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Inf luence of Weld BeadArea on Weld Meta lMechanical Propert ies

B Y B . L. S H U L T Z A N D C . E . J A C K S O N

The yield strength of weld me tals in multipass weldsvaries inversely with the cross-sectional area of eachweld bead

A B S T R A C T . V a r i a t i o n s i n w e l d i n gco n d i t i o n s h a ve b e e n kn o w n t o i n f l u ence the p rope r t ies o f we ld depos i tsb y t h e i r e ff e c t o n th e t r a n s f o r m a t i o na n d so l i d i f i ca t i o n p r o ce sse s fo r w e l dm e t a l . T h e se ch a n g e s o ccu r d u r i n gthe coo l ing po r t ion of the we ld the r ma l cyc le ; thus , it i s imp o r ta n t toknow the coo l ing ra tes o f we ld me ta lin o rde r to adequa te ly des cr ibe i t s re sp o n se to t h e w e l d i n g p r o ce ss .In the pas t , the heat sou rce fo r ca l cu la t ing we ld me ta l coo l ing ra tes hasbeen measu red by the pa ram e te rs o fthe we ld in g a rc ; th i s hea t inpu t hasbeen ca lcu la ted in un i ts o f j ou les o rwa t t -seconds pe r un i t l eng th o f w e l d .S ince va r ious coo l ing ra tes ca n be ob ta ined for a g iven we ld ing e ne rgy in pu t , the ene rgy inpu t conc ep t cann o tadequa te ly p red ic t mechan ica l p rop e r t ies .Recen t l y i t has been p roposed tha tthe ca lo r i f i c hea t con ten t o f the we ldme ta l is a mo re app rop r ia te sou rcef o r d e t e r m i n i n g w e l d m e t a l co o l i n gra tes . The calorif ic hea t con ten t i s de p e n d e n t u p o n t h e vo l u m e o f m o l t e n

B. L. SHULTZ is W elding Engineer, General American Transportation Corporation, Sharon, Pa., and C. E. JACKSON isProfessor, Department of Welding Engineering, The Ohio State University,Columbus, Ohio. Based on paper presented at the A WS National F all Meetingheld in Baltimore, Md., during October 6-9, 1969.

Metric C1 in.1 ipm1 l b / i n . 21 ksi1 f t- lb1 k J / i n .1 in.2

1 mm1 mm/sec1 M N / m 21 joule1 k j /mm1 m m 2

onversion25.4 mm0.423 mm/sec6894.76 N/m 26.895 MN/m 21.35582 joules0.03937 kJ /mm645.16 m m 2

0.03937 in.2.362 ipm0.145 ksi0.737561 f t- lb25.4 k J / i n .0.00155 in .2

we ld me ta l and is p ropo r t iona l to thewe ld me t a l a rea . In th i s inves t iga t io nthe ca lo r i f i c hea t con ten t o f a un i t v o l ume of a s ing le w e ld bead was theb a s i s f o r m e a su r e m e n t . F o r co n ve n ience , such un i t vo lu me has b eent e r m e d "n u g g e t " vo l u m e , a n d i tsc r o ss - se c t i o n a l a r e a "w e l d n u g g e ta rea . " These te rms shou ld no t be c o n f u se d w i t h t h e A W S d e f i n i t i o n s f o rnugge t and nugge t s i ze w h i ch re fe rspec i f i ca l l y to res is tance we ld ing .Th is pape r a t temp ts to p resen t a

w o r k i n g r e l a t i o n sh i p b e t w e e n m u l t i p a ss sh i e l d e d m e t a l - a r c w e l d m e t a lm e ch a n i ca l p r o p e r t i e s a n d t h e w e l d

nugge t a reas fo r h igh s t reng th s tee l .The e f f ec t o f we ld nugge t a rea onwe ld me ta l coo l ing ra tes and the e f f ec ts o f va r ia t ions in we ld ing t e c h n i q u e o n r e su l t a n t w e l d n u g g e t a r e a sa re a lso p res en te d . A un iqu e coo l in gra te has been ob ta ined fo r each comb i n a t i o n of p r e h e a t a n d w e l d n u g g e ta rea . W e ld m e ta l coo l ing ra tes andwe ld ing ene rgy inpu ts a re a lso in ves t iga ted fo r poss ib le re la t ions h ipst o m e ch a n i ca l p r o p e r t i e s .I n t r o d u c t i o nThe p rope r t ies o f many we ld de pos i ts have been known to be s ign i f i can t l y in f l uenced by the a rc we ld i ngt e ch n i q u e u se d d u r i n g t h e d e p o s i t i o no f the we ld m e ta l . Va r ia t ion s in w e l d i ng tech n iqu e a f f ec t the d i l u t ion ,s o l i d if i c a ti o n a n d t r a n s f o r m a t i o np r o ce sse s f o r w e l d m e t a l . T h e sep rocesses a re qu i te comp lex and a rei n f l u e n ce d by t h e r a p i d t h e r m a lch a n g e s a sso c i a t e d w i t h w e l d i n g .The a rea in the v ic in i t y o f a we ld inga rc i s sub jec te d to a com p lex the rm a lcyc l e i n w h i c h t e m p e r a t u r e s f r o m t h ein i t i a l p la te tempe ra tu re to above theme l t ing po in t o f the a l loy a re in vo lved . A f t e r t h e w e l d m e t a l h a s b e e nm e l t e d , not on ly does the ra te o f heatt rans fe r to the su r rou nd ing s de te r m ine the ra te o f so l id i f i ca t ion , bu ta lso the na tu r e and pa t te rn o f he a tf low du r ing so l id i f i ca t ion have ma jo re f fec ts in the m ic ro s t ru c tu r e an dp rope r t ies o f the w e l d . T h e t h e r m a lcyc le o f the we ld hea t -a f f ec te d zon e

2 6 - s I J A N U A R Y 1 9 7 3

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i s d e t e r m i n e d b y t h e h e a t i n g a n d c o o l i ng ra tes o f the we ld me ta l . The e f f ec ts of thes e rap id the rm a l cyc les ont h e s t ru c t u r e a n d su b se q u e n t m e ch a n i ca l p r o p e r t i e s a r e o f p r i m a r yco n ce r n i n w e l d i n g .T h e co n d u c t i o n o f h e a t t h r o u g h t h ep l a t e a n d t h e r e su l t i n g w e l d m e n tp r o p e r t i e s a re d e t e r m i n e d b y t h e r e sponse o f the me ta l to the hea t in t ro d u ce d d u r i n g w e l d i n g . T h u s , i t is i m p o r t a n t t o kn o w t h e e f f e c t i ve t h e r m a lcyc l e p r o d u ce d b y t h e w e l d i n g t e c h n i q u e , in o rde r to adequa te ly re la tet h e r e sp o n se t o t h e w e l d i n g p r o ce ss .

Fo r yea rs , inves t iga to rs have s t u d i e d t h e r a p i d t h e r m a l cyc l e s a sso c i a t e d w i t h w e l d i n g , b o t h m a t h e m a t i ca l l y a n d e m p i r i c a l l y . 1 - 6 T h eb a s ic r e l a t i o n sh i p w h i c h h a s b e e n d e r i ved fo r the coo l ing po r t ion o f t hethe rm a l cyc le in the v ic in i t y o f th ew e l d ha s t h e f o l l o w i n g g e n e r a l f o r m .The coo l ing ra te in an a rc we ld on ave ry th i ck p la te i s g iven by :

Z = e f f i c ienc y fac to r f o r he a t t ra ns fe rto t h e b a se m e t a l .The hea t sou rce fo r c a lcu la t ingwe ld me ta l coo l ing ra tes has typ ica l l yb e e n co n s i d e r e d t o b e t h e w e l d i n garc, and i ts energy, H, is usua l ly c a l cu la ted in ene rg y un i ts pe r li nea r incho f w e l d , a s f o l l o w s :H ( jou les per inch) =

V (vo l tage) * A (cu r ren t ) * 6 0S ( t r a ve l , i n . / m i n )

or: H ( jou les pe r m i l l ime te r )V (vo l tage) * A (cu r ren t )

S ( t r av e l , m m / s ) (2 )

d T _d 0

dT

2 T T K (T - T0 )2

ZH (D

Not a l l o f the ene rgy g ene r a ted bythe we ld ing a rc en te rs the base p la te ,thus the e f f ic iency factor , i .e . , Z in eq(1), i s used to accoun t f o r the va r iou shea t losses . The va lue fo r the e f f i c iency fac to r i s t yp ica l l y ass um ed tor e m a i n co n s t a n t fo r va r i o u s w e l d i n gtechn iques . Thus , i t has been a s su m e d t h a t w e l d m e t a l co o l i n g r a t e s( a n d su b se q u e n t m e ch a n i ca l p r o p e r t ies ) wo u ld be p ropo r t io na l to w e l d ing ene rgy inpu t , as f o l l ows:

w h e r e = co o l i n g rate on the c e n t e r -l ine o f the w e l d ; K = t h e r m a l co n d u c t i v i t y o f t h e m e t a l ; T = t e m p e r a t u r e a twh ic h the coo l ing ra te i s to be d e te r m i n e d ; T0 = i n i t i a l u n i f o r m t e m p e r a tu re o f the p la te ; H = a rc ene rgy inpu t ;

dT S ( t rav e ldO V (vo l tage ) * A (cu r ren t )

(3 )

In th i s exp res s ion the coo l ing ra te i sd i rec t l y re la ted to t rave l speed a ndinve rse ly re la ted to cu r ren t and a rc

T a b l e 1Typical 5 N i - C r - M o - V P l a t e a n d W e l d D e p o s i t C h e m i c a lC o m p o s i ti o n s , % a n d M e c h a n i c a l P r o p e r t ie s

I t emCN iCrM oSiM nVT iPSAlCuY.S. , 0 .2%

ks iM N / m 2

T.S. , ksiM N / m 2

E l o n g a t i o nR e d u c t i o n

o f f se t :

, %o f a rea , %

C h a r p y V - n o t c h a t0 F( -17 .8C) :f t - lbj o u l e s

P la te5 N i - C r - M o - V

0 1 24 . 9 10 . 5 60 . 4 80 . 3 20 . 8 40.0700.0030.0050 . 0 2 11 4 19 7 21 51

1 0 4 12 0 . 05 8 . 4

9 71 3 2

W e l d deposit.5 / 3 2 i n . d i a m

E-140180 . 0 92 . 7 40 . 5 90 . 4 40 . 4 71.770.0200 . 0 3 10.0060 . 0 1 00 . 0 10 . 0 2

14 49 9 215 2

104815.85 8 . 0

4 66 2

Deposited W eld Metal

BaseMetal MeltedI

HeatAffected ZoneI /Fig. 1 The cross-sectional area of abead weld (shaded area) is the weld nugget area

vo l t a g e . V a r i o u s d e g r e e s o f su cce sshave been repo r ted by inves t iga to rsus ing th is equa t ion fo r p r ed ic t ingcoo l ing ra tes and subs eque n t m e ch a n i ca l p r o p e r t i e s i n w e l d i n g .One ex tens ive inves t iga t ion o f c o o l i ng ra tes in we ld m e ta l rece n t l y pu b l i shed by Do rshu 3 s h o w e d t h a t w e l dme ta l coo l ing ra te is app ro x im a te ly

l i nea r l y re la ted to t rave l speed ,i n ve r se l y r e l a t e d t o w e l d i n g cu r r e n t ,bu t p rac t i ca l l y una f fec ted by changesin the a rc p lasma leng th (a rc vo l tage ) .M y e r s 4 repo r ts tha t the ove ra l l resu l tso f using equat ions, such as eq (1) , fo rp red ic t ing coo l ing ra tes in we ld ing i stha t they a re inadequa te fo r f as tt rave l speeds and on ly r easo nab lysu cce ss f u l a t m o d e r a t e t r a ve l sp e e d s .I n r e v i e w i n g t h e b a s ic a ssu m p t i o n snecessa ry f o r the de r i va t ion o f c o o l i ng ra te exp ress ion s , one o f the ma jo rfac to rs f o r e r ro r i s p rec ise ly ind ica t ing the e f f ec t i ve en e rgy o f the h ea tsou rce . The ene rgy o f the hea t sou rc eis t yp ica l l y cons ide red to be the w e l d ing ene rgy inpu t mod i f i ed by a c o n s tan t e f f i c iency fac to r . The exac tr e l a t i o n sh i p b e t w e e n w e l d i n g t e c h n ique and e f f i c iency o f ene rgy u t i l i za t i o n h a s n o t b e e n d e t e r m i n e d . H o w ever, i t has been po in ted out tha te f f i c iency depends upon the spec i f i cw e l d i n g t e ch n i q u e u se d f o r d e p o s i t i ng we ld me ta l . Th is v ie w has b eenestab l i she d by a qua l i ta t i ve s tudy bvJ a c k s o n et al.7 f o r the suomerged a rca n d sh i e l d e d m e t a l - a r c w e l d i n g

p rocess . The ene rgy u t i l i zed inf o r m i n g t h e w e l d m e t a l w a s sh o w n t orange f rom 2 0 to ove r 5 0% o f the to ta lene rgy o f the a rc sou rce . Thus , i t wasfoun d tha t the to ta l ene rgy ge ne ra tedby the we ld ing a rc , modified by a c o n s tan t e f f i c iency fac to r , i s no t necessa r i l y an accu ra te measu re o f thehea t sou rce fo r ca lcu la t ing coo l ingra tes . Th is i s because the e f f i c ienc yo f ene rgy util ization i s no t cons tan tove r a w id e range o f we ld in g co nd i t i o n s .One meas u re o f the e f f ec t i ve hea tsou rce fo r we ld me ta l coo l ing ra tes ,

wh ich has been p roposed by J a c k son , 8 i s the ca lo r i f i c hea t con te n t o ft h e m o l t e n w e l d m e t a l . T h e h e a t c o n -W E L D I N G R E S E A R C H S U P P L E M E N T ! 2 7 - s

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9 0 0

8 0 0

7 0 0

6 0 0

5 500 -XEZ5. 1 4 0 0XD

50

< 300

2 0 0

100

Nugget Area (mm 2 )100 150 200 2 5 0 3 0 0 3 5 01

oS L A\

-> ,V

V_ MoV J

1

oo

1

K E Y :A C 0 2 S o l i d WireV S ub m e r g e d A r cD S t i c k E l e c t r o d e TigO C0 2 Cored WireA E l e c t r o n - B e a m Mig

AV AO

1o1

-

67

- 6 0

5 0

4 0

3 0

20

orr

0.2 0 3 0 4Nugge t A rea (i n2) 0.5 06Fig. 2 The relation of nugget area to maximum hardnessin the heat-affected zone for an AISI 1045 carbo n steel 9

1,000

9 0 0

8 0 0

* 700

Input-Kilojoule Per L inear Mi l l im eter1 2 3 4 5

6 0 0 5 0 0E

ES 400 -N