1- A Study Concerning Intercritical HAZ Microstructure and Toughness in

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W E L D I N G R E S E A R C HSUPPLEMENT TO THE WELDING JOURNAL, DECEMBER 1991Sponsored by the American Welding Society and the Welding Research Council

All papers published in the WeldingJourna/ s Welding Research Supplement undergo Peer Review beforepublicat ion for: 1) or iginality of the contr ibut ion; 2) technical value to the welding community; 3) pr iorpublicat ion of the mater ial being reviewed; 4) proper credit to others working in the same area; and 5)just if icat ion of the conclusions, based on the work performed.The names of the more than 170 individuals serving on the AWS Peer Review Panel are publishedperiodically. All are experts in specific technical areas, and all are volunteers in the program.

A Study Concern ing In tercr i t ica l HAZMicrostructure and Toughness in HSLA Steels

Two steels, although similar in composition, proved to havemarked differences in intercritical HAZ toughnessB Y D . P. F A I R C H I L D , N . V . B A N G A R U , J. Y . K O O , P. L. H A R R I S O N A N D A . O Z E K C I N

mar tens i t e -aus ten i t e (M-A) c o n

(ICHAZ) and then t e r c r i t i c a lly reheated coarse-gra inAZ ( IRCG) . Prev ious work has shownn the I R C G ; however , in t he p resen t ICHAZ

sing the Charpy and crack t ip opening

sed to study base metal and ICHAZ miOne of the steels suf fered severe

D. P . FAIRCH ILD is with the Department ofWelding Engineering, The Ohio StateUni-versity, Columbus, Ohio. N. V. BANGARUis with The Ferrous Wheel Group, Annan-ale,N.J.J.Y. KOO is with PohangIronteelC o., Ltd.,PohangCity,South Korea. P.L. HARRISON is with Brit ish Steel Corp.,with ExxonResearchand Engineering Co.,

measured by the CTOD test . I t was dete rm ined by TEM tha t t he on ly s ign i f i cant low- toughness feature in the ICH AZwas the presence of M-A is lands. Theformat ion of the M-A was believed to becaused by a h igh amo unt o f van ad i uma n d s i l i c o n i n s o l i d s o lu t i o n , wh i c h i n creased hardenabi l i ty .The Charpy da ta showed no d i f f e r ence be tween the ICHAZ toughnessesof the steels; whereas, the CTOD resultsshow ed a d is t inc t d i f f e renc e . Cha rpytest ing may be insensi t ive when the m i -

K E Y W O R D SHSLA SteelsM ic r o a l l o y i n gIntercr i t ica l HAZHAZ Trans fo rmat ionHardenab i l i t yM-A Const i tuentElect ron MicroscopyHAZ ToughnessCTOD Test ingCharpy Test ing

crostructure var ies over small distances,as is the case for weld HAZs.I n t r o d u c t io n

Prev ious work has shown tha t lowf racture toughness can ex is t in thecoarse-gra ined HAZ (CGHAZ) o f someh igh-s t reng th low-a l loy (HSLA) s tee ls(Refs. 1-6). The low toughness is causedby cer ta in microst ructural features, onebe ing ide n t i f ied as is lands o f h igh-car b o n , mar tens i t e -aus ten i t e (M-A) c o n st i tuent in which the mar tensi te has atwin ned subst ructure. The M-A is locatedin a re la t ive ly co n t inu ous pa th a longpr ior austeni te gra in boundar ies (Refs.2, 7). The M-A is pr imarily a result of intercr i t ica l reheat ing, which is producedby a subseq uent we ld pass. Spe cif ically ,the M-A is located in a subarea of theCG HA Z c a l l e d t h e i n t e r c r i t i c a l l y r e hea ted coarse-gra ined HAZ ( IRCG)(Ref .1) . Var ious HAZ regions are ident i f ied in Fig. 1.

Whi le t he in te rc r i t i ca l t herma l cyc lecan produce M-A in the IRCG, it is alsopossible that M-A islands can be createdin t he in te rc r i t i ca l H AZ ( IC HAZ ) i t se l f.In mult ipass welds, these are the ICHAZareas that are unaffected by subsequent

W E L D I N G R E S EA R CH S U P PL E M E N T I 321-s


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HM

1 : U n a l t e r e d c o a r s e - g r a i n HAZ (C GH AZ )2 : F i n e - g r a i n H AZ (F GH A Z)3 : I n t e r c r i t i c a l HAZ ( IC HA Z)4 : S u b c r i t i c a l HAZ ( SC HA Z)5 : I n t e r c r i t i c a l l y r e h e a t e d CGHAZ ( IR C G)6 : S u b c r i t i c a l l y r e h e a t e d CGHAZ (S RC G)

SB W e l d M e t a lHAZ in weld metal

Note: The thermal profiles of previous HAZ s are drawnonly up to the FGHAZ of subsequent HAZ s to indicatethat point at which re austenitization has occurred.The SCHAZ boundary is shown as a dashed line becausethe SCHAZ in the base metal does not etch.

Fig.1 Various HAZregionsin a m ultipass weld.

we ld passes . The ICH AZ fo rms d ur in gpeak tempera tu re hea t ing be tween approx imately 727C (1341F) and 850C(1562F) Fig. 2. The low- temp eratureboundary o f t he ICHAZ is t he Ac l 7 andthe h igh- tempera tu re boundary is t heA c 3 .Uchinoand Ohno (Ref. 8) report f ind in g low-ICHAZ toughness in expe r imen tal heats of HSLA steels. They producedICHAZ microst ructures by thermal s imu la t ion , and measured toughness us ingthe Charpy test. The low toughness wascaused by M-A is lands, and the volumefract ion of the M-A increased with va na

d ium con ten t .Th is paper doc ume nts a s tudy c o n cern ing ICHAZ toughness in two com

merc ia l HSLA steels . Whi le the steelshave subt le d i f ferences in composi t ion,they have marked di f ferences in ICHAZtoughness . Mu l t ipass we lds were p ro duced using the submerged arc process.Charpy and c rack - t ip open ing d isp lacement (CTOD) tes t s were conduc ted toassess toughne ss. Base metal and ICH AZmic ros t ruc tu res were s tud ied us ing opt ica l ,scan ning elec tron (SEM), and transmiss ion e lect ron microscopy (TEM).E x p e r im e n t a l P r o c e d u r e sMaterials

Tab le 1 l i s t s t he chemica l compos i t ions of the mater ia ls used. Both steels

Carbon (wt.%)

Fig. 2HAZregions superimposedon the iron-carbonphasediagram.

were 50 mm (2 in . ) t h ick , norma l ized ,ca lc ium- t rea ted and comm erc ia l l y m a n ufac tu red to meet BS 43 60 G rade 50Especi f icat ions. The tota l microal loy con tent for both steels was 0.08 wt-%. SteelA c o n t a in e d 0 . 0 6 w t - % v a n a d iu m a n d0.019 wt -% n iob ium, wh i le s tee l Bc o n t a i n e d 0 . 0 8 w t - % v a n a d iu m . M e c h a n i cal property data for these mater ials areg iven in Tab le 2 . The Charpy tes t p ro cedures used for the base metal wereconsistent with those used for the ICHAZand are descr ibed below.Welding

To produce specimens for toughnesstest ing, a single-V edge preparat ion wasused. The ICHAZ along the square edgeof the single-V preparat ion was the focusfor toughness test ing. Figure 3 schematical ly shows the weld geometry and theICHAZ reg ions a re dep ic ted in so l idb la c k . Th e I CHAZs f o r m a s o m e wh a tcont inuous path f rom one plate sur faceto the other. I t was desired that the l ineo f ICHAZ areas a long the square edgebe as st ra ight as possib le and perpendicular to the p late sur face. These features a l low adequate fat igue crack sampl ing of the desired microst ructure wh enthe through- th ickness notch or ientat ionis used for C TO D test ing (Ref.9). In orde rto accompl ish t he des i red ICH AZ geo me t r y , a spec ia l we ld p roced ure was deve lop ed a nd is de ta i le d in F ig . 4 . Theheat input for weld ing was 3 kj/mm (75kj/ in.) .To prov ide specimens for metal lurg ic a l e v a lu a t i o n , s i n g le - p a s s b e a d - i n -g roove we lds were made us ing parameters ident ical to those l isted in Fig. 4.

Toughness TestingCharpy and CTOD tes t s were c o n ducted with the notch ( fat igue crack forCTOD spec imens) loca ted in t heth rough- th ickness o r ien ta t ion as shownschemat ical ly in Fig. 5. I t was intendedthat the root of the notch be posi t ionedd i r e c t l y i n t h e I CHA Z . A p p r o x im a t e l yten Charpy tes t s were conduc ted fo r

each Steel at var ious temperatures to determine the t ransi t ion curve. The valuesof the 35 J (26 f t - lb) and 50 J(37 f t - lb)t ransi t ion temperatures are l is ted inTable 3. The 35-J or 50-J transit ion t e m perature is the temperature cor responding to the point on the t ransi t ion curvewh ose energy valu e is 35 J or 50 J, respect ively .Three CTOD tes t s were conduc tedfor the ICHAZ of each steel And the resul ts are l is ted in Table 3. The CTODtes ts were conduc ted accord ing to BS5 7 6 2 . The B X 2B geom et ry was used ,

the c rack dep th was 50% o f t he spec i men thickness, and the test temperature

3 2 2 - s I D E C E M B E R 1 9 9 1


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1Chemical Composit ion (wt-%)

A 0.14B 0.10 0.0040.011 0.012 0.240.015 0.46

M n1.431.42

N i0.240.28

Cr0.040.05

M oNDN D

Cu0.010.20 0.060.08

N b0.019ND


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-*8Sih mryt-KK- 'I - ^ \ v.--'

f/'g. 9 T E M micrographs of th e ICHAZferrite inSteelsA left)a nd B (right).

st i tuent of the second-phase was ident i f ied as mos t ly upper b a in i t e ; how eve r ,a sma l l amo un t of M-A is a lso present .The upper b ain i te is show n in Fig. 10B.Figure 10 shows that wi th in the pear l i te ,the fer r i te phase between the cement i tep la tes is re la t ive ly d is loca t ion f ree ;whereas , in t he ba in i t e , t he d is lo ca t iondensi ty in the fer r i te is s igni f icant . Thepresence o f t h is d is loca t ion dens i t y in d ica tes tha t t he ba in i t e t rans fo rmat ionhas a shear component and that the ferr ite has been strained.Figure 11 s h o ws a TEM m ic r o g r a p h

of the second phase in the ICHAZ ofSteel B. The microstructure is mainly M-A const ituent. The martensite is the predominant phase of the M-A const i tuentc o m p r i s i n g a b o u t 9 6 t o 9 7 v o l - % . TheM-A is lands are on the order of to 0mic ro ns in s ize . The mar tens i t e possesses a tw in ne d su bst ruc ture and th is

fea tu re is ind ica t ive o f a h igh-carbonconten t ; > 0 .5% (Ref . 11) . This type ofM-A con s t i t uen t i s kn ow n to be qu i t ebr i t t le . On ly a m inor f rac t ion o f t heICHAZ m ic ros t ruc tu re in t h is s tee l wasident if ied as pearlite.D i s c u s s i o n

T h e d i f f e r e n c e i n f r a c t u r e t o u g h ness between the two steels is causedb y t h e d i f f e r e n t I C H A Z m i c r o s t r u c tu res . The ma jo r d i f f e rence in t he m i c ros t ruc tu res was in t he na tu re o f t hes e c o n d p h a s e : S t e el A b e in g p r im a r i l y pear l i t i c a nd Stee l B be in g p r ima r i l y M - A c o n s t i t u e n t . To u n d e r s t a n dh o w th e s e d i f f e r e n c e s i n s e c o n d -p h a s e m ic r o s t r u c t u r e a r i s e , t h e t im e -t e m p e r a t u r e - t r a n s f o r m a t i o n p h e n o m e n a o c c u r r i n g i n t h e I CHA Z m u s t b eu n d e r s t o o d .

Intercrit ical HAZ FormationW i th a base meta l m ic ro s t ruc tu recomposed p r imar i l y o f f e r r i t e -pear l i t e ,t ransformat ion to austeni te begins whenthe heat ing por t ion of the HAZ thermalcyc le reaches the eutecto id temperature(727C/ 341 F). The first areas to transform to austeni te are the regions ofpear l i te . However , in re lat ive ly low-car bon steels , such as used in th is s tudy,the vo lume f rac t ion o f pear l i t e is lowand is consu med ear ly in the in te rc r i t i cal range. As the temperature increases,

other nucleat ion s i tes for the austeni teappear , mos t no tab ly a t f e r r i t e - f e r r i t egrain boundar ies. Because HAZ thermalcyc les a re rap id , t he tempera tu re mayonly be in the intercr it ical range for threeto s ix seconds as shown in Fig. 12 (der ived f rom Ref . 2). For a heat input of3 kj/mm (75 kj/ in.) and a peak tempera-

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t u re o f about 780C (1436F), a largenumber of austenite nucleat ion sites wil ldeve lop , bu t very l i t t le g rowth occursbecause coo l ing beg ins qu ick ly . Theausteni te is , therefo re, l im i ted to sm al lislands at pr ior pearlite sites and ferr ite-ferr ite grain boundaries.Upon coo l ing , t he aus ten i t e is landsd e c o m p o s e a n d b e c o m e t h e s e c o n d -phase of the ICHAZ. The pr imary phase,the fer r i te , is that of the or ig inal basem e t a l . The fac to rs t ha t de te rm ine thest ructure that forms f rom the austeni teare the ICHAZ coo l ing ra te and theaus ten i t e hardenab i l i t y . I n t he p resen tp r o g r a m , both steels exper ienced ident i c a l we ld in g c o n d i t i o n s ; i.e., the sameICH AZ therma l cyc le. Therefore, thedifference in second-phase microst ructurebetween Steels A and B is only a factoro f aus ten i t e is land hardenab i l i t y . Because the second phase of Steel B wasthat of a lower temperature t ransformat ion product , i t is bel ieved that the

austenite islands possessed higher hardenabil i ty than for Steel A.The dif ference in the hardenabil i ty ofthe austeni te is lands (and thus theICH AZ second phase) is caused by theconcent ra t ion o f m ic roa l loy ing and a l loy ing e lements in so l id so lu t ion . Themost no tab le d i f f e rences be tween thetwo steels with respect to elements thathave a s igni f icant ef fect on hardenabi l ity are the V, Si, and C content.Vanadium. Ord inar i l y , m ic roa l loy ingelements do not have a signif icant ef fecton hardenabi l i ty because they are t iedup in the form of stable precipitates. Thisis the case for Steel A where Nb and Vare largely in the form of carboni t r ides.For Steel B, however , few prec ip i tateswere observed in both the base metaland the ICHAZ, and this is evidence thatmuch of the V is in sol id solut ion. Thehardenabi l i ty of s teels is great ly enh a n c e d wh e n t h e m ic r o a l l o y i n g e le ments (especia l ly V) are in sol id so lu t ion (Ref. 13). It is interesting that Ref. 8repor ts that the volume f ract ion of M-Aincreases in s imulated ICHAZs with increasing V content.

The reason for the d i f ference in thes o l i d s o lu t i o n m ic r o a l l o y c o n t e n t b e tween the two steels is not prec iselyk n o w n , bu t some exp lana t ions can beo f fe red . Bo th s tee ls we re no rma l ize d ,and they con ta ined approx imate ly t hesame tota l microal loy content . The so l ubi l i ty of Nb(C,N) in austeni te is muchlower t han tha t o f V(C,N) . Thus , t hemix ed N b,V (C,N ) in Steel A may haveproduced re la t ive ly inso lub le carbon i t r ides , while in Steel B, the pure V(C,N)may have read i ly d isso lved dur ing thenorma l iz ing t rea tment . I t a lso appearsl ike ly that Steel B exper ienced a fastercoo l ing rate dur in g i ts norma l iz ing t reatm e n t , wh i c h wo u ld h i n d e r t h e f o r m a -

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. :

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LSr ' *' *

t ion of microal loy prec ip i tates.The speci f ic cause of the microal loydis t r ibu t ion in Steel B is not , h ow eve r ,the focus of at tent ion. The important factis to re alize that the ICHAZ of steel weldsc a n d e v e lo p u n d e s i r a b le m ic r o s t r u c tures as a result of the original base metalc o n d i t i o n ( wh i c h is c o n t r o l l e d b y t h es tee l manufac tu r ing techn ique) . TheICHAZ thermal cyc le, unl ike that of theCGHAZ, is not of suf f ic ient in tensi ty toe l im ina te a l l t r aces o f base meta l p ro cess ing . I CHAZ f r a c t u r e p e r f o r m a n c ecan be signif icant ly af fected by the steelmanufac tu r ing techn ique .

Silicon. The Si co nte nt of Steel B ishigher than that for Steel A (0.46 vs. 0.26wt -%) and is be l ieved to con t r ibu te t othe d i f ference in austeni te is land ha rd enabi l i ty . I t has been shown (Ref . 13)that Si content in the range of 0.5 wt-%,in the presence of a s ign i f icant am oun t

Fig.11 TEMmicrographof theICHAZsecond phase inSteelB showing twinnedmartensite arrow)andM-A constituent.of V (about 0 .08 wt -%) , p romotes theformat ion of twinned mar tensi te and M-A cons t i t uen t dur ing in te rc r i t i ca l hea t ing.Carbon. Steel A has a high er ca rbo ncontent than Steel B (0 .14% vs .0.10 )and this dif ference is contrary to the theory o f h igher aus ten i t e is land hardenab il i ty in Steel B. W ith respect to C con tent , the chemical analys is resul ts maybe s l ight ly mis lea ding. Some of the C inSteel A is t ied up in the form of microalloy prec ip i tates and does not af fectaustenite island hardenabil i ty. For SteelB, few precipitates exist ; thus, the C (asis the V) is available to increase hardenabi l i ty . Even cons ider ing th is C t ie-upeffect, i t is l ikely that Steel A had moreC ava i lab le t o in f luenc e h arde nab i l i t ythan di d Steel B. The obse rved m ic ros t ruc tu re shows, however , t ha t t hedif ference in C was overcome by the ef-

88C D

a8

20 30 40 50 60 70 80T im e [ s e c o n d s ]

Fig.12 ICH AZ thermal cycles with the time abovetheAcl highlighted.

90 100

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feet of V and Si to the extent that thelower C steel displayed higher austeniteis land hardenabi l i ty in the ICHAZ.Carbon also p lays an impor tant ro lein de te rm in ing the subs t ruc tu re mor pho logy o f any ICHAZ mar tens i t e t ha tdoes form. By refer r ing to Fig. 2 andusing the lever rule, it can be seen thatdur ing the in te rc r i t i ca l por t ion o f t heICHAZ therma l cyc le , t he aus ten i t e is lands are enr iched in carbon far abovethe level of the base metal . These enr iched is lands then exper ience a re la t i v e l y r a p id c o o l i n g r a t e , wh i c h l im i t scarbon m obi l i ty and leaves the local car bon content h igher than the bulk of themat r ix . Th is enr ichment mechan ism isrespons ib le f o r t he tw inn ing o f t hemartensite observed in Steel B.The Cu and Al content was higher inStee l B and wh i le t hese e lements mayhave had a minor ef fect on austeni te island hardenabi l i ty they are not bel ievedto be pr ima ry factors.The Significance of M -A Islands

Whi le t he po ten t ia l f o r reducedtoughness in s t ructural s teel HAZs wasrealized for decades, only recent ly havethe m ic romechan isms o f f r ac tu re in i t ia t ion in HAZs been studied. A consider ab le amount o f work has concent ra tedon CGHAZ toughness , spec i f i ca l ly t helocal br i t t le zone (LBZ) problem (Refs.1- 3 , 6 , 7 , 14 ) . A c o m p a r i s o n b e t we e nthe causes of low C GH AZ toughness andlow ICHAZ toughness w i l l p rov ide fu r ther insigh t as to the poten cy of M-A islands for reducing toughness. Before thiscompar ison is made,note that the CT ODvalues for Steel B in this study and thosefo r low toughness CGHAZs {i.e. forLBZs) are essent ially the same: CTOD

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1- A Study Concerning Intercritical HAZ Microstructure and Toughness in