FIELD OF INVENTION
The present invention relates to method of welding dissimilar metals of alloy 617 and alloy grade 91 steel plates based on optimized parameters. More particularly, the invention deals with development of a welding procedure and optimizing parameters for obtaining defect-free dissimilar metal welds of Alloy 617 (ASME Code Case: 2439) and Alloy grade 91 steel (ASTMA 387 Gr 91) plates which are approximately 25 mm thick using ASME ER NiCrFe-7a(AWS A5.14) filler material. This procedure can be applied for welding thick component sections which are primarily of interest in Advanced Ultra Super Critical Power Plants.
BACKGROUND OF THE INVENTION AND PRIOR ART
In Advanced Ultra Super Critical (AUSC) power plants, steam temperatures are of the order of 700/720oC in Superheater and Reheater sections of Boiler, High Pressure (HP) and Intermediate Pressure (IP) turbines. This requires boiler sections and valves to be made up of Ni-base superalloys for withstanding such high temperatures. But the entire components cannot be made up of Ni-base superalloys as the investment would be extremely high. Hence, moderately high temperatures of boiler sections where temperatures are of the order of 580/600oC are proposed to be made up of grade91 steel. the Therefore, welding of dissimilar materials, i.e., grade91 steel with Ni-base superalloys becomes inevitable. Further, post weld heat treatment temperature of alloy

617 and grade 91 has huge different, which makes this dissimilar jointing process highly difficult. Hence, it is proposed to develop thick section welds between grade 91 steel and Alloy 617 (Ni-base superalloy) for component applications in AUSC power plants where steam temperatures of weldment will be around 580/600oC.

Prior state of art
In US6753504B2 patent, a procedure for welding a Ni-base superalloy with a steel was

explained. This joining was carried out 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 to the steel using Metal Active Gas process or Tungsten Inert Gas welding process. This is employed for obtaining crack-free welded rotors for turbo-machinery applications.
In US20130301309A1 patent, a procedure for obtaining dissimilar metal welds using buttering technique is discussed in detail for applications related to welding large components such as turbine rotor. In the method disclosed by this patent, dilution ratio of buttering layer to metal is less than 50%. This can be employed for welding large components such as turbine rotor.
In US8308051B2 patent, welding method for welding dissimilar metals (i.e., one high melting point material with one lower melting point material) were discussed using friction stir welding process. The method involves presence of rotary tool which makes the interface softer and later joins them to obtain a sound weld joint. Joints between steel and aluminium were successfully obtained using this technique.
In US200880067214A1 patent, a novel method of having a transition for superheater or reheater tubes by mechanical forming and hot isostatic pressing (HIP) is discussed. This method eliminates the need for arc welding or other fusion welding processes. The transition is from ferritic T11 or T22 steel (ASME A213 type) to TP347 or TP 304 type austenitic steels (which resist higher temperatures).

WO2001083151A1 patent discusses joining dissimilar metals such as steels and aluminium using new technique. In this invention, Aluminium is retrogressively heat treated locally at the joint prior to electromagnetic forming (EMF) or magnetic pulse welding (MPW). This technique led to development of stronger welds.
EP2617512A1 patent discusses the procedure for producing weld joints between Ni-base superalloy impeller and steel shaft. The welds are obtained by fusing the metals (mixing ratio by weight) in the ratios of 0.5 to 0.8. The welds obtained are sound and crack-free.
With reference to the above patents, all of them are successful in development of dissimilar metal welds which are defect free. But they require sophisticated techniques such as HIP or EMF or MPW. None of them can employ techniques as such buttering with ER NiCrFe- 7a filler using Tungsten Inert Gas welding or Gas Tungsten Arc Welding directly and sequential post buttering heat treatment to avoid filler weldment heat treatment to obtain a dissimilar weld joint. In the present work, GTAW employed with commercially available ER NiCrFe- 7a filler butting in show welding with recommended heat treatment cycle for obtaining final weld on-site without any heat treatment processing , i.e., between grade 91 steel and Alloy 617 materials.
OBJECT OF THE INVENTION
Therefore, it is an object of the invention to propose a method of welding dissimilar

metals of alloy 617 and alloy grade 91 steel plates based on optimized parameters, which is capable of achieving a defect free dissimilar metal welds of thick component sections.
SUMMARY OF THE INVENTION
According to the present invention, a new procedure with optimized welding parameters was invented for obtaining dissimilar welds of Alloy 617 and grade 91 steel with ER NiCrFe-7a filler material using conventional, Gas Tungsten Arc Welding process. This method was observed to be repeatable as 10 pairs of base metal plates were successfully welded using the procedure laid out in the present invention. Hence, this technique can be relied on for reproducibility. Moreover, the weldments produced as a result of this welding procedure have qualified side bend tests without showing up any cracks. The welding process is hence, applicable not only for plates but also for tubes, pipe, forged sections, etc. where weldments of grade 91 steel and Alloy 617 are envisaged. These weldments can be used for Advanced Ultra Super Critical Power Plant component applications.
According to this procedure, edge preparation design as shown in Fig. 1 has to be followed while buttering. Edge prepared groves of alloy 617 and grade 91 steel plates are butted with ErNiCrFe-7a using Semi-automatic Gas Tungsten Arc Welding process up to the thickness of 15 mm. Alloy 617 butter base plates are subjected to post

buttering heat treatment at the temperature of 980°C for 2 hour and allowed cool inside the furnace for stress reliving purpose. Similarly buttered grade 91steel base plates are subjected to post butter heat treatment temperature of 760°C for 2 hour and allowed cool inside the furnace for tempering of grade 91 steel heat affected zone. These are then welded using ErNiCrFe-7a filler material (0.8 mm dia) with back purging of Argon gas. No Post Weld Heat Treatment is necessary after final welding.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWING
Figure 1: This figure indicates in detail various geometric parameters used for preparation of the edge for buttering and welding. The plate on left and right are assumed to be Alloy 617 and grade 91steel respectively
DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION
The base materials considered for this work are grade 91 steel (ASTM A387 Gr 91) and Alloy 617 (ASME Code Case 2439). The filler material used for welding is ERNiCrFe-7a) grade. The chemical compositions of the base metals and the filler material were measured by Optical Emission Spectrometry technique and they all complied with the ASME specifications. The base metal plates were cut and machined as per AWS A 5.14/5.14M standard. Schematic of the arrangement of edge machined plates before welding is indicated in Fig. 1. Edge prepared groves of alloy 617 and grade 91 steel

plates are butted with ErNiCrFe-7a using Semi-automatic Gas Tungsten Arc Welding process up to the thickness of 15 mm. Alloy 617 butter base plates are subjected to post buttering heat treatment at the temperature of 980°C for 2 hour and allowed cool inside the furnace for stress reliving purpose. Similarly buttered grade 91steel base plates are subjected to post butter heat treatment temperature of 760°C for 2 hour and allowed cool inside the furnace for tempering of grade 91 steel heat affected zone. Further, buttered layer of both base metal are machined up to 7 mm to meet the edge preparation design for given weld joint. The setup for welding was prepared with 2 Deg revered pre-bending to reduce the distortion. Run-on and run-off plates made of stainless steel were used during welding. There is no backing plate used during welding as Argon gas back-purging provision was given during root pass welding. During root pass welding and subsequent hot pass welding, the base metal plates were not clamped while during the remaining passes, the plates were clamped using a fixture to avoid distortion. Welding was carried out using semi-automatic Gas Tungsten Arc Welding (GTAW) system. A gap of 1.6 mm is maintained between the buttered heat treated base metal (X, Y) at the time of welding. Between passes, the material was cleaned using acetone followed by wire-brush to remove any inter-pass oxides. During various welding trials conducted, defects such as porosity, lack of fusion, weld depression, etc. were detected by radiographic testing. These were later eliminated by optimizing the welding parameters. The optimized parameters employed during welding are indicated in Table 1. It took a total of 42 passes to successfully complete

the welding. Inter-pass temperature to be maintained <150oC except for hot and root pass. Each stage residual stress has measured using hole drilling method to ensure the post heat treatment stress levels of alloy 617 side. It is noted in table 2 for reference. All the weld plates which have passed radiographic testing only were considered for further characterization and sample preparation after discarding 15 mm from each end. Tensile testing was carried out at various temperatures (ambient and 650oC) for cross-weldment specimens and results are reported in Table 3. The cross-section of weldments were cut from the radiographically qualified portions. These were subjected to 180o side bend testing. The weld regions did not develop any cracks during bend testing. This indicates the successful development of welding procedure for these welds.

WE CLAIM
1) Method of welding dissimilar metals of alloy 617 and alloy grade 91 steel plates based on optimized parameters, the said method comprising the steps of:
preparing the edges (E) of alloy 617 (X) and alloy grade 91 steel (Y) to obtain a V-groove (V) at the point of contact during welding;
butting the edges (E) of both base metal (X, Y) with Semi automatic Gas Tungsten Arc welding using Nickel-chrome-iron filler material up to a thickness of 15 mm (B);
subjecting buttered base plate of alloy 617 to post buttering heat treatment to a temperature of 980o for two hour and allowing the said plate to cool inside the furnace for achieving stress relieve;
subjecting buttered grade 91 steel base plate to post buttering heat treatment to a temperature of 760oC for 2 hour and allowing the said plate to cool inside the furnace for tempering grade 91 steel heat affected zone;
machining the buttered layer of both base metal up to 7 mm to meet required edge preparation;
wherein,
an optimized parameters are established conducting few trials when buttered heat treated base metals (X, Y) are welded using Nickel-chrome-iron filler material with GTAW system with back purging of Argon gas during root pass welding of

unclamped base metal plates (X, Y) wherein, the said plates (X, Y) are clamped during the remaining passes to avoid distortion while between passes, the material is cleaned with acetone followed by wire brush to remove any inter-pass oxides when inter pass temperature is maintained at less that 150oC except for hot and root pass to obtain a defect free welding of alloy 617 (X) and Grade 91 steel (Y) plates.
2) The method as claimed in claim 1, wherein a gap of 1.6 mm is maintained between the buttered heat treated base metal (X, Y) at the time of welding.