?HEEFFEKTOFHEATINTUl?ONMI cYRcmRJmAND CSYCKUE IN ALLOY 625 WELD OVERLAYS

I.L.W. Wilson, R.G. Gourley, R.M. Walkosak, and G.J. Bruck*

Westinghouse Electric Corporation El-& Division

CheswickAvenue cheswick, Pennsylvania 15024

Abstract

Weld overlays of alloy 625 have been deposited on AISI Type 304 stainless steelusingPTA,GIlAWandlaserweldingpmcesses. crackingwas-ed inthewelddeposit&cnm~laywithhigherheatin@. Thelcwerheat inputs resulted in sour12 weld overlays. Thecrackingwasfoundtobe associat&iwithsecondphaseparticles intheweldmnt. Metallography and xanningelectronmicrmcopy with EDXA were used to analyze the particles. Particle analyses and nrxphological characteristics revealed thepresence of Lavesphase. !l%e size and distribution of this phase is affected by the heat iqmt during welding and can be controlled to avoid pmblemswithcracking.

*WestinghouseScie.nce&TechnologyCenter 1310 Beulah Road Pittsburgh, Fennsylvania 15235

Superalloys ‘718,625 and Various Derivatives Edited by Edward A. lm-ia

The Minerals, Metals & Materials Society, 1991

735

Introduction

Inconel625 isacorrosiollresistant~~basealloyused~ively for its wear resistance in aggressive, chloride bearing, environmmts. wrouFplt,cast,pawdermetdLlurgyand~dcwerlayp~~havebeen used. Alloy625has alsodemonstratede.xcellentperfonnance in nuclear pcwerplantoperationenvimxmmts (Ref. 1)wfreretheopecating chemistriesarelessaggressive~~~veryreliablel~-~ perfonmnceismquimd. Forthis reason, thealloyhasbeen investigated forapplications inthereactorqstem The~ofthepresmt investigation is to develop a method for producing reliable, defect-free weldoverlays of alloy 625 onAISIType 304 stainless steelusing prcductionweldingtechnigues. lXri.rqthe initial trials, crackingwas observed inthecverlay. Metallographic examinationrevealedthatthe crackirg was asscciated with secmti phase particles which were identified as Laves phase, (Refs. 2,3,4). This paper reports the investigation of the-of Lavesphase inalloy625weldoverlaysusingthree prcxkctionweldirqpmcesses withvaryingheatiqnks.

'Ihreeweldpmcesses wereusedtodepositlixonel 625 onto AISI 304 stainless steel. meseprocesseswereplasnatransferredarc(m),gas imqstenarcweldingwithautcanaticccldwire feed (GIAW) a&laser cladding. TheseproGesseswereselectedbecausetheyrepresented significant variations of welding heat input, the by process variable.

TheETApmcess erqloyed Nistelle 625 powder, mesh 100/325 and an AISI 304 stainless substate. Theequi~tusedwasaLindePSM-2SurfaceWelder withaPT9torch. TheAIS 304 bsen&erialwasa ring measuTing 2.0 in&esthick, with anoutsidediamter of 10.9 b-&and an insidediameter of 9.3 inch. Theweldoverlaywasplacedonone face of the ring. The welding was ccnducted in two layers of 0.1 inch thickness intherootand 0.125 inch thickness in the second layer. Priortoweldingthe seccti layer, the firstlayerwas machimdto remve any surface oxidation. The seccndlayerwasma&inedtogenera teasmothsurfacemdpenetrant testingwasparformedtoverifysoundness of the weld.

736

TheGTAWprocess was used with 0.045 inch dian&er ERNiCt%o-3 filler wire: its chemistry is given in Table 1. Theequi~usedwasaMillerpower .source With a LiWe wire drive unit aMi an autmatic voltage contml (AVC) headwithaHW27 torch.

TheAIS 304 stainless steelbasematerialwasaring measuring 2.2 ties thick with an outside diameter of 10.5 in& and an inside diameter of 8.5 in&. Theweldoverlaywas depositedonone oft&two faces ofthe ring. The first layer was welded follmed by interpass/bead grimking and apenetranttest ofthe root layer. Thethicknessoftherootlayerwas 0.08 inch. The second layer was deposited creatiq a build-up of 0.115 in&,ar-ditalsowasgrourdandpenetranttested.?heweldparameters are defined in Table 2. Eightweldbeadswexdepcsited inthe mot layer arvl 7 were requimd for the secoti layer. Theweldbeadswemdeposited startingfromthe insidediamterandworkirqoutwazd~theoutside diameter"

Table 1: ChemistryofInconelFmderandWim

C cr Fe m MO Ni P S Si 5.Ta Al Ti

Nistelle 625 0.05 22.17 2.30 0.40 9.22 61.1 .005 .002 .39 3.71 0.19 .29

FYx&x

-045 Dia. Irkxmd 0.05 22.04 3.75 0.15 9.16 60.86 -008 c.001 0.11 3.48 0.13 .26

625 Wire

737

Table 2: WeldProcess Parameters aMHeatIqmt(KJ/in2)

Flr.wer-

Polarity

Volts

Travel Speed

WireFeedspeed

Max. Interpass Temp.

Oscillation Width

Oscillation speed

ScanAmplitude

-Frequency

--Depth

Passoverlap

Atmosphere

DeliveredRwer

Shield Gas and Flew

Bead Width

No. of Beads

He&Input

Note:

PICA

lx!

DCSP

165

30

2.25 IFM

-

350" F

0.750 - 1.0"

24-32 OEM

-

-

-

-

-

-

75% He 25% Ar (50 CFH)

1.0"

1

132 (KJ/in2)

DC

DCSP

70

12

2 IFM

93 IFM

150" F

-

-

-

-

-

-

-

-

Aryan (45 CFH)

0.375"

7-8

67 (W/in2)

-

-

-

8 IFM

-

-

-

-

0.75"

15 Hz

0.075"

0.375"

Arson

5.6 KW

-

0.750"

1

56 (W/in2)

IFN= InchesPerMinute OFM = Oscillations Per Minute IXSP=Direct Current Straightmlarity

738

lhelasercladded sampleswereproducedusinganAvcoHpLcontinuouswave carbon dioxide laser rated at 15 kilowatts output pmer. Nistelle 625 ~~waspreplacedonthe3O4substrateandthelaserbeamwasusedto meltthepreplaceand fuse itwiththe substrate. Eachpasswas positioned with a 0.375 in& overlap of the preceding pass. Two cmplete layers were applied. The 304 stainless steel samplewas inan unrestrained position, in a controlled atxmsphere chamber and oriented such that its major axis was parallel to the direction of carriage n&ion. A narrm slot in the chamber coverpermittedlaserbeamaccess. Pure argonpurgegaswas suppliedthroughamanifold systmenclcsed ina plenumbeneaththeporousnketalbase of the cha&er. Anoxyg~mmitor wasusedtownfinnthatshieldingwasadequate inpreventing contaminationdurixqprocessirq. Mterweldingeachpass, light grinding was applied to remve oxidation prior to depositirq the next pass. flrbsequentpasseswere alsogrourdard finally, the entirelayerwas ground 0.020 inch to reeve any surface irregularities arki oxidation. The processparamete~~usedare illustrated inTable 2. Thispmcedure and aforementioned precautions regarding shielding are routine, recmmrded practices for laser beam cladding. The size of the plate clad was 1.0 inch x 6.0 inch x 54.0 inch. The Chemistry of the Nistelle 625 powder can befouMinTable1. Its msh size was 100/325.

All welded samples were cm5s-s.ectioned,mountedandmetallographically prepared for examination using standard light micmscopy andahnraymodel 1645 seaming electron mi croscope fittedwithKevexseries 82 Energy Dispersive X-ray Analysis (EEA) system. The spot size used for analysis was l/2 microaneters. The et&ant& forallspecm was a 6% solution of &manic acid in water.

739

The initial atteqk at applying an allay 625 wald overlay was conducted

usingthepIlAtechniqlle. Cra~wasKwealedbypenetrant testing of

thesurfaceafterma~ csoss-sections~thatthecrackingwas

presentintheoverlaydmntothe304sMnlesssteel substrate; see

Figs. 1, 2, and 3. The crackbq was associatedwiththecnmekrationof

secxxxlphaseparticles inthem 'e, (Fig. 3). EDXA analyses of

theseparticles revealed increased levels ofNb,Mo, andsiccsnparedtc

thecnncekmtions inthemerlaymatrix; seeTable 3. This segregation

istypicalofthatf~in~higNyall~~~materialsandhas

been identified as Iaves @mse (Ref. 5). lhe cuqosition of the alloy 625

pumler, Tablel, shcwsNb,Mo, andsitobe sufficientlyhighsuchthatit

wouldresultinLmesphase formation (F&f. 5).

Althmti- content is high, no indications of carbide

precipitateswere observed. TheEDXAanalyseswexxclosetothe 304

stainlesssteelinterfacewhichaaxnmts forthehighFecontenti.nthe

weld overlay.

Fig. 1: Fh&xnie through Cross-Section of Plasma Transferred Arc (Pm) weldment. Magnification 200X

740

Fig. . 2:scarmngElectronMi~ofmwelMm-won. Magnification 500X

Fig. 3:LzaumhgElectronMicrogra33h ofPTAWeldmntCro6s-Section. Magnification 2000X

741

Table 3: EDXAAnalysisofweldmen~

Wt%

tziDecimt?.n Nb MO Si cr Fe Ni

PI24 Particle 27.51 17.10 9.69 12.46 12.92 20.31

P-m Matrix 3.73 8.08 2.25 21.40 27.51 37.03

cXAW Particle 13.40 13.58 6.12 18.35 8.02 40.54

GlYw Matrix 4.89 9.47 3.93 21.76 9.57 50.38

laser Particle 12.26 18.57 8.23 17.35 6.89 36.70

Laser Matrix 4.27 9.84 4.82 21.99 7.42 51.67

Nocrackingwas observed intheGI7Wweldwerlay; seeFigs. 4 and 5. The

ccmposition of the wire consumable is again relatively high for Si, Mo, ard

Nb, indicatingthatIaves@asecouldbe~dily formed. Itcanbe seen

from the cmss-section through the werlay, Figs. 4 and 5, that the size of

thesecomlphaseparticlesis smallerthanwasobervedinthePTA

werlay. TheEDXAanalyses frcantheprticles,Table 3, shmsless

segrqationrmparedtothePTAwerlay.

Fig. 4: FhotoMi~~m-SectionofGasTLlngstenArr= (GI1Aw) weldment Magnification 500X

742

. Fig. 5: Zkannmg fin Micrograph of GTAW cmss-Section. Magnification 2000X

!theap$eammeofthelaserweldcverlay,usingpowder,isverysimilarto

theGI!AWwelduverlayusingwire. ThesecoM@aseparticlesarxscmzwhat

finerarxImreevenlydistributed;seeFigs. 6 and7. l'hesegregationof

Si,MoandNb intotheparticlesis similartothatfaund intheGTAWweld;

see Table 3. Saanesegregationwitkinthematrixisp~,asirdicated

bytheli~tareaofthematrixadjacenttotheparticles inthe

back-scatMSEMphotomicrcqraphsof Figs. 6and7. ExamplesoftheEDXA

~framtheLavesphaseparticlesardthematrix~shcrwninFigs.8A

and 8B.

743

Fig. 6: scanning Electron Mix@ of Iaser c!mss-section (Back-scatter -) l

Magnification 500X

Fig. 7: sodming Electron Mix* of Laser aross-section (Back-scatter Mode) . Magnification 2000X

744

Theweldingparamters anIthecalculat&heatinputs foreachpmcess are sham in Table 2. ThehirPlheat~tofapproximately132W/in2 with the350" F. interpasstemperatureused inthispreliminary investigationof thePTApxcess wouldmsultinrelativelyslcwcoolirqth.mughthe solidification rarqe. Slcwcoolingwould allowtime for the rejection of theSi,Mo, andNb intothe intekiemlritic spaces resultinginalaqe volumeoftheteminal Lavesphaseardagreaterchance forcrackingto occur. Asimilarhighheatinputwasusedby Cieslack for GDNweldingof alloy 625 Plus inwhi&he ckservedcracking (Ref. 2). Theheatinput in theGI?AWpxcess inthis studywas significantly less at67 KJ/in2 with an interpass temperature of 250" F. Thisheatinputwouldresultinmre rapid cooling and less time available for segregation. Rapidcoolingwould prcmtealmerdegreeof segregation of the alloy constitue.& and the smallerparticle size oftheteminal phase. The fastexcooling ratewould alsoleadtoa finerdemlritespacing (Ref. 6), resultingina finer dispersionoftheteminal phase. Thisdemnstrates thattheheatinpkis axmreimportantfactorinthecracking,thanotherweldprocess

parameters. Thelaserp- was co~toproducethelowestheat input; seeTable 2, justbelowthatfortheGTAWpmcess. Thedqreeof segregation; see Table 3, is similar for the two pmcesses. Thereisa difference in the size and distribution of the termbal solidification phase; see Figs. 4 and 6. This suggests that, at these lower levels, the reducedheatinputand fastercoolingrate, hadagreatereffecton distribution of the teminal phase than on the degree of segregation leading to it.

Thedifferencebetweeslthehighheatinput~welds,~~cracked, and thelcrwerheat~t~andlaserwelds~~didnotcracJc, is significant. No signofcrackixqwas &served intheGTAWpmcess weldments ti it is possible that higher heat inputs and deposition rates arepcssiblebefore a crackingthresholdis ached. Alternately, alcwer heat input FTAprocess could be used to achieve higher deposition rates than the GTAW. Acceptable deposition rates were achieved for cur application using parameters close to those listed for the GTAW process.

746

Conclusions

Alloy 625 is susceptible tc cracking during welding due tc the formation of raves phase. ~~canbeavoidedbyreducingheat~tzturingthe weldiIqpmcess. Ameptabledepositionrateswere achievedatthe reduced heat input.

Iheauthoxswouldliketoackncwl&gethe follcwingpecple fortheirexpert assistanceintheweldingandpreparationofthesamplesusedinthis investigation: G.M. Bumin, A.M. Delenne, V.J. Friscare lla, T.A. Mullin an3 J.A. Rosepink.

References

1. Ccpson, H.R. and Econcmy, G. 1968. Effect of Scane Enviromtal Coalitions on Stress Corrosion Behavior of Ni-Cr-Fe Alloys in Pressurized Water. Corrosion 24 (3), No. 3: 55.

2. Cieslack, M.J., Headley, T.J., and Frank, R-B., 1989. The Welding of Custan Age 625 Plus Alloy. The Welding Journal 68 (12): 473-s to 482-s.

3. E3mst, S.C., Baeslack III, W.A., and Lippold, J.C., 1989. Weldability of High-Strength Im-Expansion Superalloys. The Welding Jcurnal 68 (10): 418-s to 430-s.

4. Th- Jr., R.D., 1984. HA2 Cracking inI¶xickSections of Austenitic Stainless Steels - Part II. The Weldiq Jom-nal 63 (12): 355-s to 368-s.

5. Cieslack, M.J., 1991. The Welding and Solidification Metallurgy of Alloy 625. The Weldiq Journal 70 (2): 49-s to 56-s.

6. Cblmers, B., 1967. Principles of Solidification. John Wiley and sons, Inc., p. 120, New York.

747