THE CRYSTALLIZATION OF IRON AND STEEL

BY THE SAME AUTHOR

HIGHER MATHEMATICS FOR STUDENTS OF CHEMISTRY AND PHYSICS,

With Special Reference to Practical Work.

With 142 Diagrams.

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THE CRYSTALLIZATION

OF

IRON AND STEEL

AN INTRODUCTION TO THE STUDY OF METALLOGRAPHY

BY

J. W. MELLOR, D.Sc.

LONGMANS, GREEN, AND CO.

39 PATERNOSTER ROW, LONDON NEW YORK AND BOMBAY

1905 All rights reserved

DEDICATED TO MY FRIEND

A. E. BONE, ESQ.

DUNEDIN, NEW ZEALAND

14064-3

PREFACE

THIS course of six lectures— delivered to the Engineer- ing Students of the Staffordshire County Technical Classes at the Newcastle High School, in November and; December, 1904 — was intended to summarize the results of the more important researches which have been made during the last ten years upon the constitution of alloys of iron and steel.

The claims of the new science — metallography — which has revealed the internal structure of the metallic alloys, have been too long overlooked. The far-reaching importance of this knowledge is begin- ning to dawn upon the most conservative minds. Metallurgists are now rending the veil which has so long concealed the internal structure of the metals and their alloys; and many phenomena connected with the industrial treatment of the metals, so long inexplicable, are now yielding up their secrets to the indefatigable methods of modern scientific research.

Unfortunately, much of this important work is disseminated throughout various scientific journals,

vii

vlii PREFACE

and the facts are so frequently obscured beneath a mass of controversial matter that it is difficult for the uninitiated to get into touch with the work. In consequence, I have been lead to reproduce my lectures in a form suitable for publication, in the hope that they may help those who have not specially studied the subject to appreciate the remarkable chapter which metallurgists have recently added to physical chemistry.

I desire to thank Profs. Arnold, Beilby, Cohen, Ewing, Heyn, Newth, Osmond, Popplewell, and Stead for permission to use their micro-photographs ; Dr. G. T. Beilby, the Secretary of the Iron and Steel Institute, the proprietors of The Philosophical Transactions and of The Iron Age for the loan of blocks; and Mr. H. Fowler for help with the diagrams.

J. W. MELLOR.

March 4, 1905.

CONTENTS

(The bracketed numbers refer to pages.)

PAGE

THE SOLIDIFICATION AND COOLING OF ALLOYS ... 1

§ 1, Atoms and molecules (1) ; § 2, The degradation of energy (3) ; § 3, Passive resistance (4) ; § 4, Allotropy (5) ; § 5, Transi- tion temperatures (6) ; § 6, Velocity of transformation (7) ; § 7, Cooling curves (9) ; § 8, Surfusion and recalescence (11) ; § 9, The cooling curve of pure iron (12) ; § 10, The freezing of salt water (16) ; § 11, The solidification of copper-silver alloys (18) ; § 12, The solidification of copper-antimony alloys (20) ; § 13, The cooling of iron-carbon alloys (22) ; § 14, The colour names for high temperatures (24).

THE CONSTITUENTS OF IRON AND STEEL 26

§ 15, Eutexia (26) ; § 16, The relative proportions of ferrite, cementite, and pearlite (27) ; § 17, Graphitic, hardening, and cement carbon (30) ; § 18, Compounds, mixtures, and solutions (31) ; § 19, The solidification of molten iron (33) ; § 20, Martensite, hardenite, austenite (39) ; § 21, Sorbite, troostite (42) ; § 22, The phase rule (43).

THE HARDENING, ANNEALING, AND TEMPERING OF STEEL 49

§ 23, General properties of hypo- and hyper- eutectic steels (49) ; § 24, The influence of rate of cooling (51) ; § 25, The allotropic modifications of iron (54) ; § 26, Annealing, tempering, and hardening of steel (58) ; § 27, The law of mass action (60) ; § 28, Theories of annealing and hardening (62).

ii

CONTENTS

PAGE

THE CRYSTALLIZATION OF IRON AND STEEL .... 67

§ 29, The crystallization of iron (67) ; § 30, The development of crystalline grains (68) ; § 31, Grain size and fracture (70) ; § 32, Influence of mechanical work (77) ; § 33, Influence of other elements (79).

THE INFLUENCE OF STRESS AND STRAIN 82

§ 34, Intercrystalline or intergranular weakness (82) ; § 35, Intracrystalline or cleavage weakness (85) ; § 36, Birth, growth, and structure of crystals (88) ; § 37, Effects of progressively augmented strain (90) ; § 38, Effects of repeated alterations of stress (92).

HOW TO PREPARE A SPECIMEN FOR THE MICROSCOPE 95

§ 39, The cutting of a sample (95) ; § 40, Filing and rough polishing (96) ; § 41, Fine polishing (96) ; § 42, Polishing in relief (97) ; § 43, Etching (98) ; § 44, Osmond's polish attack (102) ; § 45, Heat tinting (104) ; § 46, Mounting (104) ; § 47, Preservation of polished specimens (105) ; § 48, The microscope and its accessories (105) ; § 49, Photography (107) ; § 50, Miscellaneous (107).

APPENDIX— GLOSSARY Ill

INDEX 135

THE '.f.-.\<

CRYSTALLIZATION OF IRON AND STEEL

THE SOLIDIFICATION AND COOLING OF ALLOYS

§ i. Atoms and Molecules

CHEMISTS have invested matter with an imaginary constitution, which explains very well the various transformations which matter undergoes. Matter is supposed to be made up of extremely small particles, called molecules. No successful attempt has been made to describe how the molecules associate together except in the case of crystalline substances. Here all the evidence points to a symmetrical and fixed mode of arrangement, which finally produces regular geometrical figures called crystals.

More or less approximate attempts to calculate the size of the molecules show that, if a drop of water were magnified to the size of the earth, the size of the molecules of water would be between that of small shot and of cricket balls. Molecules lie quite outside the range of observation, and we must accept here, in

i B

2 CRYSTALLIZA TION OF IRON AND STEEL § i

good faith, the large mass of circumstantial evidence accumulated by the chemist.

By analysis it. has been found that the infinite • variety of substances known to man can all be reduced to. about .eighty simple forms, called elements. No analyst ha§ -ever .separated from an element anything but itself. Pure iron will yield nothing but pure iron. Iron is, therefore, an element. Pure copperas, on the other hand, will furnish iron, sulphur, and oxygen. Copperas is not an element. Facts like these seem to indicate that molecules are made up of still smaller particles. These are called atoms. Atoms of the same kind make up the molecules of elements; atoms of different kinds make up the molecules of compounds.

All substances known to man are supposed to be made up from different combinations of some eighty different kinds of atoms. Eecent investigations seem to show that the different atoms are different com- binations of still smaller particles, called corpuscles, or, if they be charged with electricity, electrons. All corpuscles are the same. Matter, whether it be a mummy, a piano stool, or a toothpick, is essentially one universal substance. Variety enters when the corpuscles arrange themselves in groups of atoms, when the atoms unite into molecules, and when the molecules aggregate into masses of matter. The stages are —

Corpuscles — > atoms -> molecules — > matter en masse.

The molecules of a gas lead a more or less inde- pendent existence. This is illustrated by the rapidity with which the molecules of, say, ammonia gas travel from one end of a room to the other and affect the

§ 2 THE COOLING OF ALLOYS 3

sense of smell. In liquids, however, the molecules are much less mobile. This can easily be proved by dropping a small grain of aniline dye into a tumbler of clear still water. The water will be uniformly coloured in a few days. The molecules of a solid substance have practically lost their mobility. But not all. Carbon laid in contact with pure, hot, solid iron will diffuse into the mass of the metal ; and gold in contact with lead will, in a few years, diffuse into the lead in appreciable quantities.

§ 2. The Degradation of Energy

The infinite variety of changes continually taking place in the properties of bodies around us is often said to be due to the action of an external agent, called energy, upon matter. Just as water will always run down from a high to the lowest level that circumstances will permit, so will energy at a high potential always run down to energy at a low potential. And one of the most interesting phenomena in connection with all natural changes is this constant running down or degradation of energy. Still keeping the same analogy, just as water may descend from the top of a hill in many ways — rivers or rain, underground channels, glaciers, or avalanches— so may energy give rise to electrical, thermal, or chemical phenomena in its descent from a high to a low potential. But I need say little to engineers on this subject. The electric light, steam engine, electric tramcars, gas engines, water-wheels, watches, and clocks all bear testimony to the ubiquity of the law. An ancient philosopher has said that all things are in motion, and we might add

4 CRYSTALLIZA TION OF IRON AND STEEL § 2

that the motion always involves & degradation of energy. Motion only ceases when energy has run down to the level of its surroundings. The system is then said to be in a state of equilibrium.

§ 3. Passive Resistance

There is also another remarkable law — the law of passive resistance. Equilibrium may be apparent. The running down of energy may be resisted in some way. It is a common thing to find energy at a higher potential than we should expect. Energy does not always, of itself, run down to its lowest level. Just as the throttle-valve of a steam-engine must be moved before the degradation of high-pressure energy com- mences, and the engine can start on its journey, so may a preliminary impulse be required to set the process of degradation in motion.

We therefore distinguish between two states of equilibrium. The one is stable, the other unstable. The one is a real state of equilibrium, the other is only apparent. When you see water in a liquid state at a temperature below its normal freezing-point, 0° C., you know that some agent must be at work which prevents the freezing of the water. This unknown agent is called passive resistance.

Sodium thiosulphate is a convenient substance to illustrate these facts. At ordinary temperatures this salt is a white crystalline solid. On heating to 56° it melts to a clear liquid. This is also the freezing-point of the liquid. But it is possible to cool the molten salt down to the temperature of the room without solidification. The sodium thiosulphate is then said to be in a state of apparent or false equilibrium,

FIG. 1.— Octahedral Sulphur. (G-. S. Newth.)

FIG. 2. — Prismatic or " Needle-shaped " Sulphur Crystals, (fl. S. Xewth.)

[To face p. 5.

§ 4 THE COOLING OF ALLOYS 5

and it can be kept in this state an indefinite time. Now put a crystal of sodium thiosulphate into the liquid mass. The passive resistance is overcome in some way, for now the liquid assumes the stable crystalline condition, and during the transition from the liquid to the solid states energy is degraded.

§ 4. Allotropy

Sulphur, at the temperature of this room, is a pale yellow crystalline solid. The crystals are shaped like octahedrons (Fig. 1). If sulphur be heated above 96°, these pass into needle-shaped crystals (Fig. 2). Still further, if sulphur be heated to near its boiling-point, and suddenly quenched by pouring into cold water, an amorphous, non- crystalline, plastic, and elastic mass is produced. Here, then, you have the element sulphur existing in three different forms — plastic, octahedral, and needle-shaped crystals. Each form is said to be an allotropic modification of sulphur.

This word allotropic. How can the same substance exist in different forms ? Just as the builder can with the same kind of bricks build up various structures, so can Nature with the same kind of atoms build up molecules with very different properties. The atoms of sulphur, for instance, may form molecules which crystallize as octahedral or as needle-shaped crystals ; atoms of carbon form three allotropic modifications — diamond, graphite, and amorphous carbon. Allotropy occurs when a substance exists in two or more forms which differ in some of tlieir properties. The term is not usually applied to the different states of aggrega- tion of a substance — solid, liquid, or gaseous. Allo- tropic transformations are usually accompanied by

6 CRYSTALLIZA TION OF IRON AND STEEL § 4

changes in the internal energy of the substance concerned. Energy is at a higher potential in one allotropic form than in another. The one form which has energy at the higher potential must be unstable.

§ 5. Transition Temperatures

The octahedral crystals are alone stable at ordinary temperatures. Both plastic sulphur and the needle- shaped crystals pass spontaneously into octahedral crystals at the temperature of the room. But above 96° the needle-shaped crystals are stable, while the plastic and the octahedral crystals slowly assume the needle-like form. You might just as well try to prevent water running down a hill as to prevent these changes taking place at these temperatures. Plastic sulphur will crystallize in octahedrons at the tem- perature of the room because the process involves the running down of energy. Sulphur, therefore, has two crystalline forms, one of which is stable above 96°, and the other below 96°. The critical temperature, 96°, is called the transition temperature.

Mercury iodide also exists in two allotropic forms, one of which is red and the other yellow. The red form is stable at ordinary temperatures, and passes into the yellow modification when heated above 126°. The yellow form is stable above 126°, but passes into the red form when cooled below the transition temperature, 126°.

After a particularly cold winter, 1867-68, some blocks of tin stored in the Customs House, and some tin buttons in the Military Stores at St. Petersburg had mysteriously crumbled to a grey powder. It has since been proved that tin exists in two allotropic

FIG. 3.— Surface of Diseased Tin. (E. Cohen.)

[To face p. 7.

§6 THE COOLING OF ALLOYS ^

modifications — white malleable metal, and grey powder. The transition temperature is 20°, just a little above the average temperature of the air. The grey powder is the stable form below 20°. Hence it follows that all the malleable tin in the world, except on the hottest summer days, is in an unstable condition. It is only passive resistance of some kind which prevents all the tin vessels in the world slowly crumbling to powdered grey tin.

§ 6. Velocity of Transformation

We have just seen that a crystal of sodium thio- sulphate will make the unstable liquid thiosulphate pass into the stable form, so will the presence of a little grey tin facilitate the transformation of white into grey tin. This crumbling of tin to a grey powder is known as the tin pest. The disease is, therefore, infectious. The surface of a piece of diseased tin is shown in Fig. 3. The change is slow at ordinary temperatures. But articles of tin which have been buried a few hundred years are in almost every case in a more or less advanced state of disintegration.

The comparative rigidity or immobility of the molecules of a solid offers a kind of frictional resistance to change, analogous to the action of a brake upon the wheels of a car. If it were not for passive resistance the speed of transformation from one allotropic form to another would be faster -the more distant the temperature away from the transition-point. Experi- ment shows that the rate of transformation of white into grey tin increases as the temperature is reduced below the transition-point, 20°. At — 50°, for instance, the transformation is very rapid. As a general rule,

8 CRYSTALUZA TION OF IRON AND STEEL § 6

passive resistance increases as the temperature falls below the transition-point. The one effect works against the other. If 6 be the prevailing temperature, we may write —

transition temperature less & Velocity of change at e° = pa88ive resi8tance at go

Or, if V denotes the velocity of transformation, R the magnitude of the passive resistance at 0°, and E the difference of the temperature between the transition point and 0°, we have —

" R

a result which bears a close formal analogy with Ohm's well-known law. It may be assumed that E also represents the amount of energy to be degraded in the process. This formula states in symbols the observed facts that the greater the value of E, the greater the velocity of transformation ; and the greater the value of R, the less the velocity.

These experiments teach us four important facts which must be clearly understood : —

(1) A substance may exist in two or more forms having different properties.

(2) Only one of these forms is, in general, stable at any given temperature.

(3) The transformation of a substance from its unstable to its stable form occupies time.

(4) The transformation from the unstable to the stable form may be hindered or even arrested by passive resistance for an indefinite time.

The phenomena are not always so obtrusive as the changes which take place with sulphur, tin, and

§7

THE COOLING OF ALLOYS

mercury iodide. We naturally ask, how can we tell whether a substance is capable of existing in different allotropic forms ? As a matter of fact, we select some physical property of the substance and measure it at different temperatures : if there is a sudden change in the physical property of the substance at any particular temperature, we infer that there is some drastic change going on in the internal structure of the substance.

§ 7. Cooling Curves

Let the temperature of a cooling copper bar at 200° be measured every ten minutes. Let distances at right angles to the line 0°-200° (Fig. 4) represent

200* ISO"

50°

w 0 20 40 60 Time FIG. 4. — Cooling Curve of Solid Copper.

time, and vertical distances from the line 0 - 60, the corresponding temperatures of the bar. We thus obtain the series of points shown in Fig. 4. Draw a line so as to lie most evenly among the points. The result is a so-called cooling curve. The simple form of the cooling curve in Fig. 4 gives no evidence of any sudden change in the nature of the cooling copper. If a curve is drawn for water cooling down from

io CRYSTALLIZATION OF IRON AND STEEL § 7

20° to — 20° C., we get a terrace in the cooling curve, as shown in Fig. 5. This tells us that some change has taken place in the nature of the substance at 0°. We

cv 10' 0° -vr -4	v
\	Freezit	W
	\	^
		X	^
} 20 40 60 Time

FIG. 5.— Water.

see directly that this change corresponds with the passage of water from the liquid to the solid state of aggregation.

Now draw the cooling curve of molten sodium thiosulphate. We know that the molten liquid " ought" to freeze at 56° (Fig. 6). But the cooling curve goes

90°

70

56° 50'

30°

10

0 20 40 60 Time FIG. 6.— Sodium Thiosulphate.

on quite normally below that temperature until, at length, there is a great evolution of heat, and the liquid

§8 THE COOLING OF ALLOYS u

solidifies. The temperature may even rise above 56°. The cooling curve of the solid is quite normal. The amount of heat evolved as the molten liquid solidifies corresponds with the " latent " heat absorbed as the solid melts.

§ 8. Surfusion and Recalescence

The molten sodium thiosulphate as it cools down the " surfusion " curve is at a lower temperature than its normal point of solidification or freezing. The liquid is then said to be in a state of superfusion or surfusion. The system is in unstable equilibrium. We may get a similar state of things when a saturated solution of a substance is slowly cooled. More salt may be in solution than the true solubility of the salt. The result is a supersaturated solution. Agitation, or the addition of a trace of something which will serve as a nucleus for crystallization, will generally suffice to start the system on its passage to a state of stable equilibrium. But when the trans- formation does set in, it usually takes place very rapidly, and is accompanied with a rise of temperature. The cooling curve is distorted in a corresponding manner (Fig. 6).

It is interesting to put a little ether in a small bulb blown at the end of a piece of glass tubing, placed in supercooled sodium thiosulphate (Fig. 7). Drop in a crystal of sodium thiosulphate. The evolution of heat as the liquid solidifies raises the temperature high enough to vaporize the ether. The vapour of ether will burn at the mouth of the tube with a steady flame when ignited.

When a steel bar is cooling, an evolution of heat

12 CRYSTALLIZATION OF IRON AND STEEL § 8

occurs at about 690°. The amount of heat evolved is so great that the metal visibly brightens in colour.

IUUU 800" 600* W

\

^v

^

A

""

Ar,

\

^

^

FIG. 7.

20 40 60 80 100 TIME FIG. 8. — Kecalescence.

The phenomenon is called recalescence. The cooling curve is shown in Fig. 8.

§ 9. The Cooling Curve of Pure Iron

The cooling curve of iron from the molten condition is shown in Fig. 9. The iron was practically pure. It

•M	^	fccxtn \	7
WMT IGOO' 600' ?firt		\	An
	8&F ft	*9 ^	i
			^

0 20 40 60 Time. FIG. 9.— Iron.

only contained O'Ol per cent, of carbon. F. Osmond,

or § 9 THE COOLING OF

a celebrated French metallurgist, maintains that the existence of the transition-points, or discontinuities, Ara and Ar2, in the cooling curve of the solidified metal, points to the existence of three allotropic modifications of solid iron : —

i. Alpha Iron. — Below Ar2, that is 750°, we have what he calls a-iron, or alpha iron.

ii. Seta Iron.— Between Ar2 and Ar3, that is between 750° and 860°, we have what he calls /3-iron, or beta iron. Beta iron is non-magnetic. Heat is evolved when iron passes from the /3- to the a-state, and mag- netic properties are developed at the same time.

iii. Gamma Iron. — Above the Ar3 critical point, namely 860°, we are supposed to have y-iron, or gamma iron. This variety is non-magnetic.

Each critical point is found to be associated with a change in the mechanical properties, the microscopic appearance, the electrical conductivity, the magnetic properties, and the specific gravity of the metal.1

The changes which occur during the cooling of a substance are reversed when the substance is heated. The cooling curve of steel, with 1-2 per cent, of carbon, shown in Fig. 10, is reversed on heating, as shown by the heating curve in the same diagram. There is only one critical point at about 690°, called the Ar! critical point.

The critical points Aci, Aca, Acs on the heating curve of mild steel are generally a few degrees higher

1 O. Boudouard, Jaurn. Iron and Sted Intt., 63. i. 229, 1903 ; H. le Chatelier, Compt. Rend., 128. 1444, 1899; 129. 299, 331, 497, 1899; Metallographist, 2. 334, 1899 ; 3. 38, 152, 1900; G. E. Syedelius, Phil. Mag., [5], 46. 173, 1898 ; G. Charpy and L. Grenet, Compt. Bend., 124. 540, 598, 1902; MetaUographist, 6. 240, 1903; S. Curie, ibid., 1. 107, 229, 1898.

14 CRYSTALLIZA TION OF IRON AND STEEL § 9

than the corresponding points Ari, Ar2, and Ar3 respectively. There seems to be a kind of molecular inertia, or lag, which prevents the y to )3, the |3 to a, or the reverse changes taking place sharply. The critical 900ir

soo*

700° 600* 50(T

Ac.

20 40 60 Time FIG. 10.— Steel.

points on the cooling curve are, in consequence of this lag, a few degrees below the true critical point. The lag induces a state somewhat analogous to surfusion in molten sodium sulphate. The critical points on the heating curve are a little too high, and for a similar reason.1

The critical points of iron really represent ranges of temperature, although, for the sake of inconvenience, we call them points. The Ar3 with soft steel commences at 845°, and finishes at 800° ; it is most marked at 820°. The Ar2 extends from 755° to 710° ; and the Ari from 680° to 645°.

The "r" of "Ar" comes from the French word refroidissant, for cooling; the "c" of "Ac" from chauffant, heating. This notation is due to D. Tscher- noff,2 the Russian metallographist.

1 F. Osmond, Metallographist, I. 270, 1893; 2. 169, 1899; H. M. Howe, ibid.,2. 257, 1899; M. Aliament, La £ledricien, 49, 1903.

2 Otherwise spelt " D. Chernoff."

§9 THE COOLING OF ALLOYS 15

Now y-iron is said to be hard, a-iron soft. If, therefore, y-iron be quickly cooled past the Ar2 critical point, the passage of the hard y-iron to the soft a-iron is retarded ; the iron is then in an unstable hardened condition, ready on the least provocation to pass into the stable soft ,a-form.

We naturally ask is there any method of helping the passive resistance so that the iron will not readily change from, say, the y to the a modifications at ordinary atmospheric temperatures? It is supposed that the presence of many foreign substances, like carbon, nickel, and manganese, augment the passive resistance so as to render the hard y-iron more stable and permanent at low temperatures. On the other hand, the presence of chromium, tungsten, aluminium, silicon, phosphorus, arsenic, and sulphur facilitate the passage of hard beta 1 iron to the soft alpha iron.

The influence of minute traces of foreign substances upon the properties of the metals is a most important subject. The effects seem inexplicable. The presence of 0*05 per cent, of tellurium alters the properties of bismuth so much that we seem to be dealing with a totally different substance; a few hundredths of one per cent, of sulphur will determine the success or failure of iron; and the presence of O'l per cent, of bismuth in copper lowers its conductivity so much that if copper so contaminated had been alone avail- able, it would have been fatal to the success of the Atlantic cable.

1 Either "gamma" or "beta." We are not sure which. Both are supposed to be hard.

16 CRYSTALLIZATION OF IRON AND STEEL § 10

§ 10. The Freezing of Salt Water

I have been speaking of pure or almost pure iron, and now we naturally turn to alloys of iron with carbon. Cast iron and steel are, as you well know, alloys or solidified solutions of carbon in iron. These alloys are so complex that it will be profitable for us to examine some other solutions which do not present such complications as occur in the case of the iron- carbon alloys. It is a most interesting fact to find that the same general laws hold good for the cooling of metallic alloys, for the separation of ice when sea water is frozen, the separation of crystals in the glazes of the potter, the devitrification of old glass, for alloys of carbon and iron, and the formation of rocks when the world was a-building. True enough, with the iron- carbon alloys other phenomena are superposed upon, and hence modify the course of the simple phenomenon as it occurs during the freezing of sea water.

The freezing-point of a 5 per cent, solution of sodium chloride is below that of pure water. If more salt be added, the freezing-point is reduced still more ; and this goes on until the solution contains 23^ per cent, of sodium chloride, when further additions of salt raise the freezing-point. The experimental results are shown in Fig. II.1

But the experiment reveals something more in- teresting than this. If the solution contains, say, 5 per cent, of salt, pure ice separates out as the solution freezes, and, in consequence, the solution which remains unfrozen has more than 5 per cent, of salt dissolved in it. The freezing-point of the

1 F. Guthrie, Phil. Mag., [5], 1. 354, 1876.

§ 10

THE COOLING OF ALLOYS

mother liquid is therefore lower than that of the original solution. This separation of pure ice, and the lowering of the freezing-point, goes on along the curve AP (Fig. 11), until the solution contains 23 J- per cent, of salt. After that, the residual mixture freezes en masse at — 22°.

If the solution contains more than 23J per cent, of salt, then pure salt separates from the solution, and

10 20 23i 30 %SaU FIG. 11. — Freezing Curves of Aqueous Salt Solutions.

the separation of salt, and the lowering of the freezing- point of the solution goes on along the curve BP (Fig. 11), until the solution contains 23J per cent, of salt, and the whole residue then solidifies at - 22°.

If the solution contains just 23J per cent, of sodium chloride, it freezes en masse at — 22°. No other mixture of salt and water freezes at a lower temperature than this. Hence this mixture is called a eutectic mixture. Guthrie used to think that the mixture which separated at this temperature was a definite chemical compound of water with salt, which he called a cryohydrate. Ponsot calls the mixture a "cryosel." The term eutectic mixture is to be preferred. We know now that Guthrie's cryohydrate is nothing but a mechanical

i8 CRYSTALLIZATION OF IRON AND STEEL § 10

mixture of ice and salt. The one is entangled with the other. Under the microscope the crystals of ice can be seen lying in a matrix of salt.1

§ ii. The Solidification of Copper-Silver Alloys

A like phenomenon occurs when molten mixtures of silver and copper are allowed to cool. At 770°, when the alloy has the composition 28 per cent, of copper and 72 per cent, of silver, the whole solidifies en masse. If the mixture contains less than this per- centage of copper, pure silver separates at temperatures along the " silver " line (Fig. 12) ; while, if the molten

1100"

1000'

000

700°

A

^

Lute

0 25 50 75%COPPER FIG. 12. — Fusibility Curves of Copper-Silver Alloys.

mixture contains more than 28 per cent, of copper, pure copper separates, and continues separating along the "copper" line until the mixture has the above composition, when the whole solidifies as a eutectic mixture at 770°. No other alloy of silver and copper

1 A. Ponsot, Ann. Chim. Pliys., [7], 10. 79, 1897 ; T. Andrews, Proc. Boy. Soc., 40. 544, 1890; 48. 106, 1890; J. Y. Buchanan, Proc. Roy. Soc. Edin., 14. 129, 1888.

FIG. 13.— Polished Surface of Cu-Ag Alloy. (F. Osmond.)

'FiG. 14.— Polished Surface of Cu-Ag Alloy. (F. Osmond.)

[To face p. 19.

§ii THE COOLING OF ALLOYS 19

melts at so low a temperature. The resulting alloy is a network of the two metals, pure silver and pure copper, as shown in Fig. 13, where the heterogeneous nature of the alloy is clearly seen. In Fig. 14 we have an alloy of 15 per cent, of copper and 85 per cent, of silver. The alloy has a greater percentage of silver than the eutectic alloy, and in consequence silver separates out until the residue has the eutectic com- position. This is in harmony with the microscopic appearance of the alloy, which shows large masses of silver embedded in a network of the eutectic alloy.

An alloy of these metals appears to possess two freezing-points: (i.) The temperature at which the mass begins to solidify ; and (ii.) the temperature at which the whole is solidified. The pasty condition of solder — tin with 66 per cent, of lead — is due to the fact that there are two freezing-points. Solid lead separates first, and on this fact depends the facility with which a joint can be wiped with plumber's solder.

Wa get similar results with binary alloys of antimony and lead, tin and lead, tin and bismuth, tin and zinc, lead and silver, zinc and aluminium, and with copper and gold.1

The fusibility curve is very much simpler if the one constituent is mutually soluble in the other in all proportions. The fusibility curve is then approxi- mately a straight line (AB, Fig. 15). This is the case

1 W. Campbell, Jonrn. Franklin Inst, 154. 1, 131, 201, 1902; Metallographist, 5. 286, 1902; J. E. Stead, {bid., 5. 110, 1902; H. M. Howe, {bid., 5. 166, 1902 ; A. W. Kapp, Drude's Ann., 6. 754. 1901 ; W. C. Roberts- Austen, Proc. Roy. Soc., 23. 481, 1884 ; 63. 452, 1898 ; G. T. Heycock and F. H. Neville, Phil. Trans., 189. 25, 1897 ; A. Dahms, Wied. Ann., 54. 486, 1895 ; H. le Chatelier, Compt. Rend., 118. 350, 415, 800, 1894.

20 CRYSTALLIZATION OF IRON AND STEEL § n

with alloys of silver and gold. The same thing occurs with alloys of antimony and bismuth.

1200°

1100°

1000' 900°

^0 25 50 75%SILVER FIG. 15.— Fusibility Curve of Gold-Silver Alloys.

§ 12. The Solidification of Copper-Antimony Alloys

Silver and copper do not form a chemical compound. Many metals, however, do form compounds. Copper and antimony, for example, form a compound having the chemical formula SbCu2, or, according to H. le Chatelier, Sb2Cu8. This behaves as if it were a single and distinct element. The addition of either copper or antimony lowers the melting-point of the compound SbCu2 in the ordinary way. In Fig 16, G represents the melting-point of the pure compound SbCu2. The line CPi represents the effect of the addition of antimony to the compound ; while the line GP2 repre- sents the effect of additions of copper. There are two eutectic points, the one, P2, corresponding with the eutectic mixture of the compound SbCu2 with copper,

1 A. Gautier, Bull. Soc. d' Encouragement, [5], 1. 1293, 1896 ; W. C. Roberts-Austen, Proc. Roy. Soc., 67. 105, 1900; with T. K. Roso, 71. 161, 1903.

§12

THE COOLING OF ALLOYS

21

and the other, PI, corresponding with a eutectic mixture of SbCu2 with antimony.

The results depicted in Figs. 11, 12, and 16 can be represented graphically in another way.1 Take the more complicated case (Fig. 16). Let the horizontal

nor

800°

TOO* A

500

F>

25 50 75% COPPER FIG. 16.— Fusibility Curve of Copper-Antimony Alloys.

line (Fig. 17) represent the ultimate composition of the alloy in terms of copper; the vertical lines, the structural composition in terms of the various con- stituents— copper, SbCua, antimony, or the two

10 20

30 40 50 60 70 FIG. 17. (After A. Sanveur.)

80 90%COPPE(

eutectics. As an example, an alloy with 10 per cent, of copper will have 40 per cent, of the first eutectic

A. Sauveur, MetallograpMut, 1. 103, 1898.

22 CRYSTALLIZATION OF IRON AND STEEL § 12

and 60 per cent, of antimony ; an alloy with 33 per cent, of copper will have 30 per cent, of the first eutectic, and 70 per cent, of SbCu2.

Alloys of gold and antimony present similar phenomena.1

§ 13. The Cooling of Iron-Carbon Alloys

But we can go a step further. Let us consider what takes place when an iron bar containing, say, 0*6 per cent, of carbon and 99*4 per cent, of iron cools from 900°. The cooling curve shows nothing very remarkable until a temperature of about 720° is attained. There is then a sudden evolution of heat. The critical points, Ar3 and Ar2, of pure iron coalesce into one. At this point pure iron, or ferrite, as

1200°

0 0-5 1-0 I-5%CARBON Ifrj. 18.— Cooling of « Solid Iron."

Howe calls it, separates from the solid solution. The separation of ferrite goes on along the curve AP (Fig. 18) until the temperature reaches about 660°,

1 C. T. Heycock and F. H. Neville, Proc. Boy. Soc., 68. 171, 1901 ; J. E. Stead, Journ. Soc. Cham. Ind., 17. 1111, 1898; Metallographist, 1. 179, 1898; 2. 314, 1899; G. Charpy, ibid., 1. 87, 192, 1898.

PIG. 19.— Surface of Lamellar Pearlite. (F. Osmond.)

FIG. 20. — Surface of Granular Pearlite. (E. Heyn.)

[To face p. 23.

§ 13 THE COOLING OF ALLOYS 23

when another recalescence point occurs (Ari). No other noteworthy change occurs as the system cools down to the normal temperature of the atmosphere. Other alloys containing different amounts of carbon furnish a set of curves quite analogous to the freezing curves of salt water and of silver-copper alloys. But with iron these changes take place in the solid cooling metal.

If the alloy contains less than 0'89 per cent, of carbon there is a separation, or, better, segregation of ferrite ; if the alloy contains more than 0*89 per cent, of carbon, there is a separation, not of carbon, but of a chemical compound of carbon with iron, called normal iron carbide, or cementite, and represented in chemical symbols by Fe3C. Cementite contains 6*9 per cent, of carbon. The separation of cementite occurs along the curve BP (Fig. 18). We are there- fore dealing with a mixture of ferrite and of cementite. The eutectic alloy contains 13 per cent, of cementite (i.e. 0-89 per cent, of carbon) and 87 per cent, of ferrite — roughly, six of ferrite to one of cementite. The microscopic appearance of the eutectic reminds one forcibly of other eutectic mixtures. The eutectic mixture of cementite and ferrite is called pearlite, owing to the fact that it generally shows the rainbow tints of mother-of-pearl under microscopic treatment. Sorby called it the " pearly constituent " of steel.

Fig. 19 shows a fine specimen of lamellar pearlite obtained from a steel containing 1/0 per cent, of carbon. The black streaks are ferrite. Fig. 20 shows a specimen of what is sometimes called granular pearlite, from a

1 According to H. le Chatelier, Journ. Iron and Steel Inst., 61. i. 40, 1902, there are two or three allotropic forms of cementite.

24 CRYSTALLIZATION OF IRON AND STEEL § 13

forged bar of crucible steel containing 0'92 per cent, of carbon. It is magnified 1240 times.

§ 14. Colour Names for High Temperatures

I always speak of temperatures on the centigrade scale. For reference purposes I will give a scale of corresponding centigrade and Fahrenheit tempera- tures, and also the colour names as determined by White and Taylor with the Le Chatelier pyrometer.1 These results are said to be more accurate than Pouillet's old numbers.

The colour names do not really correspond with any particular temperature, but rather with a certain range of temperature. The results, too, depend upon so many external factors — physical and physiological — that different numbers might be obtained by different observers, and by the same observers at different times.

Colour names.

532

566 635

682 746 835

941

QQfi

«/yo 1079 1205

990

1050 1175 1250 1375 1550

1650 1725 1825 1975 2200

Dark blood red, black red, incipient red, rouge

naissant.

Dark red, blood red, low red, rouge sombre. Dark cherry red, incipient cherry red, cerise naissant. Medium cherry red. Cherry, full red, cerise. Light cherry, bright cherry, scaling heat,2 light red,

cerise clair.

Orange, salmon ; free scaling heat, orange foncee. Light orange, light salmon, orange clair. Yellow. Light yellow. White, blanc.

1 M. White and F. W. Taylor, Metallographist, 3. 41, 1900 ; H. M. Howe, ibid., 3. 43, 1900; C. S. M. Pouillet, Compt. Eend., 3. 784,1836.

2 Scale forms and adheres, i.e. does not fall away from the piece when cooled