designed. I think you will
agree with me that they are beautiful, if they are imperfect, and I can
assure you that the pleasure of watching their formation fully repays
one for the trouble, if for no other reason than the mere gratification
of the senses. From the earliest times and by all races of men, the
crystal has been admired and imitated, or improved by cutting and
polishing into faces of various substances. I have also procured
specimens of steel and iron which show the effect of crystallization,
which was produced (perhaps) under known conditions, so that the
conclusions which we arrive at from their study will have a fair chance
of being logical, at least, and perhaps of some practical value.
When we examine inanimate nature we find two grand divisions of matter,
_fluid_ and _solid_. These two divisions may be subdivided into, the
former gaseous and liquid, the latter amorphous and crystalline; but
whether one or the other of these divisions be considered, their
ultimate and common division will be the ATOM. By the atom we understand
that portion of matter which admits of no further division, which,
though as inconceivable for minuteness as space is for extent, has still
definite weight, form, and volume; which under favorable circumstances,
has that power or force called cohesion, the intensity of which
constitutes strength of material, which every engineer is supposed to
understand, but which lies far beyond the powers of the human mind for
comprehension or analysis. When we apply a magnet to a mass of iron
filings, we observe the particles arrange themselves in regular order,
having considerable strength in one direction, and very little or none
in any other. Now, although we understand very little about the force
which holds these particles in position, we do know that it is actual
force applied from without and maintained at the expense of some of the
known sources of force. But the force or power or property of cohesion
seems to be a quality stored within the atom itself, in many cases
similar to magnetism, having powerful attraction in some directions
and very little or none in others. A crystal of mica, for instance, or
gypsum may be divided to any degree of thinness, but is very difficult
to even break. This property of crystals is termed cleavage. Cohesion
and crystallization are affected variously by various circumstances,
such as heat or its absence, motion or its absence, etc. In fact, almost
every phenomenon of nature within the range of ordinary temperatures
has effects which may be favorable to the crystallization of some
substances, and at the same time unfavorable to others; so it will be
seen that it is impossible to lay down any rule for it except for named
substances, like substances requiring like conditions, to bring its
atoms into that state of equilibrium where crystallization can occur.
If we examine crystals carefully we find, not only that nature has here
provided geometric forms of marvelous beauty and exactness, with faces
of polish and quoins of acuteness equal to the work of the most skillful
lapidist, "but that in whatever manner or under whatever circumstances a
crystal may have been formed, whether in the laboratory of the chemist
or the workshop of nature, in the bodies of animals or the tissues of
plants, up in the sky or in the depths of the earth, whether so rapidly
that we may literally see its growth, or by the slow aggregation of its
molecules during perhaps thousands of years, we always find that the
arrangement of the faces is subject to fixed and definite laws." We find
also that a crystal is always finished and has its form as perfectly
developed when it is the minutest point discernible by the microscope as
when it has attained its ultimate growth. I might add parenthetically
that crystals are sometimes of immense size, one at Milan of quartz
being 3 feet 3 inches long and 5 feet 6 inches in circumference, and is
estimated to weigh over 800 pounds; and a gigantic beryl at Grafton, N.
H., is over 4 feet in length and 32 inches in diameter, and weighs not
less than 5,000 pounds; but the most perfect specimens are of small
size, as some accident is sure to overtake the larger ones before they
acquire their growth, to interfere with their symmetry or transparency.
This you will see abundantly illustrated by the examples which I have
prepared, as also the constancy of the angles of like faces. Chemically
speaking, the crystal is always a perfect chemical body, and can never
be a mechanical mixture. This fact has been of great value to the
science of chemistry in developing the atomic theory, which has
demonstrated that a body can only exist chemically combined when a
definite number of atoms of each element is present, and that there is
no certainty of such proportions existing except in the crystal. I
hold before you a crystal of common alum. Its chemical symbol would be
Al_{2}O_{3},3SO_{3}+KO,SO_{3}+24H_{2}O. If we knew its weight and wished
to know its ultimate component parts, we could calculate them more
readily than we could acquire that knowledge by any other means. But the
elements of this quantity of uncrystallized alum could not be computed.
Then we may define crystallization to be the operation of nature wherein
the chemical atoms or molecules of a substance have sufficient polarized
force to arrange themselves about a central attracting point in definite
geometrical forms.
Fresenius defines it thus: "_Every operation, or process, whereby bodies
are made to pass from the fluid to the solid state, and to assume_
certain fixed, _mathematically definable, regular forms_." It would be
folly for me to attempt to criticise Fresenius, but I give you both
definitions, and you can take your choice. The definition of Fresenius,
however, will not suit our present purpose, because the crystallization
of wrought iron occurs, or seems to, _after_ the iron has acquired a
_solid state_.
Iron, as you all know, is known to the arts in three forms: cast or
crude, steel, and wrought or malleable. Cast iron varies much in
chemical composition, being a mixture of iron and carbon chiefly, as
constant factors, with which silicium in small quantities (from 1 to
5 per cent.), phosphorus, sulphur, and sometimes manganese (e.g.
spiegeleisen) and various other elements are combined. All of these have
some effect upon the crystalline structure of the mass, but whatever
crystallization takes place occurs at the moment of solidification, or
between that and a red heat, and varies much, according to the time
occupied in cooling, as to its composition. My own experience leads me
to think that a cast iron having about 3 per cent. of carbon, a small
per centage of phosphorus, say about 1/2 of 1 per cent., and very small
quantities of silicium, the less the better, and traces of manganese
(the two latter substances _slagging_ out almost entirely during the
process of remelting for casting), makes a metal best adapted to the
general use of the founder. Such proportions will make a soft, even
grained, dark gray iron, whose crystals are small and bright, and whose
fracture will be uneven and sharp to the touch. The phosphorus in this
instance gives the metal liquidity at a low temperature, but does not
seem to influence the crystallization to any appreciable extent. The two
elements to be avoided by the founder are silicium and sulphur. These
give to iron a peculiar crystalline appearance easily recognized by
an experienced person. Silicium seems to obliterate the sparkling
brilliancy of the crystalline faces of good iron, and replace them with
very fine dull ones only discernible with a lens, and the iron breaks
more like stoneware than metal, while sulphur in appreciable quantities
gives a striated crystalline texture similar to chilled iron, and very
brittle. Phosphorus in very large quantities acts similarly. The form of
the crystal in cast iron is the octahedron, so that right angles with
sharp corners should be avoided as much as possible in castings, as the
most likely position for a crystal to take would be with its faces along
the line of the angle. Steel, to be of any value as such, _must_ be made
of the purest material. Phosphorus and sulphur _must_ not exist, except
in the most minute quantities, or the metal is worthless. If either of
these substances be present in a bar of steel, its structure will
be coarse, crystalline and weak. The reason of this is unknown, but
probably their presence reduces the power of cohesion; and, that being
reduced, gives the molecules of steel greater freedom to arrange
themselves in conformity with their polarity, and this in its turn again
weakens the mass by the tendency of the crystals to cleavage in certain
directions. Carbon is a constant element in steel, as it is in cast
iron, but is frequently replaced by chromium, titanium, etc., or is said
to be, though it is not quite clear to me how it can be so if steel is
a chemical compound. However this may be, we know that a piece of good
soft steel breaks with a fine crystalline fracture, and the same piece
hardened when broken shows either an amorphous structure or one very
finely crystalline, which would indicate that the crystals had been
broken up by the action of heat, and that they had not had sufficient
time to return to their original position on account of the sudden
cooling. The tendency of the molecules of steel after hardening to
assume their natural position when cold seems to be very great, for we
have often seen large pieces of steel burst asunder after hardening,
though lying untouched, and sometimes with such force as to hurl the
fragments to some distance. If a piece of steel be subjected to a bright
yellow or white heat its nature is entirely changed, and the workman
says it is burnt. Though this is not actually a fact, it does well
enough to express that condition of the metal. Steel cannot be burnt
unless some portion of it has been oxidized. The carbon would of course
be attacked first, its affinity for oxygen being greatest; but we find
nothing wanting in a piece of burnt steel. It can, by careful heating,
hammering and hardening, be returned to its former excellence. Then what
change has taken place? I should say that two modifications have been
made, one physical, the other chemical. The change chemically is that
of a chemical compound to a mixture of carbon and iron, so that in a
chemical sense it resembles cast iron. The change physically is that of
crystallization, being due partly to chemical change and partly to the
effect of heat. I have procured a specimen of steel showing beautifully
the effect of overheating. The specimen is labeled No. 1, and is a piece
of Park Brothers' steel (one of the best brands made in America). It has
been heated at one end to proper heat for hardening, and at the other is
what is technically called "burnt." It has been broken at intervals
of about 11/2 inches, showing the transition from amorphous or proper
hardening to highly crystalline or "burnt." Malleable or wrought iron
is or should be pure iron. Of course in practice it is seldom such, but
generally nearly so, being usually 98, 99, or even more per cent. It is
exceedingly prone to crystallization, the purer varieties being as much
subject to it as others, except those contaminated with phosphorus,
which affects it similarly with steel, and makes it very weak to cross
and tensile strains. I have never estimated the quantity present in any
except one specimen, a bar of 11/2 round, which literally fell to pieces
when dropped across a block of iron. It had 1.32 per cent. of phosphorus
and was very crystalline, though the crystals were not very large. Iron
which has been, when first made, quite fibrous, when subjected to a
series of shocks for a greater or less period, according to their
intensity, when subjected to intense currents of electricity, or when
subjected to high temperatures, or has by mechanical force been pushed
together, or, as it is called, upset, becomes extremely crystalline.
Under all of these circumstances it is subjected to one physical
phenomenon, that of motion. It would seem that if a bar of iron were
struck, the blow would shake the whole mass, and consequently the
relative position of the particles remain unchanged, but this is not the
case. When the blow is struck it takes an appreciable length of time for
the effect to be communicated to the other end so as to be heard, if the
distance is great. This shows that a small force is communicated from
particle to particle independently along the whole mass, and that each
atom actually moves independently of its neighbor. Then, if there be
any attraction at the time tending to arrange it differently, it will
conform to it. So much for theory with regard to this important matter.
It looks well on paper, but do the facts of the case correspond? If
practically demonstrated and systematically executed, experiments fail
to corroborate the theory, and if, furthermore, we find there is no
necessity for the theory, we naturally conclude that it is all wrong,
or, at least, imperfectly understood. Now there is one other quality
imparted to iron by successive shocks, which, I think, is independent
of crystallization, and this quality is hardness and consequent
brittleness. One noticeable feature about this also is, that as
"absolute cohesion" or tensile strength diminishes, "relative cohesion"
or strength to resist crushing increases. Specimens Nos. 2, 3, and 4 are
pieces of Swedish iron, probably from the celebrated mines of Dannemora.
Nos. 2 and 3 are parts of the same bolt, which, after some months' use
on a "heading machine" in a bolt and nut works, where it was subjected
to numerous and violent shocks, (perhaps 50,000 or 60,000 per day),
it broke short off, as you see in No 2, showing a highly crystalline
fracture. To test whether this structure continued through the bolt, I
had it nicked by a blacksmith's cold chisel and broken. The specimen
shows that it is still stronger at that point than at the point where
it is actually broken, but the resulting fracture shows the same
crystalline appearance. I next had specimen No. 4 cut from a fresh
bar of iron which had never been used for anything. It also shows a
crystalline fracture, indicating that this peculiarity had existed in
the iron of both from the beginning.
I next took specimen No. 3 and subjected it to a careful annealing,
taking perhaps two hours in the operation. Although it is a 1-1/8 bolt
and has V threads cut upon it we were unable to break it, although bent
cold through an arc of 90 deg., and probably would have doubled upon itself
if we had had the means to have forced it. Now what does this show? Have
the crystals been obliterated by the process of annealing, or has only
their cleavage been destroyed, so that when they break, instead of
showing brilliant, sparkling faces, they are drawn into a fibrous
looking mass? The latter seems to be the most plausible theory, to which
I admit objections may be raised. For my own part, I am inclined to the
belief that the crystal exists in all iron which is finished above a
bright red heat, and that between that and black heat they are formed
and have whatever characteristics circumstances may confer upon them,
modified by the action of agencies heretofore mentioned.
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