FIELD OF THE INVENTION
This invention generally relates to a procedure for welding of stubs to a header by waveform controlled by GMAW process.
BACKGROUND OF THE INVENTION
For welding stubs of thickness ranging from 5 - 10 mm and varying diameter from 58 - 63 mm size to medium sized diameter pipe (header) at present, Shielded Metal Arc welding SMAW is used with typical set on joint configuration. With multi pass SMAW to complete the stub welding on pipe joints, a lot of care has to be exercised to avoid possible defects like slag inclusion, porosity, etc which affect both the art time as well as non-arc time of the welders. Low rate of production, chances of rejection / repair during fabrication, are the disadvantages of prior art practice.
A traditional power source uses a transformer to turn high voltage, low amperage primary power into a low voltage, high amperage power necessary for welding. An inverter power source, takes input power, filters it to DC and using multiple fast solid-state switches, increases its frequency to 20,000 to 100,000 Hz. Then, it transforms the current into an useable welding power through an

advanced level of control over the arc. With these digital power sources, several improvements were accomplished in the prior art, besides better process control and reproducibility. These modem power sources blend the advances in power electronics and sophistication of high speed computing with the power of inverter technology to generate the desired welding current and voltage parameters.
JP 20100307429 by Mitsubishi Heavy Industries, discloses a welding structure of tube stubs and tube headers. This invention describes a welding structure of a tube header and tube stubs without interposing a component durability thereof against creep and fatigue damage of the tube stubs without interposing a component of a different material between the tube header and the tube stubs. The welding structure of the cited invention comprises: a tube header made of ferritic heat resisting steel; a plurality of tube stubs which are welded onto an outer surface of the tube header, each of the tube stubs having a bended section, in which the plurality of tube stubs are made of austenite stainless steel, and welded to the tube header by using nickel base alloy as a welding material. Moreover annular chases are respectively formed on the outer surface of the tube header to surround welding portions of the tube header; joining portions

are formed on an inner side of the annular chases (10); butt grooves are formed by joining surfaces of the tube stubs and the joining portions (14) in a continuous manner and welded by using the nickel base alloy as a welding material, the tube stubs having a diameter substantially same as those of the joining portions.
US 3262497 A teaches a fitting for connecting a tube to a header and to the assembly of a plurality of said fittings and tubes with said header. More specifically, the invention relates to a fitting for connecting a plurality of tubular elements to a pipe header and to the assembly of a plurality of said fittings and tube with said header.
OBJECTS OF THE INVENTION
It is therefore an object of the invention to propose an improved process of a deeper penetration welding of stub to header joints for high temperature and high pressure applications like boilers and reducing the weld cycle time.
Another object of the invention is to propose an improved process of a deeper penetration welding of stub to header joints for high temperature and high pressure applications like boilers and reducing the weld cycle time which achieves full penetration.

A still another object of the invention is to propose an improved process of a deeper penetration welding of stub to header joints for high temperature and high pressure applications like boilers and a reheater of a boiler and reducing the weld cycle time, which eliminates the steps of back grinding / back gouging after root pass welding.
A further object of the invention is to propose an improved process of a deeper penetration welding of stub to header for high temperature and high pressure applications like boilers and reducing the weld cycle time, which eliminates the liquid / dye penetrant testing to ensure the soundness of root run.
SUMMARY OF THE INVENTION
The present invention provides a welding structure of a tube header and tube stubs, especially those suitable for use in high temperature and high pressure applications like boilers installed in a thermal power plant, wherein a fitting for connecting a plurality of tubular elements to a pipe header.
The invention serves the purpose of effective and uniform welding on the perimeter of the stubs to be welded and also serves the purpose to avoid the stick electrode welding where care has to be exercised to avoid possible defects like slag inclusion, porosity, etc, and eliminate the grinding process.

More particularly, the invention relates to an improved process of a deeper penetration welding of stub to header joints for high temperature and high pressure applications like boilers and reducing the weld cycle time.
As the manual welding is difficult and time consuming, the defect rate is significant. Figure 3 shows a typical header assembly.
To try the stub joints in GMAW procedure, a conventional mode of transfer of GMAW power sources a Short-circuit transfer operates at relatively low voltages and amperage ranges. Due to low heat input, the short-circuit transfer is not recommended for plate materials, as penetration and fusion capabilities are limited.
Because of the violent nature of the short-circuiting event, the process mode is characterized as having high spatter levels. The short-circuit mode also has a tendency for incomplete fusion discontinuities when welding thicker materials and is not allowed by several codes/standards.
It is known that high voltage and amperage settings of a spray transfer arc and the fluidity of the weld pool provide for deep penetration, flat wide bead contours, and high welding speeds. These characteristics are suitable for welding

on thicker plate materials. Excellent bead profiles and elimination of spatter, are the factors that makes the use of spray transfer desirable. But due to weld puddle fluidity, the use of spray transfer welding is limited in the flat position for grooves and flat horizontal positions for fillet welds.
It is not only the full penetration weld, it requires rather a deep penetration mode to weld the stubs. For the stub welding it requires a very short arc with longer short circuit phases since the welding torch has to be taken to a distance of minimum of 12 mm to reach the root of the stub profile. To achieve this, it requires a power source which can deliver a required voltage and current to reach the stub root. And also it would be required to have either a torch which travels upto the depth of the stub or a process which is capable to deliver the desired welding parameters to ignite the arc even at longer contact to tube distance.
With modem digital control power sources, it became possible to intervene in the process at a fast control speed with a very short arc with longer short-circuit phases. The current is run down very quickly on re-ignition until the programmed nominal arc voltage is reached. This drastically reduces the power time period of the short-circuit phase and reduces the spatter formation to a tenable minimum.

A resulting constant arc length control demonstrating the ability to fine tune the system to adjust the random variations in Contact Tip to Work Distance (CTWD) with changing the arc length.
The improved fusion penetration characteristics of deep penetration modes, are particularly advantageous in root formation especially in tight and narrow joints. The fusion penetration is in the root area but much deeper than with a normal spray arc which interalia increases in productivity by 100%. The welding time can be reduced by avoiding the slag removal, grinding and reduced number of layers and passes as in the SMAW process.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Figure 1 - shows welding of stubs to headers.
Figure 2 - shows a welding structure for welding structure for welding of a tube header to a tube stub.
Figure 3 - shows a typical header assembly consisting of header and tubes.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION
According to prior art, welding of stubs to header (Figure 1) comprises beveling and weld preparation of the stubs, tack-welding to the main body, fully welding the stubs with header and checking the weld joints under hydraulic pressure varying from 1000 to 3000 psi.
Headers stubs are probably getting strained during transportation , improper handling, erection (overloading) resulting into failures taking place at various stages from boiler hydro test to boiler going into operation.
As per the known practice, the Header to stub joints is welded by employing Shielded Metal Arc welding (SMAW). Because of its typical joint design (set on joint) as shown in Figure 2, the root is not accessible to penetrate and due to these reasons, the problem of leakage arises which increases the rework.

WE CLAIM:
An improved process of a deeper penetration welding of stub to header joints for high temperature and high pressure applications like boilers and reducing the weld cycle time, the improvement comprising the steps of :-
adapting a deep penetration mode to weld in stubs with a short arc weld having a longer short-circuit phases with the welding torch traveling to a distance to reach the root of the stub profile;
- alternatively implementing a process capable to deliver the desired welding parameters to ignite the arc even at longer contact compared to the tube
distance;
intervening the said process at a faster control speed with a shorter arc with longer short-circuit phases, wherein the current is run down quickly on re-ignition until the programmed nominal arc voltage is reached, which interalia reducing the power time period of the short-circuit phase including a minimum spaller formation; and

- fine tuning the resultant arc length control to adjust the random variation in CTWD with the changing arc length.