FIELD OF INVENTION
The present invention relates to an improved method of root pass welding for
pressure part application in power plant. More particularly, the method relates to
an improved gas tungsten welding system for welding root pass in conjunction
with automatic wire feeding system.
BACKGROUND OF THE INVENTION
Root pass generally comprise a pair of spaced apart edges in a plate or pipe or
tube etc, which are then joined by a weld. Root pass often arise when joining
adjacent sections. The weld is usually made in several steps. First, a root pass is
made where at least the inner edges or lands of the edges are fused and the gap
between the lands filled with weld metal. Thereafter, several filler passes are
made so that the weld metal is at least flush with the outer surface of the pipe.
In Conventional Gas Tungsten Arc Welding (GTAW), when weld metal is
deposited, a welding rod is held in the left hand, and a Gas tungsten arc welding
(GTAW) torch is held in the right hand, during welding is performed. On the
other hand, when performing the advanced GTAW welding, the filler metal is
supplied by a wire feeding device for automatically feeding the filler wire by use
of the motor. Both the wire guide and the TIG torch are fixedly held by a stand.
Because the root pass is the initial pass that adjoins and secures the opposing
edges, the root pass is critical to ensure the quality of the joints. If the root pass
is not made properly then defect like excess of penetration, burn
through,leakages, crackes etc will occur. Therefore, during the root pass, a
100% sound weld bead should be made.
Soundness of the weld bead means the complete fusion of both the edges and
the complete filling of the gap between the adjoining sections with the weld
metal.
It is also necessary that the molten weld metal does not protrude inwardly of
the weld section to any substantial distance, as the inner surface should be
substantially smooth and free of any protrusions that may prevent the travel of
any pig, inspection device, or any other cylindrical devices through the pipe,
and/or initiate turbulent fluid flow or otherwise disrupt the flow of any fluid
travelling through the pipe.
Another consideration is that the heat of the open root weld cannot be too high
that would cause metal shrinkage and, thus, suck back into the gap forming the
open root.
To accomplish a quality root weld, without substantial inward protrusion of
molten metal or metal suck back, advanced gas tungsten arc welding system has
been developed and used.
The advanced GTAW pipe welding process controls the initial welding pass of the
pipe welding procedure to fill the root. Advanced GTAW uses straight argon and
has the lowest possible weld oxidation potential producing cleaner welds on any
parts.
OBJECT OF THE INVENTION
Therefore, it is an object of the invention to propose an improved method of root
pass welding for pressure part application in power plant which is capable of root
pass welding for pipe/tubes/plates having thickness in the range of 5 mm to 16
mm.
Another object of the invention is to propose an improved method of root pass
welding for pressure part application in power plant which is capable of manual
root pass welding in 5G position i.e. vertical welding when the pipe will be us
horizontal axis and welding being done around the periphery.
A further object of the invention is to propose an improved method of root pass
welding for pressure part application in power plant which is able to enhance the
productivity of root pass welding by reducing the cycle time.
SUMMARY OF THE INVENTION
The present invention has been developed to eliminate the limitation of
conventional GTAW welding and providing GTAW welding equipment, wherein
the wire is automatically fed and welding can be easily performed. Conventional
GTAW welding is well known from last decade about its quality but with poor
productivity.
In this advanced gas tungsten arc welding system the wire is feed continuously
which gives the productivity comparable to Shielded Metal Arc Welding (SMAW)
at GTAW welding quality.
By automatic wire feeder and control arc it facilitates the welder to perform the
welding. Skill level is not as much as required in conventional GTAW welding
process.
According to the present invention, the present operation performed while the
filler metal is supplied by a wire feeding device for automatically feeding the filler
wire by use of the motor. Both the wire guide and torch are fixedly held on a
stand.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Fig.l - Shows an arrangement of the component of the semiautomatic
(Tungsten Inert Gas) TIG welding equipment used in common
practice. Reference numeral 1 denotes the power source, 2 denotes
the pipe, 3 is the roller that gives support for rotating the pipes, 4
is the torch holder, 5 denotes the torch with wire feeder, according
to the invention.
Fig.2 - Shows a semiautomatic welding of the pipe to pipe in 5G position.
Fig.3 - Shows a semiautomatic welding of the pipe to pipe in 2G position.
DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE
INVENTION
The advanced GTAW welding equipment comprise a GTAW torch and wire guide
unit attached to the torch. A separate wire feeder feeds the wire through the
wire guide as shown in fig 1. Item no 5 indicates the GTAW torch with automatic
wire feeder.
In figure 1, advanced welding equipment comprises of welding torch, tungsten
electrode, base metal, filler wire, wire holder attached to the torch. The wire
holder attached to the torch in such a way that the filler wire may be parallel to
the tungsten electrode as much as possible.
In figure 1 the GTAW torch having two knob controls to adjust the current and
wire feed to facilitate the welder.The sequence of activity prior to welding, a
typical joint groove is first tack welded and kept on the free roller, the welder
can start the welding from the tack point and slowly rotate the pipe.
5G stands for vertical welding i,e,. pipe will be in horizontal axis and welding will
be done around the pheriphery, as shown in Fig.2.
There are the various welding positions. 2G stands for horizongal welding i.e.,
pipe will be along vertical axis and welding will be done all around, as shown in
Fig.3.
Improved root pass welding is carried out with the following steps:
i. A 'V' OR 'J' groove edge preparation as per requirement with root gap of
1.00 mm to 5.0 mm is prepared,
ii. Root pass welding of the butt joints is carried out by advanced OTAW at 2
to 3 times higher welding speed than conventional TIG root pass welding,
iii. The root pass welding as described above is a manual root pass welding of
pipe in down hand position by rotating the pipe or in all position while
keeping the pipe stationary
iv. The mechanized root welding of pipe in down hand position is carried out
by rotating the pipe or all position while keeping the pipe stationary,
v. The automated orbital root pass welding is carried out in all position
keeping the pipe stationary.
The pipes are tack weld before actual root pass welding. The tack is ground to
match the surface. The welding starts from the tacked point only. As the wire is
fed automatically along with the torch, welder needs to control the welding torch
only. An additional arrangement is added to the welding torch which feed the
welding wire into the weld puddle. It is an automatic wire feeder GTAW welding
process. The fire follows the GTAW torch. This improved torch design improves
the productivity and quality also.
At present root pass welding is done by using SMAW process or conventional TIG
welding and with these processes, the chances of welding defects like slag
inclusion, porosity, poor visual quality, non repeatability and hydrogen
entrapment if not backed property etc. are more.
Advantages of the improved method according to the invention:
i. Improved quality - No such defects appear as mentioned above in the
present process. All the weld gives repeated radiography quality which
means all joint become radiography pass.
ii. Improved productivity - higher production is possible with higher speed of
root pass welding,
iii. Supply of filler metal by a wire feeding device for automatically feeding the
filler wire by use of a motor also improves the productivity,
iv. Both the wire guide and torch are fixedly held on a stand,
v. Properly selected welding parameter such as current, of 135-150 Amp wire
feed rate 2.0 to 2.2 m/mm and torch angle of 60 degree and speed of
welding as 55-70 mm/mn helps in improving the quality and productivity.
WE CLAIM
1. An improved method of root pass welding comprising;
preparing the groove edge for 'V' or 'J' groove;
maintaining a root gap of 1.0 to 1.5 mm between the pipes for welding;
putting the pipes to be welded on the supporting structure with roller for
facilitating the pipes to rotate;
tack welding the pipes before actual welding;
grinding the tack for matching the surface;
starting the welding from tacked point only;
Characterized in that,
a separate automatic wire feeder feeds the wire through the wire guide
wherein the automated orbital root pass welding in all position is carried
out keeping the pipe stationary when suitable welding parameter for
current, wire feed rate and torch angle are selected for full penetration
root pass weld for meeting the required quality wherein a higher speed of
root pass welding of the butt joints is adapted.
2. An improved method of root pass welding as claimed in claim 1, wherein
the current, wire feed rate and torch angle are selected as 130-155 A,
2.0-2.2m /min and 60 Degree respectively.
3. An improved method of root pass welding as claimed in claim 1, wherein
mechanized root welding of pipe in down hand position by rotating the
pipe or for all position while keeping the pipe stationary.
4. An improved method of root pass welding as claimed in claim 1, wherein
the speed of welding is maintained at 65-70 mm / min.
5. An improved method of root pass welding as claimed in claim 1, wherein
the current is controlled by two knobs disposed on the torch.
6. An improved method of root pass welding as claimed in claim 1, wherein
the wire holder attached to the torch feeds the filler wire almost parallel to
the tungsten electrode.

ABSTRACT

An improved method of root pass welding consists preparing the 'V' or 'J' groove
on the pipes, putting the pipes on roller support, tack welding the pipes, grinding
the tack and then the root pass welding is carried out with a separate automatic
wire feeder feeding the wire through the wire guide. The wire guide unit is
attached to the torch. Current, wire feed rate, and torch angle are selected for
the root pass welding when the said welding is carried out with higher speed.