FIELD OF THE INVENTION:
The present invention relates to a portable apparatus for demagnetizing of steel
tubes before butt welding and also a methodology for removing residual
magnetism by decreasing current.
BACKGROUND OF THE INVENTION:
Heavy engineering corporations use various alloys of steel as raw materials for
their products. These raw materials are formed into finished goods by processes
like cutting, welding, cold and hot forming. Welding is an important process
that fuses similar or dissimilar jobs of metals. Alloy steel tubes of shorter
lengths are welded together to form long serpentine coils or panels which then
form an integral component of a boiler in a thermal power plant. Straight Tube
Butt Welding (STBW) machine welds tubes of lengths 10 metres to 12 metres to
form coils of lengths ranging from 90 metres to 110 metres. It employs metal
arc welding process, wherein a stationary torch welds two tubes rotating in a
lathe type machine. MIG (Metal Inert Gas) or TIG (Tungsten Inert Gas) welding
methods are usually employed in the STBW machine. In both the processes, an
arc is generated between the welding electrode and the parent metal. This arc
melts the filler wire which gets deposited in the valley formed by two opposite
chamfered edges and fuses the tubes together.
Alloys of steel are susceptible to magnetism. They gain magnetic properties
during their manufacturing or from exposure during their storage or transit. In
the case of tubes used for boiler pressure parts, magnetism is generally
induced if the material is left buried in the earth's magnetic field over time or by
residual magnetism left by magnetic inspection or by simple actions such as
plasma cutting or material stress. The magnetism in a material is measured in
air at the end of the tube and the unit of measurement is Gauss (Gs). It may
only be a few tens of Gauss. However, when two tubes with residual magnetism
are fitted up for welding, the field becomes concentrated in the gap between the
two tubes and increases in magnitude.

Arc deflection, or in many cases, extinction away from the point of welding due
to magnetism, is generally referred to as (magnetic) arc blow. This causes a
displacement of the arc relative to the joint. The continuing displacement of the
arc can cause poor quality welding or even cause weld defects. The effect occurs
because of the interaction between the magnetic field of the welding arc and the
field of the residual magnetism. Arc blow depends on the direction and
magnitude of the residual magnetic field. Magnetic arc blow causes very
expensive project delays as it results in the failure of the welding joint. The
magnitude of the residual magnetic field in the welding zone should measure
less than 30 Gauss (Gs) to ensure a high-quality weld. ISO 3183:2007(E)
requires that four readings shall be taken ≈ 90° apart around the circumference
of each end of the pipe and the average of the four readings shall be under 30
Gs, and no one reading shall exceed 35 Gs when measured with a Hall-effect
gaussmeter or equivalent values when measured with another type of
instrument. Any tube or pipe that does not meet the above requirements shall
be considered defective.
There are basically two ways to avoid magnetic arc blow. First is to remove the
magnetism altogether in the entire length of the tube. Second way is to reverse
the local magnetism using externally applied fields. To remove the magnetism
completely, the material has to be heated to the Curie Point temperature,
typically 1000 deg. C and allowed to cool in a zero field. For small components
this is feasible, but for large tubes or pipes, the energy cost and magnetic leak
back make it impractical. The second technique requires that the tube is placed
in an Alternating Current (AC) field that is slowly reduced to zero. The process,
however, is very slow. It may not be effective at all if the strength of the
demagnetizing field is too low. Finally, most degaussing systems operate at line
frequency (50/60 Hz), which has a poor skin penetration into the material, so
the process may be successful in degaussing the top layer, but underneath the
magnetism persists. Low frequency degaussing guarantees penetration into the
material.

In the prior arts, demagnetization is carried out either by supplying current
pulses of alternating polarities in a constant duration or by reducing the
current to zero, operating at line frequency. In the US Patent No. US4158873,
the inventors have disclosed methods of demagnetization by passing tubes
through a coil assembly through which pulses of current are applied to produce
magnetic field of alternating polarity and of constant durations and magnitude.
The apparatus described is bulky and not easily portable. Moreover, any
miniscule mismatch in either the magnitude of current pulse or the duration
will not remove the magnetism completely. In the US Patent No. US4360854,
demagnetizing is done again by reversing the polarity in a series of steps while
reducing the current to zero in successive steps. The apparatus described is
also bulky and requires prior knowledge of the degree of magnetism in the
material. Also, a complex control system may be required to control,
synchronize and vary the pulse frequency, alternating polarity and repetition
rate.
Hence, there is long felt need to develop an apparatus and method for couple
demagnetization of the tubes before subjected to butt welding process, where
the residual magnetism can be entirely removed.
SUMMARY OF THE INVENTION:
A demagnetizer apparatus for removing residual magnetism from steel tubes in
welding machines comprises a bobbin placed over the tube to be demagnetized;
copper coils, which are concentrically wounded over said bobbin and carries
the electrical current; a controller for supplying the varying input current which
is constantly diminishing in amplitude; a non-contact type inferred sensor
provided on the bobbin; a pair of chucks on which the tubes are mounted;
chamfered edge of said tube where the welding deposition takes place;
characterized in that when the controller is activated, the current is supplied
through the copper coil mounted over the bobbin in decreasing amplitude and
thereby the tube gets demagnetized substantially in short duration, irrespective
of the direction of the magnetic field present therein.

OBJECTS OF THE INVENTION:
It is therefore, the principal object of the present invention to provide an
apparatus for demagnetization of tubes for complete removal of magnetic
properties before butt welding.
Another object of the present invention to provide an apparatus which
demagnetize the tubes completely irrespective of the direction of the inherent or
induced magnetic fields or the polarity of the tubes.
Yet another object of the present invention to provide a method of
demagnetization of tubes before butt welding process, which can be carried out
without any problem of magnetic are blow.
Further object of the present invention to provide a method of demagnetization
of tubes, which is fast and accurate.
Another object of the present invention to provide an apparatus which is
portable, easy to handle and use.
Yet another object of the present invention to provide an apparatus which can
ensure defect free tube to tube butt welded joint.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWING:
It is to be noted, however, that the appended drawings illustrate only typical
embodiments of the present subject matter and are therefore not to be
considered for limiting of its scope, for the invention may admit to other equally
effective embodiments. The detailed description is described with reference to
the accompanying figures. Some embodiments of system or methods in
accordance with embodiments of the present subject matter are now described,
by way of example, and with reference to the accompanying figures, in which:
Figure 1 illustrates the pre-butt-weld demagnetizer apparatus along with the
controller.
Figure 2 illustrates the apparatus in use at the Straight Tube Butt Welding
machine.

Figure 3 illustrates the voltage vs time characteristics of the apparatus
operation during demagnetization.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS:
The subject matter disclosed herein relates to a novel demagnetizer apparatus
for removing residual magnetism from steel tubes before welding in butt
welding machines. The tubes are completely demagnetized in the heat affected
zone of the welding process.
As explained above, the demagnetizer apparatus has many components, such
as a hollow and cylindrical shaped bobbin (1), copper coils (2) are concentrically
wounded over the bobbin (1) and a controller (3). The copper coils (2) carries the
electrical current which is variable. The variation of current is controlled by the
controller (3). The controller (3) supplies electric current which is constantly
decreasing in amplitude. When the current from the controller (3) reaches zero,
the tube gets completely demagnetized.
Fig 1 illustrates the demagnetizer apparatus of steel tubes comprises of a
hollow, cylindrical shaped bobbin (1) which is made up of any PVC material,
copper coils (2) concentrically wound over the bobbin (1) and a controller (3)
which supplies and also controls electrical flow is in constantly in decreasing
amplitude.
The diameter [ϕ] of the hollow bobbin (1) is chosen so as to enable the bobbin
(1) to easily seat on the outer surface of tubes of different outer diameters (OD).
The thickness of the copper wire is selected so that it can easily carry the
designated current from the controller (3).
The number of turns [η] of the coil (2) depends of length [β] of the bobbin (1)
and thickness of the copper wire.
The length [β] of the bobbin (1) is chosen such that it covers the entire heat
affected zone (HAZ) of the welding process as the complete removal of residual
magnetism is the utmost important for proper welding.

In one implementation of the present subject matter, there is provided a pair of
copper power cables (4) on the controller (3) through which the copper coils (2)
receives the current.
A non-contact-type Infrared (IR) sensor [5] is provided on the bobbin (1). This
sensor [5] completes the safety of the apparatus by ensuring that the system
cannot be switched on without the presence of the tube. Switching on the
controller [3] without a tube present in the hollow bobbin (1) causes the coils (2)
to overheat which is prevented by the sensor (5). The IR sensor [5] provides a
“tube-present” feedback signal to the controller (3) via a coaxial communication
cable [6].
Fig 2 illustrates a pre-butt-weld demagnetizer apparatus at the straight tube
butt welding machine. The tubes are placed in a pair of chucks (8) which
facilitates holding the tubes (7) at a fixed position and also controls the rotation
of the tubes (7) in a desired direction such as clockwise or anti clockwise at
welding deposition. The chamfered edge (9) is the heat affected zone subjected
to demagnetization prior to welding. As the welding machine holds a pair of
tubes, the demagnetization is carried out separately for two tubes.
At first, the demagnetization of first tube takes place where it is held by one
chuck and after that the apparatus is mounted over the other tube held by the
second chuck.
Further, fig 2 also illustrates a sliding door (10) which protects the operator
from heat and light generated during the welding process. UV protective glass
[11] on the sliding door [10] offers a protective view of the entire process to the
operator.
In accordance to the other embodiment of the present subject matter, there is
provided a controller (3) for supplying the electric power to the coil mounted
over the demagnetizing head of the bobbin (1) (as illustrated in fig 1).
The current and voltage passed through the coils [2] are constantly decreased
in magnitude by the controller [3]. This is achieved by decreasing the operating

frequency of the supply. The thickness of the tube to be demagnetized serves as
the initial set point frequency. Starting frequency is kept higher for thicker
tubes. This is to achieve better skin penetration into the material.
The controller (3) also comprises a LCD screen (12) wherein the operating
frequency is displayed for monitoring.
When the start button (13) switches on, the coils (2) get charged with current of
designated frequency which can be seen on the screen (12).
By switching on the stop button (14), the demagnetization process starts
gradual decrease in frequency and consequently the current and voltage also
turns zero. When the screen (12) shows frequency as 0 Hz, the demagnetization
process of tube is complete.
Figure 3 shows the voltage vs time characteristics of the apparatus operation
during the demagnetization process. The time in seconds is plotted on the X-
axis and voltage in volts is plotted on the Y-axis. [t1] indicates the time of
starting the demagnetization process. The process gets completed at [t2]
seconds. Depending on the thickness of the tube to be demagnetized, the total
cycle time varies from 20 seconds to 30 seconds. Initially, the voltage supplied
to the coil is kept at zero. At [t1], the voltage is ramped up to the designated
value [V1]. [V1] varies from 41.5 volts to 166 volts, again depending the tube
thickness. The voltage is held at [V1] for some time to counter the effect of
residual magnetism and then gradually decreased to zero. At [t2], the
demagnetization process is complete and we get a tube free of residual
magnetism in the HAZ.
The non-limiting advantages of the present invention are given below:
1. The demagnetizer apparatus uses a cylindrical shaped bobbin (1) or head
which seats over the heat affected zone of the magnetized tubes.
2. The copper wires of the pre-butt-weld demagnetizer apparatus are
concentrically wound over the bobbin (1) which carries the power used for the
demagnetization process.

3. A controller (3) of the pre-butt-weld demagnetizer apparatus actuates the
current and voltage input to the coils by varying the operating frequency.
The bobbin (1) is placed over the chamfered edge (9) of the tubes to be welded.
The height of the bobbin (1) covers the length of the heat affected zone (HAZ) of
the welding process. The bobbin (1) can be mount over the tubes irrespective of
the direction of the inherent or induced magnetic field present in the tube.
Once the bobbin (1) is in place, the controller (3) is activated. The controller (3)
is powered by 230 V AC power supply. Switching on the controller (3) supplies
the coil over the bobbin (1) with a current of constantly decreasing amplitude.
This is achieved by decreasing the frequency of the input current. The starting
frequency varies from 20 Hz to 5 Hz depending on the thickness of the tube.
The input voltage varies from 41.5 V to 166 V at the start of the
demagnetization process. The entire process takes 20 to 30 seconds. This
constantly decreasing amplitude of the current ensures that the tubes gets
demagnetized irrespective of the direction of the magnetic field already present
in it. Same process is carried out on the second tube. The demagnetized tubes
are then sent to the STBW machine for butt welding process.
Butt welding process can then be carried out without any problem of magnetic
arc blow. The apparatus works to demagnetize the tubes irrespective of the
direction of the inherent or induced magnetic fields in the tubes. This
apparatus ensures a defect free tube-to-tube butt welded joint.

WE CLAIM:
1. A demagnetizer apparatus for removing residual magnetism from steel tubes
in welding machines comprises a bobbin (1) placed over the tube to be
demagnetized;
copper coils (2), which are concentrically wounded over said bobbin (1)
and carries the electrical current;
a controller (3) for supplying the varying input current which is
constantly diminishing in amplitude;
a non-contact type inferred sensor (5) provided on the bobbin (1);
a pair of chucks (8) on which the tubes are mounted;
chamfered edge (9) of said tube where the welding deposition takes place;
characterized in that when the controller (3) is activated, the current is
supplied through the copper coil (2) mounted over the bobbin (1) in decreasing
amplitude and thereby the tube gets demagnetized substantially in short
duration, irrespective of the direction of the magnetic field present therein.
2. The demagnetizer apparatus for removing residual magnetism as claimed in
claim 1, wherein said bobbin (1) is hollow and cylindrical shaped.
3. The demagnetizer apparatus for removing residual magnetism as claimed in
claim 1, wherein the diameter of said bobbin (1) is selected in accordance to the
outer surface of tubes of different outer diameters.
4. The demagnetizer apparatus for removing residual magnetism from steel
tubes in welding machines as claimed in claim 1, wherein said copper coils (2)
receives current from the controller (3) through a pair of copper power cables.
5. The demagnetizer apparatus for removing residual magnetism as claimed in
claim 1, wherein said IR sensor (5) provides feedback for presence of tubes to
the controller (3) through a coaxial communication cable.

6. The demagnetizer apparatus for removing residual magnetism as claimed in
claim 1, wherein the controller (3) further comprises a LCD screen (12) for
displaying the operating frequency.
7. The demagnetizer apparatus for removing residual magnetism as claimed in
claim 1, wherein the chucks (8) hold the tubes in fixed position and also
facilitates clockwise or anti clockwise rotation.
8. The demagnetizer apparatus for removing residual magnetism from steel
tubes in welding machines as claimed in claim 1, wherein the number of turns
of the coil over bobbin (1) is dependent on the thickness of the copper wire.
9. A method for complete demagnetization comprising the steps of:
placing a bobbin (1) over the chamfered edge (9) of the tubes to be welded
where the bobbin (1) is already wounded concentrically by copper wires;
activation of controller (3) by switching it on with a suitable starting
frequency and voltage;
continuous supply with current of constantly decreasing amplitude to the
coils for substantially short duration;
monitoring the frequency of current by displaying it on LCD screen (12)
and when the frequency, current and voltage shows zero, the demagnetization
process stops.
10. The method for complete demagnetization as claimed in claim 9, wherein
the controller (3) is powered with 230V AC power supply.
11. The method for complete demagnetization as claimed in claim 9, wherein
the starting frequency is between 20Hz to 5 Hz and input voltage is 41.5V to
166V.

12. The method for complete demagnetization as claimed in claim 9, wherein
the duration of the demagnetization process is 20 to 30 seconds.