Mechanical characteristics and stretch-bend failure analysis on ultra high frequency pulsed gas tungsten arc welded thin FSS 409/430 dissimilar joints

The Mechanical and Stretch-Bend Failure studies on Ultra High Frequency Pulsed Gas Tungsten Arc Welded dissimilar joints of AISI409-AISI430 Ferritic Stainless Steels were conducted. Welding was conducted with 5 ultra high frequencies (50 Hz, 150 Hz, 250 Hz, 350 Hz, 450 Hz). Mechanical characteristics evaluation on the joints included tensile strength, microhardness variations across the welds and creep. Microstructural and metallurgical investigations included weld cross section evaluation, comparing grain variations in high, medium and low thermal heat affected zones, weld zones and base material region. Stretch bend failure studies included studies on angular distortion, fracture limit strain, and coefficient of friction. Tests revealed that joints welded at 350 Hz was better, compared to other joints. Dissimilar AISI409-AISI430 joint fabricated at 350 Hz exhibited 267 ± 3 MPa as yield and 409 ± 6 MPa and as ultimate tensile strength. Its creep fracture duration was 72.7 min (highest among the joints). Microstructural studies revealed grain growth, partially coarse and partially fine grains in heat affected zones. Depending on the difference in grain sizes, on both sides of the welds, heat affected regions were identified as three distinct zones. In AISI430 side; high temperature austenitic, martensitic, delta ferrites and in AISI409 side; needle like martensitic structures, mixture of ferritic-austenitic, δ-ferrite with carbide precipitation were found in high, medium and low thermal heat affected zones, respectively. On increasing the ultra high frequency pulses, angular distortion increased, fractures changed from tensile/shear type to mixed type. In shear bend tests, on increasing the ratio of radius: thickness, fracture limit strain on outer surface, across sheet thickness, due to stretching increased.


Introduction
Ferritic Stainless Steels (FSSs) have acquired recognition in industrial use due to their high resistance to wear [1].They are cheap and exhibit high formability compared to their austenitic equivalents [2].FSSs also demonstrate enhanced thermal conductivity, making them appropriate for high-temperature applications [3].Welded joints of FSS such as AISI 409 and AISI 430 are preferred in automotive industries as they exhibit high resistance to corrosive environments and can withstand mechanical and thermal stresses [4].They are used in structural components of industrial furnaces, heating elements, domestic equipments such as ovens, commercial catering equipments used for food storage and working surfaces [5,6].FSSs are also used in decorative panels, cladding, industrial exhausts, conveyor systems and agricultural equipments [7,8].
However, issues occur during welding, inducing coarse grain growth [9], minimized ductility [10], and development of martensitic grain structures [11] which deteriorates the joints.Dissimilar material joining is very 2. Materials and methods
For identifying the chemical composition of the base materials (BMs), they were tested by using a spark spectrometer.5 mm × 5 mm specimens were prepared and placed in the spark spectrometer for testing.Sparks were ignited at different places and the chemical composition was found.The microstructural aspects of AISI409 and AISI430 were evaluated by using Optical Microscopy (OM).Using X-Ray Diffraction (X-RD) studies, the internal chemistry and phases in as-received BMs were identified.The specimens in dissimilar combination (AISI409-AISI430) were placed in butt joint configuration and fastened to fixture.Copper backing plate was used for better heat dissipation.The welding setup used for conducting Ultra High Frequency Pulsed Gas Tungsten Arc (UHFP-GTA) welding is shown in figure 1.
The welding torch was placed in a holder and held perpendicular to the direction of welding.The torch handle was connected to a servo controlled motor for moving at a constant speed.Welding was conducted with UHFPs.During pulsed times, arc burning and during the interrupted times, arc extinguishment occurs.At high frequency pulses, arc ignition using electrical discharge was done from a capacitor.The process parameters for UHFP-GTA welding were set by conducting trials and from previous investigations [29,30].At five different UHFPs, dissimilar (AISI409-AISI430) joints were fabricated.Argon gas was used as shielding gas.UHFP-GTA welding experiments were conducted according to the values indicated in table 2. The notations for the dissimilar joints at different UHFPs are also shown in table 2.

Evaluating mechanical and physical properties
The mechanical properties of BMs and the joints were tested using tensile shear, microhardness and creep tests.Using universal testing machine, the tensile studies were conducted according to ASTM E 08.Tensile tests were conducted to evaluate S y -yield strength (MPa), S u -ultimate tensile strength (MPa), e u -percentage of uniform elongation, e t -percentage of total elongation [31,32].
According to the procedures developed by Banabic et al [33] the tensile stress values such as true stress and the coefficient of hardening (n) was calculated.By studying the width to thickness strain, values of normal anisotropy coefficient (r) along rolling direction, mean normal anisotropy coefficient (r b ) and planar anisotropy coefficient (Δr) were calculated.Surface microhardness variations across the dissimilar weld regions were calculated using Vickers hardness testing as per ASTM E 384 standards.For each specimen, three hardness values were evaluated and the average of the three was recorded.
The joints and BMs were subjected to creep testing using (DIDAC-Make) creep testing equipment.Samples were prepared with gauge length of 50 mm from weld center (25 mm on either side from the weld region).At 600 °C temperature and 80 MPa induced stress, creep testing was done.Using Monkman-Grant equation [34], relationship between creep fracture duration and minimum creep rate was established.The Monkman-Grant equation is given below In the above equation, t cf denotes creep fracture duration, E s indicates minimum creep rate and MG denotes Monkman-Grant constant.

Evaluating metallurgical and microstructural
Using Optical Microscopy (OM) (Make-Ken A vision) and Scanning Electron Microscopy (SEM) (Make -XL-30 Philips), microstructural studies on cross-section of the joints were conducted.Using standard metallurgical methods, the surface of the welded joints were prepared.The samples were polished with different grades of emery sheets and polished using disc polishing equipment using diamond grit paste.For etching, Vilella reagent prepared by with 10 ml hydrochloric acid, 1 gm picric acid and 200 ml of ethanol was used.The BMs and joints were subjected to X-Ray diffraction (X-RD) (Make-RIGAKU) studies, for identifying the phase changes in the weld zones.Using Cu target and scintillating counter, X-RD studies were conducted with a step size of 0.002.

Angular distortion and stretch-bend failure analysis
Angular distortion studies, during UHFP-GTA welding were conducted, to measure the distortions induced due to different UHFPs.The schematic representation of angular distortion measurement equipment [35] is shown in figure 2. One plate was fixed and a dial gauge was fixed over the second plate.From center to periphery, the dial gauge was moved through the x-axis during welding.The variations in z-axis were measured before and after conducting UHFP-GTA welding.The vertical displacement caused due to welding, was used to identify the angular distortion value θ, according to the following equation  Dimensions of the test specimens for stretch bend failure analysis are shown in figure 3. On one side of the specimen, circular interlaced grids of 2 mm diameter were coated using electro chemical deposition method, for evaluating the variations in strain.Deformations in the circular grid were observed on subjecting the specimens to stretch bending tests.The variations in the bending stress and strain were analyzed according to the procedure developed by Keeler et al [36] In stretch bending test, Bending under Tension (BUT) method was used for identifying the draw bend fracture mechanisms of the joints [37].The schematic representation of the Draw Bend Fracture (DBF) testing process is shown in figure 4 (a) and the DBF equipment is shown in figure 4 (b).
All joints were made perpendicular to the rolling direction and the DBF tests were conducted along the rolling direction.DBF tests were intended to evaluate the fracture limit strain of the welded regions.At two testing speeds such as 3 mm s −1 (low speed) and 30 mm s −1 (high speed) DBF studies were conducted.Ten different bending pins of different radius such as 3, 4.5, 6, 7.5, 9, 10.5, 12, 13.5, 16.5 and 20 mm were used.Before starting, the interacting surfaces were cleaned with acetone and a petroleum based lubricant (SAE 20 W 40) was applied.Under each condition, three samples were tested and the average of the three was recorded.According to BUT testing principles, DBF experiments were conducted.When the joints are made to slide over the radius of curvature of the bending pin, four different components of forces were assumed.From previous investigations, the pulling force (F 1 ) around the radius of the bending pin was assumed to be equal to the sum of the basic three force components [38].The equation for F 1 is shown below In equation 3, the restraining, bending and frictional forces are indicated as F 2 , F b and F f , respectively.On the outer surface of the welded specimens, the fracture limit strain was calculated according to the following equation [39] ( ) In equation 4, the fracture limit strain developed in the outer surface of the welded specimens is denoted as E 1f , l major denotes the longitudinal deformation induced strain measured from the deformed ellipse.For accurately measuring the strains imposed in the welded samples, a digital microscope was used to capture the deformations induced during to the bending process.The equation for evaluating the reduction in thickness R f is shown below The formula for evaluating fracture limit strain across the sheet thickness The formula for fracture limit strain (FLS) due to stretching of the dissimilar joints is shown below In equation 7, L max denotes the maximum length and L o denotes the initial notch length.Using Linear Variable Differential Transformer (LVDT), the variations in length were recorded.The microstructural features of the fractured regions were investigated using SEM.The equation for calculating the coefficient of friction (μ) is shown below [41].
The bending force was calculated according to the following equation In equation 9, the welded sample width was taken as notch width (w) The sequence of operations in this experimental investigation is given as a flowchart in figure 5.

Base material properties
The chemical composition of the base materials identified using spark spectrometer studies are shown in table 3.

Mechanical properties
The tensile test results (stress-strain graphs) of BMs and joints are shown in figure 7  From tensile studies, UTS of as-received AISI430 BM and AISI409 BM were 490 MPa and 443 MPa respectively.UTS of AISI430 BM were 9.5% greater than UTS of AISI409 BM.UTS of D50 were 78% and 71% of the UTS of AISI409 BM and AISI430 BM, respectively.UTS of D150, D250 and D350 were 4.6%, 9.1% and 14.6% greater than the UTS of D50, respectively.Contrast to this, UTS of D450 was lower than D350.Similar variations in tensile properties of ferritic stainless steel welds were observed by Gupta et al [43].The total elongation percentage of AISI430 BM and AISI409 BM was 31.9% and 30.1%, respectively.Welding caused a considerable reduction in elongation.On increasing UHFP from 50 Hz to 450 Hz, consistent decrease in elongation was observed.For all joints, the values of the coefficient of hardening, normal anisotropy, mean normal anisotropy, planar anisotropy were evaluated and recorded in table 4. The variations in microhardness along the welds, for all joints were measured (figure 7 (b)).Near periphery of the joints, the microhardness values were close the microhardness of BM. 165 ± 6 HV and 213 ± 7 HV were the microhardness values observed after Heat Affected Zone (HAZ) towards the periphery for AISI409 BM and AISI430 BM.Reduction in microhardness was observed from weld center to periphery.Such variations in tensile characteristics and weld region microhardness were observed in dissimilar welding studies, conducted on ferritic steels by Anawa and Olabi [44].From BM to HAZ, reduction in microhardness and from HAZ to WZ, increase in microhardness was observed on both AISI409 and AISI430 sides.Similar variations in micrhardness values aross BM, HAZ and WZ was observed in FSS welding by Ghosh et al [45].
Creep curves were plotted for the joints and BMs.The variations in strain percentage with creep testing duration for the base materials and the dissimilar joints are shown in figure 8.
During loading, the variations in elastic strain were studied for characterizing creep.The entire creep mechanism was dived into three stages.The first stage of creep is till the resistance of the specimens against the tensile pull of the load was equal to the load.In the first stage, no deformation can be observed even though the specimen has been loaded.The second stage is when the specimen exhibits uniform deformation.The third stage is fracture of the specimen.From figure 8, creep strain was found to be more for the base metals, than the  According to the relationship developed by Abe [47], Monkman-Grant equation was used for predicting the creep life, from the data collected within a short duration of time.The validity of this relationship was found to be high in metallic specimens with standard and fine grained structures.The experimental values of Monkman-Grant constant (MG), the creep fracture duration (t cf ) and the minimum creep rate (E s ) for BMs and joints are shown in table 5.

Microstructural and metallurgical Investigations
For all UHFP-GTA weld joints, all parameters except UHFPs were maintained constant.For identifying the variations in size of bead, WZ, HAZ-AISI409 and HAZ-AISI430, the macrographs of the cross section of the joints (at low magnification) are shown in figure 9.The macrographs indicated an increase in bead width, on increasing UHFPs.
For all joints, the width of HAZ in AISI409 and AISI430 was different.Width of WZ and HAZ was identified according to the variations in grain pattern and the fusion boundary layer.For D50 (figure 9   For all joints, considerable increase in grain growth was observed from BM to WZ.Compared to rate of growth from AISI430-BM to WZ, grain growth from AISI409-BM to WZ was greater.Grains of HAZs of both AISI409 and AISI430 were significantly larger than BMs due to homogeneous distribution of enlarged ferrite grains.Even distribution of grains was observed both along and within the grains, similar to the observations made by Senol and Cam [50].For all joints, WZ exhibited larger grains, compared to HAZs.As thermal fluctuations in WZ were greater than HAZ regions, the ferrtic structures in WZ of D50, D150 and D250 were more granular, compared to their corresponding HAZs.Similar shift from fine dendrite structures in HAZ to coarse dendrites in WZ was observed by Hussein and Al-Joubori [51]. The grain patterns of AISI409-BM (figure 10(f)) and AISI430-BM (figure 10(j)) were very similar to the BM regions of D50 joints.Even though BM regions experienced thermal cycles, considerable grain growth did not occur.Figure 9 (g) shows the HAZ of D150 in AISI409 side and figure 9 (i) shows the HAZ of D150 in AISI430 side.On AISI409-HAZ (figure 10 (g)), the grain growth was coarser, compared to the grain structure in AISI430-HAZ (figure 10(i)), due to martensite formations in HAZ regions of AISI430-HAZs.Similar variations in HAZ grains were identified on conducting welding studies on ferritic stainless steels using Gas Metal Arc Welding (GMAW) by Serinag and Cam [52], Gas Tungsten Arc Welding (GTAW) by Mousazadeh and Derakhshandeh [53] and Laser Beam Welding (LBW) by Geng et al [54].
AISI409-BMs of D150 (figure 10  and figure 10 (x)), similar to studies conducted by Dong et al [55].From D50 to D450, grain growth in AISI409-HAZs was coarser than AISI430-HAZs due to the presence of increased intergranular martensites within the ferritie structure.Increase in the content of intergranuar martensites in HAZs were similar to the observations on HAZ regions of dissimilar low-carbon steel with ferritic steel joints conducted by Khorromi et al [56].
In the WZ, heterogeneous structure with fusion was expected as dissimilar joints were fabricated.Grain coarsening and growth was observed for all joints.In the WZs of D250 (figure 10 (m)) and D350 (figure 10 (r)), the non-infused characteristics of both AISI409 and AISI430 were witnessed.At certain regions in WZ, martensite phase transformations were identified.Such phase transformations induces higher residual strain [57] resulting in modified weld region hardness (figure 7 (b)).Abrupt boundary distinction, agglomerated hightemperature ferrite transformations into mixed austenites were found, similar to the WZ studies conducted on dissimilar FSS joints by Wang et al [58].
The chemical composition of AISI 409 and AISI430 were superimposed on the pseudo binary diagram of Fe-Cr-C at 17% Cr for identifying the phases at different temperatures [60].The pseudo binary diagram is shown in figure 11.This binary diagram was used for prediction of grain growth in HAZs.Three different HAZs were identified on proceeding from the fusion WZ to BM.The first is High Temperature HAZ (HTHAZ), second is the Medium Temperature HAZ (MTHAZ) and the third is Low Temperature HAZ (LTHAZ) [61].The OM images of HAZ of D50, D150, D250, D350 and D450 are shown in figures 12 (a)-(j).Particle size analysis was conducted in different zones and the average particle size was identified for LTHAZs, MTHAZs and HTHAZs.The values of average particle sizes are shown in table 6.
Grain growth was observed from LTHAZ to HTHAZ.HTHAZs are regions near WZs in which the temperature has reached above 1100 °C.From pseudo binary diagram, the predicted microstructure has to be fully δ-ferrite.
MTHAZs are regions which has experienced operating temperatures around 1000 °C-1100 °C.In AISI409-MTHAZ, in all joints, a mixture of ferritic and austenic structures were found, similar to the observations conducted by Nelson et al [65].In AISI430-MTHAZs of D50, D150, D250, grain growth and martensitic formation was observed [56].In AISI430-MTHAZs of D350 and D450 joints grain growth was inhibited due to austenitic formations around 1050 °C.High temperature austenites prevent further grain growth and it causes pinning of the boundaries [66].
LTHAZs are regions which have experienced thermal cycles between 900 °C-1000 °C.In AISI409-LTHAZs in all joints, delta ferrites were prevalent.In certain regions martensitic phases were observed [67].In AISI430-LTHAZs in all joints, a combination of δ-ferrite with carbide precipitation was found.In these regions, martensitic phases were very less [68].
For estimating the phase distributions in fusion weld region, Schaeffler diagram [69] was used.According to the relationship developed by Hull et al [70], Chromium equivalents and Nickel equivalents were calculated.The WZs of the dissimilar joints was subjected to chemical evaluation and the composition in elemental percentage was found.The chemical composition in the WZs are shown in table 7.
The intensity of the peaks detected in XRD spectrum was for detecting the phases in the weld region of the joints using Normalized Intensity Ratio (NIR) technique [74].According to the relationship developed by Peelamedu et al [75], the value of NIR at x phase was calculated according to the following equation  In equation 12, the phase intensity is denoted as I x , the total number of phases identified in the weld zone is indicated as N and the intensity of the background is indicated as I back .For evaluating the NIR value, the major peaks of a particular phase were selected and by using NIR technique, the different phases in the microstructure of the weld region were found.The values of NIR for the phases present in the weld region of the five dissimilar joints are shown in table 9.
The estimated amount ferrite -martensite was similar to the ferrite -martensite contribution indicated in the Schaeffler diagram.The OM images of WZs of the joints indicating ferritic, austenitic and martensitic phases  are shown in figure 14.In WZs of all joints, the areas of grain boundaries are higher in martenisite regions.These martenisite rich regions induce carbide precipitation, resulting in greater microhardness (figure 7 (b)).Similar variations in WZ microhardness in martensite agglomerated regions were observed by Sun et al [76].450 Hz, an increase in disorientation angle was observed.As the dissimilar welding was conducted in thin sheets of 2 mm thickness, UHFPs had an effect on non-uniform thermal expansions and contractions [78].At lower frequencies, the angular distortion was less and on increasing UHFPs, angular distortions increased.

Evaluation of bend fractures
Different types of fractures were observed on conducting Draw Bend Fracture (DBF) testing.The types of fractures were tensile, shear and mixed.The mode of fracture of the joints is shown in table 10.
In the ductile fractures, necking was observed due to bending.This necking induced tri-axial stresses at the center of the notch region.It caused plastic deformation to occur, till the pull region attained minimum thickness.Beyond a certain limit, fracture occurred.From fracture initiation, growth and rupture, tear was found to propagate from the centre to the edges of the notch, similar to the fractures of FSS joints observed by Lakshminarayanan and Balasubramanian [79].
SEM investigations were conducted on the fractured surfaces and are shown in figure 17 (a-e).In SEM image of fractured D50 (figure 15 (a)), dimples and voids were identified [80].Presence of microscopic voids and inclusions from the interfaces were identified in fracture region of D150 (figure 17 (b)).Similar voids, due to martensite induced fusion line cracking were observed by Saha et al [81].In D250 fractured region (figure 17 (c)), voids formed due to concentration of stresses in the corner of the grains [82], and ruptures were observed.For D350 (figure 17 (d)), coarse dimples due to diffusion of voids along the grain boundary and for D450 (figure 17 (e)), voids and pullouts due to grain boundary sliding and dislocation slip was observed [83,84].
3.4.3.Fracture limit strain on the outer surface of the bended specimen (E 1f ) During Draw Bend Fracture (DBF) analysis, it was found that the ratio of radius of the test pin (R) and the thickness of the plates (t) (R/t) had a significant effect on fracture.The fracture limit strain on the outer surface of the dissimilar joints (E 1f ) were observed on conducting DBF tests along the rolling direction, at two different speeds such as 3 m s −1 (low) and 30 m s −1 (high).The variations in E 1f values on increasing R/t ratio is shown in figure 18.
From figure 18, on increasing the R/t ratio, E 1f also increased.Due to strain hardening of the welded joints, yield strength increased [85].This phenomenon was attributed as the reason for increase in E 1f .On increasing UHFPs, the joints exhibited a higher E 1f till D350.On increasing UHFP from 350 Hz to 450 Hz, E 1f reduced.
DBF experiments at 3 m s −1 (low speed) resulted in consistently high E 1f values, compared to E 1f values at 30 m s −1 (high speed).In both speeds, E 1f increased till a certain of increase in R/t value and then it started to reduce.At low R/t and high R/t values, E 1f values were comparatively lower than E 1f values at mid R/t values.This was due to the changes in strain strate on using different UHFPs.As the experiments were conducted in room temperature, strain hardening occurred due to agglomeration of dislocations, similar to the observations on dissimilar ferritic-martensitic joints by Ren et al [86].This caused reduction in the ductility of joints.In DBF testing, stretching was found to dominate the plastic deformation process.From figure 18, it was found that E 1f values were highest for D350.3.4.4.Fracture limit strain across sheet thickness (E 3f ) A fracture limit strain across sheet thickness (E3f) was plotted for different R/t ratios and is shown in figure 19.
On increasing R/t ratio, E 3f increased till a particular limit and then, it started to decrease.Increase in UHFPs caused E 3f to reduce (for all R/t ratios).Due to cold work hardening, joint yield strength increased with higer E 3f values.The joints welded perpendicular to rolling direction, obeyed volume constancy law when tested using plane strain analysis method [87].Stabilization behavior of E 3f increased from D50 to D350, on increasing UHFPs.At higher UHFPs (450 Hz), the stabilization behavior reduced.This changes in stabilization behavior was due to modifications in normal anisotropy coefficient and cross-sectional volume reduction [88].Meager necking was observed at low R/t ratios.It indicates that E 3f reduced, even before the joints elongated uniformly.Increase in testing speed had a greater effect on E 3f variations.3.4.5.Fracture limit strain (FLS) due to stretching of dissimilar joints (L max /L 0 ) The variations in fracture limit strain (FLS) due to stretching of dissimilar joints L max /L 0 at different R/t ratios is shown in figure 20.
From figure 20, (L max /L 0 ) and R/t exhibited a linear relationship, i.e., L max /L 0 increased on increasing R/t ratio.At lower R/t ratio values, testing speed did not affect L max /L 0 values.On increasing R/t, the values of L max /L 0 values increased till a certain point and then, it stabilized.Separation of tensioned and compressed regions during bending occurs along the neutral line and bending in plastic deformation process occurs with shift in the neutral line [89].This caused the radius of curvature and work hardening to increase, resulting in higer L max /L 0 values.When the outer layer was stretched, compression was induced in the inner layers, providing stability to the stretching region.This improved the stretching limit of the walls The experimental investigations show that limiting strain of test specimens (under bending and stretching) depended on the deformation limit (bending and stretching) [90].At low and high values of R/t, COF values were higher, compared to those at mid R/t values.Increase in UHFP from 50 Hz to 350 Hz resulted in consistent decrease of COF (for all R/t values).On increasing the testing speed from 3 m s −1 to 30 m s −1 , significant shift in the range of COF values were observed.For all joints and R/t values, on increasing the testing speed, a reduction in COF was observed.Uneven increase in contact pressure was observed on reducing the pin radius.This increase in contact pressure is inversely correlated to the viscosity of the lubricant [91].Frictional wear during DBF tests was reduced on using SAE 20W40 oil as lubricant.COF variation patterns were similar in both testing speeds (3 m s −1 and 30 m s −1 ).This was due to formation of a hydrodynamic lubricating film [92] throughout the testing process.

Conclusions
In this investigation, AISI409-AISI430 joints were made using ultra high frequency pulsed gas tungsten arc welding, with five different Ultra High Frequency Pulses (UHFPs) (50 Hz, 150 Hz, 250 Hz, 350 Hz, 450 Hz) and the following conclusions were drawn.i.On increasing UHFPs, joint strength, microhardness, creep fracture duration and angular distortions increased, whereas percentage of elongation and weight percentage of C and Cr in fusion zone reduced.ii.The size of Heat Affected Zones (HAZs) in AISI409 side and AISI430 side of every joint was unique and different.Compared to Base Material (BM) region, grain growth was observed in HAZs and Weld Zones (WZs).
iii.Optical micrographs revealed intergranualar martensite formations in AISI430-HAZs and the grains were finer, compared to the grains (coarse ferrites) in AISI409-HAZs.In Weld Zones, presence of ferrite, martensite and austenite were observed.
v. Draw bend fracture tests showed that fracture limit strain across sheet thickness and stretching increased, whereas coefficient of friction decreased on increasing UHFPs.Among the joints, joint fabricated with 350 Hz exhibited highest yield strength (267 ± 3 MPa), ultimate tensile strength (409 ± 6 MPa) and creep duration (72.7 min).
Future Scope-Using the available results as reference, dissimilar joints between High Cr and High Ni steels can be fabricated.Instead of draw bend testing, other formability methods such as single point incremental forming process and press forming can be used.FE-SEM and TEM studies can be conducted for nanoscale analysis of the weld regions.

Figure 3 .
Figure 3. Schematic representation of stretch bending test specimens.

Figure 5 .
Figure 5. Flowchart indicating the sequence of operations in this investigation.

Figure 7 .
Figure 7. (a) Tensile test stress-strain graphs for the BMs and joints, (b) Vickers microhardness variations along welds.
(a).The variations in joint microhardness along the weld region of the joints are shown in figure 7 (b).Variations in Ultimate Tensile Strength (UTS in MPa)-S y , Yield Strength (YS in MPa)-S u , Uniform Elongation-e u , Total Elongation-e t , Hardening Coefficient-N, Coefficient of Normal Anisotropy-R, Coefficient of Mean Normal Anisotropy-r b and Coefficient of Planar Anisotropy-Δr, for BMs and the dissimilar joints are shown in table 4.

Figure 8 .
Figure 8. Variation in creep curves for the joints and BMs.

Figure 12 .
Figure 12.HAZ regions of the dissimilar joints.
EDAX spectrums of the joints are shown in figures 15 (a)-(e).The percentage of C, O, Mn, Si, Cr, Ni and Fe in the WZ of the joints are shown in figures 15 (a)-(e).In EDAX spectrum of D50 WZ (figure 15 (a)), the percentage of C, Cr and Ni was 1.87%, 13.41% and 0.32% respectively.The percentage of C and Cr decreased on increasing UHFPs.Similar reduction in Cr% was observed during WZ characterization conducted in Shojaati and Beidokhti [77].

Figure 15 .
Figure 15.SEM with EDAX graphs of the weld region of the dissimilar joints.

Figure 16 .
Figure 16.Angular distortions observed during welding of the five dissimilar joints.

3. 4 . 6 .
Variations in coefficient of friction (COF) Variations in Coefficient of Friction (COF) for the joints subjected to DBF experiments (in both low and high speeds) at different R/t ratio are shown in figure 21.

Figure 18 .
Figure 18.Variations in fracture limit strain across the sheet thickness (E 3f ) on increasing R/t.

Figure 19 .
Figure 19.Variations in fracture limit strain across sheet thickness (E 3f ) with different R/t ratios.

Figure 20 .
Figure 20.Variations in fracture limit strain due to stretching of dissimilar joints L max /L 0 at different R/t ratios.

Figure 21 .
Figure 21.Variations in coefficient of friction for joints on increasing R/t ratio.

Table 1 .
Physical properties of base materials.

Table 3 .
Chemical composition of the base materials (in wt.%).

Table 4 .
Mechanical properties of the base materials and the dissimilar joints.

Table 5 .
Creep properties of the BMs and joints.

Table 6 .
Average Grain Sizes in different HAZs of the welds.

Table 7 .
Chemical composition in WZs of the joints.

Table 8 .
Evaluated Cr eq and Ni eq values for the joints.

Table 9 .
Relative phase intensities and the corresponding NIR values for the five dissimilar joints.