4
INDUCTION FURNACE FOR HEAT TREATMENT OF AUSTENITIC AND/OR
FERRITIC STEEL WIRES AND STRANDS
I. Artuso
(1)
, S. Ghedin
(1)
, P. Siega
(1)
and A. Visconti
(2)
(1)
ATE Applicazioni Termo Elettroniche, Viale dellArtigianato 9, 36100 Vicenza, Italy
(2)
Technical Consultant, Via Spluga 80, 23853 Olgiate - Como, Italy
1. INTRODUCTION
The continuous heating of steel wire for most industrial applications is got by induction
furnaces. In these type of furnace the wire moves through a spiral copper coil. As known, this
method, in comparison with other heating techniques, offers significant advantages, e.g., good
efficiency (in particular with magnetic materials), high accuracy of the temperature
obtainable, reduced overall heating unit dimensions and above all very easy arrangement of
the heating unit in the so called in line processes.
Depending on physical characteristics of the material to heated and the wire cross-section,
the induction heating theory (as described in section 4
th
) allows to identify an optimal
working frequency value for the coil exciting. Such value is generally in the range from some
kHz to few hundreds kHz, for wires with diameters greater than 5 or 6 mm of magnetic
material or with small diameters (lower than 1 mm) for non-magnetic material.
Nowadays there are three type of inverters which cover the above mentioned working
frequency range: inverters with SCR are generally used till 10 kHz, inverters with IGBT from
10 to 50-60 kHz, inverters with MOSFET from 50 to 200-300 kHz.
The application described in this paper concerns the continuous heat treatment at 400 C of
steel strands (7 high carbon steel wires) both magnetic and no-magnetic. The requirements for
heating magnetic and no-magnetic strands in the same heating unit (with just reduced
throughput) have been fulfilled an induction heating unit with double frequency conversion
system. The heating unit is equipped with three coils: the first two supplied by a generator
400 kW - 46 kHz, are used for the heating of magnetic strands, while the third, is supplied
by a generator 250 kW - 30-50 kHz, is used for no-magnetic strands.
In this application, there isnt a great interest of using the two generator simultaneously;
this possibility, on the contrary, is very useful when the requirement is to heat over the Curie
point steel wire with diameter from 6 to about 14 mm. In this case the first two coils heat the
material 700720C up to while the third is used in the temperature range from 700720C to
9501000C.
2. PRE-STRESSED CONCRETE STEEL STRANDS LINE
Strands Composition
Pre-stressed concrete steel strands can be formed by 2, 3 or 6+1 steel wires, with properties
that will be discussed later. In accord once to ASTM A416-94 Standard the stranding pitch
can be between 12 and 16 times the strand nominal diameter or, according to ISO 6934-
4:1991 Standard, between 12 and 22, for the 2 or 3 wires strand, and from 12 to 18 times for
the 6+1 wires. The most common strand is the 6+1 wires; where the core wire, that is the
central wire around which the other six are spirally wounded, must have a diameter at least
2% bigger.
The Wires
The wires utilised to make the strand are obtained from carbon rod, Chrome micro-alloy or
Vanadium micro-alloy, cold drawn through dies. The wire has to be wound on reels, suitable
for the stranding machine, of a diameter approximately the same to the drawing block
utilised.
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4
INDUCTION FURNACE FOR HEAT TREATMENT OF AUSTENITIC AND/OR
FERRITIC STEEL WIRES AND STRANDS
Today the new philosophy is to use drawing machines with blocks diameters of 900 mm or
1.200 mm (instead of 760 mm used till some years ago), and stranders of 1.120 mm reels with
the aim of obtaining a much better cooling of the wire and a better productivity.
The Strands
Let us now have a look to the most common strands, as shown in Figure 1 and exposed in
the Table 1.
Figure 1 - The more used strands type.
These strands are made of bright (black) wires and they are utilised for the pre-tensioning
of concrete structures. In the last three rows of the Table 1 are listed the Compacted or
Dyformed strands, so called, because they are obtained from formed one and passed through a
drawing die that compacts and reduces its diameter in order to have an higher quantity of steel
having the same nominal diameter, corresponding to an higher strength of the strand.
Additionally to the above mentioned strands, there are other types of strands made of
indented wires, that means with the surface formed to allow a better grip of the concrete.
Another series of strands, utilised for the Post-tensioning of the concrete structures, or in
order to strengthen structures already installed, is constituted by strands made of bright
(black) wires or zinc plated, greased and coated with special plastic coatings.
New types of strands, produced by lines of the latest technology now on the market, are
those made of Stainless Steel Wires for very special utilisation.
2 / 9
3
4
INDUCTION FURNACE FOR HEAT TREATMENT OF AUSTENITIC AND/OR
FERRITIC STEEL WIRES AND STRANDS
Table 1 - The more used strands type list.
Strand Area Weight Pitch Speed Skip Pull Strength
inches mm mm
2
Kg/m mm m/min RPM Kg Kg
3/8" 9.3 67.9 0.408 140 112 800 5200 8800
3/8"s 9.6 72.4 0.432 140 112 800 5200 10200
7/16" 11.0 95.0 0.557 165 132 800 6800 12000
7/16"s 11.3 100.3 0.590 165 132 800 7200 13800
1/2" 12.5 122.7 0.730 190 152 800 8800 16000
6/10" 15.2 181.5 1.090 229 110 480 13500 23900
6/10"s 15.7 193.6 1.180 229 110 480 14000 26000
7/10 17.8 191.3 1.504 250 85 340 16000 30000
Dyform 14.7 mm 12.7 126.7 0.890 200 110 550 13500 20900
Dyform 18 mm 15.2 181.5 1.295 242 94 390 16000 30000
Dyform 20 mm 18 254.5 1.750 261 74 285 20000 38000
The Heat Treatment
Once the strand is made, it has to be heat treated at low temperature (380400C), with a
continuous process of pay off and take up, passing through a suitable heating system: this
process is called STRESS RELIEVING. If the strand is also pulled adequately, during the
passage through the heat treatment, by means of a system composed by two pulling units fit
one before and one after the furnace, the process is called LOW RELAXATION.
Today this is the system more commonly utilised. The treated strand has to be rewound in
coils having internal diameter sufficiently big to ensure that strand returns reasonably straight
once paid off, that means with a curve not over 25 millimetres over one metre of strand laid
onto a flat surface. This diameter is normally considered 800 mm 60 mm or 950 mm 60
mm. Strand forming operations and the stress relieving treatment shall ensure that the wires
do not unravel when the strand is cut.
Production Lines
Let us now discuss about pre-stressed concrete strand production lines, considering the
latest developed design made by an Italian producer, world leader of machinery for the wires
world, suitable to produce all types of strands above mentioned, including those made of
Stainless steel. Referring to the layout of Figure 2, we will mention the main components:
1. Skip Strander suitable to run 6 +1 reels with 1.120 millimetres flange, having a capacity of
2.500 kg of wire each, that includes the strand pre-forming system.
2. Post-former for the finishing of the produced strand, with the possibility of using the
drawing die for compacted strand.
3. First pulling unit to pull the strand out of the skip strander, giving as well the necessary
drawing pull in case of production of compacted strand and, at the same time, adequately
synchronised with the second pulling unit, (3') giving the necessary back tension needed to
allow the Low relaxation.
4. The return pulley, which not only allows to reverse the working direction of the strand for
decreasing the total length of the line, but also provides the precise pull control necessary
in the process.
5. The induction furnace, which is necessary for the needed heat treatment of the strand and
will be described in detail in a separate chapter.
6. The cooling quench, together with the connected drying unit, brings back the temperature
of the strand to the prescribed value.
3 / 9
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4
INDUCTION FURNACE FOR HEAT TREATMENT OF AUSTENITIC AND/OR
FERRITIC STEEL WIRES AND STRANDS
7. The two take-Up/pay-Off wind the complete skips load of 2.5 ton x 7 reels = 17,5 tons of
strand that is then rewound on the double automatic layer winder. While one units take-ups
the strand from the treatment line, the second one is paying off to the layer winder.
8. Double spooler with fully automatic strand layer winding, to rewind the strand at high
speed into the coils of the dimensions requested by the market. The use of the double
spooler allow the operators to set-up the second spooler to the next requested coils
dimension, chosen from a quite vast series of barrel and flange diameters together with
widths, while the first one is working, increasing enormously the lines productivity.
The speeds of those lines are, in the production and treatment area up to 160 metres per
minute, that can arrive up to 180 metres per minute in case of heat treatment off-line, and up
to 300 metres per minute in the layer winding section.
1 2 3 4
5 6 7 8 3' 4
Figure 2 - The production line lay-out is given by Mario Frigerio S.p.A. of Lecco ITALY
3. HEATING FURNACE CHARACTERISTICS
Medium Frequency Inverter
The electrical scheme used for this application, shown in Figure 3, includes:
an impressed current power supply constituted by an AC/DC
converter full controlled section followed by a DC smoothing
reactance
a Medium Frequency inverter in H bridge single-phase configuration
a resonant circuit formed by the coil and capacitor in parallel connection
DRIVER DRIVER
HEATING UNIT ELECTRICAL
CABINET
PLANT
SUPERVISORY
COMPUTER
AC/DC
CONVERTER
CONTROLLER
AND START-UP
SEQUENCES
AC/DC CONVERTER
FULL CONTROLLED
M.F. INVERTER CONTROLLER
DRIVERS SUPPLY AND
PROCTECTIONS COORDINATION
LOAD
SYNCHRONISMS
THYRISTORS
R
S
T
START-UP
SEQUENCES
M.F. INVERTER
Figure 3 - Medium frequency inverter block diagram.
4 / 9
5
4
INDUCTION FURNACE FOR HEAT TREATMENT OF AUSTENITIC AND/OR
FERRITIC STEEL WIRES AND STRANDS
This solution, the so-called impressed current parallel inverter, gives the best efficiency
conversion, a very easy construction and optimal utilisation of SCR at Medium Frequency.
Its nowadays, for rated power greater than some hundreds of kW and working frequency up
to 8 kHz, the best used configuration since it sums the advantages of high reliability and
cheapness. The AC/DC control is done through a microprocessor by means of two feedback
loops (voltage and current) and PID (Proportional Integrative Derivative) software for the
current control that delivers the M.F. inverter and its protection. The last one has a control
system that optimises the SCR firing pulses during the start-up sequence and hooks the
working frequency by means a PLL circuit with automatic matching due to the impedance
variation and load circuit Q.
High Frequency Inverter
In this case an impressed current configuration was utilised too, in order to have the same
construction as in the M.F. section. The AC/DC converter is a Graetz three-phases bridge
full-controlled with SCRs. In other cases a DC/DC chopper, with a working frequency of
about 5 kHz, which allows to reduce the DC filter size placed immediately after the AC/DC
VOLTAGE
CLAMPING
DRIVER DRIVER
HEATING UNIT ELECTRICAL
CABINET
PLANT
SUPERVISORY
COMPUTER
AC/DC
CONVERTER
CONTROLLER
AND START-UP
SEQUENCES
AC/DC CONVERTER
FULL CONTROLLED
H.F. INVERTER CONTROLLER
DRIVERS SUPPLY AND
PROCTECTIONS COORDINATION
CROW-BAR
LOAD
SYNCHRONISMS
THYRISTORS
R
S
T
Figure 4 - High frequency inverter block diagram.
converter. In the same time it assures operation times very fast and therefore a better
protection of the H.F. inverter. The principle scheme is shown in figure 4. There are two
auxiliary units, as inverter backing, the first is a crowbar, the second is a voltage clamping.
They have the scope of absorbing the overvoltages that appears at the ends of the IGBT when
there is a load anomaly. The H.F. resonant inverter suggests again the H bridge single-phase
configuration, each branch is composed by an IGBT and by a power diode in series
connection. Owing to reduced propagation times and high immunity to voltage gradients
between inverter driver and control cards, and the need of galvanic insulation, optical fibre
was utilised for the critical transmission of signals as IGBT driving signal and the protections
co-ordination ones.
The Heating Unit
The two high and medium frequency inverters are placed inside a movable strong structure
while the AC/DC converters are placed in an electrical cabinet constituted of modular
elements.
5 / 9
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INDUCTION FURNACE FOR HEAT TREATMENT OF AUSTENITIC AND/OR
FERRITIC STEEL WIRES AND STRANDS
This solution, specifically used for heat treatment strand or wire lines, has the following
advantages:
the electrical cabinet may be placed far from the heating unit since the power
connections are normal power cables, passed by DC current;
all power connections of both inverters (which require a water circulation closed circuit
distilled cooling) are placed in a very reduced space so that the furnace length is 4.5 m
only.
in particular the reduced distance between inverter and resonant circuit minimizes the
inductance of connections generally undesirable both at Medium and particularly at
High Frequency.
Automatic Control System
The automatic control system is based on an industrial PC with master function, connected
by means RS485 serial line to any slaves with microprocessor:
AC/DC digital control card
digital signals acquisition cards (running state, alarms and protections, trolley proximity
of the heating unit, breakdown strand sensor)
analog signals acquisition cards (speed and temperature strand).
The most important parameters (treatment temperature, delivered power, pull, stretching
etc.) are shown by means of graphics on PC. In case of plant stop is visualized the protection
tripped and appear the suggestion of how to solve the problem. For each wire or strand treated
is calculated the specific power to be delivered on the basis of cross section, specific heat,
treatment temperature and system efficiency. The delivered power is controlled by means of a
first feedback loop proportional to wire/strand speed. A second feedback loop makes the fine
control on the delivered power (limited to 5-10% of the maximum power in order to avoid
tracking jitters) on the basis of the temperature reading of an infrared pyrometer placed at the
coils exit. In figure 5 the components of the above described plant are shown.
Figure 5 - A view of the plant. In right side the electrical cabinet; in left side the heating unit,
in the middle the desk-control unit, in the lower left corner the heat exchanger industrial-
distilled water unit.
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INDUCTION FURNACE FOR HEAT TREATMENT OF AUSTENITIC AND/OR
FERRITIC STEEL WIRES AND STRANDS
4. INDUCTION COILS DESIGN
Frequency Selection
As known, the optimum frequency range for the heating of wires, which gives the
efficiency in the transfer of energy from the inductor coil to the wire, is given by the
relationship:
m
D
= =
2
2 5 4 5
. .
(1)
where
=
f
r 0
is the penetration depth, D is the wire diameter, is the electrical
resistivity,
r
is the permeability,
0
is the absolute one and f the exciting magnetic field
frequency. All these parameters are functions of the temperature and, in the ferromagnetic
case, both of the temperature and the exciting magnetic field intensity H
0
. Since the H
0
value
is linked to the throughput target, the permeability is a consequence and in this way is
determined the working frequency range of the induction furnace in order to do the heat
treatment for all size strands. In Table 2 are given productions and working data of an
induction furnace. The frequency values are calculated taking into consideration the heating
of single wires. Therefore, for magnetic wires, also frequencies between 1 and 2.5 kHz can be
used if the possible contact among wires is taken into consideration; however, in this case, no
particular economical advantages can be given by the reduction of the working frequency.
Table 2 - Summarising data of a throughput schedule in case of magnetic strands. H
0
and f
MF
are the magnetic field amplitude and the medium frequency value necessary to
obtain a good heat treatment.
Strand Output P
T
H
0
r
f
MF
Diam. Diam. Diam.
WIRE
(to 20C)
inches (mm) (mm) (Kg/h) (W) (A/cm) (-) (Hz)
3/8" 9.3 3.10 2578 133884 507 27.6 5028
7/16" 11.1 3.70 4187 217445 646 20.7 4706
1/2" 12.5 4.17 6176 320740 785 16.5 4648
6/10" 15.2 5.07 6013 312275 774 16.8 3088
7/10" 17.8 5.93 6215 322766 787 16.4 2313
A different situation arise when heating non-magnetic wires (e.g. inox). In this case, the
reduction of frequency corresponding to the values of Table 3 - calculated taking into
consideration the heating of single wire (column A) or the possible contact among wires
(column B) - allows the designer to obtain considerable economical advantages. Moreover, a
reduction of efficiency is acceptable in this case, taking into account the usually limited
annual production of non-magnetic wires, as compared to magnetic ones.
Table 3 - Optimal heating frequency for no-magnetic strand steel 18Cr8Ni;
20C
= 69.5
cm;.
400C
= 97.6 cm.
Strand f
AF
m m
WIRE
EQUAL
A B (to 20C) (to 400C)
inches mm mm (kHz) (kHz) - -
1/2" 4.17 11.0 182 26 3 2.53
6/10" 5.07 13.4 123 18 3 2.53
7/10" 5.93 15.7 90 13 3 2.53
7 / 9
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INDUCTION FURNACE FOR HEAT TREATMENT OF AUSTENITIC AND/OR
FERRITIC STEEL WIRES AND STRANDS
Coil Efficiency
The coil efficiency may be evaluated by means of the following formula:
=
+
1
1
2
COIL
r WIRE
i i
A k
P
(2)
where
COIL
is the resistivity of the coil,
WIRE
and
r
are the resistivity and permeability of
the wire; Ai, ki and P active power coefficients of inductor and load; coupling coefficient
between inductor and wire. In Figure 6 are shown the values for magnetic and non magnetic
strand.
3 4 5 6 7
1
10
100
mm
1
2
Figure 6 - Line 1 shows the typical efficiency of a coil when is a magnetic strand heated. Line
2 shows the efficiency for non-magnetic strand, the upper curve is referred to the heating
done at 50 kHz with good electrical contact between the wires; the lower curve is referred to
the heating done at 50 kHz with absence of a good electrical contact between the wires. In x-
axis is indicated the wire diameter that forms the strand.
50 kHz results to be the optimum frequency for the heat treatment over Curie point of the
pre-stressed steel wire. In fact in case of heat treatment of wire with diameter from 5 to 14
mm and heating temperature from 750C to 1000C the optimum working frequency is shown
in Table 4. As can be seen the 50 kHz frequency is good for wire with diameter 7 e 14 mm,
while results fair for 5 and 6 mm only.
Table 4 - Value of parameter m in case of over Curie heating for Ulbond wire at 50 kHz.
Electrical resistivity value at 750C equal to 100 cm, at 1000C is 111 cm.
f
- (mm) 5 7 9 11 14
f
AF
- (kHz) 50 50 50 50 50
m (to 750C) (-) 1.58 2.21 2.84 3.47 4.42
m (to 1000C) (-) 1.46 2.04 2.63 3.21 4.08
8 / 9
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INDUCTION FURNACE FOR HEAT TREATMENT OF AUSTENITIC AND/OR
FERRITIC STEEL WIRES AND STRANDS
REFERENCES
L.ARTUSO, F.DUGHIERO, S,LUPI, S.PARTISANI, P.FACCHINELLI: Installations for the Continuos
Induction Heat Treatment of Wire, UIE XIII Congress on Electricity Applications, Birmingham, 16-20 June
1996
J.REBOUX, B.LAPOSTOLLE, J.C.BRUNE: Contribubution du chaufagge par induction la modernisation
des lignes de traitement thermique dans les trfileries de fil dacier, Journes dEtude, Versailles, 5-6 Avril
1978
J.P.METAIL: Induction moyenne frquence applique au chauffage de fil - Etude des paramtres de
rendement nergtique, Journes dEtude, Versailles, 5-6 Avril 1978
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