FIELD OF INVENTION
This invention relates to the procedure for butt joining of Super 304 H or equivalent grade, stainless steel tubeswith full penetration welds to meet ASME or equivalent code requirements for construction of tubular components of utility boilers,industrial boilers, heat exchangers, pressure vessels, made of tubes of thickness ranging from 3– 14 mm and varying diameter from 32 – 69 mm size. More particularly, the invention relates to an improved gas metal welding arc (GMAW) process to weld super 304 H stainless steel or its equivalent tubes.
BACKGROUND OF THE INVENTION
Super 304 H, the new type of austenitic stainless steelis developed on the base of TP 304 H. This high strength, high steam oxidation resistance steel tubes is also known as 18Cr-9Ni-3Cu-Nb-N steel and the equivalent grades are METIKA-SUS304J1HTB, ASTM A 213 S 30432, ASME SA 213 CODE CASE 2328, etc. As a new type of austenitic steel, Super 304 H possesses not only excellent resistance to hightemperature corrosion and steam oxidation mainly due to high Cr content, but also superior creep strength than martensitic steels.Thus, it is widely used for superheaters and reheaters, which have the abominable service environment in super critical, ultra super critical and advanced ultra super critical utility boilers. Through the addition of about 3 wt.-% of copper increased carbon content and certain amounts of niobium and nitrogen, the elevated temperature strength and especially the creep properties are improved in the grade Super 304 H.The addition of nitrogen leads to a solidsolution strengthening of the material. This increases the allowable tensile stresses and resistance against stress corrosion cracking. The chemical composition of the steel is given in Table 1.

Welding of these tubes is necessary for making tubular components for use in boilers, heat exchangers, pressure vessels, etc.
Before taking up the tubes for welding, extensive prior art searches in the technical field was carried out by the present inventors, and the following observations were made.
• Study by Electric Power Research Institute (EPRI), reveals that EPRI prefers Gas
Tungsten Arc Welding (GTAW) process for super 304 H since it is more suitable
than the Gas Metal Arc Welding Process (GMAW).
• The known filler wires are not suitable for GTAW as it does not have no de-
oxiders.
• No filler wires suitable for GMAW process for super 304 H is available in the prior
art.
• Although some prior art indicate the use of ER 309 L filler wire with and without
purging,however the weldment failed in weld metal and cannot be used for ASME
qualification (though, the reported UTS values are higher than the specified values
of parent metal).
• WRI also experienced gas holes with purging during the experimental works by
GMAW process.
• GTAW with YT 304 H filler wire is known to provide satisfactory results meeting
the code requirements, without pores, etc.

Nevertheless, the GTAW process is having the following inherent problems.
• Low productivity.
• More time for welding
EP3064307A1 2016-09-07 describes “System and method for linking by friction welding a first piece of steel to a second piece of steel with use of ni-based alloys adapter”. According to this prior art a system (200) is provided, comprising a two-sided adapter (205), made of a Ni-based alloy, that is connected at each of the two sides with a different type of steel (201, 202), and wherein the connection (203) of the different types of steel (201, 202) with the adapter (205) is achieved at least in part by use of friction welding. A method for linking different types of steel (201, 202) by using a two-sided adapter (205) as an intermediate, wherein at least one of the adapter-steel connections (203) is made by means of friction welding, is also provided.
JP 2013040388A 2013-02-28 teaches on “AUSTENITIC STAINLESS STEEL AND METHOD FOR WELDING AUSTENITIC STAINLESS STEEL”. In this prior art, the problem addressed is to provide an austenitic stainless steel which can prevent reheat crack and composes of coarse grain structure enabling to be manufactured at low cost.
The solution provides in that the stainless steel tube is formed with stainless steel containing niobium or titanium, and a Vickers hardness of the stainless steel is ≤200. The stainless steel tube containing the niobium, is constituted with the coarse grain structure having 1 to <7 austenitic crystal grain size number. Also, the stainless steel tube containing the titanium, is constituted with the coarse grain structure having 3 to

<6 austenitic crystal grain size number. The stainless steel tube containing niobium or titanium with such formation, does not generate the reheat crack near the welded portion, even in the case of using the tube for a long time in the temperature range of 500 °C to 800 °C.
JP2005319494A 2005-11-17 discloses METHOD FOR WELDING STAINLESS STEEL TUBE AND JOINT STRUCTURE.
PROBLEM TO BE SOLVED: To provide a welding method where reheat cracks generated at new weld zones in the secularly used materials of high strength stainless steel tubes comprising small amounts of Nb and Ti can be surely and effectively prevented, and to provide a joint structure.
SOLUTION: In the method for welding stainless steel tubes, a short tube of a new material with a length of ≥20 mm is welded to the secularly used material of a stainless steel tube comprising ≥0.03% Nb or Ti and used at ≥500°C for ≥1,000 hr under an unconstrained condition, and thereafter, in the steel tube materials obtained from the obtained pair of the secularly used materials of the stainless steel tubes and the short tubes of the new materials, the new material short tubes are finally welded each other. It is also possible that, the sides of the tube inner faces in the weld zones are subjected to grinding or smoothing treatment before the new material short tubes are subjected to the final welding each other, or the new material short tubes are subjected to the final welding each other after stress removing annealing is performed to the weld zones
CN101412160B 2011-03-09 talks about the Tungsten electrode argon arc welding wire for Super304H austenite welding or simply known as GTAW process.
The invention provides an argon tungsten arc welding wire for Super304H stainless steel welding, which belongs to the field of welding in the material processing engineering. The aim of the invention is to provide the argon tungsten arc welding wire,

of which the mechanical property and the chemical composition are similar to those of the Super304H steel. The welding wire consists of the following components in weight percentage: 0.06 to 0.10 percent of C, 0.18 to 0.25 percent of Si, 0.72 to 2.5 percent of Mn, 0.004 to 0.008 percent of P, 16.6 to 20.5 percent of Cr, 8.8 to 16.1 percent of Ni,2.5 to 3.3 percent of CU, 0.0 to 1.21 percent of Mo, 0.29 to 0.58 percent of Nb, 0.08 to 0.19 percent of N, 0.001 to 0.008 percent of B, and the balance being Fe. The welding wire has the advantages of excellent performance, low cost and the like.
The patent CN101862924A 2010-10-20 describes Gas shield welding wire material for supercritical steel (en) .
The patent refers to the field of 'soldering or unsoldering; welding; cladding or plating by soldering or welding; cutting by applying heat locally; working by laser beam'. The invention relates to a gas shield welding wire material for supercritical steel, which is characterized by being prepared from the following raw materials in percentage by weight: 0.09%-0.12% of carbon (C), 0.5%-0.8% of manganese (Mn), 0.20%-0.30% of silicon (Si), less than or equal to 0.010% of sulfur (S), less than or equal to 0.010% of phosphorus (P), 8.5%-10.0% of chromium (Cr), 0.18%-0.25% of vanadium (V), 0.6%-0.8% of nickel (Ni), 0.9%-1.1% of molybdenum (Mo), 0.05%-0.08% of niobium (Nb), less than or equal to 0.20% of copper (Cu), 0.03%-0.06% of nitrogen (N), less than or equal to 0.04% of aluminum (Al), and the balance of Fe. A welding line of the invention has good combination property. Compared with the prior art, the invention has high strength, good toughness, and the like and achieves the following requirements on the mechanical property of deposited metal (a heat treatment condition: 740+/-10 DEG C*4 h): sigma b(MPa) is more than or equal to 620; sigma 0.2 (MPa) is more than or equal to 530; delta 5 (%) is more than or equal to 17; psi (%) is more than or equal to 40; Akv2 (J): 47/38 is equal to a mean value/a minimum value; and rigidity (HV) is less than or equal to 350.

From the above prior art patents it could be inferred as follows.
CN101412160B deals about the filler wire for tungsten electrode argon arc welding and wherein the present invention is about using a process without tungsten electrode.
CN101862924A talks about gas shielded welding processes, it particularly focuses on the filler wire, which is not in the scope of the present invention. Hence, none of identified prior art relates to Gas Metal Arc Welding Process and its variants for the current invention.
OBJECTS OF THE INVENTION
It is therefore and object of the present invention is to propose an improved gas metal welding arc (GMAW) process to weld super 304 H stainless steel or its equivalent tubes.
Another object of the present invention is invention is to propose an improved gas metal welding arc (GMAW) process to weld super 304 H stainless steel or its equivalent tubes, which enhances the productivity.
A still another object of the present invention is to propose an improved gas metal welding arc (GMAW) process to weld super 304 H stainless steel or its equivalent tubes
Which is enabled to produce flawless weld joints to meet the requirement of ASME or equivalent codes.
A further object of the present invention is to propose an improved gas metal welding arc (GMAW) process to weld super 304 H stainless steel or its equivalent tubes, including the process parameters as well as the shielding gas composition.

SUMMARY OF INVENTION
Accordingly, there is provided an improved gas metal welding arc (GMAW) process to weld super 304 H stainless steel or its equivalent tubes. The invention further establish the process parameters including selection of the optimum ratio of shielding gas to produce flawless joints to meet the requirements of ASME or equivalent codes, so that the welded tubes can be used for constructing components to work under high temperature and / or high pressure applications. In the disclosed process, the arc is struck between the tube and the filler wire through which the current is used. The weld pool is shielded by a mixture of argon and CO2 gas with pre-determined proposition. Conventionally in the GMAW process the gas mixture will be in the ratio of 80:20 to 98:2. In the present invention the gas mixture ratio is different. Table 2 shows the typical welding parameters of GMAW process and shielding gas mixture ratio developed through experimental works for joining super 304 H tubes.
This process is enabled to achieve full penetration welds with edge preparation in 3 to 14 mm thick Super 304 H or equivalent grade tubes by Gas Metal Arc Welding Processand its variants to fabricate tubular components as per ASME or equivalent code, etc.
The invention ensures flawless joints in butt joining of super 304 H or equivalent gradetubes by Gas Metal Arc Welding Process and its variants for increased productivity.
Theprocess is applicable for butt joining of super 304 H or equivalent gradetubes by Gas Metal Arc Welding Process and its variants.
The invention also establish new process parameters for joining of super 304 H tubes or equivalent grade to Super 304 H tubes of any size with filler wire addition by Gas Metal Arc Welding Process and its variants.
This invention reduces the production cycle time from the present method of tungsten electrode arc welding (GTAW) process.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Figure 01 shows the typical edge preparations of the tubes to be joined by welding.
Figure 02 shows the welding method followed in the invention and the set up to make the weld by the novel method.
DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION
Figure 1 shows a typical groove angle for making the joints between the Super 304 H tubes (01).
Figure 2 shows aset up having a chuck (02), a welding power source (03), atorch (04) through which the filler wire is fed to deposit the weldment. The welding arc is initiated between the filler wire which is fed through the wire feeder (05) from the wire spool (06) and the tubes (01) and the melting filler wire is deposited in the groove as weldment. The weld pool is kept surrounded by the shielding gas mixture at determined ratio, supplied by a gas cylinder (07). After completion of root pass welding, the subsequent passes of welding fill the groove as per the requirement. By this method tubes with high wall thickness can be welded by using the multipass weld technique. Table 3 shows the experimental works done to establish the GMAW process and parameters to produce flawless welds by GMAW process.
On completion of welding, the joints were evaluated by non-destructive testing and destructive methods to identify presence of flaws, if any, as per ASME or equivalent code.

ADVANTAGES OF THE INVENTION
1. The inventive process is unique for butt welding of super 304 H tubes or equivalent by Gas Metal Arc Welding Process and its variants to achieve full penetration welds to meet ASME or equivalent code requirements.
2. This method can be used to make full penetration welds for super 304 H or equivalent tubes with shielding gas surrounding the weld pool and the weld arc is struck between the tube and the filler wire.
3. The method can be used to make full penetration welds with or without root gap.
4. The method can be used to weld the super 304 H or equivalent tubes of any thickness.
5. The method can be used to weld the super 304 H or equivalent tubes of any diameter.
6. The method may be used to make the full penetration weld by any filler wire / welding consumable suitable to super 304 H or equivalent.
7. The method can be used to make the full penetration weld with or without preheating and or post heating.
8. The method can be used to make the full penetration weld with or without purging gas.
9. The method can be used to make the full penetration weld in any position of welding.
10. The method can be used to make the full penetration weld by selecting suitable welding voltage, current, travel speed and shielding gas mixture to produce flawless welds.

11. The method can be used to make the full penetration weld through manual, mechanized, semi – automated, robotic and / or automated welding modes.
12. The method can be used to make the full penetration welds with short / medium / long arc depending upon the welding parameters.
13. The method can be used to make the full penetration weld with suitable size of
filler wire.

WE CLAIM :
1. An improved gas metal welding arc (GMAW) process to weld super 304 H stainless steel or its equivalent tubes, the improvement is characterized in that a groove angle is formed between the joinable tubes, in that the welding arc is struck between the tubes and a filler wire carrying the welding current, that the weld pool is shielded by a mixture of argon and carbon-dioxide gas mixed at a pre-determined ratio, and that passes subsequent to the root pass welding fill in the formed groove.
2. The process as claimed in claim 1, wherein the pre-determined ratio of the shielding bas is 99.5% Argon and 0.5% carbon-dioxide.
3. The process as claimed in claim 1, wherein the welding position is 1G, wherein the welding torch angle is 80-90o, and wherein the start current and start voltage is 75-100 Amp and 15 to 19 voltage respectively.
4. The process as claimed in claim 1, wherein the number of subsequent passes is at least three and wherein the purging gas for the root pass is 100% Ar

5. The process as claimed in claim 1, wherein the chemical composition of super 304 H SS tube is 0.07 – 0.13%C, 17-19%cr, 7.5-10.5%qi 0.027 P%, 0.3 – 06%nb, 0.05-0.12% N, 2.5 – 3.5%cu, 0.30%max Si, and 1.00maxMn.