Claims:We Claim
1. A method for welding and testing dissimilar metals for use in fossil boiler operations, the method comprising:
welding of first base steel (1a) with second base steel (1b) using a filler material through a Hot Wire Gas Tungsten Arc Welding (GTAW) process which includes a total of seven passes;
measuring the chemical composition of the first base steel (1a) and the second base steel (1b) along with the chemical composition of the filler material using Optical Emission Spectrometry Technique;
cutting the first base steel (1a) and the second base steel (1b) by placing on a cutting machine according to the American Welding Society (AWS) 5.14/5.14M standard to prepare groves at the edges;
purging the first base steel (1a) and the second base steel (1b) with argon gas during root pass and hot pass;
clamping the first base steel (1a) and second base steel (1b) using clamps to restrict or reduce distortion during subsequent passes;
cleaning of the filler material using acetone and a stainless-steel wire brush for removal of any inter-pass oxides;
testing of resulting cross-weldments using radiographic testing process;
testing of tensile strength of the cross-weldment at various temperatures by subjecting the weldment to 180o face and root bend; and
evaluating any development of cracks if any in the cross-weldment sample by recording parameters.
2. The method as claimed in claim 1, wherein diameter of the filler is maintained at 1.14 mm during the Hot Wire Gas Tungsten Arc Welding (GTAW) process.
3. The method as claimed in claim 1, wherein the shielding flow rate is maintained at 12 litres per minute (LPM) during the Hot Wire Gas Tungsten Arc Welding (GTAW) process.
4. The method as claimed in claim 1, wherein back purging is carried out using argon gas at the rate of 4 LPM between the root pass and hot pass.
5. The method as claimed in claim 1, wherein the method further comprising maintaining inter-pass temperature setting at <150o C between the root pass and hot pass.
6. The method as claimed in claim 1, wherein flow of current is set in the range of 110-130A and voltage is set in the range of 11-12 V during the Gas Tungsten Arc Welding (GTAW) process.
7. The method as claimed in claim 1, wherein the method comprising maintaining hot wire current at 40 A during the Gas Tungsten Arc Welding (GTAW) process.
8. The method as claimed in claim 1, wherein the welding speed is maintained at a rate in the range of 70-80 mm per minute during the Gas Tungsten Arc Welding (GTAW) process.
9. The method as claimed in claim 1, wherein the wire feed speed is set at a rate of 1000 mm per minute during the Gas Tungsten Arc Welding (GTAW) process.
10. The method as claimed in claim 1, wherein the method comprising welding of the first base steel (1a) and second base steel (1b) at the edges using the filler material until a thickness of 12 mm is obtained.
, Description:FIELD OF INVENTION:
[001] The present invention relates in general to the field of welding technology, and particularly to a method using Hot Wire Gas Tungsten Arc Welding (GTAW) process for welding of two dissimilar metal alloys steel for application in Advanced Ultra Super Critical Power Plants.

BACKGROUND OF INVENTION:
[002] In Advanced Ultra Super Critical (AUSC) power plants, the steam temperatures vary in the range of 700-720oC. Such a high working temperature becomes essential in Super-heater, Re-heater sections of the Boiler and in High Pressure (HP) and Intermediate Pressure (IP) turbines in order to increase the efficiency of such thermal machines. It is preferred to use Ni-base superalloys in the manufacturing of boiler sections and valves so as to withstand such high temperatures. However, the cost of Ni-based superalloys is considerably expensive when compared to standard steels. On the other hand, the extent of exposure to heat in various sections of the boiler varies from very high temperatures to moderately high temperatures. In such cases, the cost can be optimized by producing parts of the boiler which have exposure to moderately high temperatures i.e. 640-680 oC using 22Cr25NiWCoCu steel. Welding of the metal alloys is more preferable for joining the parts rather than joining through screws because these boilers are usually exposed to high loads during their operation and thus welding would be a much safer option. Consequently, the welding of dissimilar steel becomes inevitable. It can be observed that both the steel alloys are absolutely austenitic stainless steels with a high possibility of hot cracking during the welding process and also the difference of strength between the two stainless steel alloys is very high which creates difficulty in the joining of these two metal alloys during the welding procedure.

[003] US patent No: US6753504B2 discloses a procedure for welding a Ni-base superalloy with steel and to produce turbomachine using this welding procedure. The disclosed method involves the joining by having an intermediate layer (IN 617) on the Ni-base superalloy IN 625 containing Nb which leads to weld solidification cracking and then joining this layer to the steel using Metal Active Gas process or Tungsten Inert Gas welding process.

[004] European patent No: EP2527073A1 discloses a procedure for obtaining dissimilar metal welds using buttering technique wherein the metal weld is formed by welding materials having a different composition and different refining conditions for application in the manufacturing of large components such as the turbine rotor. The dilution ratio of the buttering layer to the parent metal is 50% or less.

[005] US patent No: US8308051B2 discloses a welding method for welding dissimilar metals involving one high melting point material with one lower melting point material using friction stir welding process. The method involves the presence of a rotary tool which makes the interface softer and later joins them. However, friction stir welding cannot make weld joints in which there is a requirement of metal deposition and heavy-duty clamping setup is required to hold the workpiece or job during the welding process which is slower.

[006] US patent No: US20100028705A1 discloses a method wherein a tube joint for joining dissimilar metal sections of a superheater or reheater tube is formed using a hot isostatic press process. A first end of the tube joint is formed from a first metal which has substantially the same chemical composition as that of one section of the Super-heater or re-heater tube, and a second end of the tube joint is formed from a second metal which has substantially the same chemical composition as a metal used to form the other section of the superheater or reheater tube. Because the ends of the tube joint are made of substantially the same metal as the respective tube sections to which they attach, the welds may be performed using a standard fusion welding process. However, the equipment and tooling are more complex, the operation is inherently batch rather than continuous, and the processes overall are more expensive than the sequential approach of compaction followed by conventional sintering.

[007] WIPO patent No: WO2001083151A1 discloses joining of dissimilar metals such as steels and aluminum using a new technique. In this method, aluminum is retrogressively heat-treated locally at the joint prior to electromagnetic forming (EMF) or magnetic pulse welding (MPW) leading to the development of stronger welds. Although this technique doesn’t require heating during welding process the parts must overlap and it poses a limitation for lacking flexibility for different geometries along with the requirement of flyer materials which are electrically conducting.

[008] European patent No: EP2617512A1 discloses the procedure for producing weld joints between Ni-base superalloy impeller and steel shaft. The welds are obtained by fusing the metal using the mixing ratio by weight in the ratios of 0.5 to 0.8 to obtain welds which are sound and crack-free in a boundary between the welding metal and Ni-based super-alloy. However, this process is associated with distortion and residual stress generation as it involves melting and solidification. Additionally, palpable heat-affected zone (HAZ) exists in the welded components and the heat-affected zone is always considered as the weak portion is welded assembly. Mechanical properties of parent materials are also severely affected by intense heating. Joining dissimilar metals by fusion welding is a challenging task, especially if the metals have a substantially different melting point and coefficient of thermal expansion.
Therefore, there is a requirement of a method of welding to overcome the above-mentioned problem as disclosed in the conventional method or in the prior arts.
[009] Keywords: HOT Wire Gas Tungsten Arc Welding (GTAW), Dissimilar metal welds, Radiographic examination, Advanced Ultra Super Critical (AUSC) power plants
OBJECT OF THE INVENTION
[0010] An object of the present subject matter is to develop a method for welding of dissimilar metals using Hot Wire Gas Tungsten Arc Welding (GTAW) process

[0011] Another object of the present invention is to provide a welding procedure so as to produce a defect-free weldments/ welding element.

[0012] Yet another object of the present invention is to optimize the parameters of the welding process involved in the joining of two dissimilar metals.

[0013] Still another object of the present invention is to develop a comprehensive heat treatment sequence during the complete hot wire gas tungsten arc welding (GTAW) process.

SUMMARY OF INVENTION
[0014] The present subject matter relates to a method for welding and testing dissimilar metals for use in fossil boiler operations comprising of welding of first base steel with second base steel using a filler material through a Hot Wire Gas Tungsten Arc Welding (GTAW) process which includes a total of seven passes. Then, measuring the chemical composition of the first base steel and the second base steel along with the chemical composition of the filler material using Optical Emission Spectrometry Technique ensuring compliance with the American Society of Mechanical Engineers (ASME) specifications followed by cutting of the first base steel and the second base steel by placing on a cutting machine according to the American Welding Society (AWS) 5.14/5.14M standard to prepare groves at the edges. Further, purging of the first base steel and the second base steel with argon gas is done during root pass and hot pass. Subsequently, the first base steel and second base steel are clamped using clamps to restrict or reduce distortion during subsequent passes and cleaning of the filler material is carried out using acetone and a stainless-steel wire brush for removal of any inter-pass oxides. This is followed by testing of resulting cross-weldments using the radiographic testing process and the testing of the tensile strength of the cross-weldment at various temperatures by subjecting the weldment to 180o face and root bend and evaluation of the cross-weldment sample for any development of cracks by recording parameters. Further, there is no requirement of any post welding heat treatment after the complete welding process.

[0015] This method has been observed to be repeatable as 10 pairs of base metal plates can be successfully welded using the method described in the present invention. Hence, this technique can be relied upon for situations which demand reproducibility. Moreover, the weldments produced as a result of this welding method have the ability to qualify the side bend tests without showing up any cracks. Additionally, the method is not limited in its applicability to tubes but can also be used for welding of plates, pipes, forged sections, etc. where weldments of 22Cr25NiWCoCu steel(1a) and 18Cr09Ni03CuVNb steel(1b) are envisaged. Such resultant weldments have high applicability in Advanced Ultra Super Critical Power Plant component applications.

[0016] In an aspect, the diameter of the filler is maintained at 1.14 mm during the Hot Wire Gas Tungsten Arc Welding (GTAW) process.

[0017] In an aspect, the shielding flow rate is maintained at 12 liters per minute (LPM) during the Hot Wire Gas Tungsten Arc Welding (GTAW) process.

[0018] In an aspect, back purging is carried out using argon gas at the rate of 4 LPM between the root pass and hot pass.

[0019] In an aspect, the inter-pass temperature setting is maintained at <150° C between the root pass and hot pass.
[0020] In an aspect the flow of current is set in the range of 110-130A and voltage is set in the range of 11-12 V during the Gas Tungsten Arc Welding (GTAW) process.

[0021] In an aspect, the hot wire current is maintained at 40 A during the Gas Tungsten Arc Welding (GTAW) process.

[0022] In an aspect, the welding speed is maintained at a rate in the range of 70-80 mm per minute during the Gas Tungsten Arc Welding (GTAW) process.

[0023] In an aspect, the wire feed speed is set at a rate of 1000 mm per minute during the Gas Tungsten Arc Welding (GTAW) process.

[0024] In an aspect, the welding of the first base steel (1a) and second base steel (1b) at the edges using the filler material is continued until a thickness of 12 mm is obtained.

[0025] In order to further understand the characteristics and technical contents of the present subject matter, a description relating thereto will be made with reference to the accompanying drawings. However, the drawings are illustrative only but not used to limit the scope of the present subject matter.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
[0026] 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, the left-most digit (s) of 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 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:

[0027] Figure 1 illustrates a longitudinal view and depicts the various geometric parameters required to be met for the preparation of the edge as produced during the welding process. The tube on left and right have to be considered as 22Cr25NiWCoCu steel(1a) and 18Cr09Ni03CuVNb steel(1b) respectively.

[0028] 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 OF THE INVENTION
[0029] At the very outset of the detailed description, it may be understood that the ensuing description only illustrates a form of this invention. However, such a form is the only exemplary embodiment, and without intending to imply any limitation on the scope of this invention. Accordingly, the description is to be understood as an exemplary embodiment and teaching of invention and not intended to be taken restrictively.

[0030] Throughout the description and claims of this specification, the phrases “comprise” and “contain” and variations of them mean “including but not limited to”, and are not intended to exclude other moieties, additives, components, integers or steps. Thus, the singular encompasses the plural unless the context otherwise requires. Wherever there is an indefinite article used, the specification is to be understood as contemplating plurality as well as singularity unless the context requires otherwise.
[0031] Thus, the terms “comprises”, “comprising”, or any other variations thereof used in the disclosure, are intended to cover a non-exclusive inclusion, such that a device, system, assembly that comprises a list of components does not include- only those components but may include other components not expressly listed or inherent to such system, or assembly, or device.

[0032] In other words, one or more elements in a system or device proceeded by “comprises… a” does not, without more constraints, preclude the existence of other elements or additional elements in the system, apparatus or device.

[0033] Features, integers, characteristics, compounds, chemical moieties or groups described in conjunction with an aspect, embodiment or example of the invention are to be understood to be applicable to any other aspect, embodiment or example described herein unless incompatible therewith. All the features disclosed in this specification (including any accompanying claims, abstract and drawings), and/or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at least some of such features and/or steps are mutually exclusive. The invention is not restricted to the details of any foregoing embodiments. The invention extends to any novel one, or any novel combination, of the features disclosed in this specification including any accompanying claims, abstract and drawings or any parts thereof, or to any novel one, or any novel combination, of the steps of any method or process so disclosed.

[0034] The reader's attention is directed to all papers and documents which are filed concurrently with or before this specification in connection with this application and which are open to public inspection with this specification, and the contents of all such papers and documents are incorporated herein by reference. Post filing patents, the original peer-reviewed research paper shall be published.
[0035] The following descriptions of embodiments and examples are offered by way of illustration and not by way of limitation.

[0036] Unless contraindicated or noted otherwise, throughout this specification, the terms “a” and “an” mean one or more, and the term “or” means and/or. As used in the description herein and throughout the claims that follow, the meaning of “a.” “an,” and “the” includes plural reference unless the context clearly dictates otherwise. Also, as used in the description herein, the meaning of “in” includes “in” and “on” unless the context clearly dictates otherwise.

[0037] It may especially be noted that no animals have been harmed intentionally or unintentionally for the purposes of this invention. The animals used for the purpose of confirming the safety and efficacy of the present invention were all treated with the utmost care, and the smallest possible wound is made in a controlled manner following the treatment of the same with the outcome of the present invention.

[0038] The present subject matter relates to a method of welding two dissimilar metals for having an application in fossil boilers used in Advanced Ultra Super Critical (AUSC) Power plants. The base metal alloys preferred for this method are two types of steel namely 22Cr25NiWCoCu steel (1a) and 18Cr09Ni03CuVNb steel (1b). The filler material preferably to be used for welding is ER NiCrFe-7a grade. The chemical composition of the base metals alloys and the filler material is measured by using the Optical Emission Spectrometry technique so that it is in compliance with the American Society of Mechanical Engineers (ASME) specifications.

[0039] Optical Emission Spectrometry (OES), is a reliable and extensively used analytical technique used to establish the elemental composition of a wide range of metals. The type of samples which can be analyzed using OES include samples from the melt in primary and secondary metal production, and in the metals processing industries bolts, tubes, wires, rods, plates and so on. OES uses the part of the electromagnetic spectrum — the visible spectrum and part of the ultraviolet spectrum. That’s from 130 nanometers up to around 800 nanometers, in terms of wavelength. OES is capable of analyzing a wide range of elements from Lithium to Uranium in solid metal examples covering an extensive concentration range, providing low detection limits, high precision, and very high accuracy. The elements and concentrations that can be determined by OES analyzers depend on the material being tested and the type of analyzer used. There are three major components of OES analyzers: the first is an electrical source to trigger atoms within a metallic sample so that they produce characteristic light, or optical emission, lines, requiring a small part of the sample to be heated to thousands of degrees Celsius. This is achieved using an electrical high voltage source in the spectrometer through an electrode. The difference in electrical potential between the electrode and sample generates an electrical discharge, which enters the sample and causes the material at the surface to heat and vaporize. The atoms of the material are subsequently excited, emitting the element-characteristic emission lines. It is possible to produce two forms of electrical discharge, either an arc which is an on/off event just like a lightning strike, or a spark which is a series of multi-discharge events where the electrode’s voltage is switched on and off. Based on the element measured and the accuracy required, these two modes of operation are employed. An optical system is the second component. The light - the multiple optical emission lines from the vaporized sample called plasma — enters the spectrometer. The incoming light is split into element-specific wavelengths by a diffraction grating in the spectrometer and the intensity of light for each element-specific wavelength is measured by a corresponding detector. The intensity measured is proportional to the concentration offset element in the sample. A computer system is the third component. The computer system obtains the measured intensities and processes this data- through a predefined calibration to produce elemental concentrations. The user interface guarantees minimal operator intervention with clearly displayed results, which can be either printed or stored for future reference.
[0040] The base metal plates have to be cut and machined as per American Welding Society (AWS) A 5.14/5.14M standard. The schematic arrangement of the edge machined plates before welding has been illustrated in Fig. 1. Groves prepared at the edges of the metal, made up of 22Cr25NiWCoCu steel (1a) and 18Cr09Ni03CuVNb steel (1b) tubes, are welded with ErNiCrFe-7a using Semi-Automatic Gas Tungsten Arc Welding (GTAW) process until a thickness of 12 mm has been attained.

[0041] A backing ring during welding is not needed because there is already a provision for Argon gas back-purging during root pass welding. Argon gas purging is given during the root pass welding and subsequently during the hot pass welding. Further, purging gas is not required during the remaining passes and the tubes are clamped using a fixture to avoid distortion during these passes when there is the absence of purging gas.

[0042] Further, the welding process is carried out by using semi-automatic Hot Wire Gas Tungsten Arc Welding (GTAW) system. Between these passes, the material is cleaned using acetone followed by a stainless steel wire brush to remove any inter-pass oxides. During the various trials of this welding process, defects such as porosity, lack of fusion, weld depression, etc. have to be detected by radiographic testing.

[0043] Radiographic Testing (RT) is a non-destructive examination (NDE) technique that involves the use of either x-rays or gamma rays to view the internal structure of a component. In the petrochemical industry, RT is often used to inspect machineries, such as pressure vessels and valves, to detect flaws. RT is also used to inspect weld repairs.

[0044] Compared to other NDE techniques, radiography has several advantages. It is highly reproducible, can be used on a variety of materials, and the data gathered can be stored for later analysis. Radiography is an effective tool that requires very little surface preparation. Moreover, many radiographic systems are portable, which allows for use in the field and at elevated positions. These are later eliminated by optimizing the welding parameters. The parameters employed during the welding process been indicated in Table 1 and need to be optimized. A total of seven passes is required for the successful completion of this welding process. Subsequently, only those weld tubes which pass the radiographic testing are considered for further characterization and sample preparation.

[0045] Tensile strength testing is carried out at various temperatures, for cross-weldment specimens and the results are reported as shown in example ahead. The cross-section of weldments has to be cut from the radiographically qualified portions. These weldments have to be further subjected to 180o face and root bend testing. Upon examination of the metal surface, it may be observed and noted that the weld regions do not develop any cracks during bend testing. This indicates the successful development of a welding procedure for these welds.

[0046] The present formulation is further clarified by giving the following exhibits. It must, however, be understood that these exhibits are only illustrative in nature and should not be taken as limitations to the capacity of the invention. Several amendments and improvements to the disclosed segments will be obvious to those skilled in the art. Thus, these amendments and improvements may be made without deviating from the scope of the invention.

Example 1:
Optimized welding parameters:
[0047] For example, the best results were found by employing a fixed set of optimized parameters during welding using the Gas Tungsten Arc Welding (GTAW) process. As given in Table 1, the parameters of the welding process accomplished by using base metal alloys 22Cr25NiWCoCu steel (1a) and 18Cr09Ni03CuVNb steel (1b) and ER NiCrFe-7a grade as the filler material. Further, to initiate the process the base metal plates have to be cut and machined as per American Welding Society (AWS) A 5.14/ 5.14 M standard. The schematic arrangement of the edge machined plates before welding has been illustrated in Fig. 1. Groves prepared at the edges of the metal, made up of 22Cr25NiWCoCu steel (1a) and 18Cr09Ni03CuVNb steel (1b) tubes, are welded with ErNiCrFe-7a as a filler of diameter 1.14 mm using Semi-Automatic Gas Tungsten Arc Welding (GTAW) process until a thickness of 12 mm has been attained. The shielding gas flow rate during the welding of the metals has to be maintained at 12 liters per minute (LPM). The Pre-heat temperature must be nil which means that no pre-heating is required and the inter-pass temperature should be set at a value <150oC. The Voltage and Current flow should be maintained at 11-12V and 110-130A respectively where the hot wire current should be maintained at the value of 40A. The welding speed is to be varied in the optimum range of 70-80 mm/ min with the wire feed at the rate of 1000 mm/ min.

[0048] A backing ring during welding is not needed because there is already a provision for Argon gas back-purging during root pass welding. Argon gas purging is given at the rate of 4 liters per minute (LPM) during the root pass welding and subsequently during the hot pass welding. Further, purging gas is not required during the remaining passes and the tubes are clamped using a fixture to avoid distortion during these passes when there is an absence of purging gas.

[0049] Further, the welding process is carried out by using semi-automatic Hot Wire Gas Tungsten Arc Welding (GTAW) system. Between these passes, the material is cleaned using acetone followed by a stainless steel wire brush to remove any inter-pass oxides.

Table 1: Optimized welding parameters
Welding parameter Details
Filler diameter 1.14 mm
Welding process Hot Wire GTAW
Shielding flow rate 12 LPM
Back Purging 4 LPM
Inter-pass temperature <150oC
Preheat temperature Pre-Heating not Required
Current 110-130A
Voltage 11-12 V
Hotwire current 40 A
Welding speed 70-80 mm/min
Wire feed 1000 mm/min
Example 2:
Tensile Strength Testing:
[0050] Tensile strength testing is carried out at various temperatures preferably at an ambient temperature of 675oC, for cross-weldment specimens and the results are reported as given in Table 2. The cross-section of weldments has to be cut from the radiographically qualified portions. These weldments must further be subjected to 180o face and root bend testing. Upon examination of the metal surface, it is observed and noted that the weld regions do not develop any cracks during bend testing. The method, therefore, makes it possible to obtain crack-free development of the cross weldments through this method which is the subject matter of the present invention. The results are given in Table 2 to achieve the object of the present invention.

Table 2: Average results after tensile testing at various temperatures
Temperature (oC) 0.2% Yield Strength
(Mega Pascal) Ultimate Tensile Strength (Mega Pascal) %
Elongation %
Reduction Area
25 408 669 28 60
650 263 367 24 68

[0051] Now, the crux of the invention is claimed implicitly and explicitly through the following claims.

[0052] Each of the appended claims defines a separate invention, which for infringement purposes is recognized as including equivalents to the various elements or limitations specified in the claims. Depending on the context, all references below to the “invention” may in some cases refer to certain specific embodiments only. In other cases, it will be recognized that references to the “invention” will refer to subject matter recited in one or more, but not necessarily all, of the claims.

[0053] Groupings of alternative elements or embodiments of the invention disclosed herein are not to be construed as limitations. Each group member can be referred to a claimed individually or in any combination with other members of the group or other elements found herein. One or more members of a group can be included in or deleted from, a group for reasons of convenience and/ or patentability. When any such inclusion or deletion occurs, the specification is herein deemed to contain the group as modified thus fulfilling the written description of all groups used in the appended claims.