Description:

FIELD OF INVENTION
[001] The present subject matter described herein, relates to a process for butt joining of stainless steel tubes using a multi-pass welding process. More particularly, to a process for butt joining of 18 Cr-9 Ni-3 Cu-Nb-N (Super 304 H or Equivalent grade) stainless steel tubes by means of modified multi-pass Gas Metal Arc Welding (GMAW) process using alternate shielding gas.
BACKGROUND AND PRIOR ART AND PROBLEM IN PRIOR ART
[002] Stainless steel Super 304 H or equivalent grades of such steel is widely used for super heaters and re-heaters, 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 solid-solution strengthening of the material. This increases the allowable tensile stresses and resistance against stress corrosion cracking. Welding of tubes made of this stainless steel is necessary for making tubular components for use in boilers, heat exchangers, pressure vessels, etc.
[003] The conventional Gas Tungsten Arc welding (GTAW) process has certain inherent limitations such as Low Productivity and more consumption of time for butt joining of this type of Stainless Steel. Further, the existing state of art comprises of welding processes use a filler wire for tungsten electrode argon arc welding and also the emphasis is upon the filler wire along with gas shielded welding process.
[004] However, despite of the above described available and in order to overcome the limitations of the existing provisions, there is a need in the art to provide an improved gas metal arc welding process for joining the butt of two stainless steel tubes having a high wall thickness.
OBJECTS OF THE INVENTION
[005] It is therefore the object of the invention to overcome the aforementioned and other drawbacks in prior art.
[001] The principal objective of the present invention is to provide for a welding process that can effectively join two stainless steel tubes, having high wall thickness, by butt joining.
[002] Another object of the present invention is to improve the productivity of the gas metal arc welding process using alternate shielding gases.
[003] Another object of the present invention is to develop process parameters to ensure butt joints without any flaws for meeting the requirements of ASME code or equivalent codes.
[004] Another object of the present invention is to eliminate or minimize the occurrence of porosity in weldment of Super 304 H and equivalent steel when welded with gas metal arc welding (GMAW) process by supplying the shielding gases alternatively into the weld pool for pre-determined time periods.
[005] These and other objects and advantages of the present subject matter would be apparent to a person skilled in the art after consideration of the following detailed description taken into consideration with accompanying drawings in which preferred embodiments of the present subject matter are illustrated.

SUMMARY OF THE INVENTION
[006] One or more drawbacks of the conventional technology based on existing welding processes are overcome, and additional advantages are provided through a novel multi-pass welding process as claimed in the present disclosure. Additional features and advantages are realized through the technicalities of the present disclosure. Other embodiments and aspects of the disclosure are described in detail herein and are considered to be a part of the claimed disclosure.
[007] In an embodiment of the present subject matter, a multi-pass welding process for butt joining of stainless steel tubes having high wall thickness, the process comprising initiating welding arc between the tubes by means of a current carrying filler wire, feeding the welding arc using a wire feeder from a wire spool to a groove between two tubes, filling the groove by depositing a melting wire as weldment and enclosing the groove by two shielding gases viz. Argon and CO2 supplied by gas cylinders or any other source through gas alternator at pre-determined flow rate and for a pre-determined time period cyclically.
[008] In another embodiment of the present subject matter, the minimum number of passes required during the welding process depends upon the wall thickness of the tubes.
[009] In another embodiment of the present subject matter, the welding process includes semi-automatic Gas Metal Arc Welding Process (GMAW) with gas alternator.
[0010] In another embodiment of the present subject matter, the tubes are composed of Super 304H stainless steel.
[0011] In another embodiment of the present subject matter, the shielding gas includes argon and CO2.
[0012] In another embodiment of the present subject matter, the value of current is between 85-110 A and voltage between 20-30 V.
[0013] In another embodiment of the present subject matter, the value of welding speed is in the range of 230 mm/ min to 340mm/ min.
[0014] In another embodiment of the present subject matter, the groove is a ‘V’ shaped groove having a groove angle between 37.5 to 45o C.
[0015] In another embodiment of the present subject matter, the pre-determined flow rate is 17 litres per minute for argon and 2 litres per minute for CO2.
[0016] In another embodiment of the present subject matter, argon flows for 0.3 seconds and CO2 flows for 0.1 second through the gas alternator.
[0017] It is to be understood that the aspects and embodiments of the disclosure described above may be used in any combination with each other. Several of the aspects and embodiments may be combined to form a further embodiment of the disclosure.
[0018] The foregoing summary is illustrative only and is not intended to be in any way limiting. In addition to the illustrative aspects, embodiments, and features described above, further aspects, embodiments, and features will become apparent by reference to the drawings and the following detailed description.
BRIEF DESCRIPTION OF THE DRAWINGS
[0019] It is to be noted, however, that the appended drawings illustrate only typical embodiments of the present subject matter, and are therefore not to be considered for limiting of its scope, for the invention may admit to other equally effective embodiments. The detailed description is described with reference to the accompanying figures. In the figures, a reference number identifies the figure in which the reference number first appears. The same numbers are used throughout the figures to reference like features and components. Some embodiments of system or methods or structure in accordance with embodiments of the present subject matter are now described, by way of example, and with reference to the accompanying figures, in which:
[0020] Figure 1 illustrates a schematic view of a groove angle between the butt points of two Super 304 H stainless steel tubes in accordance with an embodiment of the invention;
[0021] Figure 2 illustrates a schematic view of welding system in accordance with an embodiment of the invention;
[0022] The figures depict embodiments of the present subject matter for the purposes of illustration only. A person skilled in the art will easily recognize from the following description that alternative embodiments of the structures and methods illustrated herein may be employed without departing from the principles of the disclosure described herein.
DETAILED DESCRIPTION
[0023] The detailed description of various exemplary embodiments of the disclosure is described herein with reference to the accompanying drawings. It should be noted that the embodiments are described herein in such details as to clearly communicate the disclosure. However, the amount of details provided herein are not intended to limit the anticipated variations of embodiments; on the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the scope of the present disclosure as defined by the appended claims.
[0024] It is also to be understood that various arrangements may be devised that, although not explicitly described or shown herein, embody the principles of the present disclosure. Moreover, all statements herein reciting principles, aspects, and embodiments of the present disclosure, as well as specific examples, are intended to encompass equivalents thereof.
[0025] The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of example embodiments. As used herein, the singular forms “a",” “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises,” “comprising,” “includes” and/or “including,” when used herein, specify the presence of stated features, integers, steps, operations, elements and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components and/or groups thereof.
[0026] It should also be noted that in some alternative implementations, the functions/acts noted may occur out of the order noted in the figures. For example, two figures shown in succession may, in fact, be executed concurrently or may sometimes be executed in the reverse order, depending upon the functionality/acts involved.
[0027] Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which example embodiments belong. It will be further understood that terms, e.g., those defined in commonly used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of the relevant art and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein.
[0028] These and other advantages of the present subject matter would be described in greater detail with reference to the following figures. It should be noted that the description merely illustrates the principles of the present subject matter. It will thus be appreciated that those skilled in the art will be able to devise various arrangements that, although not explicitly described herein, embody the principles of the present subject matter and are included within its scope.
[006] The present subject matter describes a multi-pass welding process for joining butt of stainless steel tubes having high wall thickness. Further, the present subject matter describes a multi-pass gas metal arc welding (GMAW) process with alternate shielding gases including its variants for joining and welding super 304H steel tubes or its equivalents and chemical composition of 304H stainless steel tubes has been provided in Table 1. The present process includes optimized values of flowrate and time period of shielding gases to produce butt joints without any flaws or deviations.
[0029] As illustrated in Figure 1 and Figure 2, the multi-pass gas metal arc welding process using alternate shielding gases argon and CO2 that are supplied through a gas alternator (107) so that at one point of time only a single gas is able to flow to weld pool.
[0030] The process for multi-pass welding process for joining butt of stainless steel tubes (100a, 100b) having high wall thickness comprising the following procedures beginning with initiating a welding arc between the tubes by means of a current carrying filler wire, feeding the welding arc using a wire feeder from a wire spool (105) to a groove between two tubes, followed by filling the groove by depositing a melting wire as weldment and enclosing the groove by alternatingly supplying shielding gases from gas cylinders or any other gas resources (106) through gas alternator (107) at pre-determined flow rate and for a pre-determined time period cyclically. The minimum number of passes is at-least three during the welding process and can be increased or decreased as per requirement.
[0031] The welding process is a semi-automatic Gas Metal Arc Welding Process (GMAW). The shielding gas used in the welding process includes carbon dioxide (CO2) and argon (Ar).
[0032] The present GMAW welding process maybe used to make full penetration welds with or without a root gap and is capable of welding stainless steel tubes having any value of thickness and diameter. The process can be carried out with or without preheating and post heating of the tubes. The Gas Metal Arc Welding Process reduces the production cycle time to a large extent and results into efficient joining of butt in comparison to the conventional methods of welding of stainless steel tubes.
[0033] It will be understood by those within the art that, in general, terms used herein, and especially in the appended claims (e.g., bodies of the appended claims) are generally intended as “open” terms (e.g., the term “including” should be interpreted as “including but not limited to,” the term “having” should be interpreted as “having at least,” the term “includes” should be interpreted as “includes but is not limited to,” etc.). It will be further understood by those within the art that if a specific number of an introduced claim recitation is intended, such an intent will be explicitly recited in the claim, and in the absence of such recitation no such intent is present. For example, as an aid to understanding, the following appended claims may contain usage of the introductory phrases “at least one” and “one or more” to introduce claim recitations. However, the use of such phrases should not be construed to imply that the introduction of a claim recitation by the indefinite articles “a” or “an” limits any particular claim containing such introduced claim recitation to inventions containing only one such recitation, even when the same claim includes the introductory phrases “one or more” or “at least one” and indefinite articles such as “a” or “an” (e.g., “a” and/or “an” should typically be interpreted to mean “at least one” or “one or more”); the same holds true for the use of definite articles used to introduce claim recitations. In addition, even if a specific number of an introduced claim recitation is explicitly recited, those skilled in the art will recognize that such recitation should typically be interpreted to mean at least the recited number (e.g., the bare recitation of “two recitations,” without other modifiers, typically means at least two recitations, or two or more recitations). It will be further understood by those within the art that virtually any disjunctive word and/or phrase presenting two or more alternative terms, whether in the description, claims, or drawings, should be understood to contemplate the possibilities of including one of the terms, either of the terms, or both terms. For example, the phrase “A or B” will be understood to include the possibilities of “A” or “B” or “A and B.”
[0034] It will be further appreciated that functions or structures of a plurality of components or steps may be combined into a single component or step, or the functions or structures of one-step or component may be split among plural steps or components. The present invention contemplates all of these combinations. Unless stated otherwise, dimensions and geometries of the various structures depicted herein are not intended to be restrictive of the invention, and other dimensions or geometries are possible. In addition, while a feature of the present invention may have been described in the context of only one of the illustrated embodiments, such feature may be combined with one or more other features of other embodiments, for any given application. It will also be appreciated from the above that the fabrication of the unique structures herein and the operation thereof also constitute methods in accordance with the present invention. The present invention also encompasses intermediate and end products resulting from the practice of the methods herein. The use of “comprising” or “including” also contemplates embodiments that “consist essentially of” or “consist of” the recited feature.
[0035] Although embodiments for the present subject matter have been described in language specific to structural features, it is to be understood that the present subject matter is not necessarily limited to the specific features described. Rather, the specific features and methods are disclosed as embodiments for the present subject matter. Numerous modifications and adaptations of the system/component of the present invention will be apparent to those skilled in the art, and thus it is intended by the appended claims to cover all such modifications and adaptations which fall within the scope of the present subject matter.
[0036] Reference Numerals:
Reference Numerals Description
100a, 100b Super 304H Stainless steel tubes
101 Chuck
102 Power Source
103 Torch
104 Wire feeder
105 Wire Spool
106 Gas Resource
107 Gas Alternator

[0037] Table 1:
Chemical composition of Super 304 H Stainless Steel Tubes

Alloy % C % Cr % Ni Mn P Si Nb N Cu
Super 304H 0.07-0.13 17-19 7.5-10.5 1.00 max 0.027 0.30 max 0.3-0.6 0.05-0.12 2.5-3.5

Claims:We Claim:
1. A multi-pass welding process for joining butt of stainless steel tubes (100a, 100b) having high wall thickness, the process comprising:
initiating welding arc between the tubes (100a,100b) by means of a current carrying filler wire;
feeding the welding arc using a wire feeder from a wire spool (105) to a groove between two tubes (100a,100b);
filling the groove by depositing a melting wire as weldment; and
enclosing the groove by alternatingly supplying shielding gases from gas resource (106) through gas alternator (107) at pre-determined flow rate and for a pre-determined time period cyclically.
2. The process as claimed in claim 1, wherein the welding process includes semi-automatic Gas Metal Arc Welding Process (GMAW) with gas alternator.
3. The process as claimed in claim 1, wherein the tubes are composed of Super 304H stainless steel.
4. The process as claimed in claim 1, wherein the value of current is between 85-110 A and voltage between 20-30 V.
5. The process as claimed in claim 1, wherein the value of welding speed is in the range of 230 mm/ min to 340mm/ min.
6. The process as claimed in claim 1, wherein the groove is a ‘V’ shaped groove having a groove angle (?) between 37.5 to 45o C.
7. The process as claimed in claim 1, wherein the pre-determined flow rate is 17 litres per minute for argon and 2 litres per minute for CO2.
8. The process as claimed in claim 1, wherein argon flows for 0.3 seconds and CO2 flows for 0.1 second through the gas alternator (107).
9. The process as claimed in claim 1, wherein the number of passes is dependent on the thickness (t) of the tubes (100a, 100b).