Overmatching Superalloy Consumable Inco-weld Broadens Its .

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P0468Overmatching Superalloy Consumable Inco-weld 686CPT Broadens its Applications to Include WeldingSuper Austenitic and Super Duplex Stainless SteelsAuthors:Ph.D C. Thornton,B.Sc C. CooperSpecial Metals Welding ProductsCompany, UKKeywords:Crevice corrosion, nickel alloy fillermetals, over-matching, pittingcorossion, super austeniticstainless steel, super duplexstainless steel.1. IntroductionCorrosion resistant materials suchas alloys 625 (UNS N06625) and C276 (UNS N10276) have providedexcellent corrosion resistance forequipment in the oil and gas,chemical, petrochemical, pollutioncontrol and similar industries. Newnickel-chromium-molybdenum alloys (INCONEL Alloy 686, HASTELLOYC-2000 and alloy 59) have beendeveloped, which provide increasedresistance to corrosion beyond thecapability of both stainless steelsand existing nickel based alloys. Theuse of these new alloys can providea number of benefits to operatorsincluding improved reliability,increased product life cycle, lowermaintenance and repair costs andreduced down time.The chemical compositions ofseveral nickel-chromiummolybdenum (Ni-Cr-Mo) alloys aredetailed in Table 1. Substantialadditions of the major alloyingelements provide a high level ofgeneral corrosion resistance whilstthe combination of chromium andmolybdenum improves resistance toboth pitting and crevice corrosion.Additions of tungsten also provideimproved resistance to localizedcorrosion.1.1 Localized CorrosionResistanceOne of the most commonly observedfailure mechanisms in stainlesssteels and Ni-Cr-Mo alloys is pittingor crevice corrosion. These forms ofcorrosive attack are less predictablethan general corrosion and may limitthe performance of the material. AnAbstractThe concept of using a more highly alloyed welding product to weld corrosionresistant alloys is not new. Early examples are 317LMN and 904L being welded with INCONEL FM 625 and INCONEL WE 112. There are more than five wellknown pit and crevice-corrosion resisting nickel alloys that benefit from being welded with the superalloy INCO-WELD 686CPT . In addition, there is agreater number of super austenitic and super duplex stainless steels, that maybe used to their maximum capabilities when welded with the overmatching fullyaustenitic, INCO-WELD 686CPT products.Typical applications and performance criteria include the fabrication of flue gasdesulfurisation (FGD) equipment where extreme pitting and crevice corrosionenvironments are generated by scrubbing high concentrations of sulfur andhalogens from coal-fired boilers. These environments often include chlorideconcentrations beyond 100,000 ppm and pH levels of less than 1. Superaustenitic and super duplex stainless steels are continuing to find wideacceptance in handling aggressive fluids in pulp and paper applications. Bleachplant piping and vessels are often specified in stainless steels that need muchbetter welds than matching composition can supply.It is no small task to meet the weld requirements of nickel superalloys and thestainless super austenitic and duplex alloys, but INCO-WELD 686CPT meetsthe challenge.The INCO-WELD welding products 686CPT may be deposited by a variety ofprocesses including SMAW, GMAW, GTAW, PAW and SAW. Surfacing can alsobe undertaken with the electroslag and submerged arc processes. In the case ofwelding super austenitic and super duplex stainless steels, careful adherence totightly prescribed welding procedures can be performed easily with INCO-WELDFM 686CPT because it is fully austenitic and produces optimum corrosionresistance at all cooling rates from a wide range of welding parameters.estimate of the relative pittingresistance of alloys can be madeusing the Pitting ResistanceEquivalent Number (PREN) which iscalculated using the chemicalcomposition of the alloy. Thecalculated PREN values for a rangeof Ni-Cr-Mo alloys are shown inTable 1. The standard PRENcalculation used for stainless steelscannot be used for nickel alloys, withthe equation which most closelyrepresents the performance of Ni-CrMo alloys in various media being:PREN % Cr 1.5 (%Mo %W %Nb)It has been demonstrated that alloyswith the highest PREN values havethe highest critical pitting and criticalcrevice corrosion temperatures whenevaluated using standard testmethods (ASTM G48 acidified FeCl3solution). Corrosion tests undertakenwith Ni-Cr-Mo alloys in flowing seawater and elevated temperaturestagnant sea water have also showna correlation between the PRENvalue and the depth of crevice attack[1]Stainless Steel World 2004 2004 KCI Publishing BV[1].2. Selection ofWelding Filler MetalThe selection of consumables forwelding high nickel alloys (Ni-CrMo), super duplex stainless steelsand super austenitic stainless steelsrequires consideration of both themechanical properties and thecorrosive environments to beexperienced by the welded joint. Asdeposited weld metal has a dendriticstructure with micro segregation ofalloying elements through thestructure. It has been shown thatautogeneous welds completed in316L and 317L plate have a reducedresistance to pitting corrosion inoxidizing acid chloride environments[2]compared to the parent plate . Thedecreased pitting resistance of theweld metal is due to microsegregation of alloying elementswithin the dendrites. Electronmicroprobe measurements of thechromium and molybdenum contentsshowed that the centers of the1

P0468austenitic dendrite were depleted inchromium and especiallymolybdenum. In contrast, theinterdendritic regions which were thelast to solidify, were enriched inmolybdenum and chromium.Autogeneous welds in a 316L typestainless steel containing 2.8% Moexhibited a variation in Mo content ofbetween 1.8% and 5.7%.Preferential corrosion of the dendritecores was observed in the regionswhich had suffered pitting attack.In high nickel alloy weld deposits Moand Nb and sometimes Cr areenriched at the dendrite boundaries[3, 4].in the interdendritic regionsDuring solidification of the weldmetal these elements becomeenriched in the liquid phase. Thissegregation within the dendriticstructure results in reducedcorrosion resistance compared tothe homogeneous alloy. Additionally,there is an enhanced risk ofintermetallic phase precipitation dueto the local enrichment of Mo, Cr andNb in the interdendritic regions.Work undertaken with Ni-Cr-Mo alloyfiller metals, that have additions oftungsten, has shown that tungsten isenriched in the core of the dendriteand is depleted in the interdenritic[4, 5]. As both molybdenumregionsand tungsten provide increasedresistance to localised corrosion thedifference in segregation behaviourof tungsten acts to support that ofmolybdenum.2.1 Over Alloyed FillerMetalsThe concept of using an “overalloyed” filler metal has beendeveloped in order to counteract theeffects of segregation across thedendritic structure. The selection ofan appropriate filler metal, which isover alloyed compared to the basemetal, ensures that theconcentrations of the major alloyingelements across the segregateddentritic regions are sufficient toprovide superior corrosion resistanceto that of the base material. Whensuch an approach to the selection ofwelding consumables is used it hasbeen shown that the critical pittingtemperature of the weld metal[5]exceeds that of the base material .In order to produce welds, in highalloy stainless steels, which exhibitcorrosion resistance superior to thatof the stainless steel base material ithas proved necessary to selectnickel alloy filler metals based on theNi-Cr-Mo chemistry. Weldscompleted in 904L and 254SMostainless steel using 625 and C-276(Ni-Cr-Mo) alloy filler metals havebeen shown to exhibit pittingperformance superior to that of the[6]stainless steel base material .The use of over-matching nickelalloy filler metals has also beenemployed for the welding ofcorrosion resistant nickel alloys usedin the construction of flue gas[7]desulphurisation (FGD) plant .Pitting and crevice corrosion testingof both alloys and weld deposits hasbeen undertaken in an oxidisingchloride solution (11.9%H2SO4 1.3%HCl 1%FeCl3 1%CuCl2 for 72 hours at 103 C). It has beenfound that the composition of thissolution is similar to the environmentexperienced in some areas of FGD[5]plant . Welds made in a number ofalloy base materials including C2000, alloy 59, C-22 and C-276 withmatching filler metals have beenfound to pit severely whilst weldsmade with the overmatching fillermetal INCO-WELD 686CPT (AWSA5.14 ERNiCrMo-14) showed noattack. The 686CPT alloy filler metalis based on the Ni-20%Cr-16%Mochemistry with an addition of 4% W.In nickel alloys tungsten acts in asimilar manner to that ofmolybdenum in providing resistanceto pitting and crevice corrosion.Welds made in the same materialsbut with the over matching alloy686CPT filler metal were found to beresistant to attack (Figure 1).Optimum pitting resistance isobtained when the over alloyed fillermetal 686CPT is employed tocomplete the welds.In a hot bypass duct in FGD plant inthe Seminole Electric System severecorrosion of welds in C-276 basematerial completed with C-276 filler[8]metal was observed . The weldwere repaired using the 686CPTalloy filler metal in order to providean over alloyed weld depositcompared to the base material.Replacement welds made with theover matched 686CPT filler metalshowed no corrosion whilst the C276 plate corroded. The over alloyed686CPT filler metal has beensuccessfully used for joining C-276[9]plate in other FGD plant .On the basis of the results ofcomparative pitting and crevicecorrosion performance of a numberof nickel alloy filler metals a PittingResistance Equivalent Number(PREN) has been proposed topredict the relative performance of[9]the alloys .PREN %Cr-0.8%Cu 1.5 (%Mo Stainless Steel World 2004 2004 KCI Publishing BV%W)This formula provides equalweighting to the beneficial effects ofmolybdenum and tungsten inproviding resistance to localizedcorrosive attack. The comparativeresistance of weld deposits to attackin oxidizing chloride environments isreflected by this formula.3. Welding of SuperAustenitic StainlessSteelsThe super austenitic range ofstainless steels containing 6-7% Mohave a high resistance to pitting andcrevice corrosion due to the highmolybdenum and nitrogen contents(Table 2). The addition of nitrogenimproves both the mechanicalproperties and pitting resistance ofthese alloys. The super austeniticstainless steels exhibit higher PRENvalues (PREN %Cr 3.3 (%Mo) 16 (%N)) than the duplex grades ofstainless steel. The super austeniticsteels are used in environmentswhere aqueous corrosion by chloride(and other halides) is a concern (e.g.sea water, paper and pulp bleaching,flue gas desulphurisation plant etc).However, super austenitic stainlesssteels can exhibit susceptibility tochloride ion stress corrosioncracking in hot chloride solutions.However, these steels offerresistance to crevice corrosion insea water at elevated temperatures.Alloy 27-7Mo (UNS No. S31277)offers corrosion resistance superiorto that of the 6%Mo super austeniticsteels (UNS N08926, S31254) andin some instances has corrosionresistance approaching that of nickelbased alloys such as 625 and C-276[10].The 6% Mo super austeniticstainless steels are welded with highnickel alloy welding consumables ofthe Ni-Cr-Mo type (eg. 625, 622 and686CPT). Welds completed in 6%Mosuper austenitic steels with 625 typeconsumables have been found toexhibit pitting in ASTM G48A tests at40-50 C, compared to the basematerial, which exhibits pitting at 55[11]60 C . Autogeneous welds inthese steels show a loss in corrosionperformance due to the effects ofsegregation in the as deposited weldmetal. When welding superaustenitic stainless steel it is notnecessary to impose the same heatinput controls as are applied toduplex stainless steels as there is norequirement to obtain the ferriteaustenite balance needed in duplex2

P0468stainless steels for mechanicalproperties and corrosion behaviour.In order to determine the suitabilityof high nickel alloy (Ni-Cr-Mo) fillermetal for welding super austeniticsteel the mechanical properties andcorrosion behaviour of weld depositshave been assessed. Welds werecompleted in super austeniticstainless steel using the TIG processin accordance with the proceduredetailed belowWeld preparation:Single V 60 included angleBase Material:12.7 mm thicknessPosition:Vertical upFill Current:Fill Voltage:Fill Speed:Fill Heat Input:180 A14 V, DC –115 mm/min(4.5”/min)1.3 kJ/mm(33kJ/inch)Welds in 25-6 Mo plate (UNSN08926) were completed using 622and 625 filler metals whilst welds in27-7 Mo plate (UNS S31277) werecompleted using 686CPT filler metal.A macro section from one of thesesingle sided welds is shown inFigure 2 and the compositions of thefiller metals are listed in Table 4.The tensile properties of the weldswere determined from both weldmetal tensile and cross weld tensilespecimens. The Charpy V-notchimpact toughness of the welddeposits was evaluated at -50 Cusing specimens located at the weldcenter line. Pitting corrosion testingof the weld metal was undertakenusing the acidified FeCl3 solution inASTM G48 for 24 hours.The weld metal yield and tensilestrengths (Table 5) for the Ni-Cr-Moalloy filler metals were 520-527MPa(75.4-76.4ksi) and 797-802MPa(115.6-116.3ksi) respectively. Thesestrength values exceed the specifiedminimum values for the grades ofsuper austenitic steels. All the crossweld tensile specimens exhibitedductile fracture in the base plateaway from the weld (Table 6). For allthree nickel alloy filler metals theweld metal toughness values at 50 C (-58F) exceeded 70J (52ft.lbs).The impact toughness performanceof 622 and 686CPT was similar withimpact toughness values at -50 C (58F) lying in the range 72-88J (5365ft.lbs). The toughness of the 625weld deposit was higher with 100104J (74-77ft.lbs) being achieved at-50 C (-58F).Pitting corrosion testing of the weldscompleted in 25-6Mo plate (UNSN08926) showed that no pitting wasobserved at a temperature of 50 Cwith both 622 and 686CPT fillermetals (Figure 3). However, at 60 C pitting occurred in the HAZ atthe weld root location. For the weldcompleted in 27-7Mo plate (UNSS31277) with 686CPT filler metal nopitting was observed at 60 C. At atemperature of 70 C pitting wasobserved to occur in the HAZ at theroot of the weld. There was no pittingin the weld deposit in either the capor root locations at 70 C, howeverthere was some evidence of slightcorrosive attack in the cap of theweld. The high alloyed Ni-Cr-Moweld deposits (625, 622 and686CPT) were found to exhibitsuperior pitting corrosion resistanceto that of the HAZ of the basematerial. In all instances pitting wasfound to occur preferentially in theHAZ at the root of the weld.The 686CPT filler metal has beenused successfully in a productionenvironment for welding 4.5%Moand 6%Mo super austenitic stainlesssteels. During welding procedurequalification good levels of impacttoughness have been recorded attemperatures down to -196 C andtensile properties exceeding thespecified minimum values have beenachieved. Pitting corrosion tests inaccordance with ASTM G48A for 24hours showed no evidence of pittingwith weight loss values of 3glm²being recorded.4. Welding of DuplexStainless Steelsdepends on the cooling rate duringwelding with high cooling ratesresulting in higher levels of ferritewhen welding is undertaken withduplex stainless steel filler metals.Nitrogen acts to aid the reformation[12]byof austenite in the weld regionpromoting austenite reformation athigher temperatures. Nitrogen alsoimproves the corrosion resistance,especially of the austenite phase.Practically, heat inputs in the rangeof 0.5-1.5kJ/mm for super duplex,and 0.5–2.0kJ/mm for standardduplex stainless steels are employedin conjunction with maximuminterpass temperatures of 100 C and150 C respectively dependent uponwall thickness. Interpasstemperature is often restricted onthin wall super duplex materials toprevent the precipitation of thirdphase intermetallics. Weldingconsumables for duplex stainlesssteels are similar in composition tothat of the base material but withhigher levels of nickel to ensure anappropriate phase balance (30-60%ferrite) in the deposited weld metal.Many fabrication specifications in theoil and gas industry require ASTMG48A pitting corrosion testing of thedeposited weld metal to beundertaken at 25 C and 40 C forduplex and super duplex stainlesssteels respectively. In manyinstances super duplex filler metal isused for welding standard duplexstainless steels. The use of such anover alloyed filler metal (superduplex stainless steel) in thesecircumstances produces welddeposits with enhanced pittingcorrosion resistance compared tostandard duplex stainless steel welddeposits.Duplex and super duplex stainlesssteels provide good resistance tostress corrosion cracking togetherwith higher strength levels comparedto both standard austenitic andsuper austenitic stainless steels(Table 3). These steels arecharacterized by a microstructurecontaining both ferrite and austenite.Duplex stainless steels are sensitiveto variation in chemical composition,which will influence themicrostructure and phase balance inthe weld region. Super duplexstainless steels are characterized ascontaining higher levels of Ni, Cr, Moand N compared to standard duplexstainless steels. Some super duplexstainless steels are also alloyed withtungsten (Table 2).Nickel alloy filler metals have beenused in some applications tocomplete welds between duplexstainless steels and other alloymaterials (eg. Cr-Mo steels, nickelbased alloys etc.). The use of a fullyaustenitic high nickel Ni-Cr-Mo alloyfiller metal for welding duplex andsuper duplex stainless steelsprovides potential advantages interms of welding proceduralapproach and improved pittingcorrosion resistance compared tothe use of super duplex stainlesssteel filler metals. The use of a fullyaustenitic nickel alloy weld depositremoves the requirement whichexists with duplex stainless steelfiller metals to produce welds withbalanced quantities of austenite andferrite.The amount of ferrite and austenitein the weld deposit and HAZThe variation in weld metalStainless Steel World 2004 2004 KCI Publishing BV3

P0468mechanical properties for INCONELFM 686CPT with heat input hasbeen determined by depositing allweld metal test plates with the TIGwelding process in the flat position.The variation in 0.2% proof strengthand tensile strength with heat inputis shown in Fig.4 which shows that0.2% proof strength levels in excessof the 550 MPa specified for superduplex stainless steels are achievedin the weld deposit at a heat input ofless than 1.5kJ/mm. The impacttoughness of welds completed with686CPT filler metal shows littlevariation with heat input with valuesin excess of 95J and 65J beingachieved at -50 C and -196 Crespectively (Figure 5).These mechanical property resultsfor the 686CPT wire, combined withthe pitting corrosion performance ofthis alloy, clearly demonstrate itssuitability for welding both standardand super duplex grades of stainlesssteel. To ensure that the weld metalstrength and other mechanicalproperties exceed the minimumrequirements for super duplexstainless steels appropriate controlof welding parameters will berequired.5. ConclusionsThe selection of a weldingconsumable must be based onanticipated service requirements andspecified weld metal mechanicalproperty requirements. The asdeposited weld metal microstructure,exhibits some variation in chemicalcomposition of the main alloyingelements due to the effects ofsegregation during solidification. Theselection of an over alloyed weldingconsumable compensates for thesesegregation effects. There are manyapplications where highly alloyed NiCr-Mo filler metals, including686CPT filler metal, provideenhanced resistance to pitting andcrevice corrosion compared to thebase material being welded.Examples of the applications forthese alloys include:Welding of C-276 material in flue gasdesulphurisation

improved resistance to localized corrosion. 1.1 Localized Corrosion Resistance One of the most commonly observed failure mechanisms in stainless steels and Ni-Cr-Mo alloys is pitting or crevice corrosion. These forms of corrosive attack are less predictable than general corrosion and may limit the performance of the material. An

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