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.

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.

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.

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]

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.

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, Ar₁, Ar₂, Ar₃, Ar_3·2 and Ar_3·2·1, through the points which we have arranged.

For convenience, metallurgists everywhere mark these points Ar₁, Ar₂ and Ar₃, the first being the lower, and Ar₃ 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 Ac₁, Ac₂, 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 Ar₁, Ar₂, Ar₃, 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 Ar₁, and that in all other steels this point becomes stronger as they contain higher and higher carbon. There is little doubt that the point Ar₁ exists or results from and because of the carbon of the alloy. In wrought iron there is no carbon, hence there is no point Ar₁. If the extremely low carbon steels have an Ar₁ 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 Ar₂, then, shows the temperature at which loss or gain of magnetism occurs. The electrical conductivity change comes at neither of these points, Ar₁, nor Ar₂, but at Ar₃.

However, with increase of carbon the line Ar₃, which was drawn through the points, Ar₃, rapidly descends. At about .45% or .50% carbon content, this line Ar₃, representing the changes in conductivity, joins line Ar₂. 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 Ar₂ and Ar₃ 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 Ar₂ and at Ar₃. 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 Ar₂, we have alpha iron, between Ar₂ and Ar₃, beta iron, and above Ar₃, gamma iron. Both beta and gamma iron are non-magnetic, while alpha iron is strongly magnetic. In cooling through Ar₃, 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.

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, Fe₃C.

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.

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.

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 (Fe₃C), 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.

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.)

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.

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.

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.

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.