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  • niIn

    an,-831

    Article history:

    Accepted 30 January 2014

    Keywords:Nickel base alloy

    In the present study, dissimilar welding between Inconel 718 nickel-base superalloy and 310S austenitic

    Ni3Nb precipitates. Alloy 718 is widely used in severe workingconditions such as chemical and process industries, nuclear re-actors and gas turbine engines due to its good properties includingcorrosion and oxidation resistance and appropriate strength at high

    emely resistant to

    ng between nickelection of a properg between Inconelal is not properlyion and liquationed joints. In recentd for introducingeen nickel based

    alloys and stainless steels. The studies showed that in most cases,nickel based ller metals offer better properties because theygenerally have satisfactory mechanical properties and good ther-mal stability [1,7e10]. In this regard, Sireesha et al. [7] evaluated thedissimilar welds between 316LN austenitic stainless steel and alloy800 using four types of ller metals. Their results showed thatInconel 82/182 ller materials exhibited the best thermal stability.Lee et al. [8] used two types of ller metals, Included Oxford FillerMetal 52 and Electrodes 152, and investigated the effects of various

    * Corresponding author. Tel.: 98 9133068945; fax: 98 3113915737.E-mail addresses: [email protected], [email protected]

    Contents lists availab

    International Journal of Pre

    .e

    International Journal of Pressure Vessels and Piping 116 (2014) 37e46(A. Mortezaie).benets including reduction of material costs and improvements indesign exibility. This method is often used where a transition inmechanical properties and performance in service are required[1,2]. Dissimilar joint of Inconel 718 and AISI 310S can be employedin high temperature and corrosive environments such as chemicalprocessing equipment and oil and gas industry. In addition, due tothe high resistance against oxidation and appropriate creepstrength, the use of dissimilar joint Inconel 718/310S has beenexpanded in land-based gas turbines. Inconel 718 is a precipitationhardenable nickel-based superalloy strengthened primarily by g00-

    in the chemical compositions, grade 310S is extroxidation [5,6].

    One of the main concerns for dissimilar weldibased superalloys and stainless steels is the selller metal. Reports related to dissimilar weldin718 and 310S steel show that, if the ller metselected, some weld defects such as solidicatcracks in the fusion zone may occur in the weldyears, extensive research has been conductesuitable ller metals in dissimilar welding betwDuring the past two decades, dissimilar materials welding hasbeen increasingly considered in industry because of its several

    especially in oil, gas and petrochemical industries containing hotconcentrated acids. Due to the high levels of nickel and chromiumAustenitic stainless steelDissimilar materials joints

    1. Introductionhttp://dx.doi.org/10.1016/j.ijpvp.2014.01.0020308-0161/ 2014 Elsevier Ltd. All rights reserved.Microstructural observations showed that weld microstructures for all ller metals were fully austenitic.In tension tests, welds produced by Inconel 625 and 310 ller metals displayed the highest and thelowest ultimate tensile strength, respectively. The results of Charpy impact tests indicated that themaximum fracture energy was related to Inconel 82 weld metal. According to the potentiodynamicpolarization test results, Inconel 82 exhibited the highest corrosion resistance among all tested llermetals. Finally, it was concluded that for the dissimilar welding between Inconel 718 and 310S, Inconel82 ller metal offers the optimum properties at room temperature.

    2014 Elsevier Ltd. All rights reserved.

    temperatures [3,4]. On the other hand, alloy 310S is an austeniticstainless steel for high temperature applications (about 800 C),Received in revised form21 January 2014 purpose, three ller metals including Inconel 625, Inconel 82 and 310 stainless steel were used.Received 17 October 2013 stainless steel using gas tungsten arc welding process was performed to determine the relationshipbetween the microstructure of the welds and the resultant mechanical and corrosion properties. For thisAn assessment of microstructure, mecharesistance of dissimilar welds betweenaustenitic stainless steel

    A. Mortezaie a,*, M. Shamanian b

    aDepartment of Materials Engineering, Najafabad Branch, Islamic Azad University, IsfahbDepartment of Materials Engineering, Isfahan University of Technology, Isfahan 84156

    a r t i c l e i n f o a b s t r a c t

    journal homepage: wwwcal properties and corrosionconel 718 and 310S

    P.O. Box 517, Iran11, Iran

    le at ScienceDirect

    ssure Vessels and Piping

    lsevier .com/locate/ i jpvp

  • levels of Titanium addition on the welding feasibility and me-chanical properties of dissimilar welds between Inconel 690 andSUS 304L. The results showed that with an increase in Ti in llermetal composition, the microstructure tends to change from one ofcolumnar dendrite to one of equiaxial dendrite. Dissimilar weldingbetween AISI 304 and Monel 400 using E309L and ENiCu-7 llermetals was performed by Ramkumar et al. [10]. Their resultsshowed that the tensile strength properties of dissimilar weldswith ENiCu-7 ller metal are better than those produced withE309L ller metal. Evaluation of various sources in the eld ofdissimilar materials welding indicates that no systematic researchhas been reported regarding the dissimilar welding betweenInconel 718 and 310S. Therefore, themain purpose of this studywasto investigate microstructural features, mechanical properties and

    correspond to the brittle d-Ni3Nb phase. It should be noted thatboth phases have the identical composition (Ni3Nb), but g00 has abct crystal structure and d has an orthorhombic crystal structure[11]. These precipitates were formed in the preferred crystallo-graphic orientations. Fig. 2b shows microstructure of Inconel 718after solution treatment at 1040 C for 2 h. It can be seen that themicrostructure does not contain any g00-Ni3Nb and d-Ni3Nb phases,

    Table 2TheGTAWweldingparameters used in this study; theheat inputwastakenas0.7EI/V:where E is welding voltage, I is current and V is welding speed.

    Fillermaterials

    Pass Current(A)

    Voltage(V)

    Weldingspeed (mm/s)

    Heat input(kj/mm)

    Total heatinput (kj/mm)

    Inconel 82 Root 190 14.6 1.53 1.26 5.30Hot 170 15.9 1.36 1.39Filling 150 16.4 1.21 1.42Cap 150 17 1.34 1.23

    310SS Root 190 15.4 1.72 1.19 5.61Hot 170 16.3 1.32 1.46

    A. Mortezaie, M. Shamanian / International Journal of Pressure Vessels and Piping 116 (2014) 37e4638corrosion resistance of the joints between 310S austenitic stainlesssteel and Inconel 718 nickel-based superalloy, and to evaluate theinuence of different ller materials on the individual properties.

    2. Materials and methods

    The base materials used in this study were in the form of 10 mmthick plates of wrought 310S stainless steel and cast bulkmaterial ofInconel 718. The former material was received in the solution-annealed condition whereas the latter was obtained initially inthe precipitation-hardened state. In order to provide better weld-ability behavior before welding, a solution treatment was carriedout on the Inconel 718 base metal at 1040 C for 2 h, followed bywater quenching. Afterwards, rectangular plates with dimensions200 45 10 mm were prepared from each base metals for thewelding experiments. Plates were machined to create a single Vgroove butt joint conguration with a 70 groove angle. The rootface and the root opening were 1 and 2 mm, respectively. Toperform the dissimilar welding; three ller metals Inconel 625,Inconel 82 and type 310 SS were employed. The chemical compo-sitions of the base and ller materials are given in Table 1. Weldingprocedure was performed in four passes by the manual gas tung-sten arc welding process with Direct Current Electrode Negative.The welding parameters and the heat input in each welding passare given in Table 2.

    After welding, a series of tests were conducted to determine theproperties and microstructure. Transverse sections of the weldswere metallographically characterized after etching in marble so-lution (10 g CuSO4 50 ml H2O 50 ml HCl) using the opticalmicroscope and scanning electron microscope (SEM) equippedwith energy dispersive spectroscopy (EDS) point analysis. In orderto determine the tensile strength of the base and weld metals, thetensile test was performed with a nominal loading rate of 1 mm/min. Tensile test specimens were prepared from the transversesection of the welds according to AWS B4.0 standard. Fig. 1 shows

    Table 1Chemical compositions of base metals and ller metals (wt.%).

    Elements Base metals Filler metals

    Inconel 718 310S Inconel 625 Inconel 82 310 SS

    Cr 18.4 25 21.5 18 26Ni Rem. 20.1 Rem. Rem. 21Fe 18.3 Rem. 5 3 RemMo 2.8 0.36 9 e 0.2Mn 0.15 1.95 0.5 3 2Nb 5.1 e 4 3 eAl 0.53 e 0.4 e eTi 0.93 0.09 0.4 0.75 eC 0.06 0.04 0.1 0.1 0.1Cu e e 0.5 0.5 0.7

    Si 0.05 1.7 0.5 0.5 0.5the dimensions of the tensile test specimens. Charpy V-notchimpact test was conducted on the base metals and welds at roomtemperature by an impact tester. The impact test specimens withthe dimensions of 55 mm 10 mm 10 mmwere prepared basedon AWS B4.0 standard. After performing the impact test, fracturedsurfaces of impact test specimens were examined by scanningelectron microscope. In order to obtain statistically reliable results,impact and tensile tests were carried out using three specimens foreach individual material testing.

    Potentiodynamic polarization test was performed to evaluatethe corrosion behavior of the base and weld metals by a poten-tiostat/galvanostat (PARSTAT 2273) in the 3.5 wt% NaCl solution atroom temperature. The polarization tests were conducted in aconventional three-electrode cell using graphite as the auxiliaryelectrode, and a saturated calomel electrode (SCE) as the referenceelectrode. Theworking electrodes were prepared from the base andweld metals, respectively. Polarization curves were plotted in thepotential range 200 to 600 mV versus Ecorr with the scan rate of1 mV/s.

    3. Results and discussion

    3.1. The microstructural characterization of the base metals

    Fig. 2a shows the as-cast microstructure of Inconel 718 in theprecipitation-hardened conditions. Accordingly, two types of pre-cipitates can be observed. Small precipitates within the austeniticgrains and also at the grain boundaries represent g00-Ni3Nb,whereas the needle-shaped and the grain boundary precipitates

    Filling 150 17.1 1.18 1.52Cap 150 17.3 1.26 1.44

    Inconel 625 Root 190 15.7 1.64 1.27 5.91Hot 170 16.6 1.19 1.66Filling 150 17.1 1.15 1.56Cap 150 17.5 1.29 1.42Fig. 1. Schematic illustration of the tensile test specimen: weld metal located in themiddle.

  • nal oA. Mortezaie, M. Shamanian / International Jourwhich indicates the correctness of performed solution heat treat-ment. Nickel base alloys can be welded in either the solution-annealed or precipitation-hardened conditions. Welding Inconel718 in the aged conditions causes liquation cracking in the heataffected zone that leads to a decrease in strength and toughness[12,13]. In this regard, Qian and Lippold [13] showed that by per-forming a solution heat treatment at 1010 C for 2 h, leading to theelimination of delta phase, the liquation cracking resistance ofInconel 718 heat affected zone was improved. Thus, it seems thatthe welding Inconel 718 in annealed conditions is a better choicecompared to the precipitation-hardened conditions.

    Fig. 3 displays the microstructure of wrought 310S austeniticstainless steel. It can be seen that the structure consists of ne andequiaxed austenitic grains. Due to performance of proper annealingheat treatment followed by water-quenching after hot rollingprocess, no precipitation of secondary phases in the microstructureis observed. It can also be seen that some residual high temperatureferrite (delta ferrite) is aligned along the rolling direction. Thisferrite results from the segregation of ferrite-promoting elements(primarily chromium) during thermomechanical processing.Although not considered deleterious in most applications, thepresence of ferrite in the microstructure can reduce the toughnessof austenitic stainless steel [14].

    3.2. The microstructural characterization of the weld metals

    The fusion zone microstructure of Inconel 625 weld metal isshown in Fig. 4a. The microstructure is completely austenitic with a

    Fig. 2. (a) Microstructure of the as-received IN-718 base metal: showing g00 andd precipitates at the grain boundaries and grains (b) optical microphotograph of IN-718base metal: The effect of solution treatment on the reduction of g00 and d phases.dendritic structure. Niobium and molybdenum contents in Inconel625 ller metal are 4% and 9%, respectively. Nb and Mo are ele-ments that show strong segregation during solidication of theweld metal. This is related to their equilibrium distribution coef-cient value, k, which is dened as k CS/CL, where CS and CL are thecompositions of the solid and liquid at the solideliquid interface,respectively [15,16]. Equilibrium distribution coefcient for Nb andMo is less than one [15], as is for Nb in the Inconel 625 k 0.54 [17].Previous studies have shown that elements with k< 1 values have astrong tendency to redistribution during solidication [9,15]. Soluteredistribution of alloying elements reduces the temperature at thefront solid/liquid interface, which leads to the occurrence of aconstitutional supercooling (factor required to change the micro-structure morphology). In these circumstances, solidication modewill change from planner or cellular to dendritic, and a concen-tration gradient is generated in the solidied weld metal [15,18].According to the dendritic microstructure of Inconel 625 weldmetal (Fig. 4a), it can be stated categorically that the redistributionof Nb and Mo has occurred during solidication. Inconel 625 llermetal also has 5 wt% Iron. Studies by Dupont et al. [19] indicate thatthe value of kNb depends on the Iron content of the alloy. The ob-tained results showed that additional Fe in the chemical compo-sition of the Nickel base alloys will decrease the solubility of Nb inaustenite. A similar trend has been observed for Molybdenum in

    Fig. 3. Microstructure of 310S base metal: austenite with ferrite stringers.

    f Pressure Vessels and Piping 116 (2014) 37e46 39Nickel base superalloys when used in dissimilar welds involvingalloys high in Fe [16,20]. Equilibrium distribution coefcient of Nbdecreases as the Iron content in the weld increases. Therefore,during solidication of the weldmetal, Nb segregates preferentiallyto the terminal liquid due to the low solubility in the austeniticphase. Similarly, Mo segregates to the liquid during solidication ofthe weld owing to the low solubility of Mo in the austenite phase,and leaves the rst solid to form depleted in Mo. In addition, thelow diffusion rate of Mo in austenite does not allow Mo to diffuseback towards the dendrite cores to eliminate the concentrationgradient [15,20]. Thus, Nb-rich and Mo-rich regions (interdendriticregions) can be formed at the terminal stages of solidication.Fig. 4bed illustrates SEM microphotograph and EDS results fordendritic structure of Inconel 625 weld metal. These resultsconrm that during solidication, Nb and Mo have left dendriticcores and have been rejected to interdendritic regions.

    Fig. 5a shows the columnar to equiaxed dendritic transition inthe microstructure of the Inconel 625 weld metal. Equiaxed den-drites were formed in the weld center line while the columnardendrites have been created along the fusion line. The change in themorphology of dendrites is related to the ratio G/R (Temperature

  • al oA. Mortezaie, M. Shamanian / International Journ40gradient, G and solidication rate, R), that determines themorphology of solidication structure weld metal. Kou [21] hasindicated that at a constant temperature gradient, the solidicationrate is maximum at the weld center line (RCL max), whereas it isminimal (RFL min) at the weld fusion line, so that (G/R)CL < (G/

    Fig. 4. Microstructure of IN-625 weld metal: (a) optical microphotograph includes dendritic(c) EDS microanalysis result for core dendrite, and (d) interdendritic regions.f Pressure Vessels and Piping 116 (2014) 37e46R)FL. It is proved that by reducing G/R, solidication mode changesfrom columnar to equiaxed dendritic because the ratio G/R has ainverse relationship with constitutional supercooling. Therefore, itcan be said that constitutional supercooling can be inuenced bytwo factors, k and G/R [21]. Moreover, according to Table 2, the total

    structure with columnar dendrites (b) SEMmicrophotograph with higher magnication

  • nal oA. Mortezaie, M. Shamanian / International Jourheat input used for the Inconel 625 ller metal is relatively high.The welding heat input can cause signicant effects on the micro-structure changes. The use of high heat input reduces the temper-ature gradient (G) at the center line of the weld pool. In theseconditions, the ratio G/R decreases and the constitutional super-cooling in front of the advancing solideliquid interface intensies.In other words, the increase in the heat input is equal that inconstitutional supercooling [18,21]. This intensication in consti-tutional supercooling has led to the formation of equiaxed dendriticstructure in the center line of the Inconel 625 weld metal. Fig. 5bshows the equiaxed dendrites at higher magnication. It can beobserved that interdendritic regions (Nb-rich and Mo-rich regions)appear brighter compared to dendritic cores. As discussed, due tomicrosegregation of Nb and Mo, a concentration gradient has beencreated. The presence of signicant concentration gradients wouldlead to preferential corrosion in the dendrite cores [15,16]. Thismakes during etching in marble solution, corrosion occurs more fordendritic cores. For this reason, dendritic cores appear darker.

    Fig. 6a exhibits cellular structure of 310 stainless steel weldmetal. Since there are small amounts of elements with a hightendency for segregation in the composition of 310 SS ller metal,there is a minimal constitutional supercooling to change the so-lidicationmode from cellular to dendritic. Although, Fig. 6b showsthat a transition in the microstructure has occurred in 310 weldmetal, this probably occurred due to the use of high heat inputduring welding (Table 2), and is not related to segregation of ele-ments. As it was said, constitutional supercooling is dependent on

    Fig. 5. Optical microphotograph of IN-625: (a) showing transition from columnar toequiaxed dendrites (b) showing equiaxed dendrites at higher magnication.f Pressure Vessels and Piping 116 (2014) 37e46 41the heat input so that with increasing heat input, constitutionalsupercooling increases [18,21]. Fig. 6b shows Solidication Sub-grain Boundaries (SSGB) and Solidication Grain Boundaries (SGB).Generally, these grain boundaries are observed in the single-phaseaustenitic weld metals. The solidication subgrains represent thenest structure that are normally present as cells or dendrites, andthe boundary separating adjacent subgrains are considered as asolidication subgrain boundary (SSGB). While the solidicationgrain boundary (SGB) results from the intersection of packets orgroups of subgrains. These grain boundaries are the direct result ofcompetitive growth that occurs along the trailing edge of the weldpool [14,15]. In this respect, Lippold and Koteki [14] have expressedthat due to the high concentration of solute and impure elements atthe SGB, solidication cracking in austenitic weld metals almostalways occurs along the SGB. But, Fig. 7a reveals that the solidi-cation cracking can also be formed along the SSGB.

    In Fig. 7a, solidication cracking in the intercellular boundaries(or the same SSGB) is quite evident. Solidication cracks are oftenproduced when stainless steel ller metals are used for dissimilarjoints between nickel alloys and austenitic stainless steels [14]. EDSanalysis result (Fig. 7b) shows that solidication subgrain bound-aries are chromium-rich. Since the Cr is the most important activeelement in the chemical composition of 310 ller metal, formationof chromium carbides (specically Cr23C6) in the microstructurewas predictable. According to EDS result, it can be concluded thatduring solidication, Cr is diffused to the SSGB and has led to the

    Fig. 6. Microstructure of 310 weld metal: (a) fully austenitic microstructure withcellular morphology (b) showing Solidication Subgrain Boundaries (SSGB) and So-lidication Grain Boundaries (SGB).

  • A. Mortezaie, M. Shamanian / International Journal o42formation of Cr23C6 phases along this the grain boundaries. Sincethe chromium carbide is essentially a brittle phase [14], it does nothave sufcient strength against tensile stresses developed acrossthe adjacent grains at the end of solidication. Thus, solidicationcracking occurred in the solidication subgrain boundaries. Itshould be noted that the tensile stresses developed across theadjacent grains are caused due to prevent contraction of the weldmetal by the base metals [21].

    Fig. 8 shows the microstructure of Inconel 82 weld metal. Themicrostructure of the IN-82 weld metal is fully austenitic,

    Fig. 7. Showing the solidication cracks in microstructure of 310 weld metal: (a) SEM microalong solidication subgrain boundaries.

    Fig. 8. Microstructure of IN-82 weld metal: austenitic microstructure with granularmorphology and dispersion of Ti-rich carbides within the grains.f Pressure Vessels and Piping 116 (2014) 37e46because it does not experience signicant allotropic trans-formation during solidication. It should be noted that similar toInconel 625, Inconel 82 ller metal has considerable amounts ofNb (3 wt%) and, its Iron content (3 wt%) is less than Inconel 625weld metal (5 wt%). As mentioned previously, by reducing theamount of iron in the chemical composition of the weld metal,Niobium solubility in the austenite phase increases [19]. In theseconditions, the redistribution of Nb is reduced during solidi-cation of the weld metal and therefore, constitutional super-cooling will not happen. For this reason, a homogeneous granularmicrostructure has emerged. Furthermore, the ne particles canbe observed in the austenitic microstructure of Inconel 82 weldmetal. EDS result shows that these particles are likely to beTitanium-rich carbides. Studies conducted by Jung et al. [22]show that diffusion coefcient of titanium in nickel is a consid-erable amount (DTi in Ni 4.1104 m2/sec) so that it is higherthan the other substitution of alloying elements [15]. On theother hand, since Ti is inherently a carbide-promoting element, itcan easily diffuse in the weld metal liquid (even solid) reactingwith carbon to form TiC.

    3.3. The microstructural characterization of the interfaces

    The interface between Inconel 625 weld metal with Inconel 718and 310S base metals is shown in Fig. 9a. It can be observed that alarge number of interlocking precipitates were formed in the vi-cinity of fusion line. EDS analysis shows that precipitates aremainlyNiobium carbide (NbC). In previous studies, it was revealed that Nbaddition increases the solidication temperature range. Thiswidening in solidication temperature range provides an adequate

    photograph (b) EDS microanalysis result indicating the presence of chromium carbide

  • ig N

    nal oFig. 9. (a) Interface between Inconel 625 weld metal and Inconel 718 base metal: shown

    A. Mortezaie, M. Shamanian / International Jouropportunity for the formation of NbC [20,23]. It should also benoted that carbon can diffuse from the weld metal to the basemetal. Studies show that carbon has a high penetration rate innickel, so that activation energy term (Q) for the diffusion of carbonis low in FeeNi austenite (135 kJ/mol), while the value is almostdouble for substitutional alloying elements such as chromium andaluminum [15]. Cieslak et al. [17] also reported that the equilibriumdistribution coefcient of Carbon in Inconel 625 is less than one(kC 0.21 in IN-625). On this basis, there is a intense tendency tothe diffusion and segregation of carbon to react with other ele-ments. Thus, it appears that these factors are justiable reasons forthe formation of various carbide phases (most notably theMC type)at the end of solidication.

    The interface between Inconel 82 weld metal and 310S basemetal is shown in Fig. 9b. Grain growth can be observed in the 310Sheat-affected zone. This can be attributed to an increase in tem-perature during different passes of welding. Moreover, a narrowunmixed zone can be seen in the vicinity of Inconel 82 weld metal,which is related to the difference in melting temperatures andchemical compositions of ller material and base metal. In dis-similar welding, when the melting range of ller materials issimilar to or higher than that of the basemetal, only a small fractionof the base metal can be melted, and no dilution occurs in theresolidication stage. In these circumstances, the convection cur-rents are not able to promote adequate uid ow and mixing andan unmixed zone is formed between the two regions [1,7].

    Fig. 9c shows the interface between 310 SS weld metal and 310Sbasemetal. Due to the effect of heat input of welding, grains growthcan be seen in the heat affected zone. Grains growth also occurrednear the fusion line where grains have grown perpendicular to thefusion line. Such a growth initiation process is called epitaxialgrowth, which occurs when the liquid completely permeates the

    and 310S base metal (c) Interface between 310 SS weld metal and 310S base metal (d) Inte

    b-rich carbides in the vicinity of fusion line (b) Interface between Inconel 82 weld metal

    f Pressure Vessels and Piping 116 (2014) 37e46 43substrate [21,24]. In fusion welding, this type of growth occurswhen the base and ller material have similar chemical composi-tions [21]. In this respect, Dehmolaei et al. [24] also observedepitaxial growth in dissimilar welds between heat resistant steeland Incoloy 800 when it was welded with 309 stainless steel llermetal.

    The interface between 310 SS weld metal and Inconel 718 basemetal is displayed in Fig. 9d. There is a wide unmixed zone forInconel 718 base metal, presumably due to the difference in themelting temperatures of base and weld metal. On the other hand,thickening of grain boundaries can be seen in the heat affected zoneof Inconel 718 base metal. The precipitation of free Nb in theseboundaries and the formation of delta and laves phases may be thereasons for this phenomenon. It can be seen that the liquationcracking in the heat-affected zone of Inconel 718 has not occurred,which is certainly related to the elimination of all phases (specif-ically, delta phase) after solution treatment. Research shows that inalloy 718, the presence of a high fraction of d phase increasescracking susceptibility. This is due to the dissolution of the d phasein the heat affected zone and the subsequent segregation of Nb tothe grain boundaries where it promotes liquid lm formation[15,25].

    3.4. Assessment of mechanical properties

    The results of tensile tests for welded joints are shown in Table 3along with the properties of the two base metals. During thetransverse tensile tests, fracture occurred in all weldments in the310S heat-affected zone. As mentioned before (Fig. 9bec), occur-rence of grain growth in the heat affected zone of 310S base metalhas caused fracture to occur in these zones. It should be mentionedthat unlike the conventional tensile specimens, failure processes

    rface between 310 SS weld metal and IN-718 base metal.

  • Table 3Tensile properties of the base and weld metals at room temperature.

    Filler and base metal type Yield strength(MPa)

    Ultimate tensilestrength (MPa)

    Total elongation(%)

    Inconel 718 base metal 942 12 1170 9 23 6310S base metal 310 2 627 2 58 4Inconel 82 weld metal 420 8 671 6 34 8Inconel 625 weld metal 470 14 712 7 31 6

    Fig. 10. SEM fractograph of fractured Charpy impact specimens: (a) IN-625 weld metal,(b) 310 weld metal (c) IN-82 weld metal.

    A. Mortezaie, M. Shamanian / International Journal o44are more complicated in welded specimens, particularly in dis-similar welding which contains different zones with differentproperties, and behavior of one zone can have an effect on itsadjacent zones [26]. Results obtained from tensile tests on theweldments indicate that dissimilar welds produced by Inconel 625ller metal have the maximum yield strength and tensile. Thisresult species that the presence of concentration gradients inmicrostructure of Inconel 625 has not led to a decrease in its tensilestrength. Moreover, 310 SS weld metal showed the lowest strengthand total elongation value compared to other weld metals, this waspredictable due to the appearance of solidication cracks in themicrostructure. Overall, the average yield strengths of the threewelded joints are higher than those of 310S base metal, which in-dicates the yield strength of welded joints meets the requirementsof engineering application [1].

    Charpy impact test results for weldments and base metals arelisted in Table 4. Despite the dissolution of the d-phase after solu-tion heat treatment, a fully brittle fracture occurred for Inconel 718base metal. In contrast, 310S base metal enjoys higher toughnessthat can be attributed to the presence of wrought and annealedaustenitic grains. Charpy V-notch impact energies for the weldmetals are within the values pertaining to the base metals. Amongthe weldments, the toughness of Inconel 82 weld metal is thehighest. This can be related to the high amount of Nickel and themonolithic structure this weld metal. Inconel 625 weld metalshows lower impact energy compared to Inconel 82weldmetal dueto the high concentration of molybdenum in the interdendriticregions. Similar to the tensile test results, solidication cracks in310 SS weld metal have reduced its impact energy. SEM micro-photographs from the fracture surfaces of the impact specimens areshown in Fig. 10aec. Fracture surfaces of three types of weld metalsare ductile, which includes a combination of deep and wide dim-ples. It can be observed that for Inconel 625 weld metal, dimplessize is smaller compared to that of Inconel 82 although in the caseof Inconel 82 weld metal, dimples are more evenly distributed. Itshould be noted that due to the application of dissimilar joint IN-718/310S in high temperatures working conditions, changes inmechanical properties are unavoidable. Because transformationreactions occur in the weld and base metals at high temperature,and undesired compounds appearing in the microstructure, whichmay lead to a decrease in toughness or strength [27,28]. In thisrespect, Naffakh et al. [28] have shown that after subjecting dis-similar joint Inconel 657/310 stainless steel to an embrittling heat

    310 SS weld metal 380 18 495 14 14 3treatment at 1000 C for 100 h, fracture energy of base and weldmetals declined although tensile strength for both increased.

    Table 4The charpy V-Notch impact energy values at room temperature.

    Material Impact energy (J) Type of fracture

    Inconel 718 base metal 32 4 Fully brittleType 310S Stainless steel base metal 170 2 Fully ductileInconel 82 weld metal 166 5 Fully ductileInconel 625 weld metal 135 6 Fully ductile310 SS weld metal 118 12 ductilef Pressure Vessels and Piping 116 (2014) 37e463.5. Assessment of corrosion behavior

    The potentiodynamic polarization test results of base and weldmetals are shown in Fig. 11a and b, respectively. Furthermore, theirelectrochemical parameters are given in Table 5. It can be seen thatthe corrosion potential is almost identical for both base metals.Corrosion potential is a static indicator of electrochemical corrosionresistance, which reveals the susceptibility of materials to corro-sion. In general, materials that exhibit a high corrosion potentialoffer higher corrosion resistance [29]. It can be observed that thecorrosion potential of the weld metals is lower than that of base

  • A. Mortezaie, M. Shamanian / International Journal ometals. The corrosion potentials differences between the weld andbasemetals are nearly 0.2e0.3 V, which can lead to the formation ofeffective galvanic couples and can increase the corrosion rate. Ac-cording to Fig. 11b, Inconel 625 displays a lower corrosion potentialcompared to Inconel 82 because the concentration gradientmicrostructure reduce its corrosion resistance. Studies show thatthe concentration gradients in the solidied microstructure cancause very localized corrosion between interdendritic regions anddendritic cores, and can accelerate galvanic corrosion at themicrostructure level [20]. The passivation behavior in anodicbranch of the polarization curve of Inconel 625 can be attributed tothe high chromium contents in its chemical composition [14]. Inaddition, although 310 weld metal has lower corrosion potentialcompared to two other ller metals, its anodic branch indicates avery signicant passivation behavior. It can be seen that the pri-mary passivation potential and breakdown potential for 310 weldmetal are lower and higher, respectively, compared to the samevalues in the polarization curves of Inconel 625weldmetal. In other

    Fig. 11. Potentiodynamic polarization curves obtained in 3.5 wt% NaCl solution (a) forIN-718 and 310S base metals (b) for weld metals.

    Table 5Electrochemical data for the base and weld metals in 3.5wt % NaCl solution at 30 C.

    Specimens Ecorr (mV) Icorr (mA/cm2)

    Inconel 718 160 0.512310S 190 0.245Inconel 625 447 7.76Inconel 82 344 0.251310 SS 469 1.17[1] Standard Welding Terms and Dinitions, ANSI/AWS. Welding handbookWHB-4, Ch.12. dissimilar metals. American Welding Society Inc; 2006.

    [2] Chen HC, Pinkerton AJ, Li L. Fibre laser welding of dissimilar alloys of Ti-6Al-4V and Inconel 718 for aerospace applications. Int J Adv Manuf Technol2010;52:977e87.

    [3] Richards NL, Huang X, Chaturvedi MC. Heat affected zone cracking in castinconel 718. Mater Charact 1992;28:179e87.

    [4] Hong JK, Park JH, Park NK, Eom LS, Kim MB, Kang CY. Microstructures andmechanical properties of Inconel 718 welds by CO2 laser welding. J MaterProcess Technol 2008;201:515e20.

    [5] ASM handbook, properties and selection: irons, steels, and high performancealloys: elevated-temperature properties of stainless steels. 10th ed., vol. 1.Ohio: ASM International, Materials Park; 2002.

    [6] Tavares S, Moura V, DaCosta VC, Ferreira M, Pardal JM. MicrostructuralThe investigations performed on Inconel 718/310S dissimilarjoints by GTAW process, and using three types of ller materialsInconel 625, Inconel 82, and 310 austenitic stainless steel have ledto the following conclusions:

    Due to the redistribution of niobium and molybdenum duringsolidication, a concentration gradient was generated in themicrostructure of Inconel 625 weld metal. At the end of solidi-cation, microstructure obtained for Inconel 625 consisted ofcolumnar dendritic and equiaxed. The microstructure of 310weld metal includes solidication cracks along solidicationsubgrain boundaries.

    Due to the pre-weld heat treatment conducted on Inconel 718base metal, and the subsequent elimination of the delta phase,liquation cracking did not occur in its heat-affected zone. In theinterface between 310S base metal and other weld metals, asignicant growth in grains occurred, which led to a decrease intensile properties of 310S heat-affected zone.

    In tension tests, the yield strength of three kinds of weldedjoints meets the requirements of engineering application.Nevertheless, from the standpoint of tensile strength, 310stainless steel ller metal is not recommended for this dissimilarjoint. Among the weldments, Inconel 625 weld metal had thehighest yield strength and tensile.

    According to impact tests, Inconel 82 and 310 SS weld metalsdisplayed the highest and lowest toughness values, respectively.In addition, fracture surfaces of three types of weld metalsincluded deep and wide dimples.

    In potentiodynamic polarization test, Inconel 625 weld metaldisplays a lower corrosion potential compared to Inconel 82weld metal because the concentration gradient obtained fromthe solidication reduced its corrosion resistance.

    Due to the offer optimum properties, it is suggested that fordissimilar joints between Inconel 718/310S austenitic stainlesssteel, IN-82 ller metal to should be used.

    Referenceswords, passivation of 310 weld metal occurred at a lower potentialand remained stable at a higher potential difference range. Ac-cording to Table 1, it can be observed that the chromium content of310 ller metal is more than that in 625 and 82 ller metals. Ex-istence of this element in stainless steels increases the stability ofpassive lm [14]. On the other, due to the higher austenite contentand lack of concentration gradient in microstructure of Inconel 82,a higher corrosion potential was obtained for it. As a result, in 3.5%NaCl solution, the ranking for corrosion resistance is : IN-718BM > 310S BM > FM-82 > FM-625 > ER310.

    4. Conclusions

    f Pressure Vessels and Piping 116 (2014) 37e46 45changes and corrosion resistance of AISI 310S steel exposed to 600e800 C.Mater Charact 2009;60:573e8.

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    [8] Lee HT, Jeng SL, Yen CH, Kuo TY. Dissimilar welding of nickel-based Alloy 690to SUS 304L with Ti addition. J Nucl Mater 2004;335:59e69.

    [9] Naffakh H, Shamanian M, Ashrazadeh F. Dissimilar welding of AISI 310austenitic stainless steel to nickel-based alloy Inconel 657. J Mater ProcessTechnol 2009;209:3628e39.

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    [12] Thompson RG, Dobbs JR, Mayo DE. The effect of heat treatment on micro-ssuring in Alloy 718. Weld J 1986;65:299e304.

    [13] Qian M, Lippold JC. The effect of rejuvenation heat treatments on the repairweldability of wrought Alloy 718. Mater Sci Eng A 2002;340:225e31.

    [14] Lippold JC, Koteki DJ. Welding metallurgy and weldability of stainless steels.New Jersey: John Wiley & Sons Inc; 2005.

    [15] Dupont JN, Lippold JC, Kiser SD. Welding metallurgy and weldability of nickel-base alloys. New Jersey: John Wiley& Sons Inc; 2009.

    [16] Banovic SW, DuPont JN, Marder AR. Dilution and microsegregation in dis-similar metal welds between super austenitic stainless steel and nickel basealloys. Sci Technol Weld Join 2002;6:274e83.

    [17] Cieslak MJ, Headley TJ, Romig AD, Kollie T. A melting and solidication studyof Alloy 625. Metall Mater Trans A 1988;19:2319e31.

    [18] Winegard WC. An introduction to the solidication of metals. Institute ofMetals; 1964.

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    [20] DuPont JN, Banovic SW, Marder AR. Microstructural evolution and weldabilityof dissimilar welds between a super austenitic stainless steel and nickel-basedAlloys. Weld J 2003;82:125e35.

    [21] Kou S. Welding metallurgy. 2nd ed. Hoboken. New Jersey: John Wiley & SonsInc; 2003.

    [22] Jung SB, Yamane T, Minamino Y, Hirao K, Araki H, Saji S. Interdiffusion and itssize effect in nickel solid solutions of nickel-cobalt, nickel-chromium andnickel-titanium. J Mater Sci Lett 1992;11(20):1333e7.

    [23] Naffakh H, Shamanian M, Ashrazadeh F. Microstructural evolutions in dis-similar welds between AISI 310 austenitic stainless steel and Inconel 657.J Mater Sci 2010;45:2564e73.

    [24] Dehmolaei R, Shamanian M, Kermanpure A. Microstructure characterizationof dissimilar welds between alloy 800 and HP heat resistance steel. MaterCharact 2008;59:1447e54.

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    [26] Hajiannia I, Shamanian M, Kasiri M. Microstructure and mechanical propertiesof AISI 347 stainless steel/A335 low alloy steel dissimilar joint produced bygas tungsten arc welding. Mater Des 2013;50:566e73.

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    A. Mortezaie, M. Shamanian / International Journal of Pressure Vessels and Piping 116 (2014) 37e4646

    An assessment of microstructure, mechanical properties and corrosion resistance of dissimilar welds between Inconel 718 and ...1 Introduction2 Materials and methods3 Results and discussion3.1 The microstructural characterization of the base metals3.2 The microstructural characterization of the weld metals3.3 The microstructural characterization of the interfaces3.4 Assessment of mechanical properties3.5 Assessment of corrosion behavior

    4 ConclusionsReferences