MARCHBANKS: THE STEEL-MELTING CORELESS INDUCTION FURNACE. 509

THE STEEL-MELTING CORELESS INDUCTION FURNACE.

By M. J. MARCHBANKS, B.Sc.(Eng.).

(Paper first received 12th January, and in final form 28ih September, 1932; read before the SHEFFIELD SUB-CENTRElQth November, 1932, before the TEES-SIDE- SUB-CENTRE 20th February, 1933, and before the NORTH-EASTERN CKNTREMHh April, 1933.)

SUMMARY.

The paper aims at giving a survey of the properties,characteristics, and position, of the electric induction furnaceas used for steel-melting. Mathematical considerations arenot given in full, but sufficient are included to show the extentto which the skin effect is of importance. A J-ton furnaceis described, together with some details of the auxiliaryapparatus and the usual lay-out in steelworks.

The performance of such a furnace is indicated, withtypical wattmeter traces of runs; and an attempt is madeto show the manner of the energy dissipation, thus affordingan insight into the efficiency of induction heating.

Finally, some consideration is given to the metallurgicalaspect of the furnace, with a description of the Rohn methodof making a lining. An extension of the usefulness of thetype of melting unit is urged and its potentialities for refiningare outlined.

TABLE OF CONTENTS.

Introduction.The furnace.Supply and control apparatus.Choice of frequency and voltage.Lay-out and ventilation.Performance and energetics.Metallurgical considerations.Refining.Appendix I. Theory.

INTRODUCTION.

It took nearly a hundred years from Faraday's dis-covery of electromagnetic induction before inductionheating could be considered to be a practical means ofmelting metals; for it was not until 1916 that Dr.Northrup built an experimental furnace embodying asolenoid coil surrounding a metal charge. Both Northrupand Ribaud about this time used a frequency of approxi-mately 20 000 cycles per sec, the former employing a'mercury spark-gap and the latter a rotating equipment;but until it was found possible to run a furnace at afrequency which could be more efficiently obtained,their commercial success was uncertain.

Theoretical and practical work proceeded together toshow that lower frequencies could be employed withlarger furnaces, so that furnaces of £-ton capacity wereby 1926 being built to run at 2 000 cycles per sec.supplied from generators of the inductor type. Thismaterially opened the way to the installation in steel-works in this country and abroad of induction-furnaceequipments on a commercial basis, mainly for the pro-duction of high-quality alloy steels.

The present-day coreless induction furnace takes the

form of a cylindrical coil or inductor through which theflow of an alternating current produces a pulsatingmagnetic field. A conducting body placed in theinterior will have circulating currents induced in it byvirtue of the changing magnetic field; and dependingon its size and resistivity, as well as on the frequencyand magnitude of the magnetic field, energy will beliberated in the form of heat.

Between the inner surface of the coil and the chargea refractory heat-insulating lining has to be arrangedsuitable for containing the molten charge, which isusually of cylindrical form. The coil and refractory areset up in a case arranged for tilting the molten metalinto moulds and to protect the coil from hot metal.

The mathematical theory has been fully expressed

1 U

r

A0 2*104 4*104

Frequency, cycles per sec

F I G . 1.—Graph showing relationship between efficiency andfrequency for a given charge in a given inductor.

elsewhere* and an abridged form is given in Appendix 1.The conclusions reached may be stated as follows:—

(a) For a piece of metal of given size and resistivityplaced in a furnace coil there is a frequency abovewhich the efficiency of heating increases very slowly toan asymptotic value and below which the efficiencyrapidly decreases. Fig. 1 illustrates the variation ofefficiency with frequency in a given furnace coil.

(b) The power factor of a furnace of given size decreaseswith increase of frequency.

T H E FURNACE.

The inductor coil, being the most important part ofthe induction furnace, requires the greatest care in itsdesign and manufacture. It has to be water-cooled,and for this purpose it may be constructed of coppertube of copper strap with a water tube soldered alongits outer edge.. From design considerations it is alwaysa single-layer solenoid,, generally cylindrical. Whentube alone is used it is generally flattened to give abetter mechanical arrangement for clamping, and more

* C. R. BURCH and N. RYLAKD DAVIS: '* A>n Introduction to the Theory ofEddy-Current Heating" (Ernest" Benn, 1928).

510 MARCHBANKS: THE STEEL-MELTING CORELESS INDUCTION FURNACE.

copper for a given number of turns per unit length canbe included. Sections of both types of coil constructionare shown in Fig. 2. The advantage of the flattenedtube is that it is easier to manufacture, but with thestrap type of construction the coil may be pressed afterthe manner of a generator field-coil and axially clampedmuch more rigidly. Moreover, the thickness of copperinterposed between the molten charge and the coolingwater is considerably greater, thus giving greatersecurity in the event of the charge breaking throughthe refractory lining. With both types it is usual toapply the cooling water at more than one place alongthe coil length, the number of inlets being governed bythe quantity of water necessary, the pressure available,and the size of the water tube. In a £-ton furnace thequantity of water is of the order of 5 gallons per min.,with a temperature-rise of 20 to 30 degrees C.

The insulation of coils presents a problem ratherdifferent from that associated with the usual electricalapparatus. Occasionally the cold circulating-water willcause considerable moisture to condense from the atmo-sphere on the outside of the coil, so that insulatingmaterial as nearly waterproof as possible is necessary.Fireproof qualities are also essential, and these twoproperties rarely exist together. On account of thesteep temperature gradient from the inside of the coil,the insulation will rapidly deteriorate if wrapped to athickness much greater than T'^ inch.

Mica-silk wrapping is now used as the basic insulatingmaterial, overlapped with a layer of asbestos tape.This taping must be thoroughly dried and subjected toseveral coatings of a good insulating varnish so as toform something of the nature of a waterproof skin in

FIG. 2.—Types of coil construction.

order that the insulating material may withstand con-densed moisture.

The spacing of the turns in a coil is important, as ahigh copper space-factor leads to a high resistance lossowing to the radial component of the field. Theoreticallythe spacing should be close at the middle of the coil andopen out progressively towards the ends.

The coil framework presents certain difficulties onaccount of the heating of metallic members by the straymagnetic flux set up round the coil. With small fur-naces up to 100 lb. capacity used for laboratory workit may not be objectionable to provide wooden or asbestosslate boxes for the reception of the coil. These are notsuitable, however, for commercial operation in sizes ofseveral hundredweights or over, so that some form ofmetal container is necessary. To overcome this diffi-culty several methods are employed. One is to surveythe stray field by means of a search coil and voltmeter,with a view to the construction of a non-magnetic angle-steel box framework in which the steel members are sodisposed' that they do not heat appreciably. Since most

of the members of the box framework are either parallelto the coil axis or at right angles to it, the survey ofthe field need only be undertaken to give axial andradial components of the field, by placing the searchcoil with its plane in the appropriate direction.

Further experiments on the heating of blocks of metalhaving different values of r/j8 make it possible to deter-mine how close to the coil may be disposed pieces ofmetal in which r//J has a value greater than 3.

As an example, consider the top plate used in theconstruction of the furnace shown in Fig. 3. This plateis placed in the plane at the top of the coil at right anglesto the axis. Experiment indicated that the radial field(i.e. the field along the surface of the plate) is verystrong but the axial field is quite weak right up to theedge of the coil. Consequently it is possible to place

'','.', 'FIREBRICK BASE / /

FIG. 3.—Assembly of £-ton induction furnace.

this plate of thickness r/j8 < 1 in the position shownand to allow it to extend out to any radial distancewithout its getting very hot (say 80° C).

A framework constructed in the light of such con-siderations is shown in Fig. 3. Insulated joints are sodisposed that there are no closed circuits in which cir-culating currents may be induced, and the box is finishedby panelling with asbestos slate.

An alternative method of box construction is toconstrain the stray flux to an iron path made up ofpackets of laminated iron disposed about the coil. Out-side these a suitable sheet-iron frame may be arrangedin which little or no heating will take place.

It is usual in steelworks practice to call for inductionfurnaces to be lip-tilted in order to allow the metal tobe poured into cast-iron moulds with the smallest amountof movement of the stream during pouring. In Englandit is usual to provide each furnace with its own electric-motor tilting-mechanism. The drive from the motorconsists of two sets of worms and worm wheels con-nected to a sprocket shaft at the back, over which roller

MARCHBANKS: THE STEEL-MELTING CORELESS INDUCTION FURNACE. 611

chains pass from the furnace box to balance-weights.In America and on the Continent it appears to be moreusual to provide the furnace with a hook only at theback, for operation by the workshop crane.

Hydraulic operation of tilting may also be employedwith success. It has the advantage that inconvenientbalance-weights are eliminated, but, on account of thelarge angle of tilt required, the stroke of the ram isfrequently very long.

SUPPLY AND CONTROL APPARATUS.

For a J-ton furnace melting its charge of steel inabout one hour, a power of 150 to 160 kW is necessary.Motor-generator sets provide the most efficient meansof supplying the single-phase load required. For 500cycles per sec. it is possible to employ the usual typeof heteropolar generator, with 40 poles running at1 500 r.p.m. For" frequencies greater than this, thepole-pitch of this type of machine becomes too smalland it is preferable to use a homopolar-type machine.These machines are built with a solid steel rotor whichcarries no winding but is slotted to provide a path ofvariable magnetic reluctance for the flux. The excitingwindings are located in the stator alongside the windingsin which the voltage is induced. Such machines aremade with a high efficiency, mainly on account of thesmall power required for excitation, a 200-kW machineneeding only a 0-3-kW exciter. With one generatorsupplying one furnace the rate of temperature-rise ofthe charge is easily varied by using a field rheostat inthe exciter field-circuit.

Static condensers form the most effective method ofpower-factor correction, and a bank of oil-immersedpaper-dielectric condensers are connected in parallelwith the furnace, which is connected directly to thegenerator. It is important that these condensers shouldhave a lower power factor on account of the large numberof kVA required. Paper-dielectric condensers are nowmade with a power factor of from 0-0018 to 0-002.The bank of condensers is divided into two parts, oneof which is permanently connected across the furnace,and the other is subdivided into units which are switchedin according to the variation of power factor that takesplace as the melting of a charge proceeds.

In furnace operation it is desirable to reduce as muchas possible the manual control, in order to leave themelter a maximum amount of time to superintend themetallurgical operations consequent upon melting. Tothis end, automatic voltage control by means of avoltage relay may be fitted to keep the voltage at anydesired value. The coil of the voltage relay is suppliedfrom the high-frequency circuit, and regulation is effectedby contacts which put into operation a motor-operatedfield rheostat. To provide regulation at any desiredvoltage, the relay coil is placed across a variable resistancewhich is connected between the terminals of a potentialtransformer on the high-frequency busbars.

Automatic power-factor regulation of furnaces hasalso been put into operation. A reactive-kVA relay isconnected in the high-frequency circuit and, on contactbeing made on the lagging side, a small motor-operateddrum is set in motion'which temporarily reduces thevoltage and closes other contacts for the insertion of the

next unit of condensers. The voltage relay then takescharge,to bring back the voltage to its original value.This reduction of the voltage preliminary to switchingin a condenser is provided for on semi-automatic (i.e.voltage control only) furnaces and helps to protect thecondensers against heavy surges of current.

In another type of control two ammeters are used,one connected in the furnace circuit and the other inthe condenser circuit, and the condensers are so switchedin and out as to keep the readings of the two instrumentsequal, thus denoting that tuning is approximately correct.

CHOICE OF FREQUENCY AND VOLTAGE.It has been mentioned that from the furnace point

of view any frequency above that given by r/fi = 3

FIG. 4.—Typical lay-out of induction-furnace equipment.

will give maximum efficiency. Table 1 shows that acharge consisting of blocks of stainless steel (chosen forthe reason that it gives a higher value of frequency forr/jS = 3 than magnetic material) about 3 in. in diameterwill have a value of 2 • 8 for 500 cycles per sec. It appearsreasonable to suppose that there is no necessity formelting scrap of smaller dimensions. Accordingly, effi-cient melting takes place at 500 cycles per sec. Above1 000 cycles per sec. it appears that homopolar genera-tors are more efficient than heteropolar, but in choosingthe frequency the question of condenser cost becomes ofimportance. The reactive kVA is proportioned to thequantity of dielectric required. This is limited byvoltage breakdown in low-frequency condensers, and bydielectric loss in the case of high-frequency condensers,and this transitional frequency appears to be in the

512 MARCHBANKS: THE STEEL-MELTING CORELESS INDUCTION FURNACE.

region of 500 to 2 000 cycles per sec. All points con-sidered, therefore, a frequency not very far removedfrom that given by r//8 = 3 for the smallest reasonablepieces of charge to be heated is the most suitable forinduction-furnace operation.

With regard to choice of working voltage, the con-siderations are:—(a) safety to operators; (b) cost of con-densers ; (c) number of turns per unit length on the coil.

Taking these in order, it is obvious that the voltage

form of lay-out generally favoured is to arrange thefurnace with its platform backed against one wall ofthe substation, as shown in Fig. 4. As much of thecontrol apparatus as possible is housed in the substation,and those controls necessary for furnace operation duringmelting are centralized in a cubicle which is mountedin a wall. This cubicle is set up with its front faceflush with the wall and is usually supported on buttressesat the back, as it is 4 or 5 ft. above the substation floor-

TABLE 1.

Material

Stalloy

Stainless steelCopperCarbon

Form

Stampings, 40 mils thick

Blocks, 3 in. square0-75-in. square strapBlocks, 2 in. square

Frequency

cycles per sec.50

500500

7-08 X 103

Resistivity

e.m.u.55 000

H = 2 65072 000

17004 X 105

r

0-69

2-84-73-0

should be kept as low as possible from the point of viewof safety to operators.

The cost of condensers per kVA increases as the inversesquare of the voltage for voltages below about 600 volts,and is approximately constant for higher voltages.

With increasing voltage the number of turns on thecoil increases, and the manufacturing difficulties ofwater-cooling thin strap become excessive, as it is not

level on account of the height of the platform. Thisfurnace-control cubicle is provided with suitable instru-ments, means of voltage variation, and condenser switch-ing. Once the motor has been started in the substation,therefore, the melter has complete control of all that isnecessary for running the furnace.

It makes for easy charging if the levels of the furnacetop and the platform are the same. The furnace height

10 20 30 40 50 10 20 MinuteskW

200

100

A

/

fi

B

4

( iE

MX•

FC

UN

r

V

f i

r

M

Ixl z

FIG. 5.—Recording-wattmeter trace, for J-ton steel charge.

very practicable to make a coil having strap muchnarrower than ^ in. In a 1 000-volt J-ton furnace-coilthe strap is | in. thick and carries a cooling tube of f in.diameter. A voltage of 1 000 to 2 000 volts is in generalemployed for commercial furnaces of £-ton to 1-toncapacity.

LAY-OUT AND VENTILATION.

On account of the fine grit prevalent in foundries andof other unfavourable conditions, it becomes necessaryto house the motor-generator set and its associatedequipment in a building apart from the foundry. The

is chiefly governed by the size of the largest mould inuse, and if this involves a platform above 6 ft. in heightit may well afford suitable accommodation for the con-densers underneath. If this is adopted, the space mustbe definitely cut off from the foundry and properlyprepared with ventilation and light.

With high-frequency generators the loss is greaterthan with 50-cycle machines, so that it becomes necessaryto give special attention to their ventilation, particu-larly if the substation is small.

The most-favoured method of ventilation is to draw•the cold air through the machines into closed ducts

MARCHBANKS: THE STEEL-MELTING CORELESS INDUCTION FURNACE. 513

under the bedplate, and discharge the heated air bymeans of a suction fan through trunking to the outsideof the station. This method is to be preferred to thatof pumping cold air into the base of the machines, aswith this scheme the hot air issuing from them usuallyremains in the vicinity and either needs an extra fan toremove it or raises the temperature so that the exciterand condensers suffer from the high ambient tempera-ture. If the condensers are located near, then it becomesdesirable to employ the additional fan, especially insummer. This fan may, however, be quite small, asonly 2 or 3 kW have to be dissipated.

With forced ventilation some type of air filter isnecessary, particularly under foundry conditions, andthe former method outlined has the slight disadvantagethat doors and windows should be kept closed so as toprevent the air from being by-passed from the filter,which is usually let into the substation wall.

Other considerations in the lay-out are the busbarand cable runs. With the skin effect more marked thanat ordinary frequencies, it is economical to use a coppersection not greater than £ in. thick for heavy lead;and the clamps should be made of a non-magnetic ironfastened with brass bolts. In connecting the condensersthe disposition of the leads should be such that theyembrace no large areas which will introduce extrainductance and require further capacitance for power-factor correction.

PERFORMANCE AND ENERGETICS.

The performance of a given furnace is affected by anumber of factors, some outside the control of the melterand others depending on the type of charge being melted.Changes in the physical properties of the charge duringheating cause variations in the electrical constants ofa furnace during the melt. In 500-cycle work it isusual to provide the furnace coil with a tap in order tokeep the load on the generator within certain limits.At the beginning of a melt only part of the coil is in use,as small subdivision of the charge has the effect ofreducing the power input, and it is desirable to be ableto increase the power input by such a method. Afterthe charge has begun to weld together, the power inputincreases and the furnace tapping-switch is changed soas to put the whole of the coil in circuit and thus preventoverload of the generator. This variation is not soapparent with higher frequencies, and with 2 000 cyclesper sec. it is not usual to provide a tap on the furnacecoil, though it would enable the coil to melt a stillsmaller size of scrap were that necessary.

Apart from the variation of load outlined above,other changes in furnace load are due to:—

(a) temperature coefficient of resistivity and per-meability;

(b) change of configuration of the charge owing towelding together of pieces and formation ofmolten bath.

The power absorbed by the charge is proportional to^(mfxp). The factor m denotes the btate of subdivisionof the charge; and in theory a cylindrical charge ofradius r} may be divided into m cylinders of radiusrJ-^/m, each of the same axiad length as the original

cylinder. The value of m is not the same as the numberof separate pieces which make up the charge.

Taking the*fact~0Ts"iS"'turn, the value of rn is usuallyhigh at the beginning of a melt, and decreases as thetemperature increases, until finally with the moltenbath m = 1. The permeability also decreases with arise in temperature, and from about 750° C. it may beregarded as unity. These two factors give the chiefclue to the high power input at the beginning and itssubsequent reduction until a temperature of about600° C. to 700° C. is reached. The steady increase ofresistivity becomes more noticeable from this stageonwards and partly accounts for the increase in powershown in the later stages.

On account of the low power factor the currenttaken by the furnace is very nearly proportional to thereactance, which in turn represents a fixed inductorreactance less the reactance of the charge. This chargereactance is in turn approximately proportional to thevolume of the charge, and as it is impossible to putmore than about half the charge in the crucible a t the

200

§•% 100

v-Tap

|C iarere_1 molteii

20 10Time, minutes

60

FIG. 6.—Graph showing power taken by J-ton furnaceduring a run.

beginning, the charge reactance continuously increaseswith the subsequent additions made during the melt.The form of the charge and the magnitude of theadditions vary considerably in practice, but it may betaken that the current increases continuously throughoutthe process of melting, giving rise to the necessity ofadding condenser units as the reactive-kVA meterindicates the need.

Fig. 5 shows the trace of a recording wattmeter onthe motor side of an induction-furnace equipment, asdistinct from the ideal curve of Fig. 6. The depressionsmarked A to H indicate where the voltage was loweredfor the insertion of a condenser unit. At X the tapping-switch on the furnace coil was operated to put thewhole of the coil in circuit and thus reduce the powertaken; at the same time several of the condenser unitswere removed, to be inserted again later at I, J, and K.In the case of the curve shown, the generator voltagewas hand-regulated, and the irregularities indicated thedesirability of employing some type of automatic voltagecontrol. A voltage relay is now generally adopted, andFig. 7 gives a performance curve which shows howthis device helps to reduce the time of melting andincidentally to prevent over-voltage' running.

The bald statement of kilowatt-hours per ton of steedmelted, will enable the steel-maker to anticipate his

514 MARCHBANKS: THE STEEL-MELTING GORELESS INDUCTION FURNACE.

account for electrical energy, but a proper appreciationof the action of the induction furnace as a machine,according to the classical definition, will call for furtherparticulars of the energy distribution. A fine degreeof accurate analysis is not at present possible on accountof our incomplete knowledge of the properties of materials

10 20

per ton. For alloys of the stainless-steel variety, whichare poured in the neighbourhood of 1 500° C, an averagefigure seems to be of the order of 300 to 350 kWh perton, and A. Decary* gives 376 kWh per ton for ironhaving a melting point of 1 535° C. For the examplegiven below the author proposes to take a figure of

10 Minutes

FIG. 7.—Recording-wattmeter trace, for J-tou steel charge (voltage relay in circuit).

at high temperatures; and owing to the wide diver-gencies which occur in practice using different types ofsteels and refractories, it does not appear cogent toattempt it.

0-6

| 0 - 4

8

0-2

0

XX

N

X

o Sihcon

Car

r

Don

o -o—'-

20 80 10060Time, minutes

FIG. 8.—Carbon and silicon variation during refining.

The energy input is as follows:—

(a) Heat required to melt the charge.(b) I2R loss in furnace coil.(c) Heat required to raise the temperature of the

refractory.(d) Heat transmitted through the refractory to its

surroundings, chiefly the coil.(e) Radiation from the charge.(/) Additional stray losses such as heating of frame-

work members and losses in leads.

Items (b) and (d) above are almost entirely taken upby the cooling water, and their sum is readily calculatedfrom temperature and flow measurements.

(a) Richards quotes Ledebur in " Metallurgical Cal-culations " as giving the total heat of 1 per cent carbonsteel at itsj melting point (1 353° C.) as being 234 kWh

330 kWh per ton (1-13 X 106 B.Th.U. per ton), as thecharge consisted of stainless-steel scrap. Table 3 showsa set of readings taken during the melting of this charge,and from these and certain deductions a heat balancehas been drawn up as set out in Table 2.

TABLE 2.

(a) Total heat of molten charge(b) 12R loss in coil(e) Heat transmitted through

refractory lining[d) Heat to raise temperature

of refractory[e) Radiation and stray losses

Total

kWh

33067

34

2540

496

B.Th.U.X 101

11323

12

914

171

Per cent

66-613-5

6-8

5-08-1

100

(b) The coil copper-loss follows directly from the high-frequency resistance of the coil.

(e) The loss by conductivity through the refractorylining is taken as the difference between the loss to thewater and the coil loss (b).

(d) The heat required to raise the temperature of therefractory is calculated as 25 kWh on the followingbasis:—

Weight of refractory = 610 lb.Average specific heat = 0 - 2 9Average temperature-rise = 275 deg. C.

This temperature-rise is the estimated rise, startingwith the refractory hot from the previous melt.

(e) Radiation and stray, losses, are estimated as • adifference.

* A. DECARY: Journal du Four Eleclrique, 1929, vol. 39, p. 299.

MARCHBANKS: THE STEEL-MELTING CORELESS INDUCTION FURNACE. 515

METALLURGICAL CONSIDERATIONS.

The crucible process of steel-melting is probably themost primitive and costly of the methods used in steel-works, but the high quality of the product has justifiedits retention right up to recent times. With the intro-duction of the induction furnace, steel-makers haverealized that there is a means of superseding the crucible,and in the majority of cases where these furnaces havebeen installed the use of this type for the productionof crucible-quality steel has been the underlying motive.It is true that the first furnaces were installed in thiscountry for the manufacture of alloys that, on account

The starting of a furnace for the first charge callsfor some preparation in the matter of a refractory lining.In the early days fireclay or graphite crucibles wereemployed, lagged into the coil with a refractory sand.For steel-melting, this method proved of short durationowing to the invention by Dr. Rohn of the method offiring a refractory lining in the furnace itself. Fireclaycrucibles rarely have a life of more than 10 to 12 meltsof steel, and with graphite crucibles the increment ofcarbon into the steel was generally too great for manyclasses of steel. Crucibles are retained in use for lowmelting-point alloys where their life is long and where

TABLE 3.

Charge: 2 240 lb. of stainless-steel scrap. Refractory Lining: Silica (acid). Hot. 2J in. thick.

Time,min.

018

19

28

38

49

53

6068

75

75

Generator

Volts Amps ReactivekVA

Start2 1002 0502 1002 1002 0202 0002 0202 0602 0502 100

162

173133

145

175

195232

250289

286

0- 60+ 20

0

+ 60+ 90- 20+ 60

+ 130+ 160

Charge completely molten

Power,kW

340

350280305340

380470

505580580

Condenser,kVA

3 1403 2403 1403 6553 6203 7934 1104 2704 7304 970

Furnacecurrentamps

1 5951 550151017401 82519402 0302 1002 3702 440

Cooling water

Flow,gallons per

minute

16-2516-2516-516-816-816-816-816-816-816-816-8

Ingoingtemperature,

°C.

12121212

12

12

1212

1212

12

Outgoingtemperature,

°C.

2120-522

2525-226

28-829-730-53234

kW

46-644

52-569-570-574-889-894-598-5

107

117-5

Reading of motor kWh-meter at beginning . . . . . . . . . . 275 246Reading of motor kWh-meter at end .. . . . . . . . . . . .. 275 840

594

Energy to furnace and condensers . . . . . . . . . . .. . . 505Energy to furnace alone . . . . . . ., . . . . . . . . . . 496

of the purity demanded, could not be satisfactorilyproduced by any other method, but, as a general rule,the replacement of the crucible by this electric furnacehas been the most obvious motive.

In crucible practice it is necessary to pour the ingotswith considerable dexterity, so that the molten streamdoes not strike the walls of the mould. To simulate thiscondition, steel-makers call for the induction furnacesto be lip-pouring, so that the molten stream moves aslittle as possible from its original position during thewhole time of pouring. Unless the mould presents afairly large opening the natural curve of the stream isgenerally too -much inclined to the vertical, so thatrecourse is had to a "lander," a fireclay'funnel intowhich the molten metal is poured so that a verticalstream is easily produced.

the temperature is insufficient to produce a very well-fired lining of the Rohn type.

The bottom of the furnace is made up with brickworkcut to form a cylinder reaching from the floor of thecontaining box to a height just inside the coil, any spacebetween this brickwork and the coil being filled withasbestos wool. A layer of dry magnesite or silica sandabout 3 to 4 in. thick is then laid upon this brick founda-tion and a steel template to form the refractory liningis placed in position. This template is J in. thick andhas an outside diameter of 4 in. less than the insidediameter of the coil. The annular space so formed isfilled with sand similar to-that at the bottom, and thissand is held in at the top by a layer, of ganister clay.A steel charge may be incorporated in, the manufactureof the lining, allowance being made for the template;

516 MAftCHBANKS: THE STEEL-MELTING CORELESS INDUCTION FURNACE.

in calculating the constituents of the charge. Onapplying the current to the furnace the heat producedin the template fires the lining material to a depth ofabout | inch, and thus a refractory lining is obtainedwhich is more durable than any it is possible to producein the fireclay kiln, and which will last from 50 to 80melts. The life of the lining depends upon the typesof steel being melted, manganese steel being particularlyerosive. Messrs. Rees and Chesters in a recent paper*have given figures of lining erosion for various types ofsteel. The process of erosion of the lining should becarefully watched and measurements made of the dia-meter, to check the thickness of the lagging on the coil.A thin lagging results in excessive heat conduction tothe coil and may be detected by the high power takenby the charge consequent upon its greater radius. Asthe lagging becomes eroded the thickness of the firedportion increases, and if this gets much thicker thanf in. with a magnesite lining the stresses set up in it oncooling cause it to crack, whereupon the steel-meltershould decide to make a new lining.

Brick linings have quite recently been adopted withsuccess in induction furnaces. The bricks are airdried for ease of handling, and are built up insidethe coil. There are five or six bricks to a ring, eachtongued and rebatted, and three layers make up thelining. They are designed to leave a f-in. annular spacebetween them and the coil, for lagging purposes. It isnot essential to make the joints between the brickswith any cement, but the cracks are covered with amortar of the brick material, ground to about 30 meshand mixed with a little water to make it bind.

For electrical reasons the proportions of the chargeare such that the depth is about 20 per cent greaterthan the diameter. It is urged by steel-makers thata shallower bath would be preferable on account ofthe difficulties of bridging, that is, the formation abovethe remainder of the charge of a bridge of metal heldtight by expansion or bad charging. The danger isoccasioned by the cold metal falling into the super-heated bath below when the bridge is dislodged, andcausing a " cold boil." The experience of the author'sfirm is that bridging, occurring as it does owing to badstacking of the charge, rarely gives trouble to a melterhaving a few weeks' experience of the induction furnace.

To those who see the induction furnace for the firsttime, the electromagnetic effect on the molten chargecausing the fountain is perhaps the most striking pheno-menon, and it is to this point that much metallurgicalcontroversy has been directed. It represents a definite•departure from the crucible method of melting where aquiescent state is regarded as desirable, to allow thegases formed to be liberated. With the continual " up-welling " of the metal to the centre of the fountain andits flowing away to the sides, metallurgists foresaw acondition particularly favourable to oxidization and slaginclusion. This has not proved to be the case, for, onobservation, the slag particles may be seen rising inthe centre and flowing away to the sides to form a ringof slag, which soon spreads completely over the fountain.The- higher the frequency the smaller is the fountain,

• J. H. CHESTBRS and W. J. REBS: " Refractory Materials for tbe InductionFurnace, "journal of the'lron and Steelr Institute, 1931, vol. 123, p. 489.

and this argument has been put forward in favour ofthe use of higher frequencies. It seems to the authorthat so long as the fountain is not so turbulent as toprevent the formation of a slag covering or to be unrulythe actual movement is unimportant; in fact, ingotsmade at 500 cycles per sec. and at 2 000 cycles per sec.have shown no difference.

With regard to the mixing of alloys, the fountain isof particular significance, especially where such heavyadditions as tungsten are made. The thorough mixingof this metal takes place during the normal period ofsuperheating without recourse to external agitation.

REFINING.

It has been stated above that the employment of theinduction furnace was first considered purely as a meltingunit to replace the crucible. Experiments have, how-ever, been performed which tend to show that theinduction furnace need not be restricted to this fieldalone. The work has been attended with sufficientsuccess to justify refining being regarded as an operationwhich may well be part of the normal work of this typeof furnace. In this respect the fountain effect is of

CarbonChromium ..SiliconManganese..PhosphorusSulphur

Originalanalysis

per cent

0-3011-300-400-30——

TABLE *•

IngotNo. 1

per cent

0-3111-850-480-240-0120-023

V.

IngotNo. 2

per cent

0-3112150-400-230-0130-022

IngotNo. 3

per cent

0-3012-500-290-2100150-023

IngotNo. 4

per cent

0-3112-300-310-220-0060-023

great importance, as the surface of the charge in relationto the volume is very much smaller than with thearc furnace. New metal is constantly brought to theunder surface of the slag, so that the disadvantageof limitation of area is to a great extent eliminated.The slag surface, however, is fairly constant, so that itshould be renewed during refining for the most completereactions to take place. A further result of the fountainis that the speed of the reactions is very rapid, althoughin comparison with the arc furnace, of course, thequantity of metal dealt with is much smaller.

Before refining is considered, it is desirable to knowthat no variation in the composition of a charge takesplace during melting. To this end a 350-lb. charge ofstainless-steel scrap was melted and a 250-lb. ingot waspoured at half-hour intervals, the charge being madeup after each cast with 250 lb. of cold scrap. It willbe noted that from the results shown in Table 4 thecomposition variation may be regarded as negligible.

Oxidization of Carbon and Silicon.A charge of bar iron and pig iron was made up to give

the following percentage analysis:—

Carbon0-5

Silicon,

0-625Manganese

0-225

MARCHBANKS: THE STEEL-MELTING COR] AUCTION FURNACE. 517

This was held in the molten state in an acid (silica)lining at 1 500° C. for nearly two hours, and sampleswere taken regularly and analysed. No lid was usedon the furnace and very little slag was formed. At thebeginning the bath was fairly quiet, but at the end ofthe first half-hour it was extremely wild, but becamequieter later. Fig. 8 shows the analyses obtained. Itwill be noticed that the percentage of carbon does notbegin to fall until the silicon content is down to about0-15 per cent in this case. These reactions can beaccelerated by the addition of an oxidizing slag such asmill scale, or by directing an air blast on to the metalsurface. This latter method was employed on a high-carbon steel charge, with the following result:—

Original compositionComposition after application

of air blast for 40 mins. . . 0

Carbon,per cent

0-85

76

Silicon,per cent

0-14

0-024

Removal of Sulphur and Addition of Carbon.In this refining experiment 350 lb. of scrap steel of

composition

Carbon,per cent0-51

Silicon,per cent0-20

Manganese,per cent0-69

Sulphur,per cent0-049

were melted, with the addition of 2 lb. of coke. ."Whenmelting was complete a slag was made with lime andfluorspar and an addition of 1 lb. of CaSi. The chargewas kept well covered with the slag for 80 minutes andsamples were taken every 10 minutes. Before pouring,1 lb. of CaSi and 1 lb. of ferro-manganese were added.Five analyses are given in Table 5.

TABLE 5.

Sample 1Sample 3Sample 6Sample 8Ingot

Carbon

per cent

0-960-970-940-92 ,0-88

Silicon

per cent

0 1 50-130-190-140-23

Manganese

per cent

0-660-680-690-691-06

Sulphur

per cent

0-0570-0490-0100-0100-014

The addition of 2 lb. of coke represents an incrementof 0-57 per cent carbon, and it is to be noted that79 per cent of this is absorbed into the steel at thebeginning and that the final contains 65 per cent ofadded carbon, 14 per cent being oxidized in 80 minutes.It is clear that the coke contained at least 1 • 4 per centsulphur which appeared in sample 1, but its subsequentoxidization left the ingot with only 29 per cent of itsoriginal sulphur, showing the readiness with which this^element may be removed and the unimportance of asmall sulphur content in the coke used for carbonabsorption. In this respect it may be pointed out that0-02 per cent sulphur is low enough for practical pur-poses; this reduction was achieved in 50 minutes.

Metallurgists express the refining actions in a furnaceby means of exponential curves of carbon, silicon, phos-phorus, etc., contents against time. This work is being

done with this type of furnace and shows that the poten-tialities of induction-furnace refining are being realizedand that'its technique is in course of construction.

The author wishes to express his thanks to Mr. Bur-bridge for his advice during the writing of this paper,and to the Metropolitan-Vickers Electrical Co., Ltd.,for permission to publish it.

APPENDIX 1.

MATHEMATICAL THEORY.

The following symbols will be adopted:—

Resistivity of inductor material

Resistivity of charge material

Axial length of inductor coilAxial length of chargeInside radius of inductor coilRadius of chargeTurns per cm on inductor coil .Permeability of charge material.Frequency of supply

p1 (electromagneticunits)

p (electromagneticunits)

Zj cmI cmrx cmr cmN

O>/27T

Depth of current penetration in inductor coil

Depth of current penetration in charge

For mathematical purposes the charge and coil areassumed to be right cylinders, and the electric constantsof the charge are given with reference to the inductor.

Theory gives the following expressions for the circuitconstants:—

Inductor Resistance.27T^(2ir)N\r1'y/(a)p1)F1{r1/P1) X 10~9 ohms . (1)

The function F^rJ^j) is asymptotic and approachesunity for increasing values of rxlf$x above 3; it is plottedin Fig. 9.

Charge Resistance.

^j-Fj^) x 10-9 ohms . . (2)

For r/j8 < 1, F2(rf{}) = -7^(50) approximately. As

r/p* increases beyond the value 3, F^r/ft) approximatesto unity. It is plotted in Fig. 10.

Inductor Reactance.47r

2iV2Z1r2co2?73(r1/j81) x 10~9 ohms . . (3)

As rj/jSj increases beyond the value 3, F^rJ^) approxi-mates to unity, as shown in Fig. 11.

518 MARCHtBANKS: THE STEEL-MELTING CORELESS INDUCTION FURNACE.

Charge Reactance.

X 10-9 ohms (4)

JWjS) = 0 - 5 forr/j8= 3= 0-63 for r/j8 = 4= 0-71 for r/fi = 5

and approaches unity as r/j3 increases beyond this figure.The function F4(r/P) is plotted in Fig. 12.

The above expressions are subject to corrections due

2-2

1-8

1-4I

1-0

0-6

0-2

\\

\— —- — — —

0 2 4 r/_ 6 8 10Wt

FIG. 9.—Inductor-resistance function.

to the finite lengths of the inductor and charge, butthis does not affect their validity for the purpose ofshowing how variation of frequency alters the efficiencyand power factor of an induction furnace.

For unit current in the inductor the power absorbed

0-8

0-6

0-4

0-2 //

/

/

.—•—•. - - — • •

0 2 4 r , 6 8

FIG. 10.—Charge-resistance function.

10

by a given charge is dependent only on the frequency.Two special cases of this heating are of interest:—

(a) When the frequency is so low that the skin effectis not appreciable, i.e. rlfi < 1.

(6) When the frequency is such that the eddy currentsare mainly confined to the surface of the charge,i.e. r/P > 3.

In the first case the power absorbed by the chargebecomes

P = 27T3a>2r4 l—N2U* ergs per sec. per cm . (5)P

which represents the well-known case of the heating oftransformer cores.

To state, that this formula is applicable to low-fre-quency induction is not strictly correct, as the occurrenjce

of the skin effect is dependent also on r, fj,, and p. Thesefactors are taken into account by using the relation

commonly known as the depth of penetration. If r/fiis less than 1, the skin effect may be regarded as negligible,and expression (5) for power absorbed in the charge maybe used. It will be noticed that the term " low-fre-

2-2

1-8

1-4

1-0

0-6

0-20 2 4 ?;/ 6 8 10

FIG. 11.—Inductor-reactance function.

quency induction " is somewhat loose when used in thisconnection. The term " volume induction " has beensuggested to describe this phenomenon.

By subdividing the charge of radius r into m cylindersof radius r/y 'ra the power is reduced in the ratio m : 1.

\

— •• — •— —

1-0

0-8

0-6

04

0-2

0 2 4 r/fi 6 8 10

FIG. 12.—Charge-reactance function.

The second case mentioned above where the skineffect is marked is the more important, since, in practice,coreless induction furnaces are worked over this range.

Here the power absorbed by the charge is given by

P = 27T\/(2Tr)r-\/(o)pix)N2l1l2 ergs per sec. per cm (7)

and r/j8 > 3 decides its applicability. In this connectionit seems preferable to refer to this class of heating as" surface " rather than " high frequency." If the chargeis divided into m smaller cylinders, provided always thatr/j8 remains > 3, the heating is increased y'm times.

Now the power absorbed in the inductor coil ofradius r, of material of resistivity pv is

27T-\/(27r)r1y'(cop1)iV2Z22 ergs per sec. per cm . (8)

rilfii being very much greater than 3.

MARCHBANKS: STEEL-MELTING CORELESS INDUCTION FURNACE: DISCUSSION. 519

Taking the efficiency as that fraction of the totalpower supplied which is absorbed by the charge,we get

+. . . (9)

using expressions (5) and (8) for a charge where r/j8 < 1.From this expression it will be seen that an increase offrequency gives an increase in efficiency until r/jS becomesgreater than 3, and that

7] =

ry/{p)l(10)

which is independent of frequency. Thus there existsa certain frequency to give r/j8 = 3; above which theefficiency is a maximum and constant, and below whichthe efficiency of the furnace rapidly decreases. This isshown in Fig. 1. From the point of view of furnaceefficiency, therefore, we may choose any frequencyabove that which gives r/j8 = 3.

Power Factor.

The reactance of the inductor is approximately

47r2r2a>iV2Z1 X 10~9 ohms . . . (11)

and that of the charge

- 47r2r2coJV2Z x 10-9 ohms . . . (12)

so that the ratio of power dissipated to reactive kVAof the furnace becomes

which is proportional to lf-y/co.It becomes apparent, therefore, that to obtain the

best power factor along with greatest efficiency thefrequency should be chosen to give a value of r/j3 onlyslightly greater than 3.

Table 1 gives some examples of the values of r/j8which occur in practice.

In practice the results obtained with the above for-mulae need considerable correction. Owing to axial andradial magnetic-flux components the resistance of thecoil is actually greater than that given by the formula,and Butterworth's* work in this connection has beenfound to give results very close to actual measurements.Even so, it cannot be taken that the resistance of a coilcontaining a charge is the same as that of an emptycoil. With a molten charge the flux is reduced, andfrom the measurements which the author has been ableto make it appears that the resistance of the coil isfrom 5 to 15 per cent less with the molten charge thanwith the empty coil.

The coil reactance, as calculated from formula (4),needs correction owing to its finite length. The directuse of Nagaoka's factor (K) and a correction for the rect-angular copper section gives a very close approximation.

The charge-resistance calculation is based on manyassumptions, one being the value of the resistivity ofmolten steel. If this is taken at about 20 times thecold value, and corrections are made to the long-chargeresistance so as to take account of its actual length andalso the finite length of the inductor, the value obtaineddoes not give rise to any serious discrepancies.

• Proceedings of the Royal Society, A, 1925, vol. 107, p. G93

DISCUSSION BEFORE THE NORTH-EASTERN CENTRE, AT NEWCASTLE, 10TH APRIL, 1933.

Mr. H. V. Field: In view of the insulation difficultiesmet with in the construction of the inductor coil, wouldit not be preferable to employ an air-spaced coil, withinsulating spacers of a suitable refractory material?In the existing method, the coil is shown in contact withthe furnace lining, and in view of the very steep tempera-ture gradient in the latter there will presumably beconsiderable heat conduction from lining to coil. Theair-spaced construction would reduce this. I presumethat it is not possible to increase the lining thicknessowing to the necessity of keeping coil and charge asclose as possible, so as to increase the efficiency.Thicker linings should be possible on larger furnaces,and by means of the suggested construction the com-plication of water cooling might possibly be eliminated.Can the author give any information as to the induc-tion densities existing in the furnace and charge?Also, does hysteresis make any appreciable contributionto the heating when the charge is relatively cold ? Inview of recent developments in high-output thermionicvalves, are these likely to become competitors of theinductor alternator for lar,ge furnaces, or will theirapplication be confined to small sizes? The wave-form

of the e.m.f. affects the core loss in transformer cores:have any investigations been carried out on the influenceof wave-form on the loss in furnaces of the type describedby the author? Presumably it will have no influenceif the frequency corresponds to the lowest desirablevalue as shown in Fig. 1, because under such conditionsthe harmonics associated with distorted wave-forms areonly equivalent to operation at a higher frequency,which gives no appreciable improvement in efficiency.Distorted wave-forms of e.m.f. would appreciablyincrease the condenser current and losses, and wouldprevent the best tuning conditions being obtained whenthe two-ammeter method of control (page 511, col. 2) isemployed. Can the author give a simple explanationof the cause of the fountain effect referred to on page 516 ?

Mr. S. A. Simon: It is fairly evident that at presentthe coreless induction furnace is practically confinedto the crucible process. In this district we are moreconcerned with the steels produced by the open-hearthprocess, and we should be interested to know whetherthe induction furnace 'can be profitably applied to thesegrades. It is largely a question 'of cost &i production,and the author could no doubt give us some comparative