FIELD OF INVENTION
This invention relates to a method of and an improved system for carrying - out
Magnetically impelled arc butt welding process for welding of boiler tubes
BACKGROUND OF THE INVENTION
Magnetically impelled arc butt (MIAS) welding is an effective alternative
technique to resistance, butt, flash, induction pressure and friction welding
processes.MIAS is capable to reduce welding time by up to 90%, to cut out the
need to rotate, the weldable components, and to weld circular or non-circular
components with the same ease. Any welding which reqUires angular
orientations can be successfully achieved. The process can be used to weld
steels, stainless steel, aluminium alloys, etc.
MIAS welding uses a magnetic field to rapidly scan a welding arc along a joint.
This feature of the MIAS process makes it one of the fastest methods of welding
a tube to tube, and a tube to flange welding. This welding process offers large
cost saVingsfor a range of joint configurations. Mobile MIAS machines can be
used on site, thereby allOWingwelding of transmission pipelines with improved
productivity over other arc welding processes.
The MIAS - process is reported to have so far been applied only for welding of
automobile components in Germany and UK and only for welding of gas / oil
pipeplines in USA. However, MIAS process due to its other associated
disadvantageshas not yet been employed for welding of boiler tubes. United States Patent Application 20070108171 discloses a method and a vehiclemounted
apparatus for laying pipelines in which adjacent pipe sections are joined
by the technique of magnetically impelled arc butt welding (MIAS). A MIAS
welding head having a welding axis is mounted on a transportable platform. A
pipe guide capable of engaging a welded pipe string is located rearwardly of the
welding head. The pipe guide maintains alignment of the string with the welding
axis. The apparatus includes pipe feed means maintaining alignment of a next
pipe to be laid with the welding axis and the pipe string. The platform is
rotatably mounted on the vehicle and tilts around a horizontal axis. The vehicle is
provided with pivoting steerable tracks. However, such an apparatus can not be
adapted for welding of boiler tubes due to its constraints in the application
intended for.
United States Patent 7161115 describes a method and a vehicle-mounted
apparatus (30) for laying pipelines in which adjacent pipes (17, 28) are joined by
the technique of magnetically impelled arc butt welding (MIAS). A MIAS welding
head (13) haVing a welding axis is mounted on a transportable platform (35). A
pipe gUide(50) capable of engaging a welded pipe string is located rearwardly of
the welding head (45). The pipe guide (50) maintains alignment of the string
with the welding axis. The apparatus includes pipe feed means (47, 48, 49)
maintaining alignment of a next pipe to be laid with the welding axis and the
pipe string (17). The platform (37) is rotatably mounted on the vehicle (30) and
tilts around a horizontal axis. The vehicle (300) is provided with pivoting
steerable tracks (38). However, the prior art still is silent on adaptation of MIAS - welding for welding
of boiler tubes, in particular high thickness boiler tubes due to non-availability of
a suitable MIAS-capable machine and a corresponding process.
OBJECTS OF INVENTION
It is therefore, an object of the present invention to propose an improved
modified MIAS welding system for welding of thick walled boiler tubes.
Another object of the invention is to propose a MIAS welding process for boiler
tubes which produces better weld quality than that of the induction pressure
welding and flash butt welding.
A still another object of the invention is to propose a MIAS welding process for
boiler tubes which is enabled to an arc rotation by means of at least two
permanent magnets.
Yet another object of the invention is to propose a MIAS welding process for
boiler tubes, which is capable to produce an uniform and consistent weld joint
between the tubes and solid rods.
A further object of the invention is to propose a MIAS welding process for boiler
tubes, which produces a defect free tube to tube joints. A still further object of the invention is to propose a MIAS welding process for
boiler tubes, which reduces the cycle time for welding compared to conventional
flash butt welding or induction pressure welding.
Yet further object of the invention is to propose a MIAS welding process for
boiler tubes, which is capable of introducing an arc rotation which reduces the
total energy consumption.
Another object of the invention is to propose a MIAS welding process for boiler
tubes, which is capable of welding small diameter pipes too.
SUMMARY OF THE INVENTION
Accordingly, there is provided an improved Magnetically impelled arc butt
welding process, in which a MIAS system is adapted. The invention employs the
arc rotation by an interaction between a set of permanent circular magnets and
the welding current is passed between the tubes to be welded. Accordingly to
the inventive process, it is possible to achieve uniform and consistent weld joint
between the tubes and solid rods. It is further possible to get a defect free tube
to tube joints. The cycle time for welding is reduced as compared to the
conventional flash butt welding or induction pressure welding. The arc rotation is
introduced which reduces the total energy consumption as compared to flash
butt welding process employed for welding of thick walled boiler tubes. The
tubes joints have high efficiency (time of welding of tube of 46 mm OD 6 mm
wall thickness is approx. 35 s) and a low cycle time for welding. It is possible to
achieve narrow heat-affected zone (3-5 mm). Metal structure is fine grained inthe joint zone and no grain coarsening is found. The strength and ductility
properties of the welding joints remains at level of the base metal. In the tube
joint, there is no pore, extraneous inclusions or other volume defects, which are
typically present for other fusion welding processes. There is an insignificant
dimension (not more than half of the pipe wall thickness) of the outer and inner
reinforcement which is uniformly distributed around the perimeter being welded.
In the invented process, it is easy to control and monitor the welding
parameters. The inventors through extensive research have observed that the
Flash Butt Welding (FBW) lends itself easily to automation. The improved FBW
machine does not reqUire a highly skilled operator. The arc rotation introduced
according to the method reduces the total energy consumption as compared to
any other resistance welding process available for welding of thick walled tubes.
The inventive process can be implemented for welding of small diameter pipes
too.
BRIEF DESCRIPTION QF THE ACCOMPANYING DRAWING
Fig 1 shows an improved modified MIAB welding system according to the
invention.
DETAILED DESCRIPTION OF THE INVENTION
As shown in Fig. 1, the improved MAIB system comprises a welder unit (1), a
welding power source (6), a Pumping unit (3), a Controller (4), and a remote
controller unit (5). The welding power source (6) provides the welding current.
The pumping unit (3) gives necessaryforce for clamping the weldable boilertubes, and for upsetting the force. The magnets (2) provide the arc rotation. The
controller(4) controls the welding processtiming and sequence.
Magnetically impelled arc butt (MIAS) welding is a pressure butt welding
technique where heat is generated prior to forging by a rapid movement of an
arc over the ends of the weldments by a magnetic field. The arc is drawn
between the adjacent ends of the two weldments and is rotated around a weld
line by a force resulting from the interaction of the welding current and the
magnetic field. Two constant magnets of opposite polarity are arranged and the
radial component of magnetic induction gives the required field.
The improved MAIS system :
- a Welding power source (6);
- a Welder unit (1);
- a Hydraulic pumping unit (3);
- a Controller (4);
- a Remote control Unit (5); and
- at least two permanent Magnets (2). The boiler tubes (not shown) to be welded are clamped in the welder unit (1).
Welding current from the source (6) is passed through the tubes to be welded.
An arc is generated between the two tubes. This arc is rotated around the
periphery of the two tubes due to the interaction between the welding current
and the magnetic field as per the right hand thumb rule. Within a predetermined
time, the edges of the tubes become red hot and the forging is
initiated. During upsetting of the force, a high current is passedthrough the weld
joint to eliminate any molten metal trapped between the two tubes.
WE CLAIM
1. A method of carrying - out magnetically impelled arc butt welding (MAIB)
of boiler tubes, comprising the steps of :
(a) providing an improved system for carrying - out MAIB of boiler
tubes;
(b) clamping the weldable boiler tubes in the welder unit;
(c) passing an welding current through the boiler tubes;
(d) generating an arc between the boiler tubes;
(e) rapidly rotating the arc around a weld line by a force resulting from
an interaction of the welding current and a magnetic filed produced
by the radial component of magnetic induction due to disposition of
at least two permanent magnets of opposite polarity;
(f) implementing the forging step immediately after the boiler tube
edges becoming red-hot subsequent to step (e); and
(g) upsetting the high current and allowing it to pass through the weld
joint to eliminate any molten material if trapped between the two
welded boiler tubes. 2. An improved system for carrying out magnetically impelled arc butt
welding (MAIS) of boiler tubes, comprising:
- a welding power source (6) providing welding current which is passed
through a welder unit (1) having clamped thereon the at least two
weldable boiler tubes;
- means for generating an arc between the at least two weldable tubes;
- at least two permanent magnets (2) of opposite polarities disposed so as
allow the radial component of the magnetic induction to produce a
magnetic field;
- rapidly rotating the arc around the periphery of the tubes by means of the
force produced due to an interaction between the welding current and the
magnetic field;
- forging the two tubes via their red hot edges produced due to the rapid
rotation; and
- upsetting the high current to eliminate the molten metal trapped between
the at least two welded tubes.
3. A method of carrying - out magnetically impelled arc butt welding (MAIS)
of boiler tubes, as substantially described herein with reference to the
accompanying drawings. 4. An improved system for carrying out magnetically impelled arc butt
welding (MAIB) of boiler tubes, as substantially described herein with
reference to the accompanying drawings.