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<BB Welding In the WorldJl soudige dens le Monde Vol 36 pp 125-I 12 1995

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Printed m Great Britain All rights rcs<rvtd/)mpnmc en Grande Bretagne__

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CYcJJ ,. C1..·&dquo; Leif Karlsson

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Esab AB, Box Leif Karlsson G6teborg, SwedenD t r i ) Esab AB, Box 8004, S-402 77 G6teborg, Sweden

, ‘ v ca 1 _

(j ABSTRACT -

Dissimilar metal welding involving stainless steel base or filler metal is reviewed emphasizingpossibilities and limitations imposed by base and filler metal metallurgy. Different aspects of fillermetal selection and the ability of filler metals to accept dilution without risk of cracking areconsidered. Further, the influence of the, often hard, unmixed zone along the fusion boundary isdiscussed and service performance of dissimilar metal welds is presented. Some importantapplications are reviewed in more detail and examples of unexpected problems are included.Joining of stainless and non-ferrous metals and solid state joining are discussed briefly. Theimportance of proper design and the choice of a welding procedure suitable to the base and fillermetals involved is stressed.

INTRODUCTION

Dissimilar metal welds are common in welded constructions and their service performance is oftencrucial to the function. Whether dissimilar metal welds should be seen as stimulating challengesand considered to be a key factor in creative design or whether they should be avoided wheneverpossible is a matter of opinion. However, it is a fact that dissimilar welds are used, usually success-fully, in an increasing number of different applications (1-4).

Dissimilar metal welding involves the joining of two or more different pure metals or alloys,usually by melting and mixing and often with the addition of a filler metal. There are several typesof dissimilar metal welds including stainless steel either as base metal or as filler metal. Someimportant examples are: joining stainless steels to steels or other metals, cladding, welding of com-pound materials and welding stainless steels with Ni-base fillers. However, despite the apparentdifferen-ces these examples all fit into either of the two basic types of dissimilar welds: t) joining oftwo different metals, usually with the addition of a different filler metal (i.e., A to B with or withoutC) and 2) joining matching composition metals with a different filler metal (i.e., A to A with C) (4).

In principle an unlimited number of weld metal compositions can be obtained in dissimilar metalwelding, depending on the combination of base and filler metals, the welding process and theprocedure. It is therefore not surprising that the most commonly asked question about welding ofdissimilar metals is that of filler metal selection. Not only must the welding consumable be capableof accepting dilution from the base metals without cracking or forming deleterious phases, but the

I weld metal must also have sufficient strength, ductility and corrosion resistance for the intendedservice. Consequently, the desired properties of the final weld and the welding procedure must be

; considered already at the design stage (1).,

125

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The aim of the present paper is to briefly review dissimilar metal welding involving stainless steeleither as base or filler metal The limited space does not permit an in depth discussion of all aspectsand the selection will therefore inevitably be somewhat subjective

-

WELDING STAINLESS STEELS TO UNALLOYED OR LOW ALLOY STEEL

Dissimilar metal welds between stainless and non-stainless steels is undoubtedly the most common and most important example of dissimilar metal welds. In particular, joining of austenitic stainless;. steels and unalloyed or low alloy steels (hereafter referred to as ferritic/austenitic joints) for attach- ments or transitions are frequently occurring. The same basic metallurgical considerations apply also to cladding unalloyed or low alloy steels with an austenitic stainless weld metal, welding of

-

austenitic stainless steel/unalloyed or low alloy steel compound material and when using austenitic stainless steel fillers to make repairs in hard-to-weld ferritic steels. These cases are therefore

M, discussed separately only to point out specific details.

Ferritic/austenitic joints ,K Ferritic/austenitic dissimilar welds intended for use below approximately 350-400°C are usually M welded with austenitic fillers whereas Ni-base fillers are preferred for higher service temperatures !

m. (1, 2). The main concern, during welding, is in both cases to avoid cracking in the weld metal, in I

B base metal HAZ and in the narrow unmixed zone (UMZ) or partially mixed zone at the weld inter- !jmw face where complex, often hard, microstructures develop (1, 5-8). Cracking can be either hydrogen ; assisted cracking or hot cracking depending on base and filler metal and on the welding procedure. iB Weld metal considerations - Stainless filler metal I

Mg A stainless steel filler metal, with a total alloy content high enough to prevent the formation of}t martensite in the weld metal after dilution by the base metal, should be used to avoid hydrogen !

cracking in the weld metal. The most widely used filler metals are probably the 23Cr 12NI andJ 22Cr 12Ni 3Mo types. Other commonly used types are 29Cr 9Ni and 18-20Cr 9- lONi 3Mo. In particular 22Cr 12Ni 3Mo fillers provide adequate dilution tolerance to avoid hydrogen cracking inB. most cases. This type has the additional advantage of producing a weld metal highly resistant to hot cracking, provided dilution is controlled to give a minimum amount of ferrite of about 3 FN. TheM 29Cr 9Ni type has greater capacity for dilution but the ferrite level in undiluted weld metal is high (often >40%) which can promote embrittlement. On the other hand the 18-20Cr 9-lONI 3Mo does

JM not have sufficient tolerance for dilution to avoid cracking in all cases. A good cracking resistancej can also be obtained in cases where a fully austenitic weld deposit is required by using a Mn alloyed type such as 18Cr 8Ni 6Mn (5, 9).

, It has to be stressed that a proper choice

0 6C °.5Si ’ 1.2Mn of filler metal, such as a 22Cr 12Ni, &dquo;

K

0 IS, IlMn

3Mo, is not sufficient to guarantee a, ,

, , _ , crack free weld (see Fig. 1). Dilution

. : ::, , _, ,

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. l control and the possibility to predict’

. ’_ ;.?j&dquo;9 ’ ’ ’ &dquo; ’,&dquo;. ’, , ’

’ ’ weld metal composition is vital in. , .’ l))FylJ fl&dquo;.,,

&dquo;

J,Ju .i, ’ . u’ . choosing filler material and designing a. flt,j§, l4;:<,:&dquo; ,-... ° , . welding procedure. A sufficiently good’ °&dquo;’ &dquo;’/ < ,ø . &dquo; ??’ ’ ’ ? ’ ’ prediction of weld metal composition’NV#*’;&dquo;7-.á;,l’’f<;>;’’ and microstructure can often be obtained. ’:’,&dquo;,’..t:. .::::. &dquo;’ by using the Schaeffler diagram and, &dquo;:&#x178;1?’’;Ji<:::1¡! .:

¡. using the standard rule of thumb base. &dquo;:::.{’?l’¡&dquo;’/’o-> metal dilution levels for the different,

7 welding methods (typically MMA:25-40%, MIG and FCAW: 15-40%,TIG: 25-100% and SAW: 20-50%).’ ’ ?’’ ?&dquo; The DeLong and more recent WRC-88

M&dquo; or WRC-92 diagrams give more accurate Figure I Cracking in SAW weld between unalloyed and ferrite predictions but indicate the nsk of3t stainless steel due to excessive dilution of the martensite formation only indirectly 22Cr 12Ni 3Mo filler from the unalloyed side. (9,10)

I 17!

_

J In principle the same rules apply to welding of stainless clad materials as to any femtic/austeniticdissimilar weld Normally one of two methods is applied (9). The unclad side of the plate is

bevelled and welded with a femtic filler suitable to the non-stainless steel. A portion of thestainless steel cladding is removed from the back of the joint and a suitable stainless filler is used to

I weld the stainless side. The other alternative is to weld the entire thickness of the compoundI material with stainless steel (or Ni-base) filler metal The second method is applicable to weldingI from both sides but is less economical when the non-stainless portion of the material is compara-

I tively thick. This poses a problem in situations when the welding has to be performed from the non-

stainless side. A possibility, although.. not to be recommended as a first

’

.,a reg’°n choice, is to first weld the stainlesslayer with a suitable stainless filler; the

‘ transition to the ferritic side is thencarefully welded with a special low-hydrogen filler with a very low carbon

:: ’ content (e.g. Esab OK 55.18, 0.02C). -, using a moderate preheat and a weld-

_ ,,

Ing procedure minimizing dilution witht&dquo; ’‘ I

the stainless side (Fig. 2). Finally the’

l

nu joint is filled with a ferritic fillerA

;’.t’.. -:-- suitable to the non-stainless steel. TheO -

’ ---- transition layer between the stainless

Figure 2 Cross section showing the crack free transition and the non-stainless side will in-region of a weld In a stainless-clad plate welded evitably contain some martensite.only from the non-stainless side. A low- However, this martensite will have ahydrogen low-carbon filler (OK 53.18) was low carbon content and relatively lowused for the transition layer between the hardness and a sound crack free weldstainless (left) and the femtic (right) side. can in most cases be obtained.

Weld metal considerations - Ni-base fillersNi-base fillers such as Ni- 15Cr 2Nb or Ni- 15Cr 8Mn 2Nb have a number of advantages in ferritic/austenitic joints (1, 2). First they are resistant to dilution without the formation of martensite.Second their coefficient of thermal expansion is close to that of unalloyed and low alloy steel whilethe yield strength is relatively low. The effecave restraint applied to the base material is therebyreduced. Third, Ni-base fillers are not prone to sigma formation when postweld heat treatment(PWHT) is needed to improve HAZ mechanical properties. Ni-base fillers are normally preferredfor applications above approximately 350-400°C. Service expenences have proven that the risk ofthermal fatigue is decreased due to a good match between the thermal expansion of a Ni-base weldmetal and the ferritic steel ( 11 ). Furthermore, the use of Ni-base fillers will minimize carbonmigration into the weld. However, one drawback with Ni-base fillers is that they are inherently

8 more sensitive to hot cracking than an austenitic weld metal with a suitable ferrite content (5).

Base metal HAZ considerationsIt is well recognized that the sensitivity to hydrogen cracking in femtic steel HAZ depends on themicrostructure, the amount of hydrogen, the joint restraint and the temperature. A simple, althoughoften overly conservative, guide in making dissimilar metal welds is therefore to use the sameparameters such as preheat, interpass temperature, PWHT etc., that would be used in welding thesteels to themselves. However, a lower preheat can often be tolerated when an austenitic stainlessor Ni-base filler is used since the risk of hydrogen cracking will be decreased by the high hydrogen ’solubility and relatively low yield strength of the weld metal. ,

It should also be realised that PWHT In the range 500-700°C, that is commonly used for the ferriticsteel, can cause sensifisation of an austenitic stainless steel or weld metal, In particular for unstabi-lised grades with a high carbon content. PWHT might also cause embnttlement due to sigma phase

precipitation. This effect is more pronounced for weld metals with higher ferrite contents. A restric-

’I non to maximum 8-10 FN In the weld metal is therefore often used in cases, for example cladding) of low alloy steel, when a PWHT is required. One possible way of overcoming the deleteriousj effects of PWHT is to first surface, or &dquo;butter&dquo;, the ferritic steel with a layer of suitable austenitic j, filler metal The buttered ferritic steel can then be heat treated before the final welding to theI

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128 -

stainless steel. This procedure has the additional advantage that the portion of the weld where Idifficulties are most likely to occur are welded while there is little restraint on the weld metal (5. 9). ’

A less common, but nevertheless difficult to solve problem, can occur in welding of &dquo;dirty femtic

’

steels&dquo; with comparatively high impurity levels. Cracking or complete disbonding can take place in IIthe HAZ close to the fusion region (Fig. 3). The fracture surfaces are often discoloured indicatingthat the cracking occurs at high temperatures. This type of cracking is usually explained in terms of :

I HAZ liquation cracking in combination with local decarbunzation concentrating any deformation

&dquo;caused by shrinkage to the HAZ next to the fusion boundary. High stresses at temperatures whenliquation of e.g. S-rich inclusions can occur then promotes link-up of cracks and fracture finally

I occurs. Filler metal composition does not seem to affect this type of cracking significantly. Never- theless, dilution with a high impunty base metal will increase the nsk of weld metal hot crackingg suggesting that an austenitic stainless filler is preferred (5). Minimising the restraint during welding by using the buttering technique also has a beneficial effect on the cracking tendency.

j Unmixed zonetB Even when paying attention to all details in producing dissimilar metal welds, and ensuring that thej welds are produced as far as possible defect free, the mechanically weak point is often the micro- structures formed in the narrow unmixed or partially mixed zone at the weld interface (8). ThisM zone is always present in fusion welding and complex, often hard, microstructures are formed dueM[ to gradients in alloy elements distribution (7,12). It is not possible to completely eliminate the hard zone by modifying the welding procedure but Ni-base consumables seem more beneficial than stainless fillers since these limit the carbon migration, in particular during PWHT. Neither cang PWHT be used to eliminate the hard zone. As shown by several workers, PWHT results in carbon migration and intense carbide precipitation within the weld metal, together with formation of a decarburized zone in the steel, when stainless steel weld metal is used. This is not a problem with: Ni-base fillers but high hardnesses (500HV) can still be maintained in the weld metal adjacent to the interface due to the development of virgin martensite on cool-down after PWHT (8). ,

M* The hard zone might be a problem during welding (Fig 4) and service (see below). Fortunately, the Ii hard zone is generally not more prone to hydrogen cracking during welding than the ferritic

IK material HAZ. This is often attributed to the facts that the martensitic hard region often is dis- } continuous and surrounded by, or adjacent to, the austenitic stainless or Ni-base weld metal with a ,f high solubility for hydrogen and a comparatively lower strength (1, 5, 8, 13).

. it i

q% i’X /,, .1

50 gm - _ , , - , , ; i ;

100 gm .

Figure 3 HAZ zone cracking in a &dquo;dirty Figure 4 Hydrogen cracking in the hard zone I I ferritic steel&dquo; (0.05% S and at the interface between weld metal I 0.05% P) welded with an austenitic and base metal in an austenitic/- I

N 22Cr 12Ni 3Mo stainless filler, femtic dissimilar joint.

WELDING FERRITIC AND MARTENSITIC STAINLESS STEELS TO

UNALLOYED OR LOW ALLOY STEEL

Mt Ferritic and martensitic stainless steels are normally welded to unalloyed or low alloy steels using austenitic stainless or Ni-base filler when the construction is intended for general (not high

.

’. -

- 129 I

, -

j temperature) service. The most straightforward method is to deposit the filler metal directly on thejoint surfaces without using a separate surfacmg layer As for any dissimilar metal weld it isimportant to keep the dilution under control and use preheat and PWHT as required for the basemetals keepmg potential risks of heat treating the weld metal m mmd. The-second method is more ,

j time consuming but in many cases safer The joint surfaces are surfaced with the filler metal, using’ suitable preheat and PWHT for each base metal, the final weldmg is then made without preheat or

postheat. A filler of the type 22Cr 12Ni 3Mo is commonly used since this is sufficiently high in

I alloy content to tolerate dilution by the involved base metals (9). ’i

A filler metal similar to the stainless base matenal is usually recommended when austenitic -

stainless or Ni-base fillers cannot be used, such as applications were temperature cycling occurs or, strength matching the base metal-is required. These weld metals mostly require a PWHT to obtainI acceptable ductility and toughness.

WELDING DUPLEX STAINLESS STEELS TO UNALLOYED OR LOW ALLOY STEEL Welding duplex (ferritic/austenitic) stainless steels to, or cladding with duplex filler metal on,

: unalloyed or low alloy steels is basically straightforward. The risks of hot cracking or hydrogen

I cracking are small since dilution with the duplex base metal favours formation of ferrite and since duplex stainless steels are not very sensitive to hydrogen cracking. Suitable filler metal choices are&dquo; e.g. 22Cr l2Ni 3Mo, 23Cr l2Ni or duplex fillers. The most common duplex fillermetal type is 22Cr 9Ni 3Mo + N which is well suited

both for cladding and dissimilar joining, 1100 (14). The main concern is when a PWHT

i t000-

<t 27 J is required since the duplex stainless steels

Û _ tT and weld metals are prone to embrittle- V - ’

ment and deterioration of corrosion

8- -&horbar; 1 resistance at temperatures typical of stress

Ie 700- &dquo;&dquo; V relieving (15, 16). If allowed by specifi-600- &dquo;- cations, this can be solved by stress

I E 500- relieving after buttering or after depositing¡..;..

- only the first layer in claddingj * *’’ applications. Another possibility to¡

300 &dquo;° ’ &dquo;&dquo;°n ’ &dquo;&dquo;&dquo;’ ’ ................ achieve acceptable results, although withj 0,1

1

1 1 10 100 1000 10000 some loss of weld metal toughness, is to

j Time (mm) perform a heat treatment in the tem-I perature range 500-550°C for times not’ Figure 5 Critical aging time for embrittlement at room exceeding 10 h. As illustrated in Fig. 5,’ temperature of a 22Cr 9Ni 3Mo O.15N type heat treatment at higher temperatures will’ duplex weld metal (refs. 15 and 16). lead to unacceptable embrittlement.

SERVICE PERFORMANCE OF DISSIMILAR WELDS BETWEEN STAINLESS STEELSI AND UNALLOYED OR LOW ALLOY STEEL

Three important aspects of service performance are 1) low temperature mechanical properties,including fatigue properties, 2) mechanical properties at temperatures where creep becomessignificant and 3) corrosion resistance.

Low temperature mechanical properties ’

Dissimilar metal welds have been used successfully for decades in e.g. the chemical industry. Cr-Nitype austenitic filler metals have proved suitable for most applications up to service temperatures of

I 350-400°C (1). A general experience is that failures occurring within the first few years of serviceare often attnbutable to weld defects whereas defects are normally not significant contributors tolater failures (1, 17). Qualification and control of the welding procedure are therefore most im-

, portant factors for dissimilar metal welds. An important requirement is, furthermore, a suitable

I positioning of the weld joint within the construction. Dynamically or thenno-mechanically stressedconstructions must be particularly carefully planned and fabricated. However, the fatigue life ofdissimilar welds does not seem to be shorter than the life of the base metals to be joined provided

,

t

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.... --&dquo;’:&dquo;<;&dquo;&dquo;’-&dquo;; ’.&dquo; ..&dquo;., ’.. -&dquo;&dquo; &dquo; ; ,,’.:- ;;,. -,’_:’:,.: ,.,j

a! ’ 130

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the welds are free from defects. Fatigue life depends most on surface finish, in particular at theedges of the weld ( 17). -

h -

:’ High temperature mechanical properties Weld quality is of course vital also for high temperature application. However, it is not enough to produce defect free welds when the creep damage mechanism is likely to operate Durability of’ dissimilar welds is a function of the filler metal employed and the condition to which a weld is

j,’ { subject in terms of time, temperature cycles and system loads (11, 17). Service experience in power’ generation have shown that Ni-base fillers are-preferred for ferrinc/austenmc joints used in the

h creep regime, smce these increase service life by a factor of typically 5 by reducing the adverse1 effects of carbon migration from the ferritic side and of differential expansion strains (3, 11).

, Failure experience of ferritic/austenitic dissimilar welds in fossil boilers has shown that failure is*S usually macroscopically similar regardless of filler and service conditions and occurs close to the fusion boundary in the feriitic base metal. However, three distinct failure modes have been

. identified (11):f 1. Crack formation and propagation along prior austenite grain boundaries in the low alloy steel: about one to two grams away from the weld fusion line. Most common for stainless steel fillers.

2. Cracking along a planar array of globular carbides, that develops in service, along the weld fusion line. This failure mode is commonly observed for Ni-base fillers. 3. Failure occurring due to the propagation through the tube wall of an oxide notch formed on the, . outside of the tube at the weld/ low alloy steel junction. Both stainless and Ni-base dissimilart metal welds can suffer from this type of failure mdependent of mode 1 and 2.

M The terminal failure mechanisms generally mvolve the formation of creep cavities adjacent to carbides present at prior austenite grain boundaries and/or the fusion line. A planar array of globular carbides, which are prevalent in Ni-base dissimilar metal welds, appears to be lesse detrimental than the diffuse array of generally smaller carbides commonly found for stainless filler welds. It should be pointed out that PWHT ought to be limited to the minimum level required for qualification since the heat treatment might shorten the lifetime by initiating the metallurgical changes finally causing fracture. New experimental Fe-based fillers with acceptable thermal expansion characteristics, minimal tendency to form carbides and good thermal stability have beenp developed (3, 11). However, they have to the author’s knowledge not reached any widespread use.

; Corrosion

Corrosion is normally of little importance in dissimilar welds between stainless and non-stainless steels since the weld metal will always be more corrosion resistant than the unalloyed or low alloy‘ side. However, the sensitivity to hydrogen induced stress corrosion cracking in the hard interfacial- zone is a problem under conditions when atomic hydrogen can enter the material. This type of, cracking can occur independent of the type of filler (or cladding) applied and cannot be eliminated a by PWHT. The welds should therefore be situated outside regions in which these dangers may be‘_ encountered or at least where the hard zone is not highly mechanically stressed (1, 13, 18). It has also been observed that conventional Ni-base fillers may be sensitised to intercrystalline attack and a clear preference is emerging for higher chromium vanants for use in corrosive environments (19).

,

, JOINING DISSIMILAR STAINLESS STEELS

Welding dissimilar stainless steels to each other is quite common and in most cases fairly uncom- plicated. Usually an austenitic filler providing a suitable ferrite content and corrosion resistance and mechanical strength at least matching the poorest base metal is used. The ferrite content should be maximized to 8-10 FN for high temperature applications or material combinations requiringtJB. PWHT. It is also quite common to use Ni-base fillers, in particular for high temperature appli-N) cations. Ni-base fillers have proved to be the best choice also for joining of duplex and high-Moj austenitic stainless steels since precipitation of intermetallic phases can cause embrittlement ofP welds produced with stainless fillers (20). The Ni-22Cr 9Mo 3.5Nb is one possible choice for these stainless steels combinations. However, the newer Ni- 23-26Cr 13-15Mo Nb-free types (e.g. Esabt.. OK Autrod 19.81 and OK 92.95) have certam advantages in that the tendency of formmg Nb-rich precipitates in the weld metal has been eliminated and the probability of formation of a high femte, N-depleted zone in the duplex base metal HAZ is sigmficantly smaller (21).

7--

I. _

II _ _

_

131 1

- The use of austenitic stainless fillers, or more commonly Ni-base fillers, overalloyed m Mo is todaystandard practice for welding of highly alloyed (> 4-5 %Mo) austenitic stainless steels The over-

, alloyng is needed to compensate forj segregation In the weld metal to ensurej sufficient corrosion resistance also of

the alloy depleted regions. The unmixed--

-

or partially mixed zone at the fusionboundary is also in this case a potential

_ problem since local alloy depletion dueto segregation during solidification I

¡ cannot be avoided in this region ’

¡(FIg. 6) (12). However, practical ’

- experience has shown that the UMZ, 50 m very rarely is attacked by corrosion in

, service although laboratory tests have.

shown this to be a weak spot. Part of: Figure 6 Unmixed zone at the fusion boundary the explanation is that, fortunately, the_ between a Ni-base weld metal (top) and a UMZ is usually very narrow at the

highly alloyed austenitic stainless steel. surface. -

e An interesting possibility, presently being explored, is the use of new high strength, very corrosionresistant Ni-base fillers for welding of super duplex stainless steels. These steels are today weldedwith super duplex fillers. However, the full potential of the steels cannot be used since the weldshave a slightly lower corrosion resistance than the base metal. A Ni-base filler could also be used toimprove low temperature toughness of welds. Although further testing is required it has been

I demonstrated that the use of a Ni- 26.5Cr 14Mo +N type MMA electrode (OK 92.95) is a realisticpossibility. Welds in super duplex stainless steels produced with this filler have been shown to

I combine sufficient strength, good toughness and very good pitting resistance (21).

I WELDING STAINLESS STEEL TO NON-FERROUS METALS

I Whether two dissimilar metals or alloys can be welded together successfully is best predicted by a! combination of empirical experience and by examining the alloy phase diagrams of the metals to be

joined. Properties such as melting point, thermal conductivity and expansion, atomic sizes and for-mation of intermetallic compounds will affect the probability of success. A smvey of which pure

, metals can be fusion welded to iron might seem discouraging since most metals will not readily beJ welded to iron (22) However, a second look shows that the majonty of the important metals used’

as construction matenals (in particular Ni- and Cu-base alloys) can be fusion welded to stainlessA steels either directly or by applyng a buffer layer. Still, two important groups, Al- and Ti-alloys, cannot be fusion welded successfully to steels. Most of the combinations not producing acceptableI

fusion welds can, however, be joined to steels by solid state welding such as explosion, friction or’ ultrasonic welding (22). A possibility is therefore to use these welding methods directly for such¡ combinations or to use transition pieces, produced by solid state welding, which are then weldedI

- into the construction.Table 1 Approximate limits of acceptable alloying element

I levels in stainless steel dissimilar welds (23). Recommendations for choice offiller metal for different com-

I .

.. j bmanons can be found in e.g.

. j Diluting elements reference (23) or obtained from

i FIller metal

Fe Ni Cr Cu filler and base metal manu-,

_ _ _ facturers. However, some

Ni 25-40%* unlimited 30% unlimited general comments could beNi-Cu 5-30%* unlimited 6-8% unlimited made without going into details.

’

- The two most important factorsNi-Cr-Fe 25% unlimited 30% 15% are usually the choice of fillersCu-Ni 5-10% unlimited 5% unlimited

and designing a welding proce-,

Cu-Ni 5-10% unlimned 5% unlimned dure that will ensure defect free

/ -Dependmg on welding method and PWHT welds and at the same time

... -:·_ss,,.‘ --_ - ‘ _ .;-,-.- u U,. or . _ __ _ .._ - , _ _ .. _

. -

132 _

- I I

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i 3? -

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.

-

_

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avoiding dilution levels that cause hot cracking or embnttlement of the weld metal (see Table 1 for

r . combinations with Ni- or Cu-base alloys). I

r Joining stamless steels and Co-base alloys is not a very common combination. However, Co-basef filler metals are occasionally used for hardfacing on stainless steels. This approach seldom causes any problems but the use of Co-base fillers as buffenng on stainless before hardfacmg with1 Fe-based hardfacing alloys is not to be recommended due to the nsk of hot cracking.i _

i - -

’.i CONCLUDING REMARKS

{ Much empirical and theoretical knowledge about dissimilar welding has been collected over the ! years. However, apart from obvious problems when trying to cucumvent the laws of nature, a major problem can be finding the relevant information for a specific material combination. Another observation is that,-in particular in practical repair welding, you sometimes encounter &dquo;impossible to weld material combinations&dquo; that have been welded successfully. Not being familiar with theM. restrictions imposed by metallurgy can obviously be an advantage when trying to find ways of joining dissimilar metals. Nevertheless, use of proper welding procedures and suitable filler metals are as a rule key factors in successful welding of dissimilar metals.

. No major changes in preferred choice of filler metals can be seen when comparing dissimilar metaltt welding practice ten years ago and today. However, a trend has emerged that welding methodst minimizing dilunon with base metal are used more extensively where dilution is a major problem.Mt For example, laser and electron beam welding are used for joining and electroslag welding intg cladding applications (24). Solid state joining methods are also more widely used today than ten years ago. These changes are expected to continue although conventional fusion welding methods will certainly be dominating for dissimilar metal welding also ten years from now. Looking into the future it is likely that dissimilar metal welding will have a growing role to play in industry due to a combination of economics, rules and design criteria.

ACKNOWLEDGEMENTS

Constructive comments and suggestions by colleagues at Esab, Dr T Gooch (TWI) and Dr D I Kotecki (The Lincoln Electric Company) are gratefully acknowledged.

_

REFERENCES -

M 1 C. Pohle, 1989, Schweisstechruk, 39 (1 I)- pp 508-511. Ig 2. R.E. Avery, 1991,Chemical Engineering Progress, 87 (5): pp 70-75.

3. B. Irving. 1992,Wel&ng Journal, 71 (5) pp 27-33.! 4. C.W. Cox and S.D. Kiser, 1992,Welding Journal , 71 (5). pp 67-70

‘

’ 5. T.G. Gooch, 1980, Metal Construction, 12 (11): pp 622-631.

j 6. G. Faber and T.G Gooch, 1982, Welding In the World, 20 (5/6): pp 87-98. It 7. C. Pan, R. Wang, J. Gm and Y. Shi, 1990, J of Malenals Science, 25 (7): 3281-3285.N 8 M.F. Gutos and T.G Gooch, 1992,Welding Journal , 71 (12): pp 461s-472s -

9 J G. Feldstem, 1991, ASM Handbook. Vol. 6, ASM International, Matenals Park, OhIo’ pp 500-504

10. D J. Kotecki and T.A Siewem 1992, Welding Journal, 71 (5)- pp 171s-178s.

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