nickel welding

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The Welding Metallurgy of Nickel Alloys inGas Turbine Components

A.C. Lingenfelter

Thispaperwas preparedforsubmittaltotheRoceedings forASMInternational,JoiningandRepairofGasTurbine Components Annual Event 97

Indianapolis, INSeptember 14-18,1997

May 21,1997


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DISCLAIMERfhis document WSIS ~IVp_ as = accountof W05iftpmmred by an aWSCYof theUnited States Govermnent. Neither theUnited States Goversmsentnor the Universityof C4ifornia nor any of their employs makes any warrmty, express or imp-oraasumes anylegalliabiMy orresposssibility fortheaccnracy, ~orusefuinessof anyinformatio~ apparatus, prodx or process disclosed, or seprsaenta that itsusewouldnotinfringeprivatclyowned rights. Referenceherein toanyspeciGccommemMpruducts, process, or setice by trade nam% trademask, manufacturer, or othe~doesnotneceasarily constitute orimply itsendorsemessh swwmm endatiq or favoringby the United States Government or the Univemity of CalifonAsL The views andopinions of authom expressed herein do not necessarily state or reflect those of theUnitedStatesGovernmentor theUniversityofCaliforni~ andshalInotbeusedoradvertising or product qndorsement purposes.


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The Welding Metallurgy of Nickel Alloys in Gas Turbhe ComponentsAuthor - A. C. LingenfeiterLawrence Livermore National Laboratory

lUMU3!XMaterials for gas turbine engines are required to meet a wide range oftemperature and stress application requirements. These alloys exhibit acombination of creep resistance, creep rupture strength, yield and tensilestrength over a wide temperature range, resistance to environmental attack( including oxidation, nitridation, sulphidation and carburization), fatique andthermal fatique resistance, metallurgical stabilii and useful thermal expansioncharacteristics. These properties are exhibited by a series of solid-soiution-strengthened and precipitation-hardened nickel, iron and cobalt alloys. Theproperties needed to meet the turbhe engine requirements have been achievedby spedfic alloy additions, by heat treatment and by thermal mechanicalprocessing. A thorough understanding of the metallurgy and metallurgicalprocessing of these materials is imperative in order to successfullyfusion weldthem. This same basic understanding is required for repair of a component withthe added dimension of the potential effects of thermal cycling and environmentalexposure the component will have endured in service. This article will explorethe potential problems in joining and repair welding these materials.IntroductionThe gas turbine aircraft engine has evolved to its present form over the past 45years. Because of the critical nature of the gas turbine engine, the materialsused in its manufacture represent some of the most thoroughly tested andevaluated materials used in any application. New materials are exhaustivelytested and evaluated before being introduced into an engine design. To meetthe demanding stress, temperature and environmental requirements of theengine application, requires the science of the alloy developer and processmetallurgist be pushed to the limit. Strengthening of these materials is achievedby the addition of solid solution strengthening elements to a nickel matrix (Ref1). Nickel does not have a particularity high modulus of elasticity. It appears theability of nickel to form solid solutions with a wide variety of elements withoutundue phase stability problems as well as its its tendency with the addition ofchromium to form a chrome rich protective oxide leads to alloys which can beused up to 0.8 Tm (melting temperature). Solid solution strengthening isachieved by the addition cobalt, Iron, chromium, molybdenum, tungsten,aluminum and titanium. Alloying elements such as carbon, boron, magnesiumand zirconium tend to segregate to the grain boundaries and significantlyimprove the hot workability, creep and creep rupture properties of thesematerials. The addition of aluminum, titanium, niobium and tantalum leads to theprecipitation of a finely dispersed gamma prime phase in the gamma matrix. Theprecipitation of this phase leads to age hardening of the material with significantincreases in strength.


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Table 1 shows a representative list of the alloys which currently find applicationor have been used in gas turbine engines.Table 1 (Ref 2 )Solid solution nickel base alloysHastellyNHastelloySHastelloyXHaynes 230Inconel 600Inconel 601Inconel617Inconel 625Precipitation hardenable nickel base alloys3MR 235nconel 702inconel 706N 713Cn738n 739nconel718nconel 722nconel X-750ncoloy 901U2523ene 41Jdimet 700iVaspalloy-laynes214Solid solution iron base alloys16-25-617-14 CUMO19-9 DLIncoloy 800HIncoloy 802Precipitation hardenable iron base alloysA286DiscolloyHaynes 556Incoloy 903Incoloy 909Solid solution cobalt base alloysHaynes 25


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Haynes 188S-81 6

cd Processiga on Fusion We ld inqWe wiii iimit comments to fusion welding, although it is understood that otherprocesses such as resistance weiding, brazing and solid state bonding are usedto join materials for thisapplkation. -Several weidability problems can developwhen joining this group of alloys: fusion zone cracking, heat affected zonecracking and post weid heat treatment cracking.Fusion zone and heat affected zone crackingSoiid soiution strengthening elements, cobalt, iron, chromium, molybdenum,tungsten, aluminum and titanium have not been shown to adversely effect theweldability of these alloys. The addition of elements which segregate to the grainboundaries; boron, carbon, magnesium and zirconium, can have a significanteffect on the resistance of materials to heat affected zone cracking and fusionzone cracking. These elements are essential to the hot working and creep andcreep rupture ductility and therefore the high temperature strength of thematerials. At least one or a combination of these elements are present in most ofthese alloys. Present individually, additions of boron at 0.005 wt. Yo; zirconium at0.020 wt%; magnesium at 0.025 wFYo, will generally not adversely effect heataffected zone or fusion zone cracking resistance. Alloy content much abovethese ievels will result in fusion zone and heat affected zone microfissuring. Theeffects of these additions tend to be additive, therefore the individual addtioneiement limits wiil be less when two or more elements are present incombination. increased carbon content in combination with any of the otherthree addition elements wili increase the sensitivity to heat affected zone and andto a lesser degree fusion zone cracking.Heat affected zone cracking has been clearly linked to grain size in most of thesealloys (Ref.4). Fine grain material, grain size less than ASTM #5, is lesssensitive to heat affected zone micro fissures than coaser grain material, grainsize iarger than ASTM#5. Coarse grain size is desired for high temperaturecreep and creep rupture strength. it may result from the hot working finishingtemperatures of the material being in the grain coarsening temperature range orfinal annealing temperatures in the grain coarsening solution annealingtemperature range. Fine grain sizes are produced by iowering of the hot workingfinishing temperatures to a temperature at or slightly below the recrystallizationtemperature and/or careful selection of the subsequent final annealingtemperatures. In addition to providing fine grain material the thermo mechanicalprocessing can be used to significantly increase the strength of some alloys byretaining strain energy in the material. The welding thermal cycle will result inrecovery and recrystallization in the weld heat affected zone and thus reduce thestrength of the material in the weld heat affected zone.


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Fusion zone cracking sensitivity can be adversely effected by trace elementssuch as sulphur and phosphorous: sulphur at the 0.010 wt% level andphosphorous at the 0.025 M% level. These elements tend to be additiie in theireffect.Post weld heat treatment crackingPost weld heat treatment cracking (strain age cracking) is a problem associatedwith the age hardenable alloys. Increasing Iwels of aluminum and titanium andtherefore volume percent gamma prime, increase the sensitiv ity to post weld heattreatment cracking (Ref. 4). While the fusion zone and weld heat affected zonecracking tloted in the eartier d~ion are of a micro nature, post weld heattreatment cracking can be much more extensive and destroy the usefulness ofthe fabricated components. There are at least two factors which lead to thecracking. Yield strength level residual stresses resulting from the weldingthermal cycle are not relaxed sufficiently rapidly in highly creep resistantmaterials during the heat up portion of the heat treatment cycle, to avoid a shorttime stress rupture failure. Addng to the residual stresses, the stress developedby the rapid precipitation of gamma prime in the gamma matrix increases theproblem.Various methods have been used to heat treat materials prior to welding to avoidthe strain age cracking problem. These heat treatments involve over aging thematerial thereby reducing its creep resistance and allowing stress relaxation totake place more readily. The welding operation is followed by a solutiontreatment and age hardening cycle which recovers the desired tensile, creep andcreep rupture strength.For matetials with very high gamma prime volume fraction, such as cast IN713C, fabricators have used a weld preheat temperature in or slightly above thegamma prime precip ita tionhxatsening temperature. The preheat temperature ismainta ined thoughout the weld ing operation and is increased to the solutionannealing temperature without allowing the component to cool to roomtemperature. The solution annealing is followed by the aging heat treatment.Alloys such as Inconel alloy 718 derive their aging response from additions ofniobium. The aging response of this type alloy is sluggish in comparison thealuminum/titanium hardened alloys and tends to be far less susceptible to postweld heat treatment cracking problems. The ability to we ld and d ire c t ageharden fabrica tions of Inconel alloy 718 has made this alloy very useful for gasturbine engine applications.Effect of contamination on weld soundnessFusion zone cracking can be a problem from contamination in the weld joint or inthe vacinity of the weld joint. Heat affected zone cracking has been noted by in anumber of instances when copper was inadvertently introduced on the surface ofthe alloy in the weld heat affected zone. If it is located in the weld heat affectedzone, the copper will become liquid and result intergranular liquid metal stress


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mechanical treatment(s) were applied during fabrication. As an example, amaterial might have been supplied and welded into a subassembly in a fine graincrack resistant cond ition. The subassembly was subsequently sub jected to ahigh temperature brazing operation to join it to other components. Thetemperature of the brazing cycte caused caused grain growth and resulted inheat affected zone repair weld cracking sensitivity.

Understanding the metallurgical problems associated with joining the alloys usedin gas turbine engines requires a detailed understanding of the metallurgicalcharacteristics of each of these alloys. Most of the ailoys are readily weldable,some however, require specia l procedures to achieve the desired results. Repairwelding requires the same level of basic metallurgical understanding with theaddition complications introduced by the sewice and service environment of thecomponent.

This WOWwas performed under the auspices of the U.S. Department of Energyby Lawrence Livermore National Laboratory under contract No. W-7405-ENG-48.

1. R. F. Decker Strengthening Mechanisms in Nickel Base Superalloysw, SteelStrengthening Mechanisms Symposium, Zurich Switzedand, May 5 and 6, 19692. A. C. Lingenfetter, Generai Weldng Characteristics of High TemperatureAlloys, Volume 6, ASM Handbook, Welding, Brazing and Soldering, pg. 562-565.3. H. L. Eiselstein, Advances in Technology of Stainless Steel and RelatedAl[OyS, STP 369, ASTM, w 62-794. R. G. Thompson, Welding Metallurgy of Nonferrous High Temperature Alloys,Volume 6, ASM Handnbook, Welding, Brazinng and Soldering, pg. 566-571


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TechnicalInformationDepartmentLawrenceLivermoreNationalLaboratory

UniversityofCaliforniaLivermore,California945

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nickel welding

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