Mechanical Properties Variations and Comparative

Dinesh W. Rathod1 Department of Mechanical Engineering, Indian Institute of Technology Delhi, Hauz Khas, New Delhi 110016, India e-mail: [email protected]

Sunil Pandey Department of Mechanical Engineering, Indian Institute of Technology Delhi, Hauz Khas, New Delhi 110016, India e-mail: [email protected]

P. K. Singh Reactor Safety Division, Bhabha Atomic Research Centre, Hall 7, Trombay, Mumbai 400085, India e-mail: [email protected]

Rajesh Prasad Department of Applied Mechanics, Indian Institute of Technology Delhi, Hauz Khas, New Delhi 110016, India e-mail: [email protected]

Mechanical Properties Variations and Comparative Analysis of Dissimilar Metal Pipe Welds in Pressure Vessel System of Nuclear Plants The experimental investigations of two dissimilar metal weld (DMW) joints between SA508Gr.3Cl.1 ferritic steel and SS304LN austenitic stainless steel using Inconel 82/182 (ERNiCr-3/ENiCrFe-3) and Inconel 52/152 (ERNiCrFe-7/ENiCrFe-7) filler metals have been conducted in the present work. The integrity assessment of DMW joints and the mechanical properties variations has made pertaining to ASME Section-III and SectionIX. Mechanical tests comprising bend test, transverse tensile test (TTT), tensile test, Charpy impact test, microhardness measurement have been carried out along with microstructural evolution using the standard test specimens according to respective ASTM standards. Bend tests have shown that interfaces of the SA508–Inconel, Inconel–Inconel, and Inconel–SS304LN are free from any lack of fusion or cracks. TTTs have shown that failures of the specimens are from the SS304LN indicating integrity of the weld joint. Tensile tests confirm that tensile strength of the different regions agreed the required strength as per ASME Section-II. The weld strength mismatch and plastic instability strength (PIS) are found to be important factors during integrity assessment of joints. Based on the comparative investigations, owing to better mechanical properties, Inconel 82/182 filler metals could be an optimum choice over Inconel 52/152 filler metals for present DMW joints required in pressure vessel system of nuclear plants. [DOI: 10.1115/1.4031129]

Introduction In light water reactors, generally, pressure vessel (SA508Gr.3Cl.1 or equivalent) material is joined to piping steel (SA312 Type 304LN or equivalent) by welding using nickelbased alloys. Inconel 82/182 (ERNiCr-3/ENiCrFe-3), the Ni-based consumable, is often used to weld the SA508Gr.3Cl.1 or equivalent components to austenitic stainless steel SS304LN pipes for DMWs. Other Ni-based consumables, such as Inconel 52/152 (ERNiCrFe-7/ENiCrFe-7), are preferred for repair activities of such DMWs owing to its good corrosion resistance against Inconel 82/182. The physical and mechanical properties variation within the weldment zone always caused several problems in such DMWs. The structural integrity and design assessment of such welds are very important in consideration of safe service life. Certain failures in pressurized water reactor (PWR) plants with leak have been reported in V.C. Summer—USA (2000), Tsuruga 2— Japan (2003), and Palisades—through-wall crack in the HAZ, not in weld USA (1993). The weld materials used for the joints in these plants were Alloy 82/182 [1,2]. The PWR plants with cracks/flaws in weld of Alloy 82/182 were also reported at Ringhals 3&4—Sweden (2000), Three Mile Island-1—USA (2003), Tihang 2—Belgium (2002), Calvert Cliffs 2—USA (2005), and Biblis-A—Germany (2000) [1,2]. Several other problems have been associated with DMWs that reduce the design life of joint [1–7]. Mismatch in coefficient of thermal expansion of the austenitic and ferritic steel across the weld joint leads to the development of cyclic thermal stresses [3,7–9]. Formation of carbon 1 Present address: Department of Mechanical Engineering, Manav Rachna International University, Sec-43, Faridabad 121001, Haryana, India. Contributed by the Pressure Vessel and Piping Division of ASME for publication in the JOURNAL OF PRESSURE VESSEL TECHNOLOGY. Manuscript received April 3, 2015; final manuscript received July 1, 2015; published online September 7, 2015. Assoc. Editor: Marina Ruggles-Wrenn.

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denude soft zone and carbon-enriched hard zone can exist due to migration of carbon from ferritic steel [3,5,8,10,11]. These joints have varying mechanical and fracture toughness properties across the weld joints. Integrity assessment of the components requires to identify the lowest properties of the regions in the weld joints for conservative assessment. Some researchers have investigated properties of DMW joints between ferritic steel and austenitic stainless steel using Inconel 82/182 [9,12,13] and Inconel 52/152 filler metals [13–17]. The comparative investigation of mechanical properties of both Ni-based filler metals is required for the best choice of consumables for the DMW joints. In the present paper, two DMW joints of SA508Gr.3cl.1 ferritic steel and SA312 Type SS304LN austenitic stainless steel pipe materials have been prepared using Inconel 82/182 and Inconel 52/152 consumables. Both joints have welded as per the requirement of ASME Section-III and Section-IX. Qualified weld joints have been investigated by tensile test, TTT, Charpy V-notch test, bend test, microstructure evolution, and microhardness measurement across the weld joint. The detailed analysis of the mechanical properties is discussed thoroughly in the present paper for comparison between the weld joints of Inconel 82/182 and Inconel 52/152 filler metals.

Experimental Details Welding Procedure and Materials. SA508Gr.3Cl.1 in quenched and tempered condition and SA312 Type S304LN in solution-annealed condition have used in pipe form to prepare DMW joints. The size of the pipe was 324 mm outer diameter and 25 mm wall thickness. For the present study, pipe pieces of 160 mm length were used for welding. The filler metal rods of Inconel 82 (2 mm dia.), Inconel 52 (2.4 mm dia.), and Inconel 182 and 152 (4 mm dia.) electrodes were used as consumables. The

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chemical composition of base metals and filler metals used in the study is given in Table 1. All pipe pieces have machined as per compound bevel joint design. According to AWS D1.1, the 10 deg angle was used on top side for 6 mm thickness while for remaining 19 mm at root side the angle used was 37.5 deg. This provides the 75 deg included angle for joint design. The joint design can be seen in Fig. 1 as the schematic presentation. Four buttering layers were deposited on ferritic steel pipe piece with manual tungsten inert gas (TIG) welding process using Inconel 82 and Inconel 52 (TIG rods) filler metals for the respective joints. The 3 mm diameter Tungsten electrode was used for buttering and welding process (root joint) with straight polarity using 6–9 L/min argon gas shielding. The as-welded DMW pipe joints using Inconel 82/182 and Inconel 52/ 152 consumables are shown in Figs. 1(a) and 1(b), respectively. The average thickness of approximately 7 mm for buttering deposits was confirmed for both weld joints. The welding parameters during buttering are given in Table 2. For DMW joints preparation, two root passes have been employed using TIG process and Inconel 82/52 TIG rods for the respective weld joints. The close chamber purging was provided during root passes of the

both joints. The subsequent fill passes applied with shielded metal arc welding process using Inconel 182 and Inconel 152 electrodes at reverse polarity. The pipes welding position was 5 G (axis horizontal) for both joints and they were not rotated during welding. The welding procedure is adopted according to ASME SectionIX (welding procedure) and ASME Section-III (acceptance criteria) for both Inconel 82/182 and Inconel 52/152 weld joints. Welding parameters during root pass and fill pass are summarized in Table 3 for both DMW joints. Both DMW joints were examined 100% by radiograph test as per requirement of ASME Section-V (inspection procedure) and both joints were found to meet the acceptance criteria of ASME Section-III. The axial and circumferential shrinkage in both DMW joints was measured during welding. The Inconel 82/182 joint was noticed with 4.16 mm axial shrinkage and 2 mm circumferential shrinkage. This shrinkage is significantly less than the axial (4.87 mm) and circumferential (3 mm) shrinkage noticed in Inconel 52/152 joint. This indicates that the intensity of residual stresses would be more with Inconel 52/152 than with the Inconel 82/182 joint. The specimens for the tests were machined from the different sections of both joints.

Table 1 Chemical composition of base metals and filler metals in weight percentage Composition in wt. % Materials and element Base metals Filler metals

SA508Gr3cl1 ferritic steel SS 304LN Inconel 82 Inconel 182 Inconel 52 Inconel 152

C

Ni

Cr

Fe

Mn

Mo

Si

Nb

Cu

Ti

0.191 0.024 0.017 0.034 0.013 0.024

0.53 8.22 70.47 67.17 56.08 56.69

0.12 18.09 19.86 13.09 30.91 28.05

96.93 70.83 1.41 6.84 10.44 7.92

1.30 0.83 3.43 8.51 0.38 3.75

0.43 0.33 0.45 0.43 0.38 0.39

0.24 0.04 0.24 0.49 0.16 0.25

— — 2.09 1.46 0.06 1.44

0.19 0.94 0.03 0.04 0.10 0.12

— — 0.44 0.71 0.25 0.47

Fig. 1 As-welded DMW pipe joints using (a) Inconel 82/182 and (b) Inconel 52/152 consumables Table 2 Process parameters during deposition of buttering layers Joint

Layers

Passes

Current (A)

Voltage (V)

Welding speed (mm/s)

Heat input (J/mm)

Thickness (mm)

Inconel 82/182

1 2 3 4

6 6 6 7

98

8.4–9.5

0.60–1.05 0.64–0.97 0.72–0.98 0.67–0.97

900–1400 900–1500 800–1300 900–1400

2.5 2 2 2.5

Inconel 52/152

1 2 3 4

6 6 7 6

98–100

8.2–9.8

0.63–1.05 0.64–0.97 0.72–0.98 0.67–0.97

900–1400 900–1600 980–1380 900–1320

2.5 2 2.5 2.5

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Table 3 Welding parameters during deposition of root and fill passes of welding for both the joints Joint

Passes

Type of passes

Current (A)

Voltage (V)

Welding speed (mm/s)

Heat input (J/mm)

Thickness (mm)

Inconel 82/182

1 2 3–8

Root pass Root pass Fill pass

118–120 110–114 110

8.1–9.8 7.5–9.6 25–29

0.56 0.99 0.67–0.80

1910 973 3400–4100

3 2 20

Inconel 52/152

1 2 3–7

Root pass Root pass Fill pass

102–106 114–118 100

8.1–10.6 8.6–10.4 26–31

0.70 0.99 0.40–0.60

1395 1200 5000–7000

3 2 20

direction of weld joints with 3 mm thickness. The schematic of extracted specimens for TTT is shown in Fig. 2. The tensile test specimens in circumferential direction were machined from different materials regions of weld joints, such as HAZ of ferritic steel, buttering (Inconel 82/52), weld metal (Inconel 182/152), and HAZ of SS304LN. Three subsize specimens (ASTM E8M07) of 3 mm thickness were machined from each weldment regions of both DMW joints. The schematic position of extracted specimens of tensile specimens is shown in Fig. 3. Charpy V-notch test was performed for base metals and weldment regions of joint by keeping the notch orientation in circumferential direction of weld within the required weldment region. The schematic positions of extracted specimens are shown in Fig. 3. The Charpy test for every specimen was conducted at ambient temperature (24  C) following the ASTM E23-07 standard. For bend test, two specimens were machined for side bend test from both DMW joints. The specimens and testing procedure were followed as per the instructions given in ASTM E190-03 standard. Fig. 2 Schematic of TTT specimens, across the direction of weld

Results and Discussion Weld Qualification Tests Bend Test. The bend test was conducted on side bend test specimens by adjusting the weld, so that the weld joint regions come under complete tension. The test was performed until the specimens attained complete U-shape. The defect-free specimens were confirmed with dye penetrant test and qualified the bend test criteria for both DMW joints. This signifies that both DMW joints possess the required ductility in the weldment regions. The qualified bend test specimens are shown in Figs. 4(a) and 4(b), the Inconel 82/182 and Inconel 52/152 joints, respectively. Fig. 3 Schematic of Charpy specimens from HAZ of ferritic steel, buttering, weld metal, and the tensile specimens in the circumferential direction of welds

Mechanical Testing and Procedure. Specimens for mechanical tests were machined from different sections of both DMW joints. TTT was conducted on three standard sheet type rectangular specimens (ASTM E8M-07) extracted from transverse

TTT. The engineering stress strain curves obtained from the TTT are shown in Fig. 5 for both the DMW pipe joints. All specimens of both the joints were fractured from the weaker portion of SS304LN within gauge length, and the tensile strength of all specimens is almost equal with marginal variations in elongation. SS304LN side of the specimens has major contribution in the elongation of all specimens. The fractured specimens are shown in Figs. 6(a) and 6(b) of Inconel 82/182 and Inconel 52/152 joints, respectively. The obtained results signify that both joints possess

Fig. 4 Side bend test specimens showing no cracks or defects: (a) Inconel 82/182 and (b) Inconel 52/152 joint

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Fig. 5 Stress–strain curves from TTT specimens for both DMW joints

Fig. 6 TTT specimens after test: (a) Inconel 82/182 joint and (b) Inconel 52/152 joint

the required strength and ductility for the DMW joint requirement and structural integrity. Metallurgical Testing Welds Microstructure. The weld deposits of Inconel 82/182 and 52/152 have been primarily austenite with two phase solidification structure of dendrite and dendrite cores [12,18,19]. The microstructures showed solidified dendrite structure with recrystallized feature [8]. Figure 7(a) shows the buttering interior of

Inconel 82 with dendrites and dendrite cores. The dendrite growth is in transverse direction of weld joint, which is in the direction of cooling during buttering as shown in Fig. 7(a) by long arrow. Secondary phase precipitates in dendrite and dendrite cores are indicated by small arrows in the figure. Solidification grain boundaries (SGBs) are clearly evidenced and indicated in the figure. The location of microstructure on cross section of joint can also be seen in the figure. Similarly, the buttering interior of Inconel 52 can be seen in Fig. 7(b). The dendrite size and spacing are significantly smaller than the Inconel 82. The dominant columnar and cellular dendrite structure (small arrows in Fig. 7(b)) are present in the direction of cooling, which is in the transverse direction of welds and is indicated by long arrow in Fig. 7(b). The composition mismatch in both consumables especially Nb, Cr, and Fe contents caused to increase the dendrite size and spacing owing to the effect on dilution ability of these elements in weld deposits [6]. The dilution of filler metal with ferritic steel base metal also changed the weld deposit chemistry from the filler metal composition. The weld metal microstructure of Inconel 182 is shown in Fig. 8(a). The fully austenitic structure was observed with the presence of SGBs and migrated grain boundaries. The considerable amount of secondary phase particles with laves phases [9,18,19] can be seen in dendrite cores and is indicated by arrows. Moreover, the interior of Inconel 152 weld metal is shown in Fig. 8(b). The austenitic microstructure is observed with tiny NbC and TiC precipitates with significant amount of elongated harder r-phases (bright) at the grain boundaries. Similar to buttering, grain size in weld metal of Inconel 152 is also significantly smaller than Inconel 182 grain size. Figure 9(a) shows the microstructure of HAZ of SS304LN for the Inconel 82/182 joint. The austenite grain size marginally reduced than the corresponding base metal with minor increase in twin grain gap. The cross section of joint for microstructure location is also shown in Fig. 9. Similar thing was also noticed with the Inconel 52/152 joint as shown in Fig. 9(b). The grain size in HAZ of SS304LN is marginally more in Inconel 82/182 joint than Inconel 52/152 joint. The increase in austenite grain size of HAZ has been reported by Mathew and Latha [20] for SS316LN welds. However, owing to temper bead deposition during multipass welding, the reduction in austenite grains could be possible and was evidenced in the present study. Microhardness Variation. The microhardness variations across the weld of both DMW joints are given in Fig. 10. The high hardness in HAZ of ferritic steel is observed due to faster cooling rate and formation of reformed martensite [21]. The hardness near interface in HAZ ferritic steel was observed considerably lesser in Inconel 52/152 joint than in Inconel 82/182 joint. This may be caused because of the formation of soft zone (carbon depleted) due to carbon migration [13]. The hardness in buttering region is lesser at ferritic steel side than weld metal sides. The microhardness in Inconel 52/152 is

Fig. 7 Interior of buttering with dendrites and dendrite cores of (a) Inconel 82 and (b) Inconel 52

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Fig. 8 Interior of weld metal with general grain structure of (a) Inconel 182 and (b) Inconel 152

Fig. 9 HAZ of SS304LN representing (a) Inconel 82/182 and (b) Inconel 52/152 DMW joints

Fig. 10 Microhardness variations across both the DMW pipe joints

significantly more than the Inconel 82/182 in buttering and weld metal regions. The higher chromium content in the composition and increased Fe content due to dilution resulted in the formation of harder r-phases, which was confirmed in the microstructure of weld metal (Fig. 8(b)). The hardness in HAZ of SS304LN for Inconel 52/152 joint is more than Inconel 82/182 joint. This is consistent with the reduction in austenite grain size in the HAZ of SS304LN (Fig. 9). The hardness recorded in the present study is in agreement with the earlier studies on Inconel 82/182 [9,22] and Inconel 52/152 welds [16]. The favorable hardness profile and better combination of hardness values across the weld joint were noticed in Inconel 82/182 compared to Inconel 52/152 joint.

The stress–strain curves exhibited the almost similar patterns with marginal variations within the same materials zone of both DMW joints. The average properties are calculated from the stress–strain curves of all specimens belonging to different material zones of both DMW joints. The calculated properties yield strength (YS), ultimate tensile strength (UTS), uniform elongation (UE), total elongation (TE), and PIS are given in Figs. 12(a) and 12(b) for Inconel 82/182 and Inconel 52/152 joints, respectively. The material properties like UTS and UE govern the deformation behavior and type of failure (ductile or brittle) in material. These properties considerably varied across the weldment regions between two dissimilar metals involved in DMW joints. PIS variations in each material zone are estimated using the below equation:

Mechanical Testing Tensile Properties Variations in Weldment Materials Zone. The engineering stress–strain curves of the weldment regions are very important in integrity assessment. The typical stress–strain curves of the specimens are given in Figs. 11(a) and 11(b) for Inconel 82/ 182 and Inconel 52/152 joints, respectively. Journal of Pressure Vessel Technology

PIS ¼ UTS 

   UEð%Þ þ1 100

(1)

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Fig. 11 Typical stress–strain curves of base metal and weldment regions of (a) Inconel 82/182 joint and (b) Inconel 52/152 joint

Fig. 12 Average tensile properties of base metal and weldment regions of (a) Inconel 82/182 joint and (b) Inconel 52/152 joint

temperature but not truly on stress concentration. It represents resistance to local necking initiation and generally used as local failure criteria for ductile materials [12]. The UE(%) was converted into uniform strain in Eq. (1) for estimating the PIS. The YS and UTS in HAZ of ferritic steel have increased than corresponding base metal while UE and TE were reduced. The increase of hardness in this region is consistent with tensile properties due to the coarse and fine grains in HAZ of ferritic steel some fraction of reformed martensite [21]. The variation is almost similar for both joints. The dominant columnar dendrite and cellular growth are significant in Inconel 52 than in Inconel 82 (Fig. 7). This caused to decrease the strength and elongation in Inconel 52 buttering and the resulting PIS than in Inconel 82. The fracture surface observations of tensile test specimens of Inconel 82 buttering (Fig. 13(a)) and Inconel 52 buttering (Fig. 13(b)) are consistent with tensile properties. The comparatively larger dimples (encircled) are observed in Inconel 82 due to the larger dendrite size (Fig. 9) than in Inconel 52. The secondary phase particles nucleated microvoids (indicated by arrows) and dendrite structures with complete ductile dimpled fracture are clearly visible in Figs. 13(a) and 13(b) of Inconel 82 and Inconel 52 buttering, respectively. The presence of r-phase particles in Inconel 152 weld metal causes the strength and elongation to decrease marginally than the Inconel 182 weld metal. Fine slag inclusion and second phase particles nucleated microvoids are shown by arrows in Figs. 14(a) and 14(b) of Inconel 182 and Inconel 152 weld metals, respectively. The existence of r-phase 011403-6 / Vol. 138, FEBRUARY 2016

at grain boundaries and the second phase particles can be traced in Fig. 14(b) of Inconel 152 weld metal fracture surface. The strength in HAZ of SS304LN of Inconel 52/152 joint is marginally more than the Inconel 82/182 joint, but the elongation is reduced significantly. This observed to be consistent with the microstructure and microhardness variations. The austenite grain size in HAZ of SS304LN for Inconel 52/152 joint is smaller than Inconel 82/182 joint, which also caused to reduce the PIS of HAZ of SS304LN in Inconel 52/152 joint than Inconel 82/182 joint. Comparatively, the tensile properties of different material zones (weldment regions) of Inconel 82/182 joint are more favorable than Inconel 52/152 DMW joint. The strength mismatch between base metals and welds has the significant influence on crack-driving force and the crack growth resistance. The weld strength mismatch can be indicated with yield strength ratio (YSR). Varying tensile properties across the different material regions lead to varying YSR, which is a very important concern to regulate the strain concentration location in weld joints. Therefore, the YSR can affect the crack growth resistance in complete plastic deformation [12]. The YSR in terms of weld strength mismatch [12] is calculated using the below equation: Yield strength ratio ðYSRÞ ¼ YSWM =YSBM

(2)

The YSWM and YSBM represent the YS of weld metal and base metal, respectively. YSR more than one is desirable in terms of Transactions of the ASME

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Fig. 13 Fracture surface of tensile test specimens: (a) Inconel 82 and (b) Inconel 52 buttering

Fig. 14 Fracture surface of tensile test specimens: (a) Inconel 182 and (b) Inconel 152 buttering

structural integrity. However, if it is less than one, then the preexisting defects if any present in welds could lead to the fracture initiation [12]. The calculated YSR for ferritic steel and SS304LN side with average values and the obtained range of YSR are given in Table 4 for both DMW pipe joints. The tensile properties of both base metals and filler metals are very different so weld strength mismatch exists. However, the effect of mismatch is considered significant when the strength of mismatched materials region exceeds 10% [12]. This suggests the consideration of YSR and its effect while assessing the integrity of DMW joints [6,13]. The average YSR and its range for different materials zone for both DMW joints can be observed in Table 4. The YSR between weld metal and base metal SS304LN is overmatched in both joints, while it is undermatched with HAZ of SS304LN, which is not very significant in both joints compared to ferritic steel side. The YSR between weld metal and ferritic steel base metal is undermatched in both joints. However, the significant reduction in YSR is attributed for buttering and HAZ of ferritic steel in both joints. The obtained results indicate that plastic strain concentration can occur in SS304LN base metal, between the buttering and HAZ ferritic steel and at weld metal after initial

plastic deformation in SS304LN. This is more critical with Inconel 52/152 joint than with Inconel 82/182 joint due to very less YSR value between buttering and HAZ of ferritic steel. Similarly, the lower value of PIS is also observed within buttering region, which is significantly less in Inconel 52 than in Inconel 82. For integrity assessment, the buttering region is observed with higher risk of strain concentration and crack tip stresses during complete plastic deformation. Charpy V-Notch Impact Toughness. To measure Charpy V-notch impact toughness of different materials zone of DMW joints, standard (10  10  55 mm) specimens with 2 mm V-notch as per ASTM E190-03 standard have been machined from both pipe joints as shown in Fig. 3. Out of five specimens in each weldment region, the maximum and minimum values were discarded and the average of remaining three specimens is accounted for analysis. The impact toughness of different weldment zones of both DMW joints in circumferential direction of welds is given in Fig. 15. The impact energy absorbed by base metals and regions of both joints is more than the minimum prescribed value of 80 J [23,24]. The HAZ of ferritic steel region was observed with

Table 4 Estimated YSR for different sides of the DMW joint YSR-Inconel 82/182 joint Location

YSR-Inconel 52/152 joint

Zone

Average

Range

Average

Range

SA508Gr.3cl.1 ferritic steel side

YSRWM-BM508 YSRBT-HZ508

0.71 0.63

0.66–0.80 0.58–0.68

0.71 0.55

0.70–0.73 0.52–0.59

SS304LN side

YSRWM-BM304 YSRWM-HZ304

1.83 0.91

1.68–2.08 0.87–0.99

1.85 0.98

1.82–1.90 0.91–1.06

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The mismatch in mechanical properties has significant effect on fracture resistance and crack growth path [26]. The varying microstructure and mechanical properties exist in DMW joints and distinctive variations in same material regions are reported by Wang et al. [14–16] and Jang et al. [9]. Resistance to crack initiation and propagation can be greatly affected by heterogeneity in mechanical properties across the weldment regions [29]. Therefore, the varying local mechanical properties across welds with consideration of HAZ and interfacial regions are required to be investigated. These properties could be used in more complex integrity assessment of DMW joints and the modified methods based on local mechanical properties across the weldment zones of joints can be developed for DMW joints in nuclear plants.

Fig. 15 Impact toughness in base metals and weldment regions of both DMW joints

almost same impact toughness in both DMW joints. Moreover, the impact energy absorbed by Inconel 52 buttering is significantly more than by Inconel 82 due to significant transverse columnar and cellular dendrite growth across the direction of notch in Inconel 52 buttering (Fig. 7). The impact toughness in buttering is more than the weld metal due to the absence of fine slag inclusions from the coated electrodes, which nucleates the microvoids in weld metal and leads to low stress field ahead of cracks. This justifies the lowest impact toughness in weld metal region of both the DMW joints. The impact toughness of Inconel 152 weld metal is marginally less than Inconel 182 weld metal owing to the presence of harder r-phases at grain boundaries. The fracture surface observation was noticed to be consistent with impact toughness and not very distinctive from the fracture surface observed in tensile test. The obtained results are considerably more than the results reported by Hajiannia et al. [18] and in agreement with the results reported by Sireesha et al. [23]. The impact toughness of Inconel 52 is favorable against Inconel 82 and Inconel 182 weld metal is preferred over Inconel 152 weld metal. Considerably, larger material zone of weld metal in weld joint requires better toughness properties. Hence, owing to better impact toughness of Inconel 182 weld metal and reasonably acceptable toughness of Inconel 82, this joint can be preferred over the Inconel 52/152 joint. Influence of Mechanical Properties on Integrity Assessment of DMW Joints. The accurate method for structural integrity assessment of DMW joints in nuclear plant does not exist at present [16]. The data and information of similar metal welds are adopted for the design of DMW joints. The structural integrity assessment in the present form depends on the results of several years of experience and strength analysis of materials [25]. In reference to integrity assessment procedure for weld joints in existing codes and literature such as R6 [26], European method SINTAP [27], and FITNET FFS [28], the dissimilar joints are considered as sandwich composite combination of different materials comprising the base metals and weld metals. The effects due to interfacial regions and HAZ are not given required consideration in these codes and procedure, which was also agreed by Wang et al. [16]. The defects in welds can appear anywhere in the base metals, interfacial regions, buttering, weld metal, and HAZ regions. Considering the different materials zone in DMW joints, undermatched (unsafe) and overmatched results can be obtained owing to the variations in mechanical properties across the weldment regions. The fracture mechanism and deformation behavior are difficult to be estimated whether the crack will be positioned at interface or at weldment regions due to varying mechanical properties. This can deviate the crack from one material to another material. Hence, the integrity assessment methods in present codes and procedure cannot be used with desirable accuracy for DMW joints. 011403-8 / Vol. 138, FEBRUARY 2016

Conclusion The experimental investigations of Inconel 82/182 and Inconel 52/152 joints have been carried out and analyzed in the present study. Some derived conclusions from the study for pressure vessel system of nuclear plants are listed here (1) The axial and circumferential shrinkage is more in Inconel 52/152 joint than in Inconel 82/182 joint. This could suggest the more susceptibility of residual stresses in Inconel 52/152 joint than in Inconel 82/182 joint. (2) The desirable and favorable microstructure and microhardness profile is observed in Inconel 82/182 joint, as the significant fraction of harder r-phase exists in Inconel 52/152 joint. (3) The YS of Inconel 82/182 and Inconel 52/152 is overmatched with SS304LN, but undermatched with SA508Gr.3Cl.1 ferritic steel. The minor undermatched is observed with weld metal to the HAZ of SS304LN. While significant undermatched has observed with buttering and HAZ of ferritic steel. The YSRWM-BM508 for Inconel 82/ 182 is 0.66–0.80, while for Inconel 52/152 it is 0.70–0.73. Similarly, YSRBT-HAZ508 for Inconel 82/182 is 0.58–0.68 and for Inconel 52/152 it is 0.52–0.59. This suggests the comparatively better YSR with Inconel 82/182 joint than with Inconel 52/152 joint. (4) PIS is a very important concern for weld integrity. The lower values of PIS are observed in Inconel 52 buttering and HAZ of SS304LN of Inconel 52/152 joint. Hence, based on the PIS, the Inconel 82/182 joint could be preferred over Inconel 52/152 joint. (5) Tensile properties are more favorable in Inconel 82/182 joint compared to Inconel 52/152 joints. (6) Impact toughness of buttering Inconel 52 is more than Inconel 82 owing to significant columnar and cellular dendrites across the notch orientation. While Inconel 152 (weld metal) toughness is marginally less than Inconel 182 due to the presence of r-phases at grain boundaries. Considering the larger material zone of weld metal in weld joint and reasonably good impact toughness of Inconel 82 buttering, the Inconel 82/182 joint can be preferred over Inconel 52/152 joint.

Acknowledgment The authors thank the Board of Research in Nuclear Sciences, Department of Atomic Energy, for the financial support to the research work (2008/2036/107-BRNS/4038A).

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