Enhancing microstructure and mechanical properties of dissimilar TIG welded duplex 2205 and Ni-based inconel 718 superalloy through post weld heat treatment

In demanding applications, maintaining structural integrity in dissimilar welded joints like those between duplex stainless steel and Ni-based superalloys requires achieving the best possible mechanical properties. This work examines the impact of post-weld heat treatment (PWHT) on the mechanical characteristics of joints made via double-sided tungsten inert gas (TIG) welding. The dissimilar welded joint was investigated by exposing it to the PWHT for 12 h at 650 °C. Reducing the negative impact of heat generated by welding on the mechanical characteristics and microstructure of the fusion zone was the main goal. To study for changes in the microstructure before and after PWHT, optical microscopy and scanning electron microscopy methods were used for microstructural analysis. To determine the effect of PWHT on the welded joints, mechanical properties such as tensile strength, toughness, and ductility were also assessed. The mechanical properties showed significantly enhanced characteristics and refined grain structures in the fusion zone.


Introduction
Dissimilar welding of nickel (Ni) based alloys can be used for diverse environmental conditions as well as at higher temperatures ascribed to their austenitic matrix.Power plants, aerospace, petrochemicals are the common applications for these alloys.These alloys exhibit attractive toughness and ductility at cryogenic temperatures as well as superior strength and corrosion resistance upto 1200 °C [1].On the other hand, duplex stainless steel (DSS) is from the category of stainless steel characterized by the equal amount of austenite and ferrite in their matrix.These alloys exhibit remarkable combination of superior mechanical and corrosion properties and are widely used in oil-gas industry [2].
The dissimilar welding is generally used to join two different materials having different mechanical and compositional characteristics and has gained significant attention over the years due to its potential for cost savings.Ni-based alloys are usually very costly and therefore, the dissimilar welding of Ni-based alloys with comparable mechanical and corrosion properties seems to be an attractive solution, and dissimilar welding of Ni-based alloys with DSSs are one of the most viable options.The dissimilar welding of Ni-based alloys with DSSs are becoming more prevalent and is replacing the convectional ferritic and austenitic stainless steel [3].This type of weld joint is in demand where the high corrosion resistance and high strength are essential, like oil and gas, chemical processing, and maritime industry [4].One common application where these dissimilar combination of materials is used in the manufacturing of components for offshore oil and gas platforms [5].These platforms operate in harsh environments where they are exposed to corrosive seawater, high pressures, and temperatures.In such applications, Ni-based alloy could be used for its excellent corrosion resistance, high temperature strength, and ability to withstand harsh environments, while DSS could be selected for its combination of high strength and corrosion resistance, particularly in chloride-containing environments.Dissimilar welding of DSS and Ni-based welds generally have good mechanical qualities, including outstanding resistance to corrosion and stress corrosion cracking, as well as good strength and toughness [6].Ahmad et al [7] welded the Inconel 625 with DSS 2205 using ytterbium fiber laser but welding with varying energy input and concluded that the with decrease in energy input, the weld bead width reduced which resulted in better mechanical properties.Tumer et al [8] studied the dissimilar welding properties of Inconel 625 and DSS S32205 using metal inert gas technique.They reported that austenite grain coarsening was observed in the heat affected zone (HAZ) of Inconel 625 side whereas ferrite grain coarsening along HAZ was observed in DSS S32205 side.The authors further reported that the Nb and Mo rich laves phases were observed in the weld metal and Inconel 625 HAZ.Maurya et al [9] studied the dissimilar welded super-DSS 2507 with Inconel 625 using gas tungsten arc welding (GTAW) technique with ER2594 filler electrode.The authors observed that the Mo and Nb rich phases segregated in interdendritic region of the weld zone, and its content was higher for the high heat input weldment.Ramkumar et al [10] studied the dissimilar welding of Inconel 625 and DSS S32750 via multi pass continuous and pulsed current GTAW using ER2553 and ERNiCrMo-4 filler electrodes.The authors reported that the brittle mode of fracture was observed for the ER2553 filler electrode for both the continuous and pulsed current GTAW weldments.On the other hand, ERNiCrMo-4 filler electrode weldments exhibited superior toughness and ductility.Madhankumar et al [11] performed the dissimilar laser butt welding of Inconel 718 and DSS 2205 with varying parameters and concluded that the tensile strength is a function of laser power, welding speed and focal position.
The versatility, precision, and high-quality welds that tungsten inert gas (TIG) welding produces on a variety of materials, including titanium, steel, stainless steel, and aluminum, make it extremely effective in plant industries [12].More control over the welding process is possible with TIG welding, which produces precise, clean welds with little distortion or spatter.However, because after welding involves high heat input and quick cooling rates, it can also alter the microstructure and mechanical characteristics of the weld zone, HAZ and different reason of the welding plate [13].The strength, ductility, and impact toughness of the welded joint can all be negatively impacted by these microstructural alterations [14].Different welding techniques may use Post Weld Heat Treatment (PWHT) to lessen the residual internal stresses and dislocation densities that come from the welding process in order to lessen these effects.PWHT contributes to the integrity and long-term usefulness of the welded joint by reducing stresses, improving the microstructure, and enhancing the mechanical characteristics of the joint [15].Gope and Chattopadhyaya [16] performed the PWHT on dissimilar GTAW welded Inconel 625/SS 316 at 700 °C for 2 h and concluded that the weldment after PWHT exhibited substantial enhancement in microstructure and microhardness.
Based upon the above literature, it can be concluded that dissimilar welding is essential, and it offers numerous advantages and the literatures on dissimilar welding of Inconel 718/DSS 2205 is scanty.Therefore, the aim of the current work is to conduct post-weld heat treatment (PWHT) on dissimilar DSS 2205 and Inconel 718 weld joint.In this research, dissimilar DSS 2205 and Inconel 718 were welded with double sided TIG welding technology.The double sided TIG is performed due to the speciality of low heat input of TIG welding which results in the improvement in the depth of penetration and reduce the distortion of the plate to be joined.This study also examines the changes in microstructural parameters like precipitation and grain formation before and after PWHT.The purpose of this study is to examine the impact of PWHT in improving the microstructural and mechanical characteristics of dissimilar DSS 2205 and Inconel 718 weld joints.

Experiments and method
The chemical compositions of base metal (BM), i.e., DSS 2205 and Inconel 718 used in this study are shown table 1.Both samples were procured in the form of 300 × 300 mm metallic sheet having 6 mm thickness, cut in the dimensions of 150 mm × 70 mm × 6 mm and annealed at 1050 °C for 1 h before welding.The TIG process was employed with a double V-groove configuration over single pass.Welding consumables ERNiCrMo-4 was chosen in this experiment, and the chemical compositions are listed in table 1.The experimental optimized welding parameters included a current of 160 A, voltage between 15 to 16 V, and a welding speed of 120 mm min −1 were used in this study.The heat input during the double sided TIG welding of dissimilar DSS 2205 and Inconel 718 plate of 6 mm thick was computed to be 0.28 kJ mm −1 .The efficiency during the welding was considered to be 75% [17].Figure 1 shows the weld plate photograph of double side dissimilar TIG welded alloy Inconel 718 and DSS 2205.To eliminate internal compressive stress developed due to double sided TIG, a PWHT was conducted at a temperature of 650 °C for a duration of 12 h and followed by furnace cooling at the rate of 100 °C per hour.The above PWHT parameters were selected based on prior literatures [15,[18][19][20].
To examine its microstructural and formation of various phases, the samples were cut in the dimension of 30 mm × 10 mm × 6 mm using a wire-cut electric discharge machine.The samples were sliced at an angle with respect to the weld interfaces.After that, the cross-sections of these joints were polished using silicon carbide polishing paper with grit sizes ranging from 400 to 2000, and then samples were cloth-polished using a suspension of 75 μm alumina.After that, the samples were ultrasonicated for 5 min in distilled water to get rid of the dirt and alumina particles.The samples were electrolytically etched in 10% chromic acid for 30 s to study the microstructural evolution.An optical microscope (Leica DMi8C) and a scanning electron microscope (SEM, Model: JEOL 6380A) were used for the microstructural analysis.X-ray diffraction (XRD, Model: XRD-6000 fitted with a Cu Kα tube) examination was performed to determine the phases and compounds formed in fusion zone of the weldments.The XRD was operated at a voltage of 40 kV and an amperage of 40 mA while maintaining a step size of 0.2°.The measured XRD patterns ranged from a 20°incidence angle to 95°.For mechanical characterization, tensile test, microhardness and impact test were performed.Tensile testing was performed in Instron universal testing machine (model − 4467) at a strain rate of 3 × 10 −4 s −1 as per the ASTM E8/8 M standard [21].The sub-sized tensile test specimen was prepared, measuring 100 mm×10 mm × 6 mm, featuring a gauge length of 25 mm.The microhardness tests (Shimadzu microhardness tester Shimadzu Corp., Kyoto, Japan) were conducted on a transverse section of the welded samples measuring dimension of 30 mm × 10 mm with a 200 g load applied for 15s.To determine the reproducible result of the hardness value of the samples under each condition, the experiment conducts a minimum of five indentations.The impact test (Charpy V-notch Impact Testing (Pendulum Impact Tester Model-IT 30)), specimen was prepared according to ASTM E23, measuring 55 mm × 10 mm × 6 mm, featuring a V-notch with a depth of 2 mm at a 45°angle and impact tests were performed at room temperature [22].After the tensile and impact test, the fractrography analysis was performed using SEM.The TIG welding parameters used in the present study are presented in table 2.

Results and discussion
The cross-sectional macrographs of the dissimilar DSS 2205 and Inconel 718 weld bead using double-sided TIG is displays in figure 2(a).These macrographs demonstrate how welding current affects both depth to width ratio and penetration depth.Complete penetration at the welding current of 160A was deduced.

Microstructure
The double-sided TIG welding procedure successfully bonded the plates based on the macrograph results.Figure 2(a) shows the cross-section macrograph of the double sided dissimilar TIG welded DSS 2205 and Inconel 718 material.The current value of 160A was used and maintained a constant welding speed (120 mm min −1 ), which allowed for full penetration of the as shown in figure 2(a).It was observed through physical means that the weldment produced by TIG welding was completely free of any spatter or undercut.Further, the nondestructive test was carried out to confirm the quality of the weld.The result showed that the weld joints were also free from porosity, lack of penetration or any other defects.No solidification or HAZ liquid cracking occurred after double sided TIG welding.The optical microstructures of the BMs, i.e., Inconel 718 steels and DSS 2205 after solution annealing at 1050 °C are shown in figures 2(b), (c).The Inconel 718 alloy revealed an almost austenitic matrix without a ferrite phase.Typically, the grain boundaries of the superalloy Inconel 718 contain carbides or carbonitrides [23].However, the DSS 2205 revealed a mixed microstructure made up of ferrite and austenite, roughly equal volumetric fraction of ferrite and austenite [24].Ferrite, the dark phase, contributes to strength while austenite, the light phase, enhances toughness and corrosion resistance.This balanced combination of phases grants DSS its exceptional blend of strength, toughness, and corrosion resistance [25].

Scanning electron microscope (SEM) analysis
The microstructural analysis of double-sided TIG welds between dissimilar materials, DSS 2205 and Inconel 718, reveals significant changes before and after post-weld heat treatment (PWHT), as depicted in figures 3-5 and the quantification of different phases formed is shown in figure 6. Figure 3 presents the interface microstructures of the welds before and after PWHT.Figures 4 and 5 show the weld zone microstructures before and after PWHT, respectively.It can be observed from the figures 3-5 that PWHT resulted in a significant changes in the microstructures of the weldments.Slight grain coarsening in both BMs matrix near the interface of BM and fusion zone were observed after PWHT treatment.The fusion zone microstructure of both conditions showed the presence of columnar grains formed at the edges of the fusion zone, whereas the formation of an equiaxed grain was observed at the center of the weld zone.The formation of columnar grains near the fusion boundary could be the reason of heat extraction in that place is more rapid, and the temperature  gradient is steeper [26].However, the formation of equiaxed grains at the center of the weld is attributed to the weld pool experiences a slower cooling rate compared to the fusion boundary resulting in slower cooling rate, which allows for the growth of grains in all directions.Manikandan et al [27] reported that during solidification, solute elements can accumulate ahead of the solidifying interface, causing constitutional supercooling led to  promote the nucleation of equiaxed grains in the center of the weld pool.Further, as compared to the as-welded condition, the cellular grain and columnar grain are denser and more compact after PWHT in near Inconel 718 weld interface (figures 4-5).And, in the weld center region, the increase in grain size of equiaxed grains can be observed and the discrete microstructure is more evenly distributed in mixed grain microstructure region after PWHT (figures 4-5).Figures 4-5 illustrates the significant number of irregular form phases (white spat) and laves phases that precipitated in the interdendritic areas of fusion zone of dissimilar welded Inconel 718 and DSS 2205 joints.The XRD examination was carried out to confirm the formation of these precipitates.The phases are recognized as the metallic carbide (MC) particles, niobium carbide (NbC) and laves phase from XRD results (figure 6(a)).During the dissimilar welding process, the refractory elements Nb and Mo segregate under non-equilibrium solidification conditions, resulting in the creation of the laves phase and MC carbides [28].Avinash et al [29] also documented in their study that during the solidification process of Inconel 617 alloy, there is the occurrence of laves phase and metallic carbide formation within the interdendritic regions of the weld zone attributed to the  segregation of Nb and Mo.Further, it can be noticed that the secondary phases (figures 4-5) have been reduced after PWHT, as evident from XRD analysis (figure 6(b)).PWHT can cause these precipitates (MC, NbC) to dissolve back into the matrix, resulting in their disappearance from the XRD pattern.Similar results were obtained by Saini et al [30].

Tensile test
The tensile test findings for the BMs and weld samples, showing the tensile strength and elongation is summarized in figure 7. It is evident that the BM Inconel 718 depicted the highest tensile strength (839 MPa) along with the highest elongation value of 24.8%.The strengthening precipitates in Inconel 718, formed by the ordered body-centered tetragonal phase of Ni 3 Nb also known to be Gamma precipitates having face-centered cubic Ni 3 (Al, Ti) precipitates that also contribute to the strengthening of the alloy [31].And the BM DSS 2205 depicted the least tensile strength (691 MPa) and elongation (19.8%).
For the weldments, the weldment before PWHT depicts lower tensile strength and elongation as compared to the after PWHT weldment (figure 7).The lower tensile strength (756 MPa) and elongation (20.63%) for before PWHT weldment can be attributed to the potential development of carbides.After PWHT, the dissolving carbides and laves phase could be the reason for this tensile strength enhancement.Zhao et al [32] previously observed an improvement in tensile strength following the PWHT procedure, while investigating the impact of PWHT on the mechanical parameters of structural steel welded plate in grade S690.They have reported that the reduced sensitivity of the material to defects such as weld discontinuities or microstructural imperfections, act as stress concentration points and decreased tensile strength in the case of welded sample.However, after PWHT the weld discontinuities decreased along with grain refinement while relieving the internal stresses of the grain.The fracture region during the tensile testing of the weldments was observed to be BM DSS 2205, which was the weaker BM (figure 7).This observation suggests that the dissimilar weldments possess superior strength compared to the BM DSS 2205, which also shows the proper mixing filler electrode and BMs resulting in successful joint.However, the tensile results of the weldments were inferior to the BM Inconel 718.
The fractrograph results of tensile tests before and after PWHT are shown in figure 8.A ductile mode of fracture was also indicated by the presence of dimpled facets and macro voids with ductile ridges (figure 8) of the weldments using ErNiCrMo-4 filler that were subjected to tensile testing before and after heat treatment.This indicates that the material underwent plastic deformation before fracturing, which is characteristic of ductile materials.

Microhardness test
The measured microhardness values of the BMs and weldments before and after PHT is summarized in figure 9.It was observed that the microhardness of the BM Inconel 718 differed from that of BM DSS 2205.The hardness of the BM Inconel 718 (380 ± 7 HV) and BM DSS 2205 (292 ± 4 HV) was shown to be impaired by the attained fusion zone microhardness values (before PWHT (356 ± 9 HV) and after PWHT (341 ± 12 HV)).The heat treatment performed following the dissimilar weld, resulted in dissolving formed carbides and cause secondary phase conversion due to increased temperature, causing a significant drop in microhardness of the weldment.The reduction in the intensity of carbides can be observed from XRD analysis (figure 6).Olden et al [33] reported that the high-temperature exposure during PWHT causes the highly strained and hardened structure to undergo tempering.During heat treatment, the carbon atoms trapped in the hard structured lattice diffuse and precipitate as fine carbides, reducing the internal stresses and distortions in the microstructure resulted decrease in hardness and an increase in toughness.

Impact test
Figure 10 displays the impact energy of BMs and weldments of before and after PWHT.With an impact energy of 141 ± 3 J, the BM DSS 2205 depicted the highest value among all BMs and welded samples (both prior to and following heat treatment) while the BM Inconel 718 depicted the lowest impact energy of 86 ± 5 J.The weldment after PWHT depicted the larger impact energy (109 ± 2 J), whereas the sample welded before PWHT has a lower impact energy (96 ± 3 J).The uneven elemental composition of the fusion zone, caused by the diffusion of elements from the BM and filler electrode, along with the formation of precipitates due to high temperature during welding, notably impacts the toughness values of the weldments.The presence of carbides  resulting from the welding process has led to a reduction in the impact of the toughness values of the weldment.It is well determined that the PWHT improves the impact energy of the weldments as compared to the as-welded weldment due to the carbide precipitates dissolution and refined grain structure.A more refined grain structure makes the material more resilient by preventing dislocation motions.The hardness and brittleness at the junction decreased upon the removal of carbides from the weld joint microstructure.As a result, the toughness increased.
The fractographs of the before and after PWHT weldments that were subjected to impact testing is shown in figure 11.The presence of dimpled facets with cracked boundaries indicates a ductile mode of fracture can be observed.Ductile mode of fracture implies that the material deformed plastically before fracturing, indicating a higher toughness and resistance to sudden brittle failure, which is often desirable in structural applications subjected to impact loading [34].

Conclusion
A dissimilar welding study was conducted on DSS 2205 and Inconel 718 alloys to investigate the effects of PWHT on microstructure and mechanical properties.The investigation yielded several significant conclusions:  1.The double-sided TIG welding process could be used to successfully weld the dissimilar DSS 2205 and Inconel 718 BMs.The grain size of the fusion zone after PWHT were found to be increased as compared to the as-welded joints.Further, both welded joints nearby weld interfaces showed no signs of a mixed zone or HAZ formation, according to microstructure investigations.
2. The intermetallic phases, such as, MoC, MC, NbC, and laves phases formed during welding in the fusion zone, however, the phases dissolved after the PWHT.
3. The mechanical characteristics of the weldment improved by PWHT.Following PWHT, the laves-phase precipitates decreased, which played a key role in enhancing the mechanical properties.
4. Tensile test illustrates that the PWHT joint depicted higher strength as well as higher % elongation over the as-welded joint.In both the weldments, the fractrography results the dimple facets along with few macro voids, thus showing the ductile mode of fracture.
5. The fusion zone of weldments of as-welded joint depicted higher microhardness (356 ± 9 HV) as compared to fusion zone of weldments after PWHT (341 ± 12 HV).This decrease in the microhardness value may be the result of reduced carbide precipitation in the fusion zone due to PWHT.
6.The PWHT showed the improvement in toughness of the weldment.The toughness values of of 96 ± 3 J was observed for as-welded joint which increased to 109 ± 2 J after PWHT.

Figure 2 .
Figure 2. Microstructures of (a) SEM macrograph of dissimilar weld joint, (b) optical micrographs of DSS BM and (c) optical micrographs of Inconel 718 BM.

Figure 3 .
Figure 3. SEM interface microstructures of the dissimilar DSS 2205 and Inconel 718 welded joints.

Figure 4 .
Figure 4. SEM images of fusion zone showing the different regions of the as-welded joint.

Figure 5 .
Figure 5. SEM images of fusion zone showing the different regions of the PWHT joint.

Figure 6 .
Figure 6.XRD plots of different phases in the welded sample for (a) before PWHT and (b) after PWHT.

Figure 7 .
Figure 7. Tensile test results of the BMs and weldments before and after PWHT.

Figure 8 .
Figure 8. SEM fractographs showing the fractured surface of the (a) as welded and (b) PWHT sample.

Figure 9 .
Figure 9. Microhardness test results of the BMs and weldments before and after PWHT.

Figure 10 .
Figure 10.Impact test results of the BMs and weldments before and after PWHT.

Figure 11 .
Figure 11.SEM fractographs of the impact tested samples of (a) as-welded and (b) PWHT sample.

Table 1 .
Chemical composition (wt%) of BMs and filler material.
Power source DCEN (Direct current electrode negative)