Induction Furnace

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 dell’Artigianato 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 4th) 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 - 4÷6 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 isn’t 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 700÷720°C up to while the third is used in the temperature range from 700÷720°C to 950÷1000°C. 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 69344:1991 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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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.

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INDUCTION FURNACE FOR HEAT TREATMENT OF AUSTENITIC AND/OR FERRITIC STEEL WIRES AND STRANDS

Table 1 - The more used strands type list. Strand ∅ inches 3/8" 3/8"s 7/16" 7/16"s 1/2" 6/10" 6/10"s 7/10 Dyform 14.7 mm Dyform 18 mm Dyform 20 mm

mm 9.3 9.6 11.0 11.3 12.5 15.2 15.7 17.8 12.7 15.2 18

Area mm2 67.9 72.4 95.0 100.3 122.7 181.5 193.6 191.3 126.7 181.5 254.5

Weight Kg/m 0.408 0.432 0.557 0.590 0.730 1.090 1.180 1.504 0.890 1.295 1.750

Pitch mm 140 140 165 165 190 229 229 250 200 242 261

Speed m/min 112 112 132 132 152 110 110 85 110 94 74

Skip RPM 800 800 800 800 800 480 480 340 550 390 285

Pull Kg 5200 5200 6800 7200 8800 13500 14000 16000 13500 16000 20000

Strength Kg 8800 10200 12000 13800 16000 23900 26000 30000 20900 30000 38000

The Heat Treatment Once the strand is made, it has to be heat treated at low temperature (380÷400°C), 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 wire’s 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.

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INDUCTION FURNACE FOR HEAT TREATMENT OF AUSTENITIC AND/OR FERRITIC STEEL WIRES AND STRANDS 7. The two take-Up/pay-Off wind the complete skip’s 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 coil’s dimension, chosen from a quite vast series of barrel and flange diameters together with widths, while the first one is working, increasing enormously the line’s 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.

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8 7 3' 6 5 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 ELECTRICAL CABINET

HEATING UNIT

THYRISTORS AC/DC CONVERTER FULL CONTROLLED

LOAD

R S T

AC/DC CONVERTER CONTROLLER AND START-UP SEQUENCES

M.F. INVERTER START-UP SEQUENCES

DRIVER

DRIVER

SYNCHRONISMS

PLANT SUPERVISORY COMPUTER

M.F. INVERTER CONTROLLER DRIVERS SUPPLY AND PROCTECTIONS COORDINATION

Figure 3 - Medium frequency inverter block diagram.

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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. It’s 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 ELECTRICAL CABINET THYRISTORS AC/DC CONVERTER FULL CONTROLLED

HEATING UNIT

CROW-BAR

LOAD

R S T

AC/DC CONVERTER CONTROLLER AND START-UP SEQUENCES

DRIVER

DRIVER

VOLTAGE CLAMPING

SYNCHRONISMS

PLANT SUPERVISORY COMPUTER

H.F. INVERTER CONTROLLER DRIVERS SUPPLY AND PROCTECTIONS COORDINATION

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.

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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 industrialdistilled 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= where δ =

D = 2.5 ÷ 4.5 2δ

(1)

ρ is the penetration depth, D is the wire diameter, ρ is the electrical π f µr µ0

resistivity, µr is the permeability, µ0 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 H0. Since the H0 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. H0 and fMF are the magnetic field amplitude and the medium frequency value necessary to obtain a good heat treatment. Diam. inches 3/8" 7/16" 1/2" 6/10" 7/10"

Strand Diam. Diam.WIRE (mm) (mm) 9.3 3.10 11.1 3.70 12.5 4.17 15.2 5.07 17.8 5.93

Output

PT

H0

(Kg/h) 2578 4187 6176 6013 6215

(W) 133884 217445 320740 312275 322766

(A/cm) 507 646 785 774 787

µr (to 20°C) (-) 27.6 20.7 16.5 16.8 16.4

fMF (Hz) 5028 4706 4648 3088 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; ρ20°C = 69.5 µΩcm;. ρ400°C = 97.6 µΩcm. Strand ∅ inches 1/2" 6/10" 7/10"

∅WIRE mm 4.17 5.07 5.93

∅EQUAL mm 11.0 13.4 15.7

fAF

A (kHz) 182 123 90

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B (kHz) 26 18 13

m (to 20°C) 3 3 3

m (to 400°C) 2.53 2.53 2.53

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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

η= 1+ α

(2)

ρ COIL A i k i µ r ρ WIRE 2 P

where ρCOIL is the resistivity of the coil, ρWIRE and µ r are the resistivity and permeability of the wire; Ai, ki and P active power coefficients of inductor and load; α coupling coefficient between inductor and wire. In Figure 6 are shown the values for magnetic and non magnetic strand. 100

η

1 2

10

1

mm 3

4

5

6

7

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 xaxis 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 750°C to 1000°C 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 750°C equal to 100 µΩcm, at 1000°C is 111 µΩcm. ∅f fAF m m

(to 750°C) (to 1000°C)

(mm) (kHz) (-) (-)

5 50 1.58 1.46

7 50 2.21 2.04

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9 50 2.84 2.63

11 50 3.47 3.21

14 50 4.42 4.08

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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 à la modernisation des lignes de traitement thermique dans les tréfileries de fil d’acier”, Journées d’Etude, Versailles, 5-6 Avril 1978 J.P.METAIL: “Induction moyenne fréquence appliquée au chauffage de fil - Etude des paramètres de rendement énergétique”, Journées d’Etude, Versailles, 5-6 Avril 1978

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