In the induction heating of thin (e.g. 0.5 to 3 mm diameter) steel wires (W) in continuous heating processes, relatively low field strengths of e.g. up to 20,000 Am.sup.-1 are employed but are chosen so as to set the relative magnetic permeability .mu.r at 40 or more. The wire are passed through coil blocks (1), which are wound flat around a row of ceramic tubes (3) to guide and insulate the wires. The coil blocks are energized at frequencies typically up to 30 kHz. The direction of winding and/or the phase of the current is changed between adjacent coil blocks (1), or groups of blocks, to reduce voltage build-up along the wires.

This invention relates to induction heating and particularly to the 
induction heating of elongate metal articles such as steel wires by 
passing them through induction coils. 
There is frequently a need to heat steel wires to low or intermediate 
temperatures. Examples of thermal treatments are thermal diffusion, stress 
relieving, tempering, partial or full annealing, and so forth. The heating 
step may be carried out separately or in line with other stages such as 
surface treatments, coating processes, deformation processes etc. The 
present invention is especially concerned with the heating of steel wires 
of a diameter less than 5 mm, and preferably in the range 0.5 to 3 mm, to 
temperatures not exceeding the Curie temperature (750.degree.-760.degree. 
C.). The invention may be applied to other processes, however. For 
production purposes, it is generally desirable to heat a plurality of 
wires simultaneously while they move continuously in parallel paths and 
the invention is particularly applicable to such a context. 
Conventional devices in use for simultaneously heating a plurality of steel 
wires include for example fuel-fired or electrical furnaces, molten lead 
or salt baths and resistive heating lines making use of direct 
electrocontact wire heating and the like. 
The performance of conventional furnaces is limited with regard to high 
speed precise heating of wires up to 750.degree. C., because of rather 
slow heat transfer rates in this low to intermediate temperature range. As 
a consequence, furnaces of considerably increased length are necessary for 
achieving high wire throughput speeds, which is both technically and 
economically an unattractive solution. Direct immersion heating in molten 
salt and lead, to the contrary, is rapid and more effective, but the major 
drawbacks here are wire surface contamination, demanding extra cleaning, 
and inferior working conditions. 
Direct electroresistive devices, feeding the heating current to the wires 
by means of electro-contact rollers, have been adopted by the industry for 
several wire heating applications. With increasing speed and power 
density, however, it has been found that the risks of sparking and surface 
defects may become very critical. In the production of high quality and 
fine wires, this is highly undesirably. 
A promising solution is afforded by induction heating methods which produce 
rapid non-contact heating. Induction heating is nowadays a widely spread 
technology for heating large-sized workpieces such as billets, bars and 
rods prior to forging or working etc. and for surface hardening articles 
such as axles, sucker rods, etc. The technique is already applied in some 
specific areas of the wire industry, for example for stress relieving of 
prestressed concrete wire and strands. It has also been employed to a 
given extent for the annealing of drawn copper wire in line with the wire 
drawing machine. 
Induction heating is not, however, widely used in multiwire installations, 
which at present can treat a large number of wires (5, 10, 20, 30 and even 
40 or more) simultaneously. The concept of non-contact inductive heating 
of a plurality of wires has long been regarded as being technically 
impractical and/or too wasteful in energy and capital, in particular for 
smaller wire sizes. It has been stated that induction heating is too 
expensive because of the reputed need for distinct induction heating coils 
for each wire track, and the complexity of branching all the inductors to 
current supplies of suitable frequency. Similarly, it is a common belief 
and experience that electrical and/or thermal efficiency remain inadequate 
when comparing induction heating of wires to other heating methods. In 
this respect it has been believed that inductively heating small diameter 
wires is not only highly unsatisfactory but may become even impossible 
below a certain diameter limit of, say, 1 to 1.5 mm. 
The particular problem of effective inductively heating thin to 
medium-sized steel wires, roughly below 2-3 mm in diameter, has several 
aspects the ratio of penetration depth of the induced current to wire 
diameter becomes critical, the magnetic coupling losses are large due to 
the poorer coil filling degree, i.e. the poor ratio of wire to coil 
diameter, and so on. To remedy these factors it has been tried to pass a 
plurality of wires through a single cylindrical induction coil to increase 
the effective filling degree. Other trials have been made, but none was 
really successful or capable of satisfyihg the requirements of 
practicality and economics demanded in modern multiple wire processing 
lines. 
With regard to certain known principles and theory of induction heating, 
examples of references are the following: 
"Induction heating"--Simpson P. G., Mc Graw-Hill, N.Y. 1960 
"Induction heating handbook"--J. Davies, P. Simpson, Mc Graw-Hill, U.K. 
1979 
U.S. Pat. No. 4.118.617: Process and apparatus for continuous heat 
treatment of metallic wires and bands. European Patent Application No. 
90.488: Apparatus for simultaneously heating a plurality of elongated 
workpieces. 
When considering a cylindrical workpiece in a solenoid, the basic factors 
determining effective induction heating are the frequency and density of 
the energizing current, the air gap between the workpiece and coil i.e. 
the ratio of workpiece to coil internal diameter, and the geometric 
disposition of the coil windings. 
For assessing the heating power which is active in the workpiece itself, 
one has to consider primarily the effective penetration depth (p) of the 
induced current which is given by the formula [1]: 
##EQU1## 
with p: in centimeters 
.rho.: electrical resistivity of workpiece (in ohm-cm) 
.mu.r: relative effective magnetic permeability 
f: frequency in cycles/sec. 
Because, e.g., 87% of the total heat is produced by the induced current in 
the effective penetration depth p, it is clear that the penetration depth 
should be kept sufficiently below half of the workpiece diameter (p=1/2d) 
to prevent redundancy of the induced current. The penetration depth is 
frequency dependent and for ferromagnetic steel rods for example, can vary 
from 1 mm to 10 mm at commonly used mains frequencies, i.e. f=50 Hz. The 
problem of current losses in the workpiece center becomes particularly 
acute in the induction heating of small diameter wires. For effectively 
induction heating medium to small wires, say from below 3 mm, it will be 
necessary to use current frequencies of at least a few thousand Hertz. 
The electrical efficiency of an induction heating coil is expressed by 
formula [2] 
##EQU2## 
with .rho.cu:resistivity of coil windings (copper material) 
.rho..sub.W :resistivity of workpiece (e.g. steel wire) 
.mu..sub.r : relative permeability of workpiece (e.g. steel wire) 
D : diameter of coil windings 
d : wire diameter 
L : length of induction col 
l: effective length of wire enclosed by induction coil 
k: current efficiency coefficient (k&lt;1) dependent on ratio d/p, i.e. wire 
diameter to penetration depth. k.about.1 for d&gt;p. 
The heating power Pw dissipated in the workpiece is given by [3] 
##EQU3## 
with: H (=NI/L): inductive magnetic field 
I: energizing current 
p: penetration depth 
N: number of windings 
From [3] it follows that the amount of heat generated increases in 
proportion to H.sup.2 or I.sup.2, A decreasing penetration depth p also 
raises heat input. In induction heating of bars and billets, it is common 
practice to apply high current densities (still supportable by the coil 
windings) up to saturation level of the magnetic field, Hs. In case of 
small wire diameters, it seems reasonable to seek the lowest feasible 
penetration depths (p), by using high frequencies, and to use high current 
densities. 
We have found, however, that this frequently gives erratic results and very 
poor heating efficiencies, especially when heating wires of the diameter 
range 0.5-2 mm. We have found that the heating difficulties stem to a 
large extent from the delicate balance between the electrical and thermal 
energies when inductively heating small wires. Too high frequencies and/or 
current densities may either result in overheated, or even burnt, 
surfaces, or in widely fluctuating actual thermal efficiencies. As a 
consequence a large scatter in the required wire temperatures is obtained 
and total energy efficiency remains at levels incompatible with effective 
and economic heating. The problem can become very acute in the lower wire 
range of 0.5 to 1.5 mm diameter, where magnetic coupling, pronounced skin 
effect and heat losses from the wire surface to e.g. water cooled coil 
windings can adversely affect either heating capacities or efficiency. 
The object of one aspect of the present invention is to overcome or 
alleviate these problems. Thus it is proposed to use relatively low energy 
densities (to avoid surface overheating and to reduce the amount of less 
controllable heat losses of the wire rim to the surroundings), and a shift 
towards high values of magnetic permeability for the wires. 
Such an arrangement will improve electrical efficiency (see formula [2]) 
and it should further be noted that the relative magnetic permeability for 
steel wires increases with decreasing field strengths. Rewriting formula 
[3] as the heating power dissipated per unit volume of wire, gives: 
EQU P.sub.w =4.pi.H.sup.2 .mu.o.mu.r.multidot.f.multidot.k.multidot.p/d [4] 
Thus, considering the factor H.sup.2 .mu.r it will be seen that a decrease 
in field strength does not produce an equivalent decrease in heating 
power, since there will be an increase in .mu.r. We have found that it is 
possible to adjust the parameters, so as to permit a departure from the 
conventional use of high field strengths whilst still obtaining sufficient 
heating powers. 
Thus viewed from this aspect the invention provides a method of induction 
heating an elongate steel element in an induction coil, characterised in 
that the magnetic field strength is so adjusted that the relative magnetic 
permeability of the element is at least 40. 
This is well above the level normally used, with typical values being in 
the range 10-20 for fields approaching saturation. Preferably, in carrying 
out the present invention, the realtive magnetic permeability is at least 
80, and a typically useable range is 100 to 200. 
This aspect of the invention is particularly applicable to thin steel wires 
having a diameter of less than 5 mm, and a particularly useful range is up 
to 2 or 3 mm. The lower limits may be about 0.5 mm. 
The magnetic field will generally be in the range of 3,000 to 35,000 
amperes turns per meter, and preferably 5,000 to 20,000 Am.sup.-1. Typical 
frequencies which will be used, may be up to 50,000 Hz, and a preferred 
range is 5,000 to 30,000 Hz. 
It will be appreciated that the heat input may be less than for 
conventional arrangements, although by virtue of the invention it will be 
at practical levels. This means that the length of time required for 
heating will be increased. The invention is particularly concerned with 
the heating of continuously moving wires and thus, in comparison with what 
would be the case in a conventional heating arrangement, greater coil 
lengths will be required. This results in another potential problem to be 
solved. 
The heating requirements will vary according to wire type and diameter, and 
the coil length will also depend on the wire speed which may, for example, 
be in the range of 10 to 100 m. per minute or more. A typical coil length 
may, for example, be about 2 or 3 m to 5 m. We have found that this can 
result in unacceptably high induced potential differences over the wire 
length. This is normally not a problem in induction heating, but in this 
particular case--when using long coils and continuous wire lines--the 
accumulated voltage in the longitudinal wire direction may attain a 
critical value (up to more than 50 V) and thereby cause sparking between 
the wire and the grounded components of the installation such as guiding 
members, take-up reels and the like, which may result in intolerable wire 
surface marks or defects. 
It is believed that a main cause of excessive voltage accumulation in the 
longitudinal wire direction is the fact that, due to the pitch of the coil 
windings relative to the wire axis, a spatial electric field is induced in 
the wire, which is slightly inclined to the wire direction. The spatial 
electric field can be resolved into a large radial component and a small 
axial component the vector sum of which creates a gradual potential 
increase along the wire length. 
We have found that the problem can be resolved by dividing the length of 
the induction coil into at least two parts and by reversing the winding 
direction and/or the phase of the current in the two parts. 
This may be of use in other circumstances where relatively long induction 
coils are required and thus viewed from another aspect the invention 
provides induction heating apparatus with a coil in which an element is to 
be placed, characterised in that at least two adjacent lengths of coil are 
provided, with the direction of coil winding and/or the phase of the 
energising current being reversed between adjacent lengths. 
It has been found that phase reversal has a greater effect than the 
direction of winding. However, the best results are achieved by effecting 
both and this also reduces dead spots in the magnetic field due to the 
interaction of the ends of adjacent coils. 
These arrangements may be obtained readily by having a plurality of coil 
modules which can be arranged with respect to each other, and connected to 
an energising supply, in such numbers and combinations as may be required 
for any particular application. 
It will be appreciated that, in general, the reversal of coil winding 
results in a reversal of the angle of inclination of the windings to the 
axis of the coil. 
Another aspect of the invention is concerned with the construction of the 
induction coil itself. As noted already the air gap between the workpiece 
and the coil should be kept to a minimum, i.e. from formula [2], D/d 
should be low for best efficiency. This presents problems when large 
numbers of wires are to be treated. Proposals to use individual coils for 
each wire, are not economical. On the other hand, placing a number of 
wires in a simple coil results in large values of D/d inconsistencies in 
the treatment of wires, dependent on their position. 
Thus, viewed from another aspect, the present invention provides induction 
heating apparatus including a coil in which a plurality of elongate 
elements are to be placed, characterised in that the coil has a generally 
flat cross section in which the elements are to be arranged in a row. The 
cross section may thus be in the form of an elongate rectangle, with the 
ends preferably rounded. 
It is conceivable that two or even three rows could give reasonable 
results, but most preferally the arrangement is such that a single row 
only is accommodated. There will be a planar array of parallel wires or 
other elements with the turns of the essentially flat coil closely 
enveloping the array. Preferably, the coil windings themselves are flat 
and are made of copper strip. 
Whilst the theoretical formulae given earlier are for the case of a single 
thick section enclosed by a cylindrical coil, the advantages of the 
various other aspects of the invention are retained with the use of the 
flat coil and an array of discrete, largely identical small sections, i.e. 
wires. 
A particularly preferred and advantageous arrangement involves the use of 
guides to space the individual wires (or the like) apart and guide them 
through the coil. Such guides could be in the form of open channels or the 
like, and continuous or intermittent. Preferably, however, the guides are 
in the form of parallel, continuous tubes. 
The guides themselves are preferably electrically insulating and could be 
made from ceramic or other refractory material.

Referring now to FIG. 1, giving a general view of an inductive heting 
system, there is shown a multiwire induction heating coil 1 energized by a 
medium (or high) frequency current supplied by a suitable generator 2. The 
electrical circuitry of generator 2 normally comprises a power supply 
transformer (Tr) and a 3-phase Thyristor rectifier bridge (Rf), choke 
coils (CH), a thyristor (Th1 to Th4) controlled frequency current 
generating section and a capacitor array (C) of appropriate capacitance 
(to compensate the impedance of the load circuit in relation to the 
electrical characteristics of the generator). 
A plurality of wires W travel continuously through a flat inductor 1 and 
specifically, each wire passes through a distinct heat and electrically 
insulating tubular guide 3. The guides are preferably ceramic tubes of 
suitable cross-section and are arranged here as a one-layer planar row, 
enveloped by the windings 4 of the heating coil. For round wires one 
normally uses cylindrical tubes, whereas for shaped wires such as e.g. 
flat wires, a tube with rectangular cross-section can be envisaged. The 
coil windings may be made of solid high-conductivity copper wire, cable or 
strip, orovided suitable cooling means are incorporated in the coil outer 
construction (e.g. a water mantle separated from the coil interior and 
directly contacting the windings) or of tubular copper windings with inner 
water cooling. 
FIG. 2 gives a cross-sectional view, perpendicular to the wire direction, 
of a flat multiwire heating coil clearly showing the location of coil 
windings (4), ceramic tubes (3) and wires W. 
FIGS. 3 and 4 give further details by showing a longitudinal cross-section 
of the heating coil through the wire axis (along line B--B of FIG. 2). In 
FIG. 3 an embodiment is shown wherein the coil windings consist of solid 
copper strip (5) which is water-cooled by means of a continuous copper 
tube (6) brazed on the exterior surface of the strip windings. This type 
of copper windings makes it possible to fabricate very flat heating coils 
with an interior inductor tunnel height of even below 20 mm. 
FIG. 4 shows a heating coil type formed of water cooled copper tubes (7), 
which preferably have an oval shaped, flat-rounded or flattened 
cross-section for the purpose of being coilable to low inductor heights 
(closely matching the outer periphery of the tubular wire guide array on 
the inside). In constructing and using the heating coils, it is important 
to bring inductor height as close as possible to the interior conveying 
tubes and to hold this height constant in the lateral and longitudinal 
coil direction during service. 
Among the several possibilities for fabricating a multiwire coil one may 
choose the following method. First the copper conductor is wound on a 
mandrel of suitable dimension to form a heating coil of desired geometry 
(height, width, length and number of turns per meter). The coil is stress 
relieved for form stability, and coated with a polymeric or other material 
to insulate the individual inductor turns (e.g. stretching the coil helix 
as a spring and dipping it in a suitable varnish, curing /drying, 
releasing the helix). Thereafter the bare coil is provided with 
appropriate mechanical fixing means and with the necessary electrical 
connections and cooling accessories. Alternatively the coil windings can 
also be permanently stabilized by means of a rigid mass, e.g. by 
mouldforming in a suitable plastic material (epoxy and the like) or in a 
fibrous cementitious material such as fibre cement or fibre concrete. 
The interior tunnel space of the finished inductor is provided with the 
prescribed plurality of suitable guiding channels, which could be provided 
by a multi-groove refractory plate or a multi-hole muffle but in this case 
comprises the plurality of ceramic tubes 3 to convey the wires 
individually through the inductor. 
Between the ceramic tubes and the copper windings, one can also dispose an 
extra heat-insulating barrier layer, e.g. a mica sheet, a refractory fibre 
felt or the like, for the purpose of further reducing heat loss of the 
wires to the windings. This layer also increases system security in case 
of an accidental break of a tubular guide, and elminates risk of direct 
contact between wire and coil. 
The ceramic tubes inserted in the flat rectangular inductor channel (having 
a height of a few millimeters larger than the tube diameter) are fixed at 
a coil entry and exit by appropriate clamping means. When the total 
heating length is composed of a plurality of separate adjacent coil 
modules, the ceramic tubes may either be continuous over the entire 
heating paths or they can extend over the length of one coil block only 
and then be connected in line with the subsequent modules by suitable 
flanges. The advantage of the latter arrangement relies upon the fact that 
a broken tube and/or a faulty coil module is quickly removed and replaced 
by a new component. 
FIG. 5 gives a more detailed view of an inductor comprising a continuous 
coil length subdivided in two serially connected coil sections, having 
opposite winding pitch directions (change of winding direction in the coil 
middle 8). In this embodiment the coil windings are made of copper strip 5 
bearing a brazed-on cooling pipe 6, which is shown in more detail in the 
deployed cross-section of FIG. 5b. This coil has a length of about 2.4 
meters and comprises 37 turns of copper strip 2 mm.times.55 mm. The 
interior coil height is 20-22 mm and in the inductor channel, having a 
width of less than 60 cm, 36 ceramic tubes of 15 mm outer diameter and 2 
mm thickness can be fitted. This means that in this case wire spacing is 
nearly 15 mm. When using smaller tubes, wire spacing can still be lowered 
to increase the inductor filling factor. In a production line, however, 
the minimum attainable spacing will also depend on practical 
considerations such as the difficulty of pulling the wires through very 
small holes, which can lead to wire breakages, availability of 
small-diameter ceramic tubes, possibility of wire entanglement, etc. When 
the induction heating step is integrated in a wire processing line, the 
actual wire interdistance can also be imposed by the minimum spacing 
attainable in the process step prior or subsequent to the heating step. 
In FIG. 6 a multiwire induction heating line is schematized, comprising a 
sequence of separate coil blocks (1'), each featuring a multiwire coil 
construction. The coil is made of oval shaped tubular copper windings 7 
(36 turns of roughly 10 mm width) as depicted in FIG. 6a. Here the 
conductor windings are separated from the ceramic tubes by means of a 
heat-resistant insulating layer 10 of a few mm thickness. In FIG. 6b the 
coil blocks (1') are connected in parallel to the current supply by means 
of 2 parallel bus bars 9. This arrangement is very favourable for 
maintaining the useful capacity of the generator at a high level (versus 
installed power capacity) because ohmic and inductive voltage losses over 
bus bars and heating coils can be kept low as compared to serially 
connected coil blocks featuring considerably longer current supply lines. 
FIG. 7a illustrates the dependence of relative magnetic permeability .mu.r 
on applied field strength H based on the magnetic measurements carried out 
on several ferrous wire types (band A relates to unalloyed low and high 
carbon steel wires, curve B relates to ferritic high-Cr steel wire). The 
selected working range corresponds to .mu.r values above 40-50, preferably 
above 80 and most preferably from 100 to 200. To achieve these conditions, 
coil design (turns per m) and energizing current are adapted to cover a 
broad working range from 3,000 to 35,000 A/m, preferably from 5,000 to 
20,000 Am. 
It will be noted that for .mu.r values above about 200, the rapid change in 
.mu.r for a change in H can lead to undesirable fluctuations in working 
conditions. 
This approach considerably departs from the working range close to or 
beyond the saturation magnetization Hs .mu.r far below 40 and usually 
around 10-20), which is the normal prior art procedure of induction 
heating, as indicated in FIG. 7b. 
In using the multiwire flat coils and in applying the energizing conditions 
prescribed, coil efficiencies from 60 to 90% (according to wire diameter) 
are obtainable, typically from 70 to 85% for the wire diameter range 
0.75-2 mm at generator frequencies from 8,000 to 25,000 herz. Total actual 
energy efficiencies (which also depends on actual generator efficiency and 
its efficiency versus load curve) in all cases tried exceeded 50%, and in 
a majority of practical processing situations the system total energy 
efficiency ranged from 60 to 75%, which are surprisingly high levels for 
the wire diameters concerned. 
FIG. 8 shows various heating coil arrangements for reducing voltage build 
up, whereby either a continuous long coil is subdivided into at least two 
coil sections which are wound in opposite senses (FIG. 8a), or whereby the 
total heating length is composed of a varying number of serially connected 
distinct coil blocks (FIG. 8b-e) which are either wound in opposite 
directions and have the same instantaneous current flow direction (FIG. 
8b), or interconnected to the supply in such a way that the current flow 
direction is reversed in some coil blocks (FIG. 8c-e). The coil block may 
also be reversed in terms of winding direction. In FIG. 8 the current 
direction at any instant is illustrated by arrow i. 
Another arrangement is represented in FIG. 9, showing an induction heater 
subdivided into a number of coil sections or in distinct coil modules, 
which are connected in parallel to the supply and whereby the voltage 
build-up along the wires is reduced either by reversing the current 
direction, or the coil winding direction or both. 
FIG. 9a and 9b show a coil subdivided in 2 sections with centre tapping of 
the supply, either with the same windings (9a) or with oooosite windings 
(9b). The arrow i gives the instantaneous direction of the current. In the 
modular building block concept (FIG. 9 c-d-e) one can change the current 
direction from block to block (FIG. 9c) or from block assembly to assembly 
(FIG. 9d) and maintain the same coil block winding direction, or also 
change both the winding and current direction such as represented in FIG. 
9e, showing an embodiment wherein current flow and winding direction are 
each time reversed simultaneouosly between two adjacent coil blocks. 
The most preferred embodiments of the examples shown in FIG. 8 and 9, are 
these multi-coil arrangements whereby simultaneously coil winding 
direction and current flow are reversed. In this way, potential 
differences between the wires and the earthed installation components are 
effectively suppressed to negligible levels (1 to 5 V max.) and also the 
local potential differences over the heating length are largely reduced. 
Moreover, all the ampere-turns of the successive coils remain active 
because the reversing arrangement also rules out the negative interaction 
between two adjacent coils, which interaction otherwise lowers the 
effective heating capacity of the heating line. 
In FIG. 10, a preferred embodiment of a multi-coil building block 
(connected in parallel) is shown, which is additionally improved in that 
current supply is effected by means of a novel design of bus bar 
connection, comprising two parallel conductor bars of suitable form (e.g. 
U or L-profile) at close interdistance (max. 2 mm) and separated by a thin 
insulation layer, for example in the form of a 3-layer composite bar. This 
current supply arrangement is surprisingly beneficial in that it largely 
prevents the undesirable decrease in available work voltage (terminal 
voltage of generator versus effective voltage over the heating coil 
connections), not only by reducing current supply distance and related 
ohmic voltage drop (through parallel tapping to the supply), but in 
particular by suppressing or compensating the inductive field (and related 
voltage drop) effects along the current supply bars and/or between the 
supply and the coils. 
The preferred embodiment shown in FIG. 10 is an induction heating line of 
modular building block concept, comprising a sequence of multi-wire coils 
(1'), connected in parallel to the supply (with current reversal between 
the coils) which features the improved bus bar design comprising two 
parallel U-profiled conductors 11, with insulating interlayer 12. An 
aditional advantage of the bus bar construction is that it enables quick 
and simple attachment/replacement of standard coil blocks. 
In FIG. 11 two additional induction heating arrangements are illustrated, 
aimed at carrying out specific heat treatments. FIG. 11a shows a heating 
line for heating and soaking, comprising a first set of coils with 
impedance Z1 and a second set of coils with different impedance Z2, with 
all the coils being preferably connected in parallel to the same 
generator. Impedance Z2 is adapted to hold the temperature level attained 
after rapid linear heating with coils Z1. Other temperature profiles are 
of course feasible. In FIG. 11b, a preferred embodiment is shown to 
simultaneously heat a plurality of ferrous wire above the Curie point. The 
induction line is constituted of respectively a medium-frequency heating 
path (coils C1 connected to generator G1) and of a high-frequency heating 
path (C2-G2). As shown schematically in the accompanying heating diagrams, 
effective high-temperature inductive heating is possible (provided the 
required high level of generator frequency is applied and available at 
sufficient power) and a major part of total heating heating (up to 
approximately 760.degree. C.) still occurs at optimum electrical 
efficiency. 
It is further obvious that the multiwire induction heating apparatus being 
equipped with individual ceramic tubular channels, enables the use of any 
protective atmosphere. Gas consumption can be kept low because of the 
small tube diameter and gas volumes involved. 
The following examples will further illustrate the practical advantages and 
versatility of the preferred multiwire heating apparatus and methods. 
EXAMPLE 1 
A multiwire induction coil having a total length of 4.6 m is used for the 
simultaneous heating of 36 steel wires of 1.20 mm diameter. The inductor 
(similar type as shown in FIG. 5) is coiled from copper strip 100.times.2 
mm (water cooled by a copper pipe of 10 mm diameter brazed on the strip 
windings' outer surface), enclosing an inner inductor channel with a flat 
rectangular cross-section of 22 mm (height).times.560 mm (width). It 
contains 36 ceramic tubes (in planar parallel pattern) for guiding each 
wire individually. The coil is divided into two coil sections, each with 
17 windings, respectively right-hand and left-hand windings. A 200 kW-10 
kHz generator is connected to the coil. All the wires can be uniformly 
heated to a temperature of up to 700.degree. C. and at speeds up to and 
exceeding 50 meters per minute. In a practical example the multiwire 
inductor heater was used in line with a patenting and electrogalvanizing 
process installation to carry out a thermodiffusion step on copper and 
zinc-coated steel wire at a uniform line speed of approximately 40 
m/minute. 
The heating conditions were as follows: 
induced field strength: 14-15. 10.sup.3 A/m 
relative magnetic permeability: .mu.r=95-100 
net dissipated heat in the wires: 70 kW 
generator efficiency: 90.2% 
coil efficiency: 65-72% 
total heating efficiency: 60-64% 
average wire temperature: 596.degree. C. 
standard deviation: 14.degree. C. 
(standard deviation: scatter between the wires related to their relative 
position in heating coil and as result of slight variations in wire 
surface humidity after electroplating, rinsing and drying.) 
max. potential difference in longitudinal wire direction 32 V (coil entry 
to exit) 
voltage drop from wire to earthed guides reel: 26-29 V. 
EXAMPLE 2 
A multiwire induction heating apparatus was designed to treat 
simultaneously 40 ferrous wires up to a temperature of 700.degree. C. The 
apparatus comprises two heating coils of 2.4 m length and 0.62 m width. 
Each coil has 37 copper solid strip windings of 55.times.2 mm (provided 
with water cooled copper tube) which form an inductor channel of about 20 
mm height wherein 40 ceramic tubes (14 mm outer diameter, 10 mm inner 
diameter) are inserted. The distinct coils are connected each to a 80 
kW-25 kHz generator. The 37 windings are subdivided into 18 right and 18 
left-wound turns. The induction heating installation was used to treat 40 
wires of 0.7 to 1.4 mm diameter at a speed of 35 to 65 m/min. In carrying 
out a thermodiffusion treatment on previously plated and patented wires, 
the working conditions were as follows: 
wire 0.70% C. of 1.04 mm diameter; throughput speed 48 m/min. 
applied magnetic field strength: 8500-9000 A/m 
relative magnetic permeability .mu.r of wire material: 140-160 
working frequency: 20,600 herz 
Heating Results: 
heating power dissipated in wire: 66 kW (generator 1+2) 
heating coil efficiency: 70-76% 
total heating efficiency: 58-67% 
average wire temperature: 585.degree. C. 
mean spread (40 wires): s=12.3.degree. C. 
max. potential difference on wires between entry exit of heating cols: 17 V 
max. voltage drop from wires to earthed reeling means: 15.2 V 
EXAMPLE 3 
In this example the usefulness of the multiwire induction heating system is 
demonstrated for tempering of oil quenched (martensitic) steel wire, and 
more in particular flat wire with 5.times.1 mm section. For this purpose 
an inductor building block has been designed of 2 m length, constituted of 
4 coils of 0.50 m in series, each comprising 50 windings of tubular 
copper. 
The coils were connected to a 40 kW-generator working at 25 kHz. In the 
heating coil tunnel 10 ceramic conduits of rectangular cross-section 16 a 
10 mm (wall thickness 2 mm) were arranged to pass each of the 10 flat 
wires through its respective ceramic channel. The shaped wires were 
tempered at a temperature of 480.degree. C. at a speed of 10 to 25 m/min. 
Mean spread of wire temperature was smaller than 10.degree. C. 
The system, which replaces a conventional lead tempering bath, is 
particularly advantageous in that it is compact and clean in terms of 
working environment in that the lead contamination, which sometimes 
affects wire surface quality, is absent and in that total energy 
consumption is markedly lower--i.e. negligible heat loss to surroundings; 
compact system with zero heat content, no stand-by heating, etc. 
To demonstrate the versatility of the multiwire coil provided with flat 
tubular ceramic guides, it was tested for simultaneously heating a 
plurality of multiwire tapes, each tape comprising 10 parallel wires of 
0.5 mm held together by an adhesive compound. The test revealed that it 
was perfectly possible to dry and cure the composte wire tapes at a 
constant temperature (200.degree. to 300.degree. C.) and at elevated 
velocities of up to 300 m/min. Energy efficiency amounted to 55-70% (coil 
efficiency up to 80%) according to generator type and load factor. 
EXAMPLE 4 
In this example the versatility of multiwire heating coils is illustrated 
with regard to various heating applications. Different coil winding 
materials have been used to fabricate the desired multi-wire heating 
coils. 
In a first series of tests a 10 wire induction heater was provided with one 
or more heating coils of a total heating length ranging from 1 to 2 m. 
Steel wires with a diameter from 0.5 to 2 mm were heated to different 
temperature levels below 750.degree. C. Wire throughput speed was varied 
from 20 to 100 m/min. 
Two generator types were used: a 40 kW-25 kHz device and a 80 kW-10 kHz 
device. 
Inner coil height was 22 mm, filled with one row of ceramic tubes of 15 mm 
diameter. 
Among the tested coil windings were: solid copper strip (20.times.2 mm), 
layered cooper foil (10 layers of 20.times.0.2 mm), copper cable 
(7.times.4.times.0.30 mm), copper strip (55.times.2 mm) with exterior 
cooling tube and flattened copper tube (10.times.6 mm). The volumetric 
power density Q (total heat dissipated in the wires divided by number of 
wires, coil length and wire cross-section) was varied from 0.3 to 3 
kW/m/mm.sup.2 and the actual heating efficiencies were compared, 
calculated from power supplied by the mains and from the amount of coil 
losses and generator losses (measured by determining heat content of 
cooling water). 
The general conclusions of the tests are as follows: 
the multiwire induction heating coils made it possible to carry out rapid 
heating of medium to small wire diameters with reasonably good total 
energy efficiency. According to wire diameter, velocity (applied energy 
density) and generator type the total efficiencies range from 50 to 75%. 
coil efficiencies are higher than 65% and mostly range from 70 up to 85% 
(lowest value for highest energy density of 3 kW/m/mm.sup.2). 
for highest wire speeds improved coil designs (optimum length and number of 
windings per m to allow optimum energy density conditions) allow one to 
increase coil efficiency by 5-15%. 
generator type in particular generator efficiency curve vs. load factor, is 
important to attain improved "total" energy efficiencies. 
wire velocities in the 150-250 m/min. range are perfectly sustainable. 
max. rate of wire speed in multiwire induction heating can in principle be 
raised to above 500 m/min. (still with satisfactory heating efficiency) 
when enlarging the total heating length of a modular inductor building 
block by ading additional coil modules with all the coils connected in 
parallel to a current source (one or more generators) of sufficient power. 
EXAMPLE 5 
In this example, referring also to FIGS. 8 and 9, a large number of coil 
arrangements has been investigated for the purpose of controlling the 
axially induced voltage build-up in the moving wires. A heating length of 
4.5 to 5 m was chosen and provided with either continuous coils, 
subdivided coils or modular coil blocks (either serially or connected in 
parallel to the supply), whereby the winding direction and/or the phase of 
the current were changed. The unexpected results of these investigations 
was the fact that current flow direction in the coil section or coil block 
is comparatively of greater effect and importance in controlling and 
compensating the induced voltage in the longitudinal wire direction than 
the change in magnetic flux direction by using opposite windings. In 
theory both measures should have similar effects. We observed, however, in 
imposing a current direction reversal between 2 coil sections or 2 
adjacent coil blocks, that total heating capacity is lowered to a 
significant extent. The most preferred arrangement, which we found to be 
surprisingly effective in practice when using a squence of replacable coil 
blocks (connected in parallel to the supply), is the simultaneous reversal 
of both the current flow direction and the winding direction. Some typical 
results are given below: 
__________________________________________________________________________ 
Type of coil Voltage 
(total length of 4.5-5 m) 
A B 
__________________________________________________________________________ 
serially connected 
(1) 
continuous coil up to 80 V 
50-70 V 
(2) 
subdivided coil in left and 
33 V 24-29 V 
right hand wound section 
(3) 
2 subdivided coils as in (2) 
20 V 14-16 V 
(4) 
4 coils: change in windings 
30-48 V 
5-15 V 
(5) 
idem as (4) reversal with one 
16-20 V 
1,9-9 V 
current reversal 
(6) 
idem as (4); 3 current reversals 
12-16 V 
1,8-2,5 V 
connected in parallel 
(7) 
10 coil blocks with same 
20-30 V 
2-5 V 
winding direction, current 
reversal between coils 5 and 6 
(8) 
current and winding direction 
2-9 V 0-1 V 
reversal in all coils 
__________________________________________________________________________ 
A = max. potential difference over heating length. 
B = actual measured voltage between heated wires leaving the inductor and 
the earthed spooling device. 
It is evident that many modifications are possible, such as for example in 
the choice of the most appropriate coil geometries, inductor tunnel design 
and work path configuration and the like for a given application. In 
addition the induction systems as described hereinbefore may be provided 
with suitable temperature sensors and appropriate current regulating 
devices aimed at subjecting the workpieces to automatically controlled 
heating profiles such as for example a programmed heating schedule. 
Whilst the embodiments have been particularly concerned with thin wires, 
the apparatus and processes could be used for much thicker wires, e.g. 3 
to 6 mm diameter or larger, as well as non-ferrous e.g. copper wires, 
stainless steel wires, or shaped wires of non-circular cross section. 
Additionally, high temperature heating is possible--e.g. above the Curie 
point when ferrous materials become non-magnetic. The inductor 
configuration and frequency should be adapted for each specific purpose. 
Considering now various aspects and advantages of the preferred embodiments 
of the invention, without limiting the broader aspects thereof, the 
apparatus permits heating of a number of elements in which each element is 
to be subjected to the same heating profile extending along the length 
thereof and is guided along a distinct insulated path through the 
apparatus, the individual paths being arranged in such a way as to form a 
regular array of straight and parallel channels contained in the interior 
space of a common inductor and enveloped by the windings thereof. 
In addition to being energy efficient and being capable of uniform, 
effective and economic heating up to high wire speeds and down to small 
wire sizes, the apparatus permits identical heating of each individual 
wire to an adaptable heat profile, eliminates the need to stop the whole 
line in case of an incidental wire break (even at elevated wire 
velocities) and prevents the occurrence of wire surface defects such as 
e.g. contacting and/or sparking marks. 
The preferred induction heating apparatus comprises suitable means for 
feeding and energizing the inductor with a regulable current of desired 
frequency and further comprises suitable means for conveying the plurality 
of elements at the same adjustable rate of speed longitudinally along 
their respective parallel work paths leading to and through the heating 
coil(s) of the inductor, the apparatus being provided more specifically 
with a flat shaped inductor constituted of one or more induction heating 
coils disposed adjacently in the longitudinal wire direction, which coils 
form an induction heating tunnel of adaptable length with flat rectangular 
cross-section containing all the work paths therein. The work paths are 
arranged in one or more horizontal rows of closely spaced insulated 
channels through which the metal elements are individually guided and 
along the length of which the elements are brought to the same prescribed 
temperature. 
From another aspect, the preferred apparatus comprises a flat shaped 
inductor device for inductively heating metal wires, more in particular a 
multiwire inductor for heating simultaneously a plurality of ferrous metal 
wires (for example to a temperature not exceeding their Curie point; 
approximately 750.degree. C.), whereby the wires are guided individually 
through identical refractory channels disposed in the inductor channel 
preferably as a one-layer sheet, which channels extend over the full 
length of the respective heating coils of the inductor. According to a 
specific design possibility of the multiwire coil, the channels can be in 
the form of a multihole ceramic muffle, and most preferably in the form of 
distinct (replacable) ceramic tubes of suitable cross-section, disposed as 
a closely packed one-layer configuration and tightly enveloped by the coil 
windings. 
From a still further aspect, the preferred apparatus provides an improved 
inductor system for simultaneously heating a plurality of identical wires 
with a high energy efficiency (exceeding 50%) and elevated wire capacity 
(heating up to 40 and even more wires simultaneously at velocities ranging 
from 10 to more than 100 m/min. according to wire type and process), which 
inductor line may be constituted of one continuously wound coil unit of 
desired coil length or of a number of separate coil modules (as replacable 
blocks) of shorter length arranged together (in a so-called building block 
concept) longitudinally and electrically connected in series or in 
parallel so as to form a multi-wire heating path of any desirable length 
thereby allowing progressive power input and effective heating at any 
desired rate and further allowing, by selecting the optimum combination of 
coil parameters and energizing conditions (current density and frequency), 
one to enhance energy efficiency and wire throughput well above prior art 
methods for induction heeating of wires. The coil or coil module 
construction may be made in the form of an integrated preassembled 
building block containing all necessary electrical and cooling attachments 
and having the insulated coil windings rigidized in an electrically 
non-conductive moulding mass or rigidized by other suitable mechanical 
means so as to maintain in service a constant height of the flat interior 
inductor tunnel, in which a plurality of replacable ceramic tubes is 
fitted for conveying the invididual wires in parallel straight paths from 
entry to exit, whereby the tubes may extend uninterruptedly over the 
entire heating length or are only continuous per coil module length and 
then interconnected in line from one coil block to another so as to 
provide a continuous wire path. 
In accordance with a preferred feature there is provided a specific 
arrangement of coil windings and of the electrical connection of the 
successive heating coils to the current supply so as to minimize voltage 
build-up along the length of the inductively heated wires and thereby 
avoiding possible sparking between the wires and the guidance or take-up 
members. 
A further preferred feature relates to the current supply connection betwen 
generator and working coils which connection is designed in such a way 
that ohmic losses, and in particular inductive voltage losses, are 
substantially reduced so that actual available power (ratio of coil 
working voltage to terminal voltage of generator) remains the highest 
possible. Accordingly, there is provided a bus bar connection of composite 
construction comprising two parallel conductor profiles at very close 
distance, having a thin insulation layer separating said profiles, which 
composite bus bar is disposed parallel to the longitudinal inductor 
direction. This bus bar, and particularly in combination with induction 
coils, may be used in contexts other than those disclosed herein. 
In accordance with the preferred process, there is provided an improved 
method for inductively heating a plurality of ferromagnetic metal wires, 
such as carbon steel wires, which improvement relies upon the judicious 
selection of coil geometry and coil energizing conditions, in which the 
active magnetic field is kept below a level of 30 to 35,000 A/m and the 
relative magnetic permeability of the heated wires exceeds a value of 
40-50, so as to minimize coil losses for a given wire diameter range 
without imparting heating capacity and wire throughput. 
In the preferred embodiments there are provided reliable and economic 
methods and an improved multi-wire induction apparatus for use in 
uniformly heating a plurality of carbon steel wires of 0.3 to 3 mm 
diameter at elevated travelling speeds and at high efficiencies, without 
the need to stop the line in case of an incidental wire break and without 
causing sparking defects. 
A general advantage of the preferred embodiment featuring a particular 
inductor design, a flexible coil block concept and an improved energizing 
arrangement, is the possibility of carrying out contactless, rapid and 
economic heating of ferromagnetic steel wires at high rates, which heating 
step may be carried out separately or continuously in line with a 
preceding or subsequent wire treatment stage (depending on the type of 
wire process or multiwire processing installation) and which step can be 
adapted in velocity to be consistent with the optimum processing speed of 
the entire manufacturing line.