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ALLOY 718- ALLOY OPTIMIZATIONFOR APPLICATIONSINOIL AN'D GAS PRODUCTION0.A.OnyewuenyiShellDevelopmentCompanyP. 0. Box 1380Houston,Texas77001AbstractAlloy718 is extensivelyused by the oiland gas industryfor CriticalServiceapplicationsinvolvingboth wellheadcomponentsand downholeThis studyaddressestheauxiliaryequipmentfor corrosive/sourservice.metallurgyand precipitationbehaviorof thisalloyand theireffectsonThe use of heat treatmentand microstructuralcontrolfractureresistance.to tailorthe alloyto the specificneeds of oiland gas productionappliin contrastto aerospaceapplicationsis l canonsEdited by E.A. LoriaThe Minerals, Metals & Materials Society,3451989
IntroductionIn the lastten years,Alloy718 has become the preferredmaterialforauxiliaryand downholetoolsfor oilmanufactureof wellheadcomponents,and gas wellscontainingcorrosiveand cracksensitivespeciessuch as CO,,For many such applications,the alloynow replaceschloridesand H,S.othercorrosionresistantalloys(CRA) candidatessuch as K-monel,17-4 pHThis increasedintereststemmed primarilyfrom variousand InconelX750.(a) reportedlabieldfailuresof productionfactors,namely:(b) abilityto fabricateheavyequipmentmade from the above CRAs ars', fwalland/orcomplexshaped componentsand stillachievea desiredcombinacorrosionand environmentalcrackingresistionof strength,toughness,and (c) discoverythatslightmodificationsin compositionand heattance,treatmentcan be used to tailorthe alloyto the needs of specificenvironmentalapplications.The objectivesof thispaperare to reporttypicalapplicationsofand the specialrequirementsimposedAlloy718 in oiland gas production,by its use in deep high pressuresour gas wells.The relativecharacterheat treatmentsof Alloy718isticsof underage,peak age, and overageprovidesthe basisfor preferredheat treatmentswhich optimizedesiredThe factorswhich affectfractureresistanceare highlightedproperties.and the criteriafor toughnessrequirementsof Alloy718 in criticalapplicationsare presented.ApplicationRequirementsTableI shows the typicalcomponentscommonly made of Alloy718.InFor wellheadgeneral,the strengthrequirementsfallintotwo classes.equipmentssuch as valvetrimsand valvebodiesfor AmericanPetroleumthe minimum yieldstrengthis 75Institute'sSpecification6A PSL4 service,ksi.For fasteners,downholeand auxiliaryequipmentsdesignedto connectto threadedmembers such as high strengthtubing,and/orto containhightypicalminimum strengthis 120 ksi.In allcasesinternalpressures,involvingexposureto H,S, onEngineer'sStandard(NACE) MR-01-75limitsthe maximum acceptablehardnessto HRC 40.Table I. Typical Applications of Alloy 718 in Oil and Gas ProductionTypical MinimumYield Strength, ksiMaximumHardness, HRCSubsurface Safety Valves12040Packers12040Hangers75-12040Valve Gates, Seats, and Stems7540Fire Safe Valves7540Blowout Preventers7540Side Pocket Mandrels7540Fasteners120346
Auxiliarycomponentssuch as tubinghangers,packers,and subsurfacesafetyvalvebody components,are consideredparticularlycriticalsincethey can be highlystressedwithtensileloads up to 90,000psi,and withinternalpressuresof 15,000psi,whilebeingexposedto corrosivewellfluidcontainingH,S, CO,, and aqueouschlorides.Typicalservicetemperaturescan range from ambientduringacidizingtreatmentsto 350 F atnormalproduction.Failureof such componentscan resultin substantialand releaseof deadlytoxicgases such as H,S.monetaryrisks,Thus suchconcernsdemand nmanufacturingof Alloy718 onin deep sour gas wells,the criticalrequirementsare (1) resistanceto generaland localizedcorrosion,(2)to environmentallyinducedcrackingfor both sulfidestressand anodicstresscorrosioncracking,based on a maximum hardnessof HRC 40, (3) highstrengthtypicallyabove 120 ksi for both tubinghangers,safetyvalvesand packercomponentswhich must be compatiblewiththe strengthand threaddesignof the productiontubing,and (4) fracturetoughness.The goal of the toughnessoptimizationis to ensurethataleak-before-breaktype of failureis achievedfor pressurecontainmentboundarycomponents,or as a minimum,a catastrophicbrittlefractureisprecluded.A furtherconsiderationis thatthe metallurgicalfactorswhichoptimizetoughnessat a givenstrengthalso promotecorrosionand crackingresistance.These factorsincludespecificcontrolson alloycomposition,meltingand refiningpractices,degreeand uniformityof hot working,andheat treatment.MetallurgicalCompositionaland MicrostructuralConsiderationsEffectsThe roleof the variousalloyingelementsas wellas the(3) .microstructuralphasesin Alloy718 have been extensivelystudiedThese such y"-orthorhombicstructure(NisNb),and Laves phase[Fe,(Ti,Nb)].By controllingthe timeand temperatureat whichthe alloyis exposed,the amount of each phase ora combinationof phases whichform can be variedto meet specificrequireThe predominanthardeningphase in Alloy718 is y",thoughsome y'ments.is also present.Carbidephasessuch as MC, M,C, Cr,C,,and Cr,,C,where Mrefersto metallicelementslikeniobiumor titaniumcan also form alongwiththe intermetallicphases,and can affectthe attainmentof the desiredproperties.In addition,dependingon oxygenand nitrogenlevelsin thesome dirtin the form of nitrideand oxidephasescan beoriginalmelt,present,but to a lesserextentin vacuum processedmelts.The mechanicalpropertiesas wellas the corrosionresistanceof Alloy718 thus depend onthe relativeamount of these phasesas wellas theirmorphology(spherical,angular,film,or needle-like)and location(withingrainsor at grainboundaries)in the microstructure.The phase volumefractions,morphologyand distributionare in turn determinedby such factorsas alloycomposition,melt practice(airmelt or vacuum melt),refiningpractice(vacuum refinedor electroslagrefined),degreeof wroughtstructureThus Alloy718achievedby hot working,and finally,heat treatment.compositionused in the currentapplicationis limitedto lowercarbongrades(0.045% max) and higherminimum levelson Ti, Al and Nb (0.808,0.40%,4.87%,respectively)comparedto the aerospacematerialAlso the maximum levelsof the metalloidelementsspecificationsAMS 5596.such as P, S, Pb, Se, and Bi werelowered,sincetheseare known to affecttoughnessand environmentalcrackingresistancedue to theirinfluenceonsurfaceactivityand grainboundaryproperties.347
Behavior.Tensilestrengthin Alloy718 in the solutionannealedand aged conditionis controlledprimarilyby 7" and 7' phases,and to a lesserdegreeby the carbides,nitrides,and the matrixsolidsolution.An annealingtreatmentis used priorto agingin orderto:(1)dissolvevariousintermetallicphasesin the wroughtstructure,(2) refinethe microstructureby recrystallizingnew and uniformgrains,and (3)providehomogeneityin the alloyso thatpropertiesdevelopedby subsequentagingtreatmentare lessdependenton priorthermomechanicalprocessingSuch annealingeffectscan be assessedby an anhistoryof the alloy.nealingresponsecurve whichshows the effectof annealingtemperatureonstrength(hardness).Figure1 shows typicalannealcurvesfor Alloy718.They show thatasthe annealingtemperatureincreased,the hardnessdecreased.The decreasein hardnessalso dependson the rateof coolingfrom the annealingtemperThe hardnessof the as-annealedproductincreasesin the slowature.This effectis uted to the reprecipitationof grainboundarysecondarycarbidessuch asThis effectisM,,C,as the alloyis slow cooledthrough1550 to 1650 F.particularlysignificantin millannealedproductswhere variabilityinThe microstructuralchangesassociatedwithpropertieshave been observed.It shows the mixed structureassociatedannealingare shown in Figure2.and the coarsegrainstructurewithlow annealingtemperature( 1650"F),associatedwith high annealingtemperature( 1950"F).An annealingtemperaturein the range 1825 to 1925 F providesa more homogeneousstructureby eliminatingthe effectsof priorprocessinghistorywhich can influenceLow annealingtemperaturescloseto 1700 F are not suffiagingresponse.cientto wipe out the effectsof priorprocessinghistorysuch as duplexgrainstructure,precipitationof deleteriousphases,and chemicalthe owever,annealingtemperatureresultsin increasedstrengthening.As a compromisebetweensufficientchemicaland microstructuralhomogeneityversusstrengthand toughness,an annealingtemperaturein the range of 1825 F to 1925 F isoftenused.5040Figure 1. Softening Behavior ofAlloy 718 Valve Stem Materials348
.\.\,.’ *‘.-:\, , i.\” ,.2 ,”‘ .I \, \“* \,. ’’*‘./c.-;.-.‘.-.’/‘“i.-*.\-;11(d) 1950 F(c) i9Oo”FFigure 2. MicrostructuralAnnealed:Changes Associated with Annealing Treatment in Alloy 718Temperature“F/l hr/WaterQuench;Magnification:100XAping Behavior.For high strengthor optimum creep rupturecommon aerospace heat treatmentsinvolve1950 F or 1725 F annealfollowedby a double aging cycle.However,ments, respectively,treatmentswere not designed to providehigh fracturetoughness,both of which are major o deep sour gas production.life,thetreatsuch heatand stressforFigure 3 shows the effectof annealingtreatmenton the age hardeningThe data shows three conditionsrepresentingresponse of Alloy718.It isunderaging(1300"F),peak aging (1400 F) and overaging(1500 F).evidentthat for a given priorhot (cold)working history,the agehardeningresponse in the underage or overage conditionsis directlyIncreasingthe anneal temperatureaffectedby the anneal treatment.resultsin softeras-aged materialin the underage or overage conditions,the anneal temperaturehas nowhereas, at or near the peak aged condition,The dependence onsignificantinfluenceon strengthof as-aged material.anneal temperatureperhaps explainsthe variabilitiesin strengthobservedfurnacetemperaturescanin underaged annealed bars, where the productionThus strengthuniformitycan bevary by up to 200 F during annealing.betterassured by annealingbetween 1825 to 1925"F, followedby singleThe proposed aging temperaturerange providestheaging at 1450 F 25 F.withoutthe need for the seconddesiredstrengthand hardness requirements,aging cycle.349
Hardness,HRAA 13OWF16 hr/AC0 14OO”F/6 hr/ACV 15OO”F/6 hr/AC401400II1600Anneal Temperature,I1800“F, T/l hrMlQ2000Figure 3. Effect of Anneal Treatmenton Age Hardening Response in Alloy 718A comparisonof singleaging heat treatmentswiththe "conventional"heat treatmentsfor Alloy718 is shown in TableII.Noticethatthe doubleagingtreatmentsresultin higherstrengthbut reducedfracturetoughness.Also the susceptibilityto stresscorrosioncrackingis expectedto behigherdue to the increasedhardnessabove HRC 40. -The dependenceoffractureresistanceon heat treatmentsis evaluatedin the nextsection.Table II. Effect of Heat TreatmentTensileStrength,Heat Treatmenton TypicalIRoom Temperatureyield Strength(0.2% Offset),psiPropertiesof Alloy718IElongation% RAHardness,HRCFractureToughnessJtC, psi-in. 17414448016119NR40546175O’F/2 hrMlQAge: 1325’F/8 hr/Cool115O”Fl0 hr/ACSolution28hrMlQ14OO”F/6 hr/ACSolutionor AC124lOO’F/hr to1925’Fll195O’F/lhr/AChr/AC14OO”F/lO hr/FC lOO”F/hr to350
EffectsofHeatTreatmenton FractureResistanceofAllov718To assessthe effectsof heat treatmenton fractureresistance,Charpyimpacttoughnessand fracturetoughnessbased on J-integraland equivalentenergytechniqueswere used.Impacttoughnessmeasurementscomply withElasticplasticfracturetoughnesstestswere based on methodsASTM E23.and K (J) toughness,and on ASTM E992 for Kof ASTM E813 for JSinglelgpecime!iCunloadingcompliancemethod was used % thattoughness.Figure4 showsboth Jand Kwere obtainedfrom a singlespecimentest.a typi%Figure5 shows theload- E8-isplacementrecordfor Alloy718.In most cases tiationoccurrednear the maximum load,and the K,, toughnesswas closeLLto the ��t. I”.20 100 08Figure 4. ComputerizedLoad-DisplacementRecordfor KEE and J-Integral Fracture Analysis of Alloy 718by Unloading Compliance TechniqueNote the absence of a linear elastic behavior commonlyobserved in low toughness (brittle) materials.Blunt Line: J 298,000 -9,O'F,N1ckslBase Alloy 718JIC lOOOin-p KIC [JIG El -173 ksxK EE 171 ksiK(350000III124I6IIIII810121416CrackGrowth-aax 10'Figure 5. J-Integral Crack Growth Resistance Curve Developedfor Nickel Base Alloy 718 by Unloading Compliance Technique3513
For the heat treatstockwere heat treatedand/orCharpy specimensto the above techniques.tureon the equivalentture 2 1925 F resultedevaluation,sectionsof Alloy718 billetin accordanceto a testmatrix.Compactwere preparedfrom the samplesand testedFigure6 shows the effectof annealingenergytoughness.Noticethathigh annealin lowertoughness.or -/11;;1, A e,,;1950“F/l/’1501350hriWQFigure 6. Effect of Annealing Temperatureon Toughness of Alloy 718I1375II14251450“F/6 hr/ACI1400Aging Temperature,Figure 7. Effect of AgingTreatmenton Toughnessof Alloy 718The maximum fracturetoughnessoccursin the annealtemperaturerangeWhen a higherannealtemperaturesuch as 1925 F isof 1825 to 1875 F.improvedtoughnesscan stillbe achievedby choiceof Thisis shown in Figure7.ment.a givencompositionand alloyfabrication,the choiceof specificheattreatparametersfor annealand singleagingtreatmentscan provideaoptimumcombinationof fracturetoughness 200 ksi /in, at yieldstrength 120 ksi,and hardness HRC 40.Thus,inferencesfrom lowing1.For the typicalannealingtemperaturerange of 1750 F to 1950"F,fractureresistancecan varysignificantlydependingon the annealingcycleand the agingtreatment.Improvedtoughnesscan be achievedbya relativelyintermediateannealingtemperaturesuch as 1875 F 25"F,whilesatisfyingthe high yieldstrengthrequirementsof 120 ka?minimum.2.With a highbe (1925"F),improvedtoughnesswithan overagingtreatment,but decreasedstrengthagingtreatmentsratherthan conventionalcan be used to achieveboth high tensile352doublestrengthcouldcan beagingtreatand high
An agingtreatmentof 1450 25"F/6hours/airfracturetoughness.This saves time,money,cool is capableof meetingsuch requirements.and complicationof waiversrelativeto more complicatedtwo tionSamvlesTo assurethatAlloy718 purchasedfor CriticalServiceapplicationhas been processedand heat treatedfor highfracturetoughnessand envimaterialsheat treatedas describedaboveronmentalcrackingresistance,are requiredto achievea minimum impacttoughnessof 40 ft-lb,in ordertoFigure8 demonachievea fracturetoughnessgreaterthan 180 ksi Jin.thisrequirementcan be readilystratesthatfor billetsizesup to 5 l/2",for largerbilletsizesor largeforgings,the fracturemet.However,resistancebecomes hrwater Quenchhr/Alr CoolTS, ksl 1 YS, ksi 1 EL, % 1 RA,%174.184 1123.136 ] 24-32 1 34.551 Hardness, HRC132-39CharpyImpactToughness,ft-lb0246Bar Diameter,8Figure 8. Effect of Heat Treat Section Size on Toughnessof ProductionSamples of Alloy 718The decreasein fracturetoughnesswithincreasedsectioncaused by factorssuch as (1) non-uniformityof hot work whichtreatresponseand microstructureand (2) densityand distributioncarbideand carbonitrideprecipitates.35310in.size can beaffectsheatof
TableIIIcomparesthe impactand fracturetoughnessvaluesmeasuredat the centerand midwalllocationsof a heat treated8" diameterstockthe midwalllocationin the hangercomponentHere,used for tubinghanger.Noticethe centerregioncorrespondsto 3/4 radiusfrom centerof the bar.shows inferiortoughnesswhichis less than the specified40 ft-lb,or KThis differenceis attributedto ERe(J) of 180 ksi ,/in for the component.Figure9 shows thedifferencesin microstructurebetweenthe two regions.Compared to the mid-walland ODthru-wallmicrostructuresin the bar.the centerregionshows more coarsegrainsize,higherdensityofregions,Even thoughtheNbC inclusions,and more grainboundaryprecipitates.fracturein the centerand midwallregionare both ductile,the abovecharacteristicsmake voidinitiationand growthmore favorable,resultingThis is supportedby the slopeof thein low energyductiletearing.This slopeis a measureof theJ-resistancecurveas shown in Figure10.Noticethatin additionto the lowerfractearingmodulusof the alloy.the centerregionalso shows a lowertearingmodulusturetoughness,comparedto materialin the mid-wallregion.Table III. Variability in Impact and FractureToughnessin 8” Diameter Forging for Alloy 718 Tubing HangerImpact Toughness1 Fracture ToughnessLocation/Orientation1Kit, Jksi 737252324156016621885214221236As statedearlier,differencesin thru wallfractureresistancecanalso be caused by the densityand distributionof carbideprecipitates,Figure11 shows how such precipitatescan nitiation.As the matrixdeforms,localstressconcentrationsand plasticstrainincompatibilitybetweenthe matrixandthe hard precipitatesresultin earlycrackingand decohesionof theinclusions.The resultingmicrocracknucleican then propagateintothematrix.Whenthey provideeffectofnormaltotop is thetheinclusionsare alignedand occurin high volumefraction,a low energyductilefracturepath.Figure12 depictsthesuch precipitatealignment.In thisfigure,the microstructurethe fractureplanein a compacttensionspecimenis shown.Theprofileof the crackgrowth.Noticethatwhen the inclusions354
X80XFigure 9. Thru Wall Microstructure in the Longitudinal Orientationof 8” Diameter Forging for Alloy 718 Tubing Hanger013551355
60 mil . Offset02400001Ibo; AA1800A@IIJ-Integral,psi-in.IIIIIIIIIII0.03q 314 Radius: Jlc 1662 psi-in.A Center of Bar: Jlc 975 pis-in.II0.060.09Crack Change, Aa, in.II0.120.15Figure 10: Crack Growth Resistance Curves in 8” DiameterAlloy 718 Tubing Hanger (X-6437) Showing Differences in FractureInitiation and Ductile Tearing Modulus at Center and 3/4 Radius of Bar013551.3356
(a)lb)Figure 11. Micrographs Showing Role ofCarbides and Inclusions in Initiating CracksAs the matrix deforms, local stress concentrationandincompatibilityresult in cracking of the inclusions.The resulting microcrack nuclei can then propagate into the matrix.07347357
C-Crack Propagation(a) Fracture parallel to inclusion stringers (dark streaks). The allignedNbC precipitates control the fracture and cause low fracture resistance.f-Crack Propagation(b) Uniform structure and little amount of inclusions. Note jaggedtexture of the crack profile indicative of high fracture resistance.Figure 12. Fracture Profile and MicrostructureinAlloy 718 Showing Effect of Alligned PrecipitatesMdgdi tk :16X; t&nt: Mixed Acid agnificstian- 16X;Etchant:*Mixed Acid07347358
In contrast,the crackprofileis more steppedand planar.are aligned,when the inclusionsare dispersedand nonaligned,the crackprofileis moreThe differencesin crackprofileassociatedjaggedas shown in Figure12b.with precipitatealignmentalsoshows up as differencesin the fractographyof ductilefracture.As shown in Figure13 precipitatealignmentresultsin "woody"textureand facetedductilefracturecomparedto the randomlydispersedcase.in both cases,the fractureis ductile,Thus, even thoughthe differencesin the micromechanicsexplainthe differencein in productionenvironmentscontain2, and chloridescan occur by cathodicor anodicmechanisms.ing H,Sconditionsat which both 'it isCo clearthatanodiccrackingpredominatesat hightemperature,and cathodiccrackingoccursat lowertemperaturewhere hydrogenembrittlementpredominates.TableIV shows the conditionsand blecantileverspecimens(DCB) were used to measurethe thresholdstressintensityfactorfor unsetof crackgrowth.C-ringsprestressedto yieldloadsassesscrackinitiationresistance,and the slow strainrate(SSR) testevaluatesthedynamiceffectsof plasticstrainingduringhydrogenabsorption.In general,Alloy718 shows good resistanceto H,S inducedcrackingunder cathodicconditions.As shown in TableIV, even the heat treatmentconditionswhichshow low fracturetoughnessdid not show cracking.Theintergranularcrackingobservedin the SSR testhas been attributedto thehigherseverityof the test.Allsamplesheat treatedin accordanceto theconditionsproposedin the previoussectionsof thispaperdid not showcracking.Table V shows the conditionsand resultsof anodiccrackingexperiments,usingU bend specimensof Alloy718 samplesheat treatedto standardaerospacespecification.Samples heat treatedas proposedin thispaperIn addition,thedid not crackunderthe testconditionsshown in Table V.resultssuggestthatfor high H,S levels,anodiccrackingsusceptibilityAlso at high strengthlevelsthe resultsincreasedwith creaseswithH,S concentration.359
irar iz4 -Crack Propagation(a) Note the “woody” and flat texture of the fracture characteristicof low energy ductile teanng. No evidence of embrittlement.FatiguePrecrack l Crack Propagation(b) Note more dimpled, ductile frac?ure, consistentwith the higher fracture resistance of thissample of 718.Figure 13. Fracture Details Just Ahead of Fatigue Precrack in Alloy 718Sample with (a) high localized coarse precipitates,(b) fine and uniformly dispersed precipitates.Magnification:20x073 47360
Table IV. Hydrogen EmbrittlementSpecimen rdness,HRCTestTemp., “F37-38RT-250 F25% NaCl 200 psi H2S,C.S Coupled, 4 WeeksNo Cracking*38-39RT-250 F25% NaCl 200 psi H2S,4 WeeksNo Cracking3832NACE Solution C*SCoupled4 WeeksNo Cracking38-41RTNACE Solution C-5 Coupled2 MonthsNo Cracking38RTNACE Solution C*SCoupled40 HoursintergranularCrackinqlX5322-1lDCBResistance of Alloy718X5058-5,7/C-Ring*tX5322-4lSSRl *ConditionResult* 1925”Fll h&I/Q 14OO”F18 hr/AC” 1950 F/l hr/AC 1325W8 hr/fCI lSO’F18 hrlACTable V. U-Bend Stress Corrosion Cracking Tests Alloy for 71825% NaCl- CDay TestsTest #lTest #4Test #5195O“Fll hrlAC1325OFl8hr/FC12OO”FllOh r/AC1950 F/1 hr/WQ1325“F/8 hr/FC11SOoF/ h r/FC195O”FIl h r/WQ1325”F/8 hr/FC11SOoF/ hr/FC-135,000-156,000-156,000Stress y.s. y.s. y.s.Temperature, OF40040040014,00014,0001500HzS, %3.53.51CD2, %101010Pti2sgpsi49049022.5SCCResults,# Failed/# Testedo/2212o/2Heat TreatmentYield Strength, psiPressure, psi361
Conclusions1.Alloy718 subjectedto anneal plus singleage heat treatcyclesprovidesthe requiredpropertiesfor applicationin deep, corrosiveoil and gas wells.2.For optimum combinationof strength,toughness,and environmentalcrackingresistancean anneal temperaturewithin1825 to 1925 Ffollowedby water quench and an aging temperatureof 1450 2 25 F isproposed.3.Conventionalaerospace heat treatmentswhich optimizestrengthandcreep rupturepropertiesare unacceptabledue to decrease in fractureand ationin fractureresistanceis relatedto the amount anddistributionof carbideinclusionsand grain boundary precipitation,both contributionsare higherin large sectioncomponents.References1.J. W. Kochera,No. 50.2.P. W. Rice,3.Ya. Fangttan,p. 555.A.K. Dunlop,Materialsand J.Performance,P. Tralmer,1978, pp.P. Deb, and M. C. 2,I&,
A comparison of single aging heat treatments with the "conventional" heat treatments for Alloy 718 is shown in Table II. Notice that the double aging treatments result in higher strength but reduced fracture toughness. Also the susceptibility to stress corrosion cracking is expected to be
