Title of Invention

AN IMPROVED METHOD OF ROOT PASS WELDING IN BUTT JOINTS WITHOUT BACKING BY DOUBLE WIRE MAG WELDING WITH SPECIAL JOINT PREPARATION AND FIT UP

Abstract The present invention relates to an improved method of root pass welding in butt joints without backing by double wire MAG welding with special point preparation and fit-up and the method consists of holding the pipe ends (2,2) on holding chucks (3,4) in a welding lathe (1) to carry out the welding operation in mechanized mode, providing near the lathe a support and holding/adjusting clamps (5) to position the torch assembly (7), then keeping the wire feed units at proper height to facilitate smooth feeding of wire and connecting the wire feed units to power sources and gas supply system (8) when the double wire torch with gas supply system (8) when the double wire torch with gas shielding is held at suitable angle with respect to joint to carry out the welding operation, wherein a stable arc is maintained with uniform welding bead and without creating any burn-through defects.
Full Text FIELD OF INVENTION:
The invention relates to an improved method of root pass welding in butt joints without
backing by double wire MAG welding with special joint preparation and fit-up. More
particularly, the invention relates to a method of root pass welding with appropriate
setting of welding apparatus including placement of torch to ensure proper fusion of the
edge at the root of pipe joints.
BACKGROUND OF THE INVENTION:
One of the most common type of welds found in industry, especially for industrial pipe
fabrication, is the open butt joint weld. Such a type of weld is most often a multipass
weld since more than one pass is generally required to fill the joint, but the first or
"root" pass is by far the most difficult to perform and only experienced certified welders
are able to achieve good quality root pass welds having


adequate penetration. Precise centering of the welding torch in the
joint and critical control of the welding parameters, i.e. heat and
mass balance, are required to maintain an adequate penetration of
the weld pool. Tolerances for weld penetration are particularly
stringent since stress raising defects at root dramatically reduce the
fatigue life of the joint. In an open butt joint, the weld pool is in a
mechanical equilibrium that changes much more rapidly than
temperature and is affected by many other factors, e.g. gap,
alignment, root face, etc. Thus, mechanization/ automation of this
type of welding needs proper root pass welding to achieve near
uniform weld penetration.
Normally, for mechanized/automatic circumferential welding of pipe
in fixed position, pipes are supported on welding lathe and torch is
held on to a separate fixture comprising of motorized cross-slide and
an oscillator to position and to provide required movement of the
torch across the joint. The torch position with proper setting of angle
and height can be adjusted depending upon the laying of weld pass
for the respective layer to fill the joint progressively. The sequence
of deposition of weld passes could be performed mechanically /
automatically or the total operation could be pre-programmed
depending upon the thickness and diameter of the pipe joint and
capability of the welding machine in terms of level of intelligence


provided by sophisticated combination of sensors and feed back
controls etc.
To produce pipes for steam generating power plants & other
industrial application, weld joints have to be made with root pass
welding by TIG welding process followed by Gas Metal Arc welding
(GMAW) or Submerged Arc Welding (SAW) process for mechanized
welding operation. It is known that the Tungsten Inert Gas welding
(TIG) process also known as Gas Tungsten Arc Welding (GTAW) in
particular is used to weld quality is superior devoid of the defects
such as iack of fusion, gas hole, etc. However, it has the
disadvantage that welding is normally carried out by a highly skilled
technician/welder employing automatic/mechanized welding machine
and it takes more time to complete, as the deposition rate is low.
Few manufacturers have replaced TIG welding process by
conventional GMAW process employing single wire for root pass
welding. However, single wire GMAW process produces root pass
weld with lack of fusion faults or burn-through defects particularly for
the initial/starting portion of the joint. More recently conventional
GMAW process employing single wire is being replaced by pulsed
GMAW process improved the performance of the process but
problems pertaining to intermittent lack of fusion defects and low


productivity have to be resolved towards an efficient welding
operation.
A prior art US patent 05576857 discloses to a new system for welding
sections of pipe together to form a long, continuous pipeline. A
welding bug is mounted on a guide track which goes around the
circumference of the pipe. The welding bug is provided with a wire
feeder, a motor to drive the wire feeder, and driving motor for
driving the bug around the track. One may vary the speed of the
bug and the wire feeder while the bug is continuously driven around
the track. The welding head on the welding bug is powered by a
surface-tension-transfer power source. The initial or root pass is
made external of the pipe.
Another prior art US patent 05981906 discloses description of
equipment is similar to previous patent except that in one
embodiment there is a dual welding wire feeder for driving two
welding wires simultaneously at the same speed.
A further prior art US patent 5030812 (EP 0402648, KR 920006416B,
CA 2018701) relates to a method for one-side root pass welding of a
pipe joint comprises the steps of forming a ring groove having a V-
shaped bottom portion thereof at end faces of both pipes whose joint


is clrcumferentially welded, attaching a backing material to a butt
portion of the pipes from an internal side of the pipes, running an
automatic welding machine along a guide rail mounted on an outer
surface of the pjpes in the circumferential direction thereof,
controlling a position of a welding torch along a seam by means of an
arc sensor, and welding root pass from the outer side of the pipes
with a predetermined welding current and at a welding speed in a
high-speed rotating arc welding of the pipes at a predetermined
rotational speed and with a predetermined diameter of rotation.
A still further prior art US patent 4733051 (CA 1276245) discloses a
method and apparatus for controlling weld pool penetration during
root pass welding in open butt joints. A visual sensor is positioned
ahead of the welding torch and in alignment with the joint for frontly
viewing the weld pool. The weld pool image is then processed
through a computer including image processor operatively coupled to
controller, the image processor analyzing the weld pool contour line
to derive there from weld pool penetration depth and comparing the
weld pool penetration depth with a predetermined value, to thereby
provide an information signal for controlling welding parameters such
as to provide a root pass weld having a predetermined penetration.


An yet further prior art US patent 373125 discloses a welding
apparatus for pipes which provides for a guide means which is to be
fixedly secured to the pipe adjacent the welding area, a first carriage
and a second carriage is movably supported upon the guide means
with each of the carriages supporting a pair of spaced apart welding
heads. Attached to the guide means is a driving chain. Mounted on
each of the carriages are motor means which, in turn, operate
through the driving chain to move the carriages in a prescribed
manner with respect to the guide means. The welding heads of each
of the carriages move within the same plane but each are movable in
a separate one hundred and eighty degree arc thereby forming a
continuous circle. Associated with each welding head is a weld
groove tracking system which automatically maintains each welding
head correctly aligned within the groove. The entire apparatus of
this invention, when combined with appropriate electronic computer
equipment, automatically welds an adjoined pair of piping sections.
OBJECTS OF THE INVENTION
It is therefore, an object of the present invention to propose a
method of root pass weld of pipe joints in mechanised/automatic
mode which eliminates the disadvantages of prior art.

Another object of the present invention is to propose a method of root pass weld of
pipe joints in mechanized/automatic mode and is more productive.
A further object of the present invention is to propose a method of root pass weld of
pipe joints in mechanized/automatic mode which ensures proper fusion of edges at the
root.
A still further object of the present invention is to propose suitable process parameters
and placement of the torch for root pass welding of pipes, which ensures proper fusion
of edges at the root.
SUMMARY OF THE INVENTION:
In welding of pipe joints, the significance of root pass welding is well known. To
produce satisfactory root pass weld, proper settings w.r.t. Internal diameter (ID)
matching, joint preparation involving adequate lip thickness, joint included angle, root
gap, proper selection of welding technique, welding arc parameters, torch, oscillation of
torch etc. are required to achieve defect free root pass weld. Particularly
mechanized/automatic welding requires adequate attention to achieve good quality
welds.

Accordingly, there is provided an apparatus and process for welding of root pass of pipe
joints of steam generating pressure piping and other industrial plants. The proposed
method provides the application of recently introduced process namely double wire
metal active gas (MAG) welding to perform the root pass welding with special joint
preparation and fit-up, proper selection of welding parameters including placement of
torch in particular fashion to ensure proper fusion of edges at root. In double wire MAG
welding, since two arcs are operating simultaneously parameter setting is critical. Both
the arcs may be operated either in short circuit transfer mode or pulsed spray transfer
mode. The pulsed current for the leading arc is set in such a way to provide sufficient
reinforcement for the weld bead. This it produces good quality root pass weld beads
smoothly merging with the parent metal.
Short circuit metal transfer is a special mode of transfer wherein droplet from the
consumable wire is transferred by repeated short circuiting between the welding wire
and weldpool. It is a low heat input mode of transfer and this is achieved by selecting
the welding current in the range from 150 to 180 A and voltage from 16 to 20 V and
keeping electrode extension in the range from 10 to 15 mm. This mode of transfer
helps in achieving good root pass weld without any burn through problem. Therefore
this mode is used for the leading arc.
Pulsed spray involves pulsing the current between a low background current typically of
50 A and high pulse current typically of 300A. The pulsing of the welding current in
such a manner produces a pulsed spray mode of transfer. The pulsed spray mode helps
in achieving good side wall fusion and root weld with substantial metal deposition.
Therefore this mode is used for the lagging arc.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS:
Figure 1 (schematic) - shows an embodiment of the apparatus according to the
invention. This welding system uses a double wire welding process using the special
joint fit-up and the positioning of the torch and pipes is illustrated.


Figure 2 (schematic) - Shows the completed root pass weld after
welding.
DETAILS DESCRIPTION OF A PREFERRED EMBODIMENT OF
THE INVENTION
As shown in fig-1 the welding set-up to produce pipe is to be made
with the least possible outlay by the process according to the
invention. The root pass of joint is completely made by making use
of only double wire MAG welding process. Commercially available
welding power source for double wire MAG (Time Twin) welding was
employed and solid wires confirming to ER 70S6 were used with
Argon+Co2 gas mixture. Welding lathe (1) is used to support and
hold the pipe ends (2) on holding chucks (3 & 4) to carry out the
welding operation in mechanized mode. The support and
holding/adjusting clamps (5) are provided near the lathe for
positioning the torch assembly. Wire feed units (6) are kept at
proper height to facilitate smooth feeding of wire. The welding torch
(7) is supported by holding & other clamps provided for proper
positioning of the torch. Wire feed units are connected to power
sources & gas supply system (8). According to this embodiment, the
double wire torch with gas shielding is held at suitable angle with
respect to joint to carry out the welding operation.


The numbers of pipe joints were welded for root pass welding during welding as stable
arc has been maintained providing sufficient balancing of the arc forces and surface
tension of the molten pool to sustain uniform welding bead and without creating any
burn- through defects. The joints were checked visually and with LPI. Few samples
were subjected to radiography and were cut across the joints to check for any weld
defects like lack of fusion at the root, sidewall lack of fusion etc. The bead shape on
other side is concave which is considered suitable for laying the further passes either by
GMAW or by SAW process to achieve higher productivity. Thus, the advantages of the
invention can be seen that it produces excellent root pass weld having adequate fusion
of edges at the root with improved bead shape and tolerating greater degree of
variations in joint fit-up. The pipe ends (2, 2) and root pass weld (9) are shown in fig.2.
The Tandem GMAW torch angle is kept at 5 to 10 degrees away from the top centre of
the pipe being welded and in this position the axis of the leading wire should be kept
focused towards the pipe axis. The angle setting is very critical to achieve uniform weld
quality without any convexity in the weld. However the torch shall be positioned in such
a way that the axis of the leading wire shall pass through the pipe centre. The 5 to 10
degrees shift specified above should be counter clockwise to the direction of pipe
rotation to achieve good results. The recommended stand- off distance is 12 to 20 mm.
Many of the pipe joints do not have required accessibility to place the backing ring
below the joint i.e., inside the pipe. This may be due to smaller diameter of the pipe or
long lengths, bends of the pipe. In such applications root pass welding technology
without backing would be greatly beneficial.
Besides the ceramic and other flux type backing materials are very expensive. After
welding these backing materials must be removed otherwise they will effect the flow
path of the liquid medium. Therefore development of root pass welding without backing
is a cost effective solution.
TIG welding is normally used for achieving root pass welding without backing. But it is a
very slow process, the welding speed possible is around 100mm/min.
Whereas tandem GMAW process is able to root pass welding speed of 400mm/min to
800 mm/min depending on the pipe diameter and wall thickness. Therefore Tandem
GMAW is a highly productive and economical solution for root pass and other fill passes
welding.
The two wires of the Tandem GMAW torch can be positioned parallel to the pipe groove
joint or it can also be positioned normal [i.e., 90 degrees] to the pipe groove joint.
Excellent results are achieved if the two wires are positioned in parallel orientation to
the joint but with a slight tilt of 2 degrees.

WE CLAIM
1. An improved method of root pass welding in butt joints without backing by
double wire MAG welding with special joint preparation and fit-up comprising:
holding the pipe ends (2,2) on holding chucks (3,4) in a welding lathe (1) to
carry out the welding operation in mechanized mode;
providing near the lathe a support and holding/adjusting clamps (5) to position
the torch assembly (7);
keeping the wire feed units at height within 12-20 mm to facilitate smooth
feeding of wire;
connecting the wire feed units to power sources and gas supply system (8);
characterised in that,
the double wire torch with gas shielding is held at 5 to 10 degrees with respect
to joint to carry out the welding operation when a stable arc is maintained with
uniform welding bead and without creating any burn-through defects.
2. The method as claimed in claim 1, wherein 5 to 10 degrees shift of the torch is in
counter clockwise to the direction of.pipe rotation.

3. The method as claimed in claim 1, wherein it produces adequate fusion of edges
at the root with improved bead shape and tolerating greater degree of variations
in the joint set-up.


The present invention relates to an improved method of root pass welding in butt joints
without backing by double wire MAG welding with special point preparation and fit-up
and the method consists of holding the pipe ends (2,2) on holding chucks (3,4) in a
welding lathe (1) to carry out the welding operation in mechanized mode, providing
near the lathe a support and holding/adjusting clamps (5) to position the torch
assembly (7), then keeping the wire feed units at proper height to facilitate smooth
feeding of wire and connecting the wire feed units to power sources and gas supply
system (8) when the double wire torch with gas supply system (8) when the double
wire torch with gas shielding is held at suitable angle with respect to joint to carry out
the welding operation, wherein a stable arc is maintained with uniform welding bead
and without creating any burn-through defects.

Documents:

00233-kol-2007-correspondence.pdf

00233-kol-2007-form-18.pdf

0233-kol-2007 abstract.pdf

0233-kol-2007 assignment.pdf

0233-kol-2007 claims.pdf

0233-kol-2007 correspondence others.pdf

0233-kol-2007 description(complete).pdf

0233-kol-2007 drawings.pdf

0233-kol-2007 form-1.pdf

0233-kol-2007 form-2.pdf

0233-kol-2007 form-3.pdf

233-KOL-2007-ABSTRACT 1.1.pdf

233-KOL-2007-AMANDED CLAIMS 1.1.pdf

233-KOL-2007-AMANDED CLAIMS.pdf

233-KOL-2007-AMANDED PAGES OF SPECIFICATION 1.1.pdf

233-KOL-2007-AMANDED PAGES OF SPECIFICATION.pdf

233-KOL-2007-CORRESPONDENCE 1.1.pdf

233-KOL-2007-CORRESPONDENCE.1.3.pdf

233-kol-2007-correspondence.pdf

233-KOL-2007-DRAWINGS 1.1.pdf

233-KOL-2007-EXAMINATION REPORT REPLY RECIEVED.pdf

233-KOL-2007-EXAMINATION REPORT.1.3.pdf

233-kol-2007-examination report.pdf

233-KOL-2007-FORM 1 1.1.pdf

233-KOL-2007-FORM 13.1.3.pdf

233-KOL-2007-FORM 13.pdf

233-KOL-2007-FORM 18.1.3.pdf

233-kol-2007-form 18.pdf

233-KOL-2007-FORM 2 1.1.pdf

233-KOL-2007-FORM 3.1.3.pdf

233-kol-2007-form 3.pdf

233-KOL-2007-FORM-27.pdf

233-KOL-2007-GPA.1.3.pdf

233-kol-2007-gpa.pdf

233-kol-2007-granted-abstract.pdf

233-kol-2007-granted-claims.pdf

233-kol-2007-granted-description (complete).pdf

233-kol-2007-granted-drawings.pdf

233-kol-2007-granted-form 1.pdf

233-kol-2007-granted-form 2.pdf

233-KOL-2007-GRANTED-LETTER PATENT.pdf

233-kol-2007-granted-specification.pdf

233-KOL-2007-OTHERS DOCUMENTS(CAN PAGES).pdf

233-KOL-2007-REPLY TO EXAMINATION REPORT.1.3.pdf

233-kol-2007-reply to examination report.pdf


Patent Number 248902
Indian Patent Application Number 233/KOL/2007
PG Journal Number 36/2011
Publication Date 09-Sep-2011
Grant Date 07-Sep-2011
Date of Filing 13-Feb-2007
Name of Patentee BHARAT HEAVY ELECTRICALS LIMITED
Applicant Address REGIONAL OPERATIONS DIVISION(ROD), PLOT NO: 9/1, DJBLOCK, 3RD FLOOR, KARUNAMOYEE, SALTLAKE CITY, KOLKATA-700091 HAVING AT REGISTERED OFFICE AT BHEL HOUSE, SIRI FORT, NEW DELHI-110049, INDIA
Inventors:
# Inventor's Name Inventor's Address
1 KANHAIYALAL ROHIRA WELDING RESEARCH INSTITUTE, BHEL, TIRUCHY-620014
2 SIVASANKARAN MANOHARAN WELDING RESEARCH INSTITUTE, BHEL, TIRUCHY-620014
3 ARASAN RAJA WELDING RESEARCH INSTITUTE, BHEL, TIRUCHY-620014
PCT International Classification Number B23K9/173,37/02,9/12
PCT International Application Number N/A
PCT International Filing date
PCT Conventions:
# PCT Application Number Date of Convention Priority Country
1 NA