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OPTIMIZATION OF PARAMETRES OF WELDING STEEL TUBES BY INDUCTION AT HIGHT
FREQUENCY
Mohamed Tahar HANNACHI a, Hamid DJEBAIL b
a
Départment of Mechanic engineering, University of Cheikh Larbi Tébessi, Algeria,
hannachimt2000@yahoo.fr
b
Départment of Mechanic engineering, University of Abbas Laghrou, Algeria
Abstract
Electromagnetic induction is a technique for heating electrically conductive materials (metals), commonly
used for many thermal processes such as fusion (welding) or hot metal. It is characteristic of generating heat
directly within the material being heated. This feature has many advantages over conventional methods of
heating more standard, including reduced heating times and high yields, or the ability to heat very locally.
The high power densities in play can achieve speeds of very rapid heating temperatures and heating time,
the magnetic flux and frequency are the parameters that govern this process.
Keywords: induction, parameters, heating, frequency, conductor, welding tubes.
1.
INTRODUCTION
Electromagnetic induction is a technique for heating electrically conductive materials (metals), commonly
used for many thermal processes such as fusion or hot metal. The induction is characteristic of generating
heat directly within the material being heated. This feature has many advantages over conventional methods
of heating more standard, including reduced heating times and high yields, or the ability to heat very locally.
The high power densities in play can achieve heating rates very fast [1].
Induction heating is a method of rapid and uniform heating for applications requiring production of paste or
change the properties of metals or other conductive materials. This method relies on electric currents
induced in a material to produce heat. Although the basic principles of induction are well known, recent
advances in technology semiconductor transformed induction heating method in a remarkably simple and
profitable applications for bonding, processing, heating or test materials. The electromagnetic induction
heating is based on two physical phenomena, electromagnetic induction and the Joule effect. Advantages of
this method are numerous: a zone controlled heating, a heating rate that can be extremely fast, low inertia,
direct heating of the room, a high performance, very good reproducibility [2]. High frequency induction
welding (HFIW) is a deformation welding process belonging to the electrical resistance welding segment,
which uses the material interface generated heat, caused by electrical current flux resistance (Joule effect)
and pressure application [3]. This process is widely applied in industrial longitudinal tube welding. Ferritic
stainless steels have been used to manufacture weld tubes through such process, particularly for automobile
exhaustion systems [4]. Although these are considered low weldability steels, during the last decades the
development of Nb and Ti-stabilized and low interstitial percentage steels improved weldability, which
increased the application of such materials [5]. HFIW equipments have a high degree of automation and are
compact, in general, and also demand a great initial investment [6]. Thus, welding studies, as well as
process operational parameters optimization tend to have high cost and material consumption.
2.
MATERIAL
The material used for welding of pipes is steel construction S235JR, whose chemical composition is shown
in Table 1. The grade is indicated by the letter E followed by a number corresponding to the minimum yield
tensile strength. Possibly a number 2, 3 or 4 indicating the quality [7]. If one looks at the issue metallurgy, we
can say that the steel metal construction are steel quality (some are of special steels) are generally not allies
(sometimes low alloy) delivered ready that is to say they already have the mechanical characteristics
expected and therefore it is not necessary for them to undergo a heat treatment
excluding the treatment of stress relieving (to reduce residual stresses) after welding, and sometimes
treatment of restoration or normalization. These steels whose carbon content varies with the desired
properties, contain (for different reasons) some elements other than carbon, namely Mn (Manganese), Si
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(Silicon), P (Phosphorus), Al (Aluminum), S (Sulfur) and other elements in small quantities (Nitrogen,
sometimes niobium and vanadium or titanium and zirconium) [8].
Tab. 1 Chemical composition of steel S235JR
C
0,10
Mn
Si
0,40
0,02
P
S
Al
0,003
0,007
0,042
Tab. 2 Equivalencies nuances between
NF EN 10025-2, NF 10025 and NF A 35-501
NF EN 10025-2 NF EN 10025
NF A 35-501
S235JR
S235JR
E 24-2
S235J0
S235J0
E 24-3
S235J2
S235JRG3
E 24-4
Tab. 3 Values calculated standardized characteristics of structural steels [9].
2
Modulus of Young
E
210 KN/mm
2
Modulus of slip
81 KN/mm
E
G
Coefficient of lateral contraction (Poisson)
Coefficient of thermal expansion
Charge volume (density)
2(1   )



0,3
-5
10 /°C
T
78,5KN/m
3
3.
PRINCIPLE OF WELDING INDUCTION
Physically, all electrically conductive body is the seat of power also known as eddy current, when immersed
in an electromagnetic field varying in time is a consequence of Lenzs law or law of moderation: The system
tends to oppose the changes imposed on him the flow of current induced in the resistive material by Joule
effect causes a temperature rise in the practice room in a heating induction heating component known as
load is immersed in an exciting current flowing through inductor in an operation of induction heating
temperature rise in the number varies more or less quickly depending on the amplitude of excitatory currents
and therefore the current induced temperature rise occurs also, more or less locally, according to the
frequency of the current source, the skin effect, and heating can be superficial or dandruff Welded pipe and
tube is made by longitudinally forming flat metal strip into a nearly complete tube and then welding the edges
together. To do this, the metal strip is fed into a forming mill that shapes the strip. As the strip passes
through the work coil, current is induced to flow in the strip, mostly in the edges (see Figure 1). The metals
resistance to the flow of electricity causes the edges to heat very rapidly to the melting point. While the
edges are still molten, they are forged together by the weld rolls, extruding the oxidized molten metal and
bonding the clean metal below it to complete the welding process (see figure 3). Optimal use of
ferromagnetic metals for the electrical design is always a compromise between these magnetic properties,
thermal and mechanical [10]. The strip passes between the rollers forming and bending steadily to become
an open tube in the longitudinal direction (see Figure 1). Before welding, the slit width is kept constant by a
roller or a blade to maintain geometrical constant during welding. The energy required for welding is supplied
to the tube (still open in the longitudinal direction) by a coil (inductor). This is done by induction, without
contact with the tube as shown in Figure 2. The inductor is traversed by a high current and must be cooled
by water. And HF induces a voltage in the open tube, which gives rise to the movement of a current whose
lines will contain the contact point edge (point welding). The net current along the edges cause the heating of
the latter on the section between the inductor and the welding point (see Figure 4). An additional heating to
the point of welding the tube leads spontaneously to the welding temperature. This heat is produced, among
others, by the output of the magnetic field outside the tube to the welding point and the concentration of
power lines to the right point of the weld itself [11]. The center of tubes edges are heated at least initially, but
they heat by thermal conduction (see figure 4) is that the currents along the surface.
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Fig. 1 forming progressive strip
Fig. 2 Welding of tubes induction [11]
Fig. 3 position of the coil
Fig. 4 The edges of the heated tube are pressed
Fig. 5 heating in surface and at center of edge of tube [12]
For industrial applications, two quantities characterize the thermal and energy efficiency of induction heating
 effect skin, which characterizes the distribution of induced currents in the room. This is the
alternating magnetic field that penetrates into the material decays rapidly disappearing and with it the
induced currents
 the power dissipated in the play that characterized the electrical phenomenon of many parameters
involved: the frequency of current, magnetic and thermal nature of the material, the field induces
coupling between the inductor and the workpiece to heat (air gap lengths) the type of inducing the
characteristic geometry (see figure 3).
The current distribution and the energy dissipated in the room can be rigorously determined using
Maxwell's laws reflecting the fundamental laws of electromagnetism in quasi-stationary regime.
4.
GEOMETRIC PARAMETERS OPTIMIZATION
4.1 The strip width and Inductive coupling and distance tube surface
Where strip is bent into the shape of a tube, its outer fibers are elongated so that its internal fibers are
compressed. The neutral axis is moved, the K factor expresses the degree of this displacement After
forming and welding, the tube is calibrated with standard dimensions.
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Fig. 6 Diagram for determining the parameter K.



5.
Fig.7 dimensions and coupling Tube – inducer.
A = D, la longueur totale de l’inducteur D doit être sensiblement égale au diamètre du tube A.
A ≤ C/2 <1,5A, la distance entre le bord avant de l’inducteur et le point de soudage.
1G ≤ B ≤1,5G, une distance courte entre le demi longueur de l’inducteur et le point de soudage
est en effet un facteur décisif pour amener rapidement les bords du tube à la température de
soudage et permettre ainsi une vitesse de production élevée.
ELECTRIC PARAMETERS OPTIMIZATION
5.1 Distribution of heat in the heating zone
The exact distribution of current in the cross section is described by a set of physical laws known as
Maxwell's equations; (eq. 1) and equations of Biot - Fourrier (eq. 2). The consequences for the first equation
are the current density, and the second equation is the heat capacity with thermal conductivity plays an
important role in heating (see figure 8). Maxwell's equations, complemented by the law of the Lorentz force
can make a complete description of all electromagnetic interaction [13].
Fig. 8 Diagram of transfer of electrical energy
2 is - j ( 2  f  σ is ) =
2
K T -
 Cp
dT
+Q=0
dt
(1)
(2)
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is the density of electrical current through the tube, f, frequency electric welding, μ the magnetic permeability
2
of the tube, σ the electrical conductivity of the tube, imaginary number j (j = -1), T temperature distribution K,
thermal conductivity of the tube, the tube density, Cp, heat capacity of the tube, Q, heat the tube The
parameters that directly govern the process of induction heating. High frequency current has two very
unusual features that distinguish it from 60 Hz line current.
First, line frequency current flows through the entire conductor where HF current flows only on the surface of
the conductor. This is called the skin effect. Second, when two conductors carrying HF current are placed
close to one another, the current concentrates on the two adjacent surfaces of the conductors. This is called
the proximity effect [14].
5.2 Induced currents and the depth of penetration
The basic concept that governs the phenomenon of induction. More supply frequency f increases, the
induced currents are concentrated at the surface. According to Lenzs law, the meaning of Eddy Current (eq.
3) is always opposite to that of the current inductor. As the inductor current, the eddy creates ran the
alternating magnetic field. The two fields, with their opposite directions, cancel each other partially intérieur
metal. Only near the edges it is a resultant field: one speaks of effet skin (eq. 4) [15]. The exact distribution of
current density in part depends on the characteristic physical heated material, shape, form and position of
inducteur, level and frequency of current in the inductor
is  e1/ d
d



0
(3)
(4)
r
f
ρ, electrical resistivity (Ω.m)
d, Depth of penetration [m]
0 ,Permeability magnetic empty
 r , Relative magnetic permeability
f , frequency [Hz]
The depth of penetration increases with increasing temperature and this increase is particularly sharp at the
temperature above the Curie point (768 ° C) by following the passage of the steel of the ferromagnetic state
to paramagnetic state even time [16]. Hile the resistivity and magnetic permeability are characteristics of
body heat, the frequency is a quantity that can be chosen by utilisateur. It has a means for controlling the
dissipation inside the body to heat and to choose the most suitable heating.
5.3
The electrical resistivity
The depth of penetration is proportional to the square root of the resistivity of the armature. This, for metals,
is generally believed with temperature. To model the evolution of the electrical resistivity, we own both
models, developed from measurements [17]
(5)
  0 (1   (T  T0 ))
With
0 Initial electrical resistivity at 20 ° C (Ω.m) ; T, temperature (° C) and To, initial temperature
5.4 Power supply
Is given by solving the Maxwell equation (1) electric
Pu  2T .C p . .(d / 2)  e.V0 
TC p eV0
f
Where
e, thickness of the tube (m)
V0, welding speed (m / min)
Y0, the length of the welding V (m)
∆T,the elevation of temperature for forging welding material (°C)
(6)
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Density of steel tube 
Cp, heat capacity of the tube
,
5.5 The magnetic flux
The magnetic flux Φ passing through a surface is equal to the number of lines of force of magnetic induction
field that penetrates a surface. It is the scalar product of two vectors
(7)
θ is the angle between field lines and the induction vector normal to the surface S. Thus, if the surface is
perpendicular to field lines, this angle is 0 and its cosine is 1, the flow is maximum. Lenzs law states that if a
flux variation dΦ (t) appears in a frame consisting of an electrical conductor, an electromotive force e (t)
appear at the terminals of this framework. This electromotive force is created to oppose the flux variation in
[18]. The unit of magnetic flux is the Weber (Wb), in honor of German physicist Wilhelm Eduard Weber
(1804-1891).
 A   TCP V0 
B   V  0 

 Aimp    y0 f  
(8)
Electromagnetic induction is the vector of heat transfer from the source to be heated. The transfer of energy
to heat the object is created by electromagnetic induction. When a loop of conducting material is placed in a
magnetic field, one sees the edges of the loop voltage induced. When the loop is short-circuit induced
voltage E will result in the emergence of a current short-circuit current flowing in the opposite direction to the
phenomenon that generates it. It is the law of Faraday Lenz. If a driver (eg a cylinder) is subjected to a
variation of magnetic flux (or placed in an alternating magnetic field), are emerging, as in the case of the
closed loop, the induced currents. These currents are called eddy currents and circulating a nonhomogeneous in the cylinder. The eddy currents through the electrical resistance of the cylinder, just heat
the driver in accordance with Joule's law.
Fig. 9 Effect of skin according to the current density Fig. 10 penetration depth depending on the frequency
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Fig 11 Variation of resistivity with temperature
Fig. 13 Frequencies according to Powers for three
Thickness;
Fig. 12 Power as a function of temperature
-1
at constant speed (V0 = 30 m.min , e = 3m
Fig. 14 Temperature function of time
6.
DISCUSSIONS
The induced current decreases exponentially, then we define a depth of penetration or skin depth d, where
the induced current is reduced by 1 / exp (approximately 0.37 or 37% is) is where is the value of the current
induced on the surface (see Figure 9). So there is concentration of the heat effect in this layer represents
87% of the energy dissipated expression of d, depending on the used frequency f is given by equation. The
skin effect is tendency of an alternating electrical current to circulate in a conductor so that the current
density near the surface of the conductor is greater than its core. It causes the effective resistance of the
conductor increases with frequency current. It is proportional to the square root of the resistivity of the
armature. This, for metals, is generally believed with temperature. Figure 8 illustrates the electrical reference
depth (skin effect). The high frequency current does not spread in the cables as the DC or low frequency.
Instead of using the entire section of the driver they are confined in the layers near the surface of the
conductor. The current density decreases exponentially as and as the distance from the surface
The electrical conductivity (reciprocal of electrical resistance) is a phenomenon that is explained by the
existence of free electrons, moving with great speed in a metal material subjected to electric voltage.
Electrons during their journey through the metal collide with ions or atoms of the grid. The electrical
resistivity, being dependent on the temperature of the material is more important than for the mean free path
given the number of collisions is high. The evolution of the overall electrical resistivity of the material is to
model the temperature fields on all surfaces of the material. The time of heating up the melting temperature
is extremely short (only 0.1 s for an average speed of 30 m / min and a thickness of 3 mm), the temperature
gradient must be extremely high with increasing power. The power is very sensitive to changes in speed of
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the welder, and for double speed, it is only increased by a factor that is to say only 41% of useful power.
Obviously in this example, the thermal mode is 1.414 times more effective than the method of electric power.
results for the powers of a tube made of a specific size three-speed welding and welding different
frequencies of 100 to 425 kHz. The welding speeds are increased by 30 m / min. 60 m / min. then to 90 m /
min. At slow speeds, the frequency has a small effect on the useful power welding By cons at high speeds,
the useful power range from approximately 25% between 200 kHz and 400 kHz
The material to be heated plays a role in reducing the power for different frequencies, this is due to the
increase of heating time and hence the crossing of significant thickness, this to a starting temperature of
heating. Heating the material regulates the temperature increase versus time is legible. The specific heat is
an example of this, the Curie temperature; the material goes through a phase transformation by using energy
as the molten steel requires much energy. In reality, this happens in a short interval of temperature. We have
matched this with another distribution almost linear (see Figure 13). Therefore, the temperature continues to
rise in this interval, continuing to be more or less constant until enough energy is absorbed to melt steel.
Therefore, induction heating has the appearance of a process very short in heater
The effect of frequency on the magnetic flux of impeder is interpreted by two methods, the first concerns the
evolution of the frequency with the reduction of flows necessary to transfer the same amount of power, the
second reason is when frequency welding is reduced, the transition process of thermal mode to mode of
electric power's demands more power for welding the same geometric data of the tube and the same
welding speed. Increased power transfer requires more flux in the magnetic core, the power increases with
time, which gives the property of heating at high frequency short duration and thus facilitate the transition to
direct temperature melting process the temperature above the melting temperature have practical
significance metal in this case fall.
7.
CONCLUSION
Induction heating is a technique well adapted to heat treatments in metallurgy. However it is necessary to
properly size the inductors to have an optimal process. Given the number of parameters calculated and
taken into account in this study, it seems that this phenomenon is heating a little clearer, the need is
adequately supplemented by use of digital technology. The example of solder in pipes at high frequency is
one method of induction heating high frequency used for the production of pipes soet pipe mills, and most
profitable in terms of cost. Since profit margins are narrow in the industrial tubes and pipes, even a small
performance improvement can be a big difference compared to the results, it will not only direct economic
costs of energy, but an improvement quality and productivity as well. recent advances in technology of
semiconductors have transformed the induction heating method in a remarkably simple and profitable
applications for bonding, processing, heating or test materials
ACKNOWLEDGEMENT
We wish to thank Miss Sam and all researchers in the world.
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