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WELDING RESEARCHS U P P L E M E N T T O T H E W E L D I N G J O U R N A L , M A Y 19 77

S p o n s o r e d b y t h e A m e r i c a n W e l d i n g S o c i e ty a n d t h e W e l d i n g R e s e a rc h C o u n c i l f f D l ,

Arc and Pool Instabil i tyin GTA Weld ing

Preliminary study relates erratic welding of stainlesssteels to anode spot movem ents and identifies weldpool surface films that influence anode spot behavior

BY J. C. METCALFE A N D M . B. C. Q UIG LEY

ABSTRACT. The we ld ab i l i t y o f s ta in less stee l w i th the GT A (gas tung stenarc) process is con side red fo r th osecases wh er e there are heat to hea tvar ia t ions in the behavior o f the arcand the we ld p o o l . Indus t r ia l expe r i ence o f lack o f penetra t ion , a rcwande r , excess ive ly w ide we lds , l acko f rep rod uc ib i l i t y and changes inwe ld ing behav io r f rom one hea t toano the r i s co l la te d and rev iewe d .I t has prev iously bee n po stu la tedt h a t t h e w i d t h / d e p t h ( w / d ) r a t i o o f abead on p la te we ld ind ica tes theweldab i l i ty o f tha t heat in te rms o fresistance to arc and poo l instab i l i t ies.Ca lcu la t ions suggest tha t most o f theheat input to the workp iece is in thereg ion o f the anode spot , and a mode lis p ropose d wh i ch re la tes the b ehav io ro f the anode spo t to the we ldab i l i t y o fthe ma te r ia l .I t is pos tu la te d tha t , w i th heats onwh ich we lds have h igh w id th to dep thra t ios, there is a mo ve me nt o f th eanode spo t wh ich d is t r ibu tes the hea tinpu t and hence p roduces a w i de ,sha l low w e l d . Surface scum seena round the pe r ime te r o f the we ld poo land wh ich appears to be associa ted

w i t h anode spo t movem en t i s fou nd tocon ta in h igh concen t ra t ions o f a lum i n u m a n d ti t a n i u m . A n o t h e r f o r m o fscum has been observed f loa t ing overthe surface o f the we ld poo l e i ther aslumps or as a f i l m .Expe r imen ts a re descr ibed in w h ic hthere were d i f fe rences in the spectraand appearance o f a rcs on d i f fe ren theats. The fo r m at i on o f s lags, the i rbehavior and the i r e f fect on the arc are

d iscussed together w i th changes in theanode spo t p roduced by d i f fe ren tconcen t ra t ions o f l ow ion iza t ion po ten t ia l mater ia ls. These exper im entsa re no t yet com p le ted , and no f i rmconclusions are drawn a t th is stagerega rd ing the re la t ionsh ip b e twee nt h e co m p o s i t i o n o f the heat and itsw e l d a b i l i t y .

/. C METCALFE and M. 8.C QUICLEY arewith the Central Electricity GeneratingBoard, Marchwood Engineering Laboratories, Marchwood, Southampton, England.Paper presented at the 57th AWS AnnualMeeting held in St. Louis, Missou ri, duringMay 10-14, 1976.

I n t r o d u c t i o nThere are th ree main we ld ing fau l tsclassi f ied together under the head ing"arc and poo l instab i l i ty in GTA w e l d i n g . " These are :1. Var iab le we ld c ross sec t ion , g e n era l ly resu l t ing in sha l lower penetrat ion than an t i c ipa ted .2. Mig ra t ion o f the we ld bead f romthe l ine o f the jo in t .3. P roduc t ion o f an a symm et r i ca lmol ten reg ion by an arc f rom an e lect rode pos i t i oned cen t ra l l y on a jo in tbe tween tw o p ieces o f app a ren t l yiden t i ca l ma te r ia l .These we ld ing fau l ts can persisteven when a l l the usua l causes, asymme t r i ca l ea r th connect ions , d is tu rbedgas f lows, e tc . , have been e l im ina ted .The re is ev idence f rom recen t inve s t i ga t ions wh ich suggests tha t th is misbehavior cou ld be due to s l igh t var ia t ions in the com pos i t i on o f d i f fe ren theats o f nomina l ly the same mater ia l .Th is paper reviews previous work onvar iab i l i ty in the GTA process, con sid ers the heat inpu t to the w ork p ie ce(anode ) and demonst ra tes the impor-

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tance o f the anode spot . I t a lsodescr ibes a ser ies o f exp er im entsca r r ied ou t a t Ma rchwood and inco l l a b o r a t i o n w i t h i n d u s t r i a l co n ce r n sin the UK.P r e v i o u s I n v e s t i g a t i o n s

One o f the f i rst reports o f var iab i l i tyi n G T A W (Table 1) was tha t o f Oyler e ta/ 1 who inves t iga ted th is p rob lem w i th20 heats o f vacu um arc reme l ted Type304L sta in less stee l . They in i t ia l ly i d e n t i f i e d t h e i r " t r o u b l e so m e " m e t a l s a sones on wh i ch the top o f the w e ldbead was w ide r than expected andwas a dark b lue co lo r instead o f be i ngs t raw co lo red . They s ta te tha t w he nthere was lack o f penetra t ion the arc"d isp layed b r igh t spa rk ing and o the rd is tu rbances o r f l i cke r ing " and a lsotha t " these d is tu rbances caused ag row th to fo rm on the tungs ten e lect r o d e t i p . "

Some d i f fe rences in the com pos i t i on o f the me ta l we re no ted and we rea t t r ibu ted to d i f fe ren t p rac t i ces in vo lved in the stee l making process.These d i f fe rences in co mp os i t i o n we rec la imed to co r re la te w i th the changesin we ldab i l i t y , and p re fe r red c o n cen t ra t ions we re p roposed fo r s i l i conand manganese , bu t no quan t i ta t i veev idence was c i ted in suppo r t o f theseproposa ls.The conclusions were tha t i f thesi l icon co nte nt is g reater than 0 .7% thef lu i d i ty o f the mol ten meta l is excess ive , caus ing the poo l to w i den andg ive a cons equen t ia l l a ck o f p ene t ra t i o n . I f the s i l icon content is less than0.3% and th e m anga nese is less than1.4%, a v iscous and slugg ish w e l d po o lresul ts w i th ox ide en t rapm en t as aposs ib le resu l t . Unspec i f i ed d i f fe r ences in the concen t ra t ion o f a lum i num, somet imes used as an arcstab i l izer in the fu rnac e, we re a lso

reported but cou ld no t be c lear lyre la ted to the var iab i l i ty in the we lds.Ludw ig 3 no ted d i f fe rences in thepene t ra t ion p roduc ed by we lds ca r r iedo u t u n d e r n o m i n a l l y i d e n t i ca l co n d i t ion s on samples of 304 stainless s teelob ta ined f rom two d i f fe ren t hea ts .Me ta l f ro m one hea t had a we ld w i th aw ide , sha l low cross sec t ion , whe reaswe lds on the o the r hea t we re deepe rand na r rowe r . When mak ing a bu t twe ld be tw een two p la tes f rom thed i f fe ren t hea ts , the we ld t end ed to beskewed towa rds the p la te o f the ma te r ia l wh ich had p rev ious ly p roduced aw e l d w i t h a h i gh w i d t h / d e p t h ( w / d )ra t io .

Further tests w i th sta t ionary arcsrevea led a t im e depe nden ce in the s izeo f the anode roo t on ma te r ia l f romeach o f the two heats. Var ia t ions in th earc vo l tage and associa ted changes inanode spo t s i ze we re obse rved . Ludwig a rgued tha t th i s was b rough t

Table 1Summary of Investigations Referenced in This Study

ObserverOyler et aP

Ludwig2

Chase and Savage3

Bennett and Mills'

Paton et aP

In weld cross section20 heats: no w /d values orcomposit ions quoted.For satisfactory welds:0.3 < Si < 0.7% andMn > 1.4%Two heats with w /d ratios:A = 3.7, B = 1.8 (no com positions quoted)In joints between A and B, arcpreferentially moved to A

w /d increase (amoun t unspecified) by adding 0.3% Al andTi to Inconel 600.

w /d varied from 1.7 to 4.8 on11 heats.Increase in w/d occurs withchange in Al from 0.003 to0.019%.Al added to low w/d weldpool causes increase in w/ d .

Two heats with w /d ratios of2.4 and 7.3.w /d decreased by adding thinflux layer of unspecified metal salts.Higher w/d on heat with higher Mn, Ni and Mo (Al contents not given).

Observed PhenomaIn arc

Arc flickered.Deposit formed on tungsten.

Size of anode spot varied withtime and decreased whenslag appeared.""Larger anode spot on A."""Extraneous radiation fromanode" in first minute of operation. ""Size of anode spot increasedand arc voltage decreased byaddition of alkali elements. ,a lDifferent arc shape (unspecified) seen when 50 mg of various additives (e.g., Al, Si,M n, etc.) inserted in weldpool (estimated 0.5%).""Arc voltage increased by additives in weld poo l . ""Arc more flared on high w /d

heats.Blue zone in arcs on high w /dheats.Mn lines brighter in spectra ofarcs with high w/d heats.Arc "f l ickered" and depositsformed on tungsten (occurrence related to gas content of heat varying from 0.2to 0.4%).Arc more flared on higher w /dheat.

On weld pool surface

Lumps of slag (rich in M g, Siand Bo) appear after 1 minin center of anode spot.

Slags seen on surface of weldpool (not identi f ied).""

Differences in pool contoursreported (not specified).""

"Arc stationary. ""Using pure nickel.

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Table 2Values Used to Calculate Electron Density in Arc RegionParameter

Density of liquid (kg/m 3)Ionization potential (V)Vapor pressure from weld pool at 2000 K (Pa)Contribution to electron density at 5000 K (n vPercentage of total electron density

Al27005.9686 7

3.3 x 10 3 '54

M n74407.4118.7 X 10 '2.7 X 10 3144

Si2420

8.124. 01.8 x 1 0 "0.3

Fe7870

7.8346.78.6 x 10" '1.4

Ti4500

6.8110 - "4.6 x 10 130.7

A r-

15.6-4.8 x 100.008

abou t by the p re fe ren t ia l evapo ra t iono f the more vo la t i l e cons t i tuen ts in thew e l d p o o l ; he associa ted th is vapori z ing p rocess w i th "ex t raneous r a d i a t i o n " seen when the arc was f i rsts t ruck . Th is phenomenon may bemore re levan t than those occu r r ingla te r , because no rma l we ld ing cond i t i ons co r respond to those wh ich ex is tin the f i rst few seconds o f a sta t ionarya rc. De l ibe ra te in t rod uc t ion o f a l ka l ie lements caused an increase in thearea o f the ano de spot and a dro p o fabou t 1 V in the arc vo l tag e.Chase & Savage3 found tha t d i f fe r ences in the concen t ra t ion o f ce r ta ine lemen ts (pa r t i cu la r l y a lum inum andt i tan ium) p roduced va r ia t ions in thedepth and cross sect iona l a rea o fwe lds on Incone l 600. In add i t i on , theyin t roduced sma l l quan t i t i e s o f va r iouse lemen ts in to we lds made on pu ren ick e l ; in most cases, these a dd i t ion scaused an increase in arc voltage, e.g.,0 .7 V fo r a lum inum. They a lso cons id ered the possib i l i ty tha t the t racee lements a f fect the surface tension o ft h e m o l t e n m e t a l , d e p e n d i n g o nwhe the r they rema in in so lu t ion o rfo rm a so l id f l oa t ing on the su r face .They suggest tha t th is wou ld lead tod i f fe ren t su r face con tou rs and cou ldpro duc e a change in the s t i r r ingpa t te rn in the we ld p o o l . Theyreported tha t such var ia t ions in sur facecon tou rs and so l ids f l oa t ing on thesurface had been o bserve d as we l l aschanges in arc shape.

D i f fe rences in the w / d ra t io o fwe lds p roduced by iden t i ca l we ldprocedures on stee ls wh ich have beenprepared by d i f fe ren t p rocesses havebeen examined by a t least two groupsof invest iga tors.Benne t t & M i l l s ' we re conce rnedwi th h igh (9%) manganese stee lswh ich had been made by th reeprocessesconventional a i r me l t ing ,vacuum a rc reme l t ing (VAR) and e lect roslag re f in ing (ESR). ESR mater ia lp rodu ced muc h h ighe r w / d ra t ios thanthe o ther t wo , and i t was suggestedthat th is was re la ted to the amount o fa l u m i n u m p i cke d u p d u r i n g t h eremel t ing . The ir e leven heats hadco n ce n t r a t i o n s of a l u m i n u m r a n g in gfrom 0 .003% to 0 .019% wi th correspo nd in g w / d ra t ios o f 1.7 to 4 .8 .(These va lues are fu l ly tabu la ted fo rcompar ison w i th ou r resu l ts in Tab le3.)

The shape o f the arc was d i f fe ren tw h e n p r o d u c i n g th e se h i g h w / d w e l d sthan i t was on those w i t h a low w / d .The a rcs on we lds w i t h a h igh w / dra t io and a h igh leve l o f a lum inu mf la red ou t a t the wo rkp iece and had adome o f deep b lue l i gh t su r round ingthe arc, bu t no desc r ip t io n was g ivenof the behavior o f the anode. Spectroscopy showed th is b lue l i gh t to beemiss ion f rom manganese . Howeve r ,they were not ab le to ident i fy theprocess by wh ich arc shape, manganese em is s ion , o r w / d ra t io we rea f fec ted by an increased a lum inumconcen t ra t ion . They made a fu r the rcheck by add ing a lum inum to a we ldpoo l on a ma te r ia l no rm a l l y p ro duc inga low w / d ra t io . The we ld p oo li n s ta n t ly w i d e n e d , an d p e n e t r a t i o ndecreased so tha t the w / d ra t ioincreased and the arc shape changedto tha t normal ly associa ted wi th an arcon a h igh w / d ma te r ia l .Paton e t a / ' have co mp are d thep ro f i l e s o f we lds on s tee ls f rom open -arc fu rnaces and on e lectroslag re

me lted stee ls. They do not p resentquan t i ta t i ve da ta on the d i f fe rences inthe we lds bu t suggest tha t fo r thesame cond i t i ons the we lds in there f ined stee l have a smal l dep th o fpene t ra t ion and a 50 to 100% increasei n w i d t h ( i .e., increase in w / d o f a tleast 50% and up to more than 100%).The ana lysis o f the two meta ls usedby Pa ton showed tha t the concen t ra t ion o f the m ino r e lemen ts was lowe rin the re f ined stee l , except fo r n ick e l ,m o l yb d e n u m a n d m a n g a n e se . N o val

ues are g iven fo r the a luminum c o n cen t ra t ions . The a rc p ro f i l e was w ide rw h e n m a k i n g t h e w e l d s ( w i t h h i g h w /d ra t ios) on the re f ined stee l . Theywere ab le to decrease the w / d ra t io o fwe lds on the re f ined stee l by add ing af lux con ta in ing an unspec i f i ed m ix tu reof "meta l oxides and sa l ts" to thew e l d , e i ther in the fo rm o f pasteapp l ied in a th i n layer to the jo in tedges or as wire .The pa ten t app l i ca t ion o f Ma je t i chand Yeo" c la imed tha t a coa t ing o fmeta l oxides can increase the penetrat ion o f the GTA process on sta in lessstee ls. A s imi la r im pro ve m en t isc la imed fo r an ox ide f i lm p roduced byhea t t rea tmen t o f the me ta l . It is notrepo r ted whe the r these t rea tmen tse l im ina te the va r iab i l i t y in GTA w e l d ing be t we en d i f fe r ent heats o f sta in less stee l . A s im i la r increase in pe netra t ion was found by S imon ik 7 wi th f l uxlayers co nta in in g meta l ha l ides.The w / d ra t io o f a we ld crosssect ion can a lso be a f fected by factorso the r than the compos i t i on o f the

meta l o r i ts sur face contaminants. Forexamp le , gas co mp os i t i on and pu r i t yhave been inves t iga ted by G l i ck s te inet a/ 8 and Savage et al" respect i ve ly ;e lectrode t ip geometry by Savage etaP " and Sp i l l e r and MacGrego r " ; andcu r ren t leve l , a rc leng th and w e ld ingspeed by Gl ickste in e t a /8. The e f fectso f these o ther factors are no t considered here as these quant i t ies havebeen main ta ined as constant as possi b le unde r the cond i t i ons cons ide red inth is paper.

Table 3 -Results of First Series ofWeld Ai

w / d6.16.16.05.95.24.84.74.24.24.23.62.7

''"Arbitrary units

lalysistr0.360.680.450.230.430.870.230.360.670.690.380.23

M n1.751.610.710.871.751.621.571.801.621.751.590.80

Exper imentsSteel AiAl

0.0200.0100.0410.0250.0330.0100.0200.0280.0580.0140.0320.004

Talysis (%)Ti

0.0250.0230.0050.0250.0240.0200.0220.310.510.0100.440.003

Si0.170.540.400.420.370.520.400.630.380.150.430.41

SpectraMnll/ M n lW

1.21.20.90.91.11.20.70.91.10.60.50.35

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The invest iga tors, wh ose w ork issummarized in th is sect ion and inTab le 1 , have suggested tha t smal lconcen t ra t ions o f t race e lemen ts canhave a la rge e f fect on the pe net ra t i onand we ldab i l i ty o f sta in less stee ls. Sofar i t has been reported tha t var ia t ionsi n t h e l e ve l s o f a l u m i n u m , t i t a n i u m ,manganese and si l icon have beenassoc iated w i th changes in the w / dra t io o f we lds. Suggested exp lanat ionsinc lude changes in the f lu id p rope r t ieso f the we ld p o o l , in the arc vo l tageand in the a rc roo t on the wo rkp iece .In the next sect ion we consider theef fect o f these e lements on the anodespot reg ion o f the arc and the i r consequen t ia l i n f luence on the ene rgytransfer f rom the arc to the we ld p o o l .The processes taking p lace in theanod e spot a re exam ined because, as iss h o w n , most o f the heat input to theweld occurs in th is reg ion , bu t i t is no tin tended in the long te rm tha t theo the r hypo theses shou ld be d is re garded.T h e A n o d e S p ot a n d H e a tT r a n s f e r

I t is we l l k no w n tha t the crosssect ion o f the arc d imin ishes c lose tothe anode and a lso tha t anode spotsa re some t imes seen.13 Where the a rcd iameter reduces very sharp ly a t theanode , the re is a s ign i f i can t redu c t ionin v i s ib le lum inous in tens i ty . U nde rth is reg ion there may then be a b lued i f fuse g low and some t imes an anodespot .

It is a lso well k n o w n 1 3 1 4 tha t most o fthe heat t ransfer to the workp iece inthe GT arc occurs in the anode spot .Qu ig ley e t a /13 ca lcu la te tha t fo r a shortarc (9.5 V arc drop) 80% of the totalheat input is t ransferre d to the w or k-p iece , and th is has been ex pe r im en ta l l y con f i rmed by Lancaste r1 ' . A lso , o fthe hea t reach ing the wo rkp iece ,abou t 80% is t rans fe r red w i th in theanode spo t reg ion ; the rema in ing 20%reaches the wo r kp ie ce by co nve ct ionor rad ia t ion f rom the rest o f the arc.The ma in consequence o f th i sco nc ent ra t ion o f heat is tha t the pos i t ion o f the an ode spot is a ma jor factord e t e r m i n i n g t h e p o i n t o f a p p l i ca t i o nof the heat . I t is reasonab le to expect ,there fore , tha t i f a me cha n ism existstha t moves the anode spot th is w i l le f fect ive ly move the po in t o f heatinpu t and hence the we ld p o o l . T h u s ,i t is pos tu la ted tha t du r ing we ld in g inwh ich the re is la te ra l mo vem en t o f th earc roo t , the we ld wi l l have a mored is t r ibu ted hea t inpu t and hence bew ide r than one on wh ich the a rc roo tstays sta t ionary re la t ive to the l ine o f

the jo in t . For th is reason, var ia t ion inthe stab i l i ty o f the arc roo t is advancedas one pos sib le cause o f the var iab i l i tyi n w / d r a t i o a n d p e n e t r a t i o n .

The arc roo t is the reg io n in w h i che lec t rode ma te r ia l , wh ich has a lowion iza t ion p o ten t ia l , ente rs the a rc.W e can ca lcu l a t e t h e co n t r i b u t i o n t othe to ta l i on iza t ion f rom these e le ments u sing a version o f Saha's e q u a t ion adop ted by Su i ts ' " :log,,, (T 1 ") - 5040 V /T =log,,, (n,.-/nJ0B) - 15.38

whe re V is the ion iza t io n p o te n t ia l , T ist h e t e m p e r a t u r e , n,, is the e lec trondens i ty and n a is the a tom ic dens i ty o fthe gas in quest ion .The la t te r may be ca lcu la ted f romthe vapor pressure o f each o f thee lemen ts , and n e may then be foundfor any tempera ture ; in th is case, anarc temperature of 5000 K (8540 F) waschosen as be in g typ ica l fo r th isreg ion13 .Tab le 2 show s tha t mos t o f the i o n i za t ion is p rov ided by a lu m i num andmanga nese, assum ing tha t a sa tura ted

vapor o f these e lemen ts is p resent justabove the p o o l . There is som e su ppor tfo r th i s assump t ion f rom Benne t t andMi l l s4 wh o say they de te rm ined tha tthe a l loy cons t i tuen t e lemen ts we represent in the i r a rc rough ly in p roport ion to the i r vapo r p ressu res . H owev e r ,the fo rma t ion o f ox ides cou ld reducethe vapor pressures; a lso , t ransien ts inthe we ld poo l cou ld d is tu rb th i sba lance in the vapo r compos i t i on .Desp i te these unce r ta in t ies , a lum i num o r manganese cou ld have anin f luence on the ion iza t ion in the a rc ,and hence the anode spo t wou ld tendto move to reg ions f rom wh ich thesee l e m e n t s w e r e va p o r i z i n g . U n e q u a lconcen t ra t ions o f the e lemen ts in twoheats pu t together to fo rm a jo in twou ld by the same process cause thearc to be b iased to one side .E x p e r i m e n t a l I n v e s t i g a t i o n sPreliminary Tests: Observation of AnodeSpot Region with a Stationary Arc

Re la t i ve ly s imp le expe r imen ts we reset up fo r the purpose o f observing theano de spot reg ion in a GT arc. Am o d e r a t e p o w e r m i c r o sco p e w i t hcamera a t tachmen ts and neu t ra l dens i ty f i l te rs was used to examine andphotograph the area in the arc c lose tothe we ld poo l su r face . In th is reg ion ab r igh t b lue g low , con ta in ing theanode spo t , cou ld be seen mo v inga round the su r face o f the p o o l . Ad is t inc t i ve behav io r pa t te rn was c lea r l y obse rved in wh ich , even w i th anappa ren t l y c lean ma te r ia l , some scumwo u ld appea r a lmost ins tan tan eous lyon the su r face o f the we ld p o o l , to befo l lowed by the anode spo t mov ing tothe edge o f the scum.Mo t ion o f the anode spo t appea redto be p rod uce d by the spot a ncho r ingi tse l f to the scum unt i l the la t te r was

cons um ed , a t wh ich p o in t the spo twou ld move on to the nex t ava i lab lepatch o f scum . Clear ly the sc umplayed an imp orta n t ro le in de term in ing the behav io r o f the anodespot .The phenomenon was inves t iga tedf u r t h e r b y m a n i p u l a t i n g t h e w o r kp i e ceto co l lec t an app rec iab le amoun t o fscum and cen te r ing i t and the anodespo t in the m idd le o f the we ld p o o l . Byswi tch ing o f f the arc the mater ia l wast rapped in the cen te r o f the so l id i f i edwe ld poo l fo r subsequen t chem ica lana lysis. The dura t ion o f each test wasabout 20 seconds.The resu l ts o f these f i rst invest igat ions showed tha t the ma te r ia l wasco m p o se d p r i m a r i l y o f t i t a n i u m w i t h ,i n add i t i on , l a rge quan t i t i e s o f vana d i u m a n d m o l yb d e n u m . A l l t h e see lemen ts we re p resen t in ve ry muchh igher p roport ions (a t least onehund red t imes g rea te r ) than wasfound in the base meta l .A secon d series o f expe r ime nts w asunde r taken in wh ich the me ta l waspo l i shed w i th ab ras ive pape r andc leaned w i th ace tone . The same expe r imen ta l p rocedu re was adop ted toscavenge a s ign i f ica nt part o f th is scumin to the cen te r o f the we ld p o o l . A n a l ysis o f the so l id i f ied scu m was the ncarr ied out as be fo re , bu t th is t im et i t a n i u m a n d a l u m i n u m w e r e t h ed o m i n a n t e l e m e n t s . S in ce a l u m i n u mwas not p resen t in the we ld m eta l , a tmore tha n 0.005%, i t is co nc lud ed tha ti t came f rom the abrasive paper;fu r the r ana lys is con f i rmed tha t thepape r con ta ined a lum inum ox ide as i t sab ras ive cons t i tuen t .The la te r expe r imen ts demonst ra tedhow su r face con tam inan ts and p repa ra t ion can in f luence the cons t i tu t ionof the so l ids on the surface o f the we ldp o o l . F igure 1 shows e lec t ro n m ic ro scope pho tog raphs wh ich ind ica te theaccumu la t ion o f the spec i f i c e lemen tsin the scum.Collaborative Investigations

Co l labo ra t i ve inves t iga t ions havebeen se t up wi th o ther industr ia les tab l i shmen ts wh ich a re conce rnedabou t the fo rm o f we ld ins tab i l i t yd iscussed here . In one ser ies o f exper i men ts i t has been found tha t whentwo mater ia ls f rom heats o f AISI 304L(each p roduc ing we lds o f d i f fe ren tw id th to dep th ra t ios) a re pu t toge th e r , an unde r -pene t ra ted o r a skewwe ld can resu l t (Goodman andG e o r g e, p r iva t e co m m u n i ca t i o n ) .

The w /d ra t io o f a num ber o f w e ldsamples was measured. I t was foundthat in genera l by pa i r ing o f f mater ia lso f r o u g h l y e q u a l w / d , g o o d w e l dj o i nt s w e r e p r o d u ce d . W h e n j o i n t swe re made f rom samp les , o f w h ic hone had a w / d , fo r examp le , o f abou t 4and the o the r 2 , unaccep tab le we lds

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Fig. 7Electron microscope photographs show ing accumulation ot aluminum and titaniumin the weld pool scumFig. 2CT arc showing anode spot on weldpool. (Reproduced on blue sensitive film toshow up anode spothence only centralcore of arc can be seen)

w e r e o b t a i n e d .Ana lysis o f the meta l f rom two heatsa t the extrem es o f the w / d rangeshowed tha t the hea t w i th the lowe rw / d (1 .9 ) had the h ighe r conc en t ra t i o n o f ch r o m i u m ( 2 0 . 3 % co m p a r e dwi th 19 .4%), manganese (0 .86% compared wi th 0 .67%) and si l icon (0.41%compared wi th 0 .39%). The heat onwh ich the tr ia l we lds had sho wn a w / dof 6 .4 had a h igher leve l o f n ick e l(12 .2% compared w i th 11.8%) and ave ry much h ighe r leve l o f a lum inum(0.037% compared wi th 0 .007%). Therewas no s ign i f i can t d i f fe rence in theconcen t ra t ions o f the o the r cons t i t uents.The chemica l ana lyses o f the meta lin the we ld beads we re a lso ob ta inedfor.comparison with those o f the baseme ta l . For a l l the const i tuents excepta lu m in um the re was no s ign i f i can td i f f e r e n ce b e t w e e n t h e co n ce n t r a t ions o f the var ious e lements in theweld bead or in the base meta l .Howeve r , the pe rcen tage o f a lum inumfor the sample w i th w / d = 1.9 was0.003 in the we ld m eta l , and fo r t hesample w i t h w / d = 6 .4 i t was 0 .028,bo th o f wh ich a re s ign i f i can t l y lowe r

than the concen t ra t ions in the basemeta l . Meta l f rom these heats isinc lude d in ou r p resen t labo ra to ryinves t iga t ions .A n o t h e r co l l a b o r a t i ve p r o g r a m c o n ce rns tube - to - tube we lds in A IS I 321 ,fo r wh ich one o f the two tubes hadbeen fo rm ed f rom a he l i ca l l y w e ld edstrip. In th is case a sa t isfactory w e l dhad been p roduced fo r the ma jo r i t y o fthe 600 mm (24 in . ) c i rcu mfe renc e , bu tfor a len gth of abo ut 75 m m (3 in.) th ewe ld bead wen t o f f - l i ne e r ra t i ca l l y andincreased in w id th . Th is dev ia t ionoccu r red in the reg ion whe re thehe l i ca l seam we ld in te rsec ted w i th thec i r cu m f e r e n t i a l j o i n t .Ma te r ia l ana lys is showed tha t the rewas re la t ive ly l i t t le var ia t ion in the

concen t ra t ions o f t i tan ium and s i l i conon e i ther s ide o f the jo in t (Thomas,p r i va t e co m m u n i ca t i o n ) , b u t s i g n i f i can t d i f fe rences we re found in theconcen t ra t ions o f manganese and a l u m i n u m . The seamless tube hadapp rox ima te ly tw ice the leve l o fa lum inum o f the seamed tube andapproximate ly 1 .4 t imes the leve l o fmanganese . The re was no o bv iouschange in these conc en t ra t ion s whe rethe we ld f ir s t we n t o f f cen te r . W he r et h e w e l d b e ca m e ske w e d , i t m o ve dtowards the ma te r ia l w i th the h ighe rleve ls o f a lum inu m and manganese .Present Investigations

Experimental Arrangements. The expe r imen ta l p rog ram wh ich we a recarry ing out a t p resent is concernedp r i n c i p a l l y w i t h m o n i t o r i n g G T Awelds carr ied out under c lose ly c o n t r o l l e d co n d i t i o n s . T h e se b e a d - o n -p la te we lds are be ing m ade on a se lect ion o f sta in less stee l heats wh ich havea range o f concen t r a t ions o f e lemen tswh ich appea r to cause va r iab i l i t y inthe we lds , i .e . , a lum inum, manganeseand t i tan ium. The samp les inc lude 10heats o f AISI 304L obta ined f rom thef i rst o f the two co l labora t ive exercisesre ferred to above and six heats (one304, tw o 316 and th ree 321) mad eava i lab le by the specia l stee ls d iv is iono f the B r i t i sh S tee l Co rpo ra t ion , andwh ich have been p roduced by e i the rthe a rc reme l t ing o r the AOD s tee lmaking process.

The same we ld ing cond i t i ons a rebe ing used fo r a l l samples. They aresuch tha t they produce on any heat agreatest pene tra t ion o f just under 50%of the th ickness o f the p la te . Thewe ld i ng cu rren t is 100 A (st ra igh tpo la r i ty) , and the e lec trod e is 2.4 m m(0.094 in . ) d iameter 2% thor ia tedtungsten , w i t h a t i p g rou nd to a 30 deginc lud ed ang le and w i th a po in t o f 0 .3

mm (0.012 in.) spherical radius.The to rch is set and m ain ta ine d in apos i t i on w i th the e lec t rode ve r t i ca l tothe wo rkp iece . The gap be tween e lect rode t ip and the workp iece is se t a t 3mm (0.12 in.) before the start of aw e l d , and the t ip p ro t rudes 3 mm f romthe nozz le o f the to rch . A new e lect rode is used fo r each w e l d . Thew el di ng s peed for a l l the tests is 4.2m m / s ( 9 . 8 i n . / m i n ) .The sh ie ld ing gas is a rgon o f nor ma lw e l d i n g q u a l i ty w i t h a n o m i n a l m a x i mu m imp u r i t y leve l o f 100 pp m.Howeve r , i n v iew o f the expe r ience

reported by Savage e f al", the p recau t ion is be in g taken o f ana lyzing t hecon tents o f each cyl in der used. So fa r ,a l though the impu r i t y leve ls ( i .e., th eoxygen and n i t rogen con ten t ) havebeen ve ry much h ighe r than spec i f i ed ,they are reasonab ly constant (Smal ley,p r i va te commun ica t ion ) . The gas l i nesto the to rch are purged before eachweld to reduce the e f fects o f anyen t ra inmen t o f a i r w i th in the hoses . Agas f lo w o f 10 l i te rs /m in (21 .2 f t vh ) isused , th rou gh a ce ramic no zz le o f 16mm (0 .63 in . ) in te rn a l d iam eter , wh ic hincorpo ra tes a gauze "gas lens " tos tab i l i ze the f low .Each we ld i s mo n i to red by a num bero f me thods. C ine -pho tog raphy i s usedto record the behavior o f the arc andwe ld po o l sur face. Th is is don e byp lacin g a camera o n the axis o f thewe ld lo ok in g a t a sha l low ang le acrossthe p o o l . The f i lm is run a t 250 f rames/s, and ne utra l de nsi ty f i l te rs a re used toreduce the l i gh t i n tens i ty w i th ou ta l te r ing the co lo r ba lance .

In add i t ion to measur ing the arccu r ren t on a mo v ing co i l am mete r(0.1% accuracy), the s igna l f rom asecond cu r ren t shun t and the vo l tagebe tween the to rch inpu t and the wo rk -p iece are mo n i to red o n a cha r t reco rd e r. The expe r im en ta l ope ra t ions (e.g.,star t and stop o f t rave l ing tab le , iso la -

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t i on o f the ins t rumen ta t ion du r ing a rci n i t i a t i o n , etc.) are car ried ou t bymeans o f a sequ ence co ntr o l le r in anendeavo r to reduce manua l va r ia t ions .L ight em iss ion f ro m a cen t ra l ve r t i ca ls l ice o f the arc is recorded on a Hi lgera n d W a t t s m e d i u m q u a r t z sp e c t r o m eter.The surfaces o f the we ld samples arec leaned in one o f two ways . A " rea sona b ly" c lean su rface is p rod uce d bymanua l wash ing o f the p la tes a t leasttw ice in ace tone , rubb ing the su r facesdur ing the f i rst wash wi th paper t issuesto remove any s tubbo rn depos i ts . Fo rthe sake o f cons is tency a "v e ry " c leansurface is p repa red by u l t ras on ic ag i ta t ion in ace tone , bo th be fo re and a f te re lec t ros t r ipp ing the samp le in a s o l u t i o n co n t a i n i n g p h o sp h o r i c a n d su l f u r i c a c i d a n d ch r o m i u m t r i o x i d e .Preliminary Results. Dur ing the f i r s tpart o f th is ser ies o f exper iments,analyses we re ob ta ined o f the f o l lo w

ing e lemen ts : C, Si, M n , P, S, Cr, M o ,Ni , A l , B, Co, C u, N b, Sn, T i , VV and 0 ,and com pared w i th w /d ra t ios . Formost o f these e lements there appearedto be no co r re la t ion be tw een the i rleve ls and w / d ra t io ; howe ve r , the reseemed to be a poss ib le c o r re la t io nb e t w e e n w / d a n d M n , A l , Ti , and S i ,and as these we re the e lem entsthough t by p rev ious wo rke rs to bei m p o r t a n t , t h e y w e r e e xa m i n e d m o r eclosely and discussed in th is paper. I t isthese la t te r e lements wh ich are themain sub ject o f p resent invest iga t ionsa l though the o the rs a re mon i to red tode tec t any s ign i f i can t va r ia t ions .The resu l ts ava i lab le a t the t ime o fw r i t i ng showed tha t the se lec t ion o fheats pro vide d a s ign i f ic ant range o fw / d ra t ios in the we lds . Tab le 3 showsthe average w / d ra t ios fo r each hea t ,the s tanda rd dev ia t ion in the obse rvedva lues , the concen t ra t ions o f manga nese, a l u m i n u m , t i t a n i u m a n d s i l i co nand deta i ls f rom the arc spectra . I t canbe seen tha t the mean va lues vary f rom2.7 to 6 .1 w i th the standard devia t ionsrang ing f rom 0 .23 to 0 .87 . The lowestand h ighes t va lues o f w / d ob ta ine d in

the tests were 2.1 and 7.0.The pho tog raphs showed tha t the rewere var ia t ions in the arc pro f i les ond i f fe r ent heats. A t one ex trem e the arcwas, apart f rom th e reg ion c lose to th eca t h o d e , a u n i f o r m w h i t e co l o r . O nthe o ther hand some records showarcs in w h i ch there was a d is t inc t b luereg ion c lose to the surface o f the we ldpo ol (Fig. 2). In some cases th is blu ezone stayed re la t ive ly centra l , in o thersi t moved around, and in some cases i texten ded as fa r as the edge o f the we ldp o o l .The f i rst in te re st ing s pectr osco p icresult is that in al l cases th e s pe ctr umnear the anode is to ta l ly d i f fe re n t f ro mthat h igher u p in the arc. A t yp ic a lspec trum appears in Fig . 3 wh i ch

2488Fel

Ari l

Ar HJ

Ar n2949-Mn n29803033 ArTPA r i l -

Fig. 3Spectrum of CT arc showing metalvapor lines near the weld pool

shows tha t in the body o f the arc thedominan t l i nes a re those o f a rgon .H o w e ve r , a p p r o a ch i n g w i t h i n a b o u t am i l l ime te r o f the wo rkp iece these fadeand meta l vap or l ines, espec ia l ly m a n ganese, sho w very st rong ly.The spectra f rom a lmost a l l the arcsconta in the same l ines, bu t there ared i f fe rences (Tab le 3) in the i r in tensi t ies, par t ic u la r ly in the ra t ios o f the l inein tens i t i es f rom the ion ized and un ion ized sta tes o f manganese. For heatsw i th a h igh w / d ra t io the ra t ios o fion ized to un - ion ized manganese l i nes(at 294.9 and 279.5 nm respectively) areh igh , whe reas fo r low w / d ra t ios theM n l l / M n l ra t ios are lower.T h is lo o se co r r e l a t i o n b e t w e e n w / dand the re la t ive in tensi ty o f theion ized manganese l ine suggests tha t ah igh w / d ra t io is assoc ia ted w i th ah igher a rc tempera ture c lose to thewe ld poo l . One consequence o f ah ighe r a rc tempe r a tu re w ou ld be a

g rea te r p ropo r t ion o f hea t t rans fe r redto the we ld by convect ion , and as th isis mo re d is t r ibu te d tha n the hea t d iss i pa ted a t the anode spot i t might beexpected to p rod uce a w id e r we l d andh e n ce t h e h i g h e r w / d f o u n d .The c ine obse rva t ions o f the we ldpoo ls have revea led d i f fe rences in thesu r face cond i t i ons . In some ins tances ,qu i te la rge lumps o f s lag have beenseen f loa t ing a round on the su r face o fthe mo l ten me ta l . Gene ra l l y on ly a fewof these " is lan ds " a re seen, and theyappear to be so l id a l though tha t has

n o t b e e n co n f i r m e d . A n o t h e r f o r m o fsu r face con tam inan t seen on o the rwe ld s consists o f a th in layer wh ic hlooks dark in contrast to the re f lect ingsu r face o f the m o l ten me ta l . Th is th inscum is mo re m ob i l e than the p re v i o u s l y m e n t i o n e d slag, and it g ivesthe imp ress ion o f be ing e i the r a l i qu ido r a f i ne pow de r w h ic h i s eas i ly mov edby the surface curren ts on the we ldpoo l . A lso , a mo re tena c ious fo r m o fscum has been seen adher ing to th esides o f the we ld p o o l , and in ourp re l im ina ry expe r imen ts the a rc roo tshow ed a tende ncy to anch o r on th isscum.So fa r , the on ly che mic a l ana lysis o fthese slags and scums has been in ourp re l im ina ry tes ts wh en they we refoun d to con ta in h ighe r than no rm a lleve ls o f a lum inum and t i tan ium. Toda te we have insu f f ic ien t ev idence toclear ly ident i fy the ro le o f thesesu r face con tam inan ts , and fu r the r in vest ig a t ion is p la nne d to c la r i fy th is .Discussion

W e have fo un d tha t , fo r the heatsavailable to us at present, our testsp r o d u ce d a r a ng e o f w i d t h / d e p t hra t ios f ro m 2.7 to 6 .1 . Ben net t andMi l l s ' cons ide red tha t the w / d ra t io isan increas ing func t ion o f the a lum i -

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n u m c o n t e n t o v e r t h e r a n g e f r o m 0 . 0 03t o 0 . 0 1 9% , a s s h o w n i n T a b l e 4 . O u rr e s u lt s h a v e n o t c o n f i r m e d t h i s r e l a t i o n s h i p , a l t h o u g h t h e s i n g l e h e a t i no u r te s t s w h i c h h a s a v e r y l o wa l u m i n u m c o n t e n t a ls o h a s b y f a r t h el o w e s t w / d r a t i o . A l l o f o u r o t h e r h e a t sh a v e a t l ea s t 0 . 0 1 % a l u m i n u m w h i c h isa t t h e h i g h e r e n d o f t h e B e n n e t t a n dM i l l s d a t a ; h o w e v e r , i t c o u l d b e i m p o r t a n t t h a t t h e i r h e a t s h a d a m u c h h i g h e rl e v e l o f m a n g a n e s e ( 9 % ) t h a n o u r s .

W e h a v e n o t f o u n d a r e l a t i o n s h i pb e t w e e n w / d r a t io a n d a n y o t h e re l e m e n t s ; t h e o n l y c o r r e l a t i o n w e h a v ei d e n t i f i e d s o f a r is a l o o s e o n e s h o w i n gt h a t w / d i s a n i n c r e a s i n g f u n c t i o n o ft h e i n t e n s i t y r a t i o o f s p e c t r a l l i n e sf r o m i o n i z e d a n d u n - i o n i z e d m a n g a n e s e w h i c h c o u l d i n d i c a t e a d e p e n d e n c e o f w / d o n lo c a l a r c t e m p e r a t u r e . W e c o n f i r m t h e p r e s e n c e o f t h eb l u e g l o w r e g i o n c l o s e t o t h e w o r k -p i e c e a n d s c u m o n t h e w e l d p o o l .B o t h c o u l d p l a y s o m e p a r t i n t h ep r o c e s s e s w h i c h o c c u r , b u t s o f a r o u rr e s u l t s a r e n o t s u f f i c i e n t t o i n t e r p r e tu n a m b i g u o u s l y t h e r o l e s o f t h e s ep h e n o m e n a .

L u d w i g 3 as w e l l as G o o d m a n a n dG e o r g e ( p r iv a t e c o m m u n i c a t i o n )f o u n d t h a t, w h e n j o i n i n g t w o h e a ts o fd i f f e r e n t w / d r a t i o s , a s k e w e d w e l dc a n r e s u l t w i t h t h e w e l d g e n e r a l l yb e i n g b i a s e d t o w a r d s t h e m a t e r i a l o fh i g h e r w / d . A n a l y s i s o f s k e w o r o f f -c e n t e r w e l d s s u g g e s t s t h a t t h e w e l dp o o l e x h i b i t s a p r o p e n s i t y t o m o v e t o ar e g i o n o f h i g h e r a l u m i n u m a n d m a n g a n e s e .

N o c l e a r p i c t u r e h a s y e t e m e r g e df r o m o u r te s t s. T h e s i m p l e m o d e lp r o p o s e d e a r l i e r m a y b e i n a d e q u a t e ,b e c a u s e it d o e s n o t t a k e i n t o a c c o u n tt h e r o l e o f t h e o x i d e s l a g s w h i c h t h e s ee l e m e n t s c a n f o r m o n t h e w e l d p o o ls u r fa c e . It is k n o w n ( W o l s t e n h o l m e ,p r iv a te c o m m u n i c a t i o n ) t h a t a l u m i n u m a n d t i t a n i u m ar e m o r e l i k e l y t of o r m s t a b l e o x i d e s t h a n m a n g a n e s ea n d s i l i c o n . If, f o r e x a m p l e , aMnO.S iOo s l a g e x i s t s o n t h e su r f a ce o ft h e w e l d p o o l , a n y a l u m i n u m s u d d e n l y a p p e a r i n g w o u l d t e n d t o f o r ma l u m i n a at t h e e x p e n s e o f t h e m a n g a n e s e s i l ic a t e . T h i s w o u l d c h a n g e t h em e t a l v a p o r p r o p o r t i o n s i n t h e a r c .

T h e S i O , , T i O , a n d M n O s la g s h a v el o w m e l t i n g p o i n t s ( a b o u t 1 9 0 0 K o r3 0 00 F ), w i t h t h a t o f M n O . S i O , b e i n ge v e n l o w e r ( 1 6 0 0 K o r 2 4 0 0 F) . A l u m i n a( A l , 0 3 ) , h o w e v e r , h a s a m e l t i n g p o i n to f 2 3 0 0 K ( 3 7 0 0 F ) , h i g h e r t h a n t h ew e l d p o o l t e m p e r a t u r e , a n d w i l l g i v e ac r y s t a l l i n e s la g r a t h e r t h a n t h e t h i ng la s sy s l a g o f M n O . S i O , . T h e r e m a y ,t h e r e f o r e , b e a n i n t e r a c t i o n b e t w e e nt h e f o r m a t i o n o f s la g s i n v o l v i n g a l u m i n u m a n d m a n g a n e s e a n d th e p r e s e n c eo f t h e s e e l e m e n t s i n t h e a r c w h e r e

Tab le 4Experimental Resul ts of Be nne t tand M i l l s (a t 200 A; Mn = 9 .0% no min a l )Al

0.0030.0030.0030.0030.0040.0180.0190.0100.0090.0040.003

w / d2.12.21.81.81.74.04.84.23.33.82.2

t h e y a f f e c t i o n i z a t i o n .A n a s p e c t w h i c h w e h a v e n o t t a k e n

i n t o a c c o u n t a t t h i s s t a g e is t h e e l e c -t r om agne t i c a l l y i n d u c e d c i r c u l a t i o n o fm o l t e n m e t a l in t h e w e l d p o o l . It isk n o w n t h a t c i r c u l a t i o n t a k e s p l a c e o nt h e s u r f a c e , a n d e x p e r i m e n t s b yW o o d s a n d M i l n e r " a n d o t h e r s h a v ei n d i c a t e d t h a t it t a k e s p l a c e b e l o w t h es u r f a c e a l s o . C i r c u l a t i o n o f t h e m o l t e nm e t a l c a n m o d i f y t h e d i s t r i b u t i o n o fe l e m e n t s s u c h as a l u m i n u m a n d a l s oa f f e c t t h e h e a t f l u x p a t t e r n t h r o u g h o u tt h e b u l k o f t h e w e l d p o o l . F o r a f i xe da n o d e s p o t t h e r e m a y b e a n u m b e r o fs t i r r in g p a t t e r n s w h i c h c o u l d g i v ed i f f e r e n t w / d r a t io s .

In o u r e x p e r i m e n t s w e h a v e s e e n am o v i n g a n o d e s p o t . It c o u l d b e a f a i r l yc o m p l e x i n t e r a c t i o n b e t w e e n s p o tm o v e m e n t a n d c i r c u l a t io n w h i c h d e t e r m i n e s t h e f i n a l w / d r a t i o . T h e o r e t i c a l w o r k o n t h i s s u b j e c t , u n t i l r e c e n t l y , h a s b e e n c o n c e r n e d w i t h c i r c u l a t i o n f r o m a s i n g l e s p o t . W o r k a t t h eM a r c h w o o d L a b o ra t o ri e s b y A n d r e w sa n d C r a i n e ( p r i v a te c o m m u n i c a t i o n ) isi n p r o g r e s s w i t h t h e i n t e n t i o n o fo b t a i n i n g a s o l u t i o n f o r t h e m o v i n gs p o t c a s e .

C o n c l u s i o nT h e r e s u l t s o f e a r l i e r w o r k e r s , a n d

o u r c a l c u l a t i o n s , su g g e s t th a t s m a l ll e v e l s o f i m p u r i t i e s i n h e a t s o f s t a i n l e s ss t e e l c a n i n f l u e n c e t h e b e h a v i o r a n dc h a r a c t e r i s t i c s o f t h e G T a r c , a n d s op o s s i b l y a f f e c t t h e w / d r a t i o o fw e l d s .

O u r r e c e n t e x p e r i m e n t a l r e s u l t s d on o t s u p p o r t t h i s l a t t e r h y p o t h e s i s . I t i ss u s p e c t e d t h a t t h e p r o c e s s e s d e t e r m i n i n g w e l d a b i l i t y a r e c o m p l e x a n dm a y i n v o l v e in t e r a c t i o n s b e t w e e n t h ee f f e c t o n t h e ar c o f m a t e r i a l o f l o wi o n i z a t i o n p o t e n t i a l , s u c h as a l u m i n u m , s la g s o r s c u m o n t h e w e l d p o o ls u r f a c e a n d e l e c t r o m a g n e t i c s t i r r i n ge f f e c ts w i t h i n t h e p o o l .

Acfcnow/edgmentT h i s w o r k w a s c a r r i e d o u t at M a r c h -

w o o d E n g i n e e r i n g L a b o r a t o r i e s a n d isp u b l i s h e d b y p e r m i s s i o n o f t h e C e n t r a lE l e c t r i c i t y G e n e r a t i n g B o a r d .

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" M o t i o n i n t h e W e l d Po o l i n A r c W e l d i n g , "Welding lournal, 50 (4), Apr i l 1971 , Res.Supp l . , p p . 163-s to 173-s.