Stainless Steelverlays

ladding and Weld

A STAINLESS-STEEL-CLAD metal or alloyis a composite product consisting of a thin layer ofstainless steel in the form of a veneer integrallybonded to one or both surfaces of the substrate.The principal object of such a product is to com­bine, at low cost, the desirable properties of thestainless steel and the backing material for appli­cations where full-gage alloy construction is notrequired. While the stainless cladding furnishesthe necessary resistance to corrosion, abrasion, oroxidation, the backing material contributes struc­tural strength and improves the fabricability andthermal conductivity of the composite. Stainless­steel-clad metals can be produced in plate, strip,tube, rod, and wire form.

The principal cladding techniques include hotroll bonding, cold roll bonding, explosive bond­ing, centrifugal casting, brazing, and weld over­laying, although adhesive bonding, extrusion, andhot isostatic pressing have also been used to pro­duce clad metals. With casting, brazing, and weld­ing, one of the metals to be joined is molten whena metal-to-metal bond is achieved. With hot/coldroll bonding and explosive bonding, the bond isachieved by forcing clean oxide-free metal sur­faces into intimate contact, which causes a shar­ing of electrons between the metals. Gaseousimpurities diffuse into the metals, and nondiffusi­ble impurities consolidate by spheroidization.These non-melting techniques involve some formof deformation to break up surface oxides, to cre­ate metal-to-metal contact, and to heat in order to

accelerate diffusion. They differ in the amount ofdeformation and heat used to form the bond and inthe method of bringing the metals into intimatecontact.

This article will review each of the processescommonly associated with stainless-steel-cladmetal systems as well as the stainless steels used.Design considerations and the welding of stain­less-steel-clad carbon and low-alloy steels arealso addressed. Additional information can befound in Ref 1 to 3.

Hot Roll Bonding (Ref 3)

The hot roll bonding process, which is alsocalled roll welding, is the most important com­mercially because it is the major productionmethod for stainless-clad steel plates. Hot rollbonding accounts for more than 90% of the cladplate production worldwide (Ref 1). It is knownalso as the heat and pressure process because theprinciple involves preparing the carefully cleanedcladding components in the form of a pack orsandwich, heating to the plastic range, and bring­ing the stainless and backing material into inti­mate contact, either by pressing or by rolling. Aproduct so formed is integrally bonded at the in­terface. The clad surface is in all respects (corro­sion resistance, physical properties, andmechanical properties) the equal of the parentstainless steel. It can be polished and worked inthe same manner as solid stainless steel.

Table 1 lists the clad combinations that havebeen commercially produced on a large scale. Asthis table indicates, stainless steels can be joinedto a variety of ferrous and nonferrous alloys. On atonnage basis, however, the most common cladsystems are carbon or low-alloy steels clad with300-series austenitic grades. The types of austeni­tic stainless steel cladding commonly available inplate forms are:

.. Type 304 (18-8)• Type 304 L (18-8 low carbon).. Type 309 (25-12).. Type 310 (25-20).. Type 316 (17-12Mo).. Type 316 Cb (17-12 Nb stabilized).. Type 316 L (17-12 Mo low carbon).. Type 317 (19-13 Mo)• Type 317 L (19-13 Mo low carbon)lit Type 321 (18-lOTi)lit Type347(18-11Nb)

The carbon or low-alloy steel/stainless steelplate rolling sequence is normally followed byheat treatment, which is usually required to re­store the cladding to the solution-annealed condi­tion and to bring the backing material into thecorrect heat-treatment condition. Table 2 liststypical mill heat treatments.

The cladding thickness is normally specifiedas a percentage of the total thickness of the com­posite plate. It varies from 5 to 50%, depending onthe end use. For most commercial applications in-

Table 1 Selected dissimilar metals and alloys that can be roll bonded (hot or cold) into clad-laminate form

Weldabililyraliog(a)Basemetal AI Carbon StainlessNo.l/No.2 Ag AI alloys Au steel Co Cn Mo Mo·N! Nb Ni PI steel Steel So Ta Ti U Zr

Ag A B BAl A C B C B B B CAlfesil D D D D D D D D D D D D D D D D D D DBe D D D D D D D D D D D D D D D D D D DCarbon steel B B BCn A B A B B B A B B A A B B BMn B B A BNi B A A B B ANb B BStainless B B B B B

steelSteel B A B BU B

(a) A, easy to weld; B, difficult but possible to weld; C. impractical to weld; D, impossible to weld. Source: Ref2

ASM Specialty Handbook: Stainless Steels, 06398GJ.R. Davis, Davis & Associates

Copyright © 1994 ASM International ®All rights reserved.

www.asminternational.org

108 / Introduction to Stainless Steels

Table 2 Typical mill heattreatments for stainlessclad carbonand low-alloysteels

Typeofcladdingmaterial

TypeofASTM-gradebackingmaterial Heat treatment(a)

metallurgical bond that is due to a sharing of at­oms between the materials. The resulting bondcan exceed the strength of either of the parentmaterials.

(a) Heattreatments listedaregenerallycorrect for thematerial combinations shown.Deviationsmay be madetomeetspecificrequirements. Procedureselected willbe onefavorable forboth cladding andbacking material. (b)Stabilized orlow-carbon typesof stainless steelshould beusedwhenthisdoubleheattreatment is involved.Source:Ref3

A204, A 302 (up to 50mm, or2 in., gage)

A204, A 302 (over 50mm, or2 in., gage).A301 (all gages)

Cold Roll Bonding

Upon completion of this three-step process, theresultant clad material can be treated in the sameway as any other conventional monolithic metal.The clad material can be worked by any of thetraditional processing methods for strip metals.Rolling, annealing, pickling, and slitting are typi­cally performed to produce the finished strip tospecific customer requirements, so that the materialcan be roll formed, stamped, or drawn into therequired part.

Clad steels prepared by this method show sub­stantially the same microstructures as those thathave been bonded by hot roll bonding processes.Because of the high power requirement in the in­itial reduction, the cold bonding process is notpractical for producing clad plates of any appre­ciable size.

The single largest application for cold-roll­bonded materials is stainless-steel-clad alumi­num for automotive trim (Table 3 and Fig. 2) (Ref6). The stainless steel exterior surface providescorrosion resistance, high luster, and abrasion anddent resistance, and the aluminum on the insideprovides sacrificial protection for the painted autobody steel and for the stainless steel.

Explosive Bonding (Ref1)

Explosive bonding uses the very-short-dura­tion, high-energy impulse of an explosion to drivetwo surfaces of metal together, simultaneouslycleaning away surface oxide films and creating ametallic bond. The two surfaces do not collide in­stantaneously but rather progressively over the in-

Anneal 1065 to 1175 °C (1950 to 2150 oF),air quench

Anneal 1065 to 1175 °C (1950 to 2150 oF),air quench, normalize 870 to 900 °C(1600 to 1650 "F) 1 hr per 25 mm (1 in.)thickness, air quench(b)

Anneal 1065 to 1175 °C (1950 to 2150 oF),air quench

Anneal 1065 to 1175 °C (1950 to 2150 OF),air quench, normalize 870 to 900°C(1600 to 1650 OF)1 hr per 25 mm (1 in.)thickness, airquench(b)

The cold roll bonding process, which isshown schematically in Fig. 1, involves threebasic steps:

" The mating surfaces are cleaned by chemicaland/or mechanical means to remove dirt, lu­bricants, surface oxides, and any other con­taminants.

" The materials are joined in a bonding mill byrolling them together with a thickness reduc­tion that ranges from 50 to 80% in a single pass.Immediately afterwards, the materials have anincipient, or green, bond created by the massivecold reduction.

" The materials then undergo sintering, a heattreatment during which the bond at the inter­face is completed. Diffusion occurs at theatomic level along the interface and results in a

A285, A201, A212 (up to 50 mm, or 2 in.,gage)

A201, A212 (over 50mm, or 2 in., gage)

304, 304L, 309, 310, 316, 316Cb, 316L,317,321,or347

304L, 316L, 316Cb, 317L, 321, or 347

304, 304L, 309, 310, 316, 316Cb, 316L,317,321,or347

304L, 316L, 316Cb, 317L, 321, or 347

volving carbon or low-alloy steel/stainless steelcombinations, cladding thickness generally fallsin the 10 to 20% range.

Hot roll bonding has also been used to cladhigh-strength low-alloy (HSLA) steel plate withduplex stainless steels (Ref 4, 5). The microal­loyed base metals contain small amounts of cop­per (0.15% max), niobium (0.03% max), andnitrogen (0.010% max) and have mechanicalproperties comparable to those of duplex stainlesssteels. Typically these HSLA base metals haveyield strengths of 500 MPa (72.5 ksi) and impactvalues of 60 J (44 ft-lbf) at -60°C (-75 OF). Theshear strength of the cladding bond can be as highas 400 MPa (58 ksi).

Other metals and alloys commonly rollbonded to stainless steels include aluminum, cop­per, and nickel. Table 3 lists properties and appli­cations of roll-bonded clad laminates.

Table 3 Typical properties of roll-bonded stainlesssteel

Tensile YieldComposite Thickness Width strength strength Elongation,

Materialssystem ratio, % mm in. mm in. MPa ksl MPa ksi % Applications

Type 434 stainless/5052 40:60 0.56-0.76 0.022-0.030 :0;610 :0;24 395 57 360 52 12 Widely used for automotive bodyaluminum moldings, drip rails, rocker panels,

and other trim components, oftenreplacing solid stainless steel oraluminum. Stainless steel providesbright appearance; the hiddenaluminum base provides cathodicprotection, corroding sacrificially tothe body sleel.

CI008 steel/type 347 45:10:45 0.36 0.014 305 12 393 57 195 28 35 Used in hydraulic tubing in vehicles,stainless steel/CI008 replacing teme-coated carbon steelsteel tubing. The outer layer of carbon steel

cathodically protects the stainlesscore of the tube, extending its lifesignificantly.

Nickel201/type 304 7.5:85:7.5 0.20-2.41 0.008-0.095 25-64 1-2.5 310 45 40 Used in formed cans for transistor andstainless steel/nickel button cell balleries, replacing solid201 nickel at a lower cost

Copper 1mOO/type 430 17:66:17, 0.10-0.15 0.004-0.006 12.7-150 0.5-6 415(a)· 60(a) 275 40 20(a) Replaces heavier gapes ofcopper andstainless steel/copper 20:60:20, bronze in buried communications10300 33:34:33 cable. The stainless steel provides

resistance to gnawing by rodents,which is a serious problem inunderground installations.

(a)20/60/20three-layerlaminate. Source: Ref2

Stainless Steel Cladding and Weld Overlays / 109

CLAPDING METAL

~

.~I

(a)

(b)

Sintering

Fig. 3 Bond zone pattern typical of explosion-cladmetals. Materials are type 304L stainless steel

and medium-carbon steel. 20x (e)

Roll Bonding

Process steps in cold roll bonding

Steel body panel

Mechanical Cleaning

Fig. 1

Chemical Cleaning

Fig. 2 Stainless-steel-c1ad aluminum automotivetrim provides sacrificial corrosion protection

to the auto body while maintaining a bright corrosion-resis­tant exterior surface.

terface area. The pressure generated at the result­ing collision front is extreme and causes plasticdeformation of the surface layers. In this way, thesurface layers and any contaminating oxides pre­sent are removed in the form of a jet projectedahead of the collision front. This leaves perfectlyclean surfaces under pressure to form the bond.Figure 3 illustrates the wavy interface that charac­terizes most explosive bonds.

Two basic geometric configurations of the ex­plosive bonding process are commonly used: an­gle bonding and parallel-plate bonding. Anglebonding is normally used for bonding sheet com­ponents and tubes, where the required bond widthdoes not exceed 20 times the flyer plate thickness.The more commonly used parallel-plate geome­try (Fig. 4) is applicable for welding larger flat ar­eas, plate, and concentric cylinders.

The energy of bonding typically creates suffi­cient deformation that flattening or straighteningis required prior to further processing. Flatteningis performed with equipment of the same designused in plate and sheet manufacture.

Explosive bonding is an effective joiningmethod for virtually any combination of metals.

The only metallurgical limitation is sufficientductility and fracture toughness to undergo therapid deformation of the process without fracture.Generally accepted limits are 10% and 30 J (22ft-lbf) minimum, respectively. Figure 5 lists thecombinations that are useful in industry. More de­tailed information on explosive bonding is avail­able in Ref? to 9.

Centrifugal Casting (Ref 1)

An entirely different approach to clad seam­less pipe production uses horizontal centrifugalcasting technology. First, well-refined moltensteel is poured into a rotating metal mold withflux. After casting, the temperature of the outershell is monitored. At a suitable temperature aftersolidification the molten stainless steel is intro­duced. The selection of the flux, the temperatureof the outer shell when the molten stainless steel isintroduced, and the pouring temperature of thestainless steel are the most important factors inachieving a sound metallurgical bond. By con­trolling these various parameters it is possible toachieve minimum mixing at the interface andmaintain homogenous cladding thickness andwall thickness.

Centrifugal casting is followed by heat treatmentto solution anneal the cladding and quench and tem-

Fig. 4 Parallel-plate explosion welding process. (a)Explosion-cladding assembly before detona­

tion. (b) Explosion-cladding assembly during detonation. (c)Closeup of (b) showing mechanism for jetting away the sur­face layer from the parent layer

per the outer pipe to achieve the required me­chanical properties. Finally, the pipe is machinedexternally and internally to remove the shallowinterdendritic porosity in the bore and achieve therequired dimensions and surface finish.

Centrifugal cast pipe is available with theouter steel made of API 5L X52, X60, or X65grades and internal cladding made of type 316Lstainless steel. Sizes range from 100 to 400 mm (4to 16 in.) in diameter, wall thickness from 10 to 90mm (004 to 3.5 in.) (minimum 3 mm, or 0.12 in.cladding), and lengths typically from 4 to 5 m (13to 16 ft), withlongerlengths above 200mm (8 in.)in diameter.

Brazing

In furnace brazing, the stainless steel claddingand the backing material, in their respective finalgages, are assembled as a multilayer sandwich,with a brazing alloy placed between each pair ofsurfaces to be bonded. The sandwich is heated un­der continuous vacuum to a temperature at which

110 / Introduction to Stainless Steels

Fig. 5 Commercially availableexplosion-cladmetalcombinations

'" '"Qi '"~ >- Q)

'" QiE .2 E .... Qi Q)E a:l

E .2 co '" ....::J ::J CD E E >- E ::J '"

Q) '"'in ::J .2 '" c '"....

c'c Q) ::J ::J .... .9! '"Q)

~::J co ]. Q) 'f 00 c C

1J....

:.0 Qi 'c c. c >- -E~ 01 ';:; Q) .... ....'" C. ::J '(ij .2

'" ] eo '0 2: 0 c '" .... .s 0 '"'" eo~

....~N :2: (f) a:: o 'w Z l- I i= z o (f) o

Carbon steels • ED • • • • CD CD • • • • • •Alloy steels • • • • • III • III ED III ED

Stainless steels " • • • It " • 4& • It

Aluminum " • " III • • • •Copper aIIoys • III ED CD CD •Nickel alloys • • ED III It • It

Titanium It • • • • •Hastelloy 4&

Tantalum III ED •Niobium • •Silver •Gold

Platinum •Stellite 68

Magnesium CD

Zirconium •

the brazing alloy liquefies and forms an intennet­allic alloying zone at the interface of the stainlessand backing material (normally carbon steels), Awide range of brazing filler metals can be used tojoin stainless steels to carbon or low-alloy steels.The most commonly used are silver-base alloys.More detailed information on brazing of stainlesssteels can be found in the article" Brazing, Sol­dering, and Adhesive Bonding" in this Volume.

Weld Overlays

Weld overlaying refers to the deposition of afiller metal on a base metal (substrate) to impartsome desired property to the surface thatis not in­trinsic to the underlying base metal. There are sev­eral types of weld overlays: weld claddings,

hardfacing materials, buildup alloys, and butter­ing alloys.

A weld clad is a relatively thick layer of fillermetal applied to a carbon or low-alloy steel basemetal for the purpose of providing a corrosion-resis­tant surface. Hardfacing is a form of weld surfacingthat is applied for the purpose of reducing wear,abrasion, impact, erosion, galling, or cavitation. Theterm buildup refers to the addition of weld metal to abase metal surface for the restoration of the compo­nent to the required dimensions. Buildup alloys aregenerally not designed to resist wear, but to returnthe worn part back to, or near, its original dimen­sions, or to provide adequate support for subsequentlayersof truehardfacing materials.Buttering also in­volves the addition of one or more layers of weldmetal to the face of the joint or surface to be welded.It differsfrom buildup in that the primary purpose of

buttering is to satisfy some metallurgical consid­eration. It is used primarily for the joining of dis­similar metal base metals, as described in thesection "Welding Austenitic-Stainless-CladCarbon or Low-Alloy Steels" in this article. Anextensive review of the weld processes and mate­rials associated with weld overlays can be foundin the article "Hardfacing, Weld Cladding, andDissimilar Metal Joining," in Volume 6 of theASM Handbook (Ref 10),

WeldCladding

The term weld cladding usually denotes theapplication of a relatively thick layer (;::3 mm, orYs in.) of weld metal for the purpose of providinga corrosion-resistant surface. Hardfacing pro­duces a thinner surface coating than a weld clad­ding and is normally applied for dimensionalrestoration or wear resistance. Typical base metalcomponents that are weld-cladded include the in­ternal surfaces of carbon and low-alloy steel pres­sure vessels, paper digesters, urea reactors,tubesheets, nuclear reactor containment vessels,and hydrocrackers. The cladding material is usu­ally an austenitic stainless steel or a nickel-basealloy. Weld cladding is usually performed usingsubmerged arc welding. However, flux-cored arcwelding (either self-shielded or gas-shielded),plasma arc welding, and electroslag welding canalso produce weld claddings. Figure 6 compares'deposition rates obtainable with different weldingprocesses. Filler metals are available as coveredelectrodes, coiled electrode wire, and strip elec­trodes. For very large areas, strip welding witheither submerged arc or electro slag techniques isthe most economical. Table 4 lists some of thefiller metals for stainless steel weld claddings.

Application Considerations. Weld claddingis an excellent way to impart properties to the sur­face of a substrate that are not available from thatof a base metal, or to conserve expensive or diffi­cult-to-obtain materials by using only a relativelythin surface layer on a less expensive or abundantbase material. Several inherent limitations or pos­sible problems must be considered when planningfor weld cladding. The thickness of the requiredsurface must be less than the maximum thicknessof the overlay that can be obtained with the par­ticular'process and filler metal selected.

Welding position also must be considered whenselecting an overlay material and process. Certainprocesses are limited in their availablewelding posi­tions (e.g., submerged arc welding can be used onlyin the flat position). In addition, when using a high­deposition-rate process that exhibits a large liquidpool, welding verticallyor overhead may be difficultor impossible. Some alloys exhibit eutectic solidifi­cation, which leads to large molten pools that solid­ify instantly, with no "mushy" (liquid plus solid)transition. Such materials are also difficult to weldexcept in the flat position.

DilutionControl.The economics of stainlesssteel weld cladding are dependent on achievingthe specific chemistry at the highest practicaldeposition rate in a minimum number of layers.The fabricator selects the filler wire and weldingprocess, whereas the purchaser specifies the sur-

Stainless Steel Cladding and Weld Overlays /111

III Amperage: Increased amperage (current den­sity) increases dilution. The arc becomes hot­ter, it penetrates more deeply, and more basemetal melting occurs.

III Polarity: Direct current electrode negative(DCEN) gives less penetration and resultinglower dilution than direct current electrodepositive (DCEP). Alternating CUITent results ina dilution that lies between that provided byDCEN and DCEP.

III Electrodesize: The smaller the electrode, thelower the amperage, which results in less dilu­tion.

III Electrode extension: A long electrode exten­sion for consumable electrode processes de­creases dilution. A short electrode extension in­creases dilution.

• Travel speed: A decrease in travel speed de­creases the amount of base metal melted and in­creases proportionally the amount of filler met­al melted, thus decreasing dilution.

III Oscillation: Greater width of electrode oscilla­tion reduces dilution. The frequency of oscilla­tion also affects dilution: The higher the fre­quency of oscillation, the lower the dilution.

III Welding position: Depending on the weldingposition or work inclination, gravity causes theweld pool to run ahead of, remain under, or runbehind the arc. If the weld pool stays ahead ofor under the arc, less base metal penetrationand resulting dilution will occur. If the pool istoo far ahead of the arc, there will be insuffi-

mum. Less than 10% raises the question of bondintegrity, and greater than 15% increases the costof the filler metal. Unfortunately, most weldingprocesses have considerably greater dilution.

Because of the importance of dilution in weldcladding as well as hardfacing applications, eachwelding parameter must be carefuIly evaluatedand recorded. Many of the parameters that affectdilution in weld cladding applications are not soclosely controlled when arc welding is per­formed:

34323028262422

carbon at a low level to ensure corrosion resis­tance. The prediction of the microstructures andproperties (such as hot cracking and corrosion re­sistance) for the austenitic stainless steels hasbeen the topic of many studies. During the lasttwo decades, four microstructure prediction dia­grams have found the widest application. Theseinclude the Schaeffler diagram, the DeLong dia­gram, and the Welding Research Council (WRC)diagrams (WRC-1988 and WRC-1992). Each ofthese is described in Ref 10 and the article" Weld­ing" in this Volume.

Although each weld cladding process has an ex­pected dilutionfactor,experimenting with the weld­ing parameters can minimize dilution. A valuebetween 10 and 15% is generally considered opti-

14 16 18 20

Deposition rate, kg/h

Submerged arc - double wire

121084

Comparison of deposition rates for various weld cladding processes. To obtain equivalent deposition rates inpounds per hour, multiply the metric value by 2.2. Source: Ref 1

;~;~;~;~;~;~ Pulsed GMAW

~;~;~;~;~;~ Spray transfer GMAW

""" ~SUbmergedarc - 60 mm strip

Submerged arc - 90 mm strip<>"'>';,"*m~""",

Submerged arc - 120 mm strip

Electroslag - 60 mm strip

Electroslag - 90 mm strip

Fig. 6

Hot wire GTAW

face chemistry and thickness, along with the basemetal. The most outstanding difference betweenwelding a joint and depositing an overlay is thepercentage of dilution:

% dilution =....£ x 100x+y

o

where x is the amount of base metal melted and y isthe amount of filler metal added.

For stainless steel cladding, a fabricator mustunderstand how the dilution of the filler metalwith the base metal affects the composition andmetallurgical balance, such as the proper ferritelevel to minimize hot cracking, absence ofmartensite at the interface for bond integrity, and

Note: Colombium (Cb) is also referred to as niobium (Nb). (a) Refer tn AWS specification A5.4. (b) Referto AWS specification A5.9.

Table 4 Stainless steel fillermetalsforweld cladding applications

E320 ER320Fig. 7 Weld cladding of a 1.8 m (6 ft) inner diameter

pressurevesselshell with SO mm (2 in.) wide,0.64 mm (0.025 in.) thick stainlesssteelstrip. Courtesy of l.],Barger, ABB Combustion Engineering

ER308ER308L

ER317

ER347ER347ER309ER310ER316

ER316L

ER317L

Subsequentlayers

E308E308L

E347E347E309E310E316

E316L

E317

E317L

Covered Bare rod oreleelrode(a) electrodetb)

Weld First laxeroverlay Covered Barerodortype electrode(a) eleclrode(b)

304 E309 ER309304L E309L ER309L

E309Cb321 E309Cb ER309Cb347 E309Cb ER309Cb309 E309 ER309310 E310 ER310316 E309Mo ER309Mo316L E309MoL E309MoL

E317L ER317L317 E309Mo ER309Mo

E317 ER317317L E309MoL ER309MoL

E317L ER317L20Cb E320 ER320

112/ Introduction to Stainless Steels

Fig. 8 Closeup view of the 25 mm (1 in.) wide by 0.64 mm (0.025 in.) thick stainless steel strip used to clad a 300 mm(12 in.) inner diameter pressure vessel nozzle. Courtesy of ).). Barger, ABB Combustion Engineering

Hardfacing Alloys

Hardfacing materials include a wide variety ofalloys, carbides, and combinations of these al­loys. Conventionalhardfacing alloys are normallyclassified as carbides (We-Co), nickel-base al­loys, cobalt-base alloys, and ferrous alloys(high-chromium white irons, low-alloy steels,austenitic manganese steels, and stainlesssteels). Stainless steel hardfacing alloys in­clude martensitic and austenitic grades, the lat­ter having high manganese (5 to 10%) and/orsilicon (3 to 5%) contents. As will be describedbelow, both cobalt-containing and cobalt-freeaustenitic stainless steel hardfacing alloys havebeen developed.

Hardfacing alloy selection is guided primarilyby wear and cost considerations. However, othermanufacturing and environmental factors mustalso be considered, such as base metal; depositionprocess; and impact, corrosion, oxidation, andthermal requirements. Usually, the hardfacingprocess dictates the hardfacing or filler metalproduct form.

Hardfacing alloys usually are available as barerod, flux-coated rod, long-length solid wires,long-length tube wires (with and without flux), orpowders. The most popular processes, and theforms most commonly associated with each proc­ess, are:

dent melting of the surface of the base metal,and coalescence will not occur.

• Arc shielding: The shielding medium, gas orflux, also affects dilution. The following listranks various shielding mediums in order ofdecreasing dilution: granular flux without alloyaddition (highest), helium, carbon dioxide, ar­gon, self-shielded flux-cored arc welding, andgranular flux with alloy addition (lowest).

• Additionalfillermetal: Extrametal (notincludingthe electrode),added to the weld pool as powder,

wire, strip, or with flux, reduces dilution by in­creasing the total amount of filler metal and re­ducing the amount of base metal that is melted.

For weld cladding the inside surfaces oflargepressure vessels, as shown in Fig. 7 and 8, widebeads produced by oscillated multiple-wire sys­tems or strip electrodes have become the means toimprove productivity and minimize dilutionwhile offering a uniformly smooth surface. Weld­ing parameters for stainless steel strip weld over­lays are described in Ref 10.

Hardfacingprocess

Oxyfuel/oxyacetylene(OFW/OAW)

Shielded metal arc (SMAW)

Gas-tungsten arc (GTAW)Gas-metal arc (GMAW)Flux-cored open arcSubmerged arc (SAW)Plasma transferred arc (PTA)Laser beam

Consumable form

Bare cast or tubular rod

Coated solid or tubular rod(stick electrode)

Bare cast or tubular rodTubular or solid wireTubular wire (flux cored)Tubular or solid wirePowderPowder

Table 5 Characteristics of weldingprocesses used inhardfacing

MinimumWelding Modeof Weld-metal Deposition thickness(a) Depositprocess application Form of hardfacingalloy dilution,% kg/h Ib/h mm in. efficiency, %

OAW Manual Bare cast rod, tubular rod 1-10 0.5-2 1-4 0.8 Y32 100Manual Powder 1-10 0.5-2 1-4 0.8 \-32 85-95Automatic Extra-long bare cast rod, tubular wire 1-10 0.5-7 1-15 0.8 Y32 100

SMAW Manual Flux-covered cast rod, flux-covered tubular rod 10-20 0.5-5 1-12 3.2 Y. 65Open arc Semiautomatic Alloy-cored tubular wire 15-40 2-11 5-25 3.2 Y. 80-85

Automatic Alloy-cored tubular wire 15-40 2-11 5-25 3.2 lis 80-85GTAW Manual Bare cast rod, tubular rod 10-20 0.5-3 1-6 2.4 %2 98-100

Automatic Various forrns(b) 10-20 0.5-5 1-10 2.4 3!:l2 98-100SAW Automatic, single Bare tubular wire 30-60 5-11 10-25 3.2 Ys 95

wireAutomatic, multi wire Bare tubular wire 15-25 11-27 25-60 4.8 3/16 95Automatic, seriesarc Bare tubular wire 10-25 11-16 25-35 4.8 3/16 95

PAW Automatic Powder(c) 5-15 0.5-7 1-15 0.8 Y32 85-95Manual Bare cast rod, tubular rod 5-15 0.5-4 1-8 2.4 %2 98-100Automatic Various forrns(b) 5-15 0.5-4 1-8 2.4 3.32 98-100

GMAW Semiautomatic Alloy-cored tubular wire 10-40 0.9-5 2-12 1.6 YI6 90-95Automatic Alloy-cored tubular wire 10-40 0.9-5 2-12 1.6 1/16 90-95

Laser Automatic Powder 1-10 (d) (d) 0.13 0.005 85-95

(a)Recommended minimum thickness of deposit. (b)Baretubular wire; extra-long (2.4m,or8 ft) barecastrod;tungsten carbide powder withcastrodorbaretubular wire. (c)Withor without tungsten carbide granules. (d)Varies widelydepending on powderfeedrateandlaserinput power

Stainless Steel Cladding and Weld Overlays /113

Typical dilution percentages, deposition rates,and minimum deposit thicknesses for differentwelding processes, along with various forms,compositions, and modes of application of hard­facing alloys, are given in Table 5. More detailedinformation on the selection of hardfacing alloysand processes can be found in Ref 10.

The buildup alloys include low-alloy pearli­tic steels, austenitic manganese (Hadfield) steels,and high-manganese austenitic stainless steels.For the most part, these alloys are not designed toresist wear but to return a worn part back to, ornear, its original dimensions and to provide ade­quate support for subsequent layers of true hard­facing materials. However, austenitic manganesesteels are used as wear-resistant materials undermild wear conditions. Typical examples of appli­cations where buildup alloys are used for wearingsurfaces include tractor rails, railroad rail ends,steel mill table rolls, and large slow-speed gearteeth. The stainless steel included in this cate­gory is AWS EFeMn-Cr, which has a hardnessvalue of 24 HRC and the following chemicalcomposition:

produced. The activated particles are incorpo­rated into the oxide layers of primary system com­ponents and contribute considerably to theoccupational radiation exposure of maintenancepersonnel during the inspection, repair, or re­placement of components. Additionally, materialloss has been found for cobalt-base hardfacingsused for control or throttle valves that are exposedto high flow velocities, indicating that this type ofalloy has a limited resistance to erosion-corrosionand cavitation attack.

Detailed investigations of candidate replace­ment cobalt-free, iron-base alloys have been per­formed since the late 1960s. In the U.S., theElectric Power Research Institute has developedcobalt-free NOREM alloys (U.S. Patent4,803,045, Feb. 7, 1989). These alloys can be de­posited successfully on stainless and carbon steelsubstrates with gas-tungsten arc welding, in anyposition and with no preheat, using controlledheat input techniques. Nominal compositions ofthe NOREM alloys are as follows:

Element

CarbonChromiumManganeseSiliconNickelMolybdenumIron

Composition, wt%

0.515.015.0

1.31.02.0

bal

Element

CarbonChromiumManganeseSiliconNickelMolybdenumNitrogenIron

Composition,wt%

0.7-1.024-26

4.0-5.22.5-3.25.0-9.01.7-2.3

0.05-0.15bal

NOREM alloys are characterized by highwear resistance and antigalling properties, andthey have a microstructure consisting of anaustenitic matrix containing eutectic alloy car­bides. The NOREM alloys meet or surpass theperformance of cobalt alloys with respect to cor­rosion, material loss due to wear, and mainte­nance of the valve's sealing function. Gallingwear data for various NOREM and cobalt-base al­loys are given in Table 6. Chemical compositionsof the alloys tested are provided in Table 7. Addi­tional information on these alloys can be found inRefll to 14.

Considerable work has also been carried out inEurope on cobalt-free, iron-base hardfacing al­loys. Everit 50 (47 to 53 HRC) , Fox Antinit DUR300 (28 to 32 HRC), and Cenium Z 20 (42 to 48HRC) are tradenames used by Thyssen Edel­stahlwerke Bochum (Germany), Vereinigte Edel­stahlwerke Kapfenberg (Austria), andL.A.M.E.E Rueil-Malmaison (France), respec­tively. Compositions of these alloys are given inTable 8. Studies have demonstrated that these al­loys have tribological, corrosion, and mechanicalproperties comparable to those of cobalt-baseStellite 6 (Ref 15).

Cobalt-containing austenitic stainlesssteels have been developed by Hydro-Quebec forthe repair of the cavitation erosion damage of itshydraulic turbines. Cavitationrefers to the forma­tion of vapor bubbles, or cavities, in a fluid that ismoving across the surface of a solid component.

Surface damage, um, at Stress,MPa (ksi)indicatedtestsin air tests in water

Alloy/form 140 (20) 275 (40) 415 (60) 140(20) 275(40) 415 (60)

NOREM Ol/solid 0.4 0.9 1.1 0.3 0.4 0.4NOREM Ol/solid 0.7 1.6 2.8 nt nt ntNOREM OI/metal- 0.7 0.4 0.6 0.7 1.1 1.3

coreNOREM Ol/metal- 1.9 2.3 4.7 1.2 1.3 1.5

coreNOREM OI/metal- 0.3 0.5 1.4 0.3 0.5 0.7

coreNOREM 04/metal- 0.6 0.7 1.0 nt nt nt

coreStellite 21/solid 1.3 1.9 2.4 0.5 1.0 1.5Stellite 6/solid 2.2 2.6 2.8 1.1 1.7 1.6

Source: H. Ocken, ElectricPowerResearchInstitute

0.312.02.01.0

bal

Composition,wt%Element

CarbonChromiumManganeseSiliconIron

Martensitic air-hardening steels (including Table 6 Galling wear of gas-tungsten arc weld overlays made from cobalt-free NOREM alloysstainless steels) are metal-to-metalwear alloys that,with care,can be applied(withoutcracking)towear­ing areas of machineryparts. Hence, thesematerialsare commonly referred to as machinery hard/acingalloys. Typical applications of this alloy family in­clude undercarriage components of tractors andpower shovels, steel mill work rolls, and cranewheels. The stainless steel in this category is AWSER420, which has a hardness value of 45 HRC andthe following chemicalcomposition:

Table 7 Chemical compositions of the NOREM hardfacing alloys listed in Table 6

Cobalt-free austenitic stainless steels havebeen developed to replace cobalt-base hardfacingalloys (Stellite grades) in nuclear power plant ap­plications. Cobalt-base alloys have been tradi­tionally used for hardfacing nuclear plant valves(check valves, seat valves, and control valves),because they generally show high corrosion resis­tance and superior tribological behavior undersliding conditions. However, even the (usuallylow) corrosion and sliding-wear rates of thesehardfacings lead to a release of particles with ahigh cobalt content. The particles are entrained inthe coolant flow through the core, and Co60

,

which is a strong emitter of gamma radiation, is

Nominal composition,\,,(%(a)AlIoylVendor C Mn Si Cr Ni Mo P S Other

NOREM Ol/Stoody 1.3 9.7 3.3 25 4.2 2 0.02 0.01 O.INNOREM Ol/Cartech 1.27 6.15 3.17 25.5 4.47 2.03 0.006 0.009 0.12N,

0.02Cu,O.OICo

NOREM 04/Anval 1.17 12.2 5.13 25.3 8.19 1.81 0.029 0.01 0.22N,0.05Cu,0.068Co

NOREMA/Anval 1.22 7.5 4.7 26.5 4.9 2.21 0.018 0.Dl5 0.236N,0.03Nb,0.007Ti,0.07Co

(a) Single valuesare maximumvalues.Source:H. Ocken,ElectricPowerResearchInstitute

114 / Introduction to Stainless Steels

Table 8 European-developed cobalt-free hardfacing alloys Studies by Simoneau (Ref 16 and 17) at the In­stitut de Recherche d'Hydro-Quebec have deter­mined that the elements most favorable tocavitation resistance, in decreasing order, arecarbon, nitrogen, cobalt, and silicon. The combi­nation of carbon and nitrogen has an equivalenteffect, whereas chromium and manganese show aneutral effect within the 8 to 12% Co range.Nickel is detrimental. Figure 9 presents the effectof carbon plus nitrogen, and Fig. 10 presents theeffect of cobalt concentration, on the steady-staterate of cavitation erosion. These results allow theformulation of alloys with the appropriate amountof austenitizer (carbon, nitrogen, cobalt, manga­nese) and ferritizer elements (chromium, silicon,molybdenum) to stabilize the austenite phase atroom temperature. Cobalt alone is not sufficientas an austenitizer, because it only very slightlylowers the martensitic transformation tempera­ture. Thus, it must be supplemented with manga­nese, carbon, or nitrogen. Inorder to increase theductility and the corrosion resistance, carbon canbe replaced by nitrogen.

The composition of cobalt-containing austeni­tic stainless steels provides a balance of elementsin such a way that an essentially austenitic yphasewith a low stacking fault energy is obtained in anas-welded and solidified weld overlay. This me­tastable face-centered cubic (fcc) y-phase trans­forms under stress to a body-centered cubic (bee)rx-martensitic phase exhibiting fine deformationtwins. The phase transformation and twinning ab­sorb the energy of the shock waves generated bythe collapsing of the vapor bubbles. Such behav­ior is similar to that of cavitation-resistant high­cobalt alloys, which exhibit a transformationfrom a fcc y-phase to a hexagonal close-packed(hcp) s-phase in addition to twinning.

In the" incubation" period of the alloy surfaceunder a cavitation condition, the hardness in­creases as deformation twins form on the surface.The metal loss during this period is generallyminimal, and the surface is smooth and hardened.Unlike the case for other alloys, such as 300-se­ries stainless steels, this incubation period is longand high hardness levels (450 HV) are reached inthe steady state.

After the surface is fully hardened, furthercavitation causes damage by initiating fatiguecracks and subsequent detachment of particulatesat the intersections of the deformation twins. Be­cause the twins are relatively small and the metalparticles also small, the result is a uniform andslow degradation of the metal surface.

The main effect of these chemical composi­tion modifications on the mechanical propertiesof austenitic stainless steels is illustrated by thetensile curves shown in Fig. 11. The work- orstrain-hardening coefficient increases markedlywhen going from 304 to 301, and in particular forthe cobalt-containing stainless steel. Decreasingthe nickel and replacing it with cobalt results in adecrease in yield strength and in an important in­crease in ultimate tensile strength. Although theinitial strain-hardening coefficient for these steelsis quite similar, it increases to a very high value atlarger strains (up to 1.26) for cobalt-containingstainless steels. This larger strain hardening is as-

0.6

0.2179.52.590.2

bal

0.5

Olher

0.5V

301304Fe-18Cr-l0CD

2.0 W, unspecifiedother elements ,,5

Composition,wt%

oo

Mo

3.2

0.2 0.3 0.4True strain

0.1

Tensile stress-straincurves of 308,301, and co­balt-containing stainless steels.Source: Ref 18

O"--_.L-_-'-_---'-_---'-_--IL...-----I

o

1600

2000

roa.~ 1200

Element

moval of small metallic particles from the ex­posed surface. This eventually results in seriouserosion damage to the metallic surfaces and is amajor problem in the efficient operation of hy­draulic equipment, such as hydroturbines, run­ners, valves, pumps, ship propellers, and so on.The damage caused by cavitation erosion fre­quently contributes to higher maintenance and re­pair costs, excessive downtime and lost revenue,use of replacement power (which is very expen­sive), reduced operating efficiencies, and short­ened equipment service life.

The outstanding cavitation erosion resistanceof cobalt-containing austenitic stainless steelscomes from a patented chemistry formulated toyield the highest work-hardening rate, with a highinterstitial carbon and nitrogen content. For thesame reason, and in order to stabilize a fullyaustenitic structure, nickel has been replaced bymanganese and cobalt, which are balanced withsilicon and chromium to give good corrosion re­sistance. The nominal composition for these al­loys is:

CarbonChromiumManganeseSiliconCobaltNitrogenIron

Fig. 11

1.1

o

8 10 12 14 16 18 20CDbait concentration, %

0.3 0.5 0.7 0.9C + N concentration, %

Effectofcobalt additions on cavitation erosionof austenitic stainless steels. Source: Ref 17

6

o00

010

00 0 Ctp 0 goo oo Cb 0

o

d90 --;:.°.::.°--"----:-0;---100

a ~

°0 0

0.6L...-_---'-__-'-__.L-_--I__-'

0.1

0.6'-----'-_-'-_.L---'_---'-_-'-_L---l

4

~_ 1.8co.~

w 1.4

Chemicalcomposition,wt%(a)Alloy C Mn Si Cr Ni

Everit50 2.5 ,,1.0 ,,0.5 25.0Fox Antinit Dur 300 0.12 6.5 5.0 21.0 8.0CeniumZ20 0.3 NR(b) NR(b) 27 18

(a)Singlevaluesaremaximum values.(b) NR, notreported. Source:Ref 15

2.6

3

These vapor bubbles are caused by localized re­ductions in the dynamic pressures of the fluid.The collapse of these vapor cavities produces ex­tremely high compressive shocks, which leads tolocal elastic and/or plastic deformation of the me­tallic surfaces. These repeated collapses (com­pressive shocks) in a localized area cause surfacetearing or fatigue cracking, which leads to the re-

<D 00 0

~ 2.2E

~co'ii)

ew

Fig. 9 Effect of carbon plus nitrogen additions on cavi­tation erosion of cobalt-containing alloys.

Source: Ref 17

Fig. 10

Stainless Steel Cladding and Weld Overlays / 115

35

30

304N304301NFe-1BCr-l0Co

5

Ou:;~~;:::[g-::::I':::=------,------,-----,-,--

o 5 10 15 20 25 30 35 40 45Elongation, %

35.00 34

30.00

25.00s:'§;E 20.00ell'§c

15.000'(;;

eUJ

10.00

5.00

0.001020 308SS 301SS CA-6NM fe-15Mn-14Cr Stellile-21 Stellile-6 fe-IOCr-lOCo fe-18Cr-8Co

Fig. 12 Deformation-induced martensitic transforma­tion measured in tensile tests.Source: Ref18 Alloy

Fig. 14 Comparison of cavitation erosion rate of various materials. Source: Ref 18

1OO.........-L~-'-.o-1~-'-~l-o._'_~.L............L............Jo 50 100 150 200 250 300 350 400 450

Depth.jim

SELF BRAZINGMATERIAL(CopperCladStainlessSteel)

Heatexchanger fabricated using clad brazing("self-brazing") materials

Fig. 15

The choice of a material for a particular applica­tion depends on such factors as cost, availability,ap­pearance, strength, fabricability, electrical orthermal properties, mechanical properties, and cor-

Designing with Clad Metals (Ref6)

thematerialsareadequateformostapplicationsinflowing river or tap waters.

The original experimental cobalt-containingstainless steels were named IRECA to denote Im­proved REsistance to CAvitation. The currentlycommercially available welding consumablesthat can be deposited on stainless and carbon steelsubstrates are 1.2 mm (0.045 in.) and 1.6 mm(1/16 in.) gas-metal arc welding wires and 3.2 mm(1/8 in.) and 4.0 mm (5/32 in.) shielded metal arcwelding electrodes. The name for these consu­mables is Hydroloy HQ9 13, which is a tradenameof Thermodyne Stoody. Additional informationon cobalt-containing stainless steel hardfacing al­loys can be found in Ref 16 to 23 and in the article"Tribological Properties" in this Volume.

SELFBRAZING MATERIAL(CopperClad StaInlessSteel)

sociated with a faster initial martensitic transfor­mation, 'Y~a', of the less stable austenite phase,as shown in Fig. 12. The higher the cavitation re­sistance, the less the plastic deformation requiredto transform the fcc "{-austenitic phase to the beea'-martensitic phase. For the cobalt-containingsteel, only 5% elongation is required to producesome 25% transformation,

Figure I3 presents the actual hardness valuesreached by the material surface exposed to cavita­tion. Almost no cavitation-deformation harden­ing could be detected for 1020 carbon steel,whereas substantial strain hardening was meas­ured for austenitic stainless steels and the cobalt­base alloy, in good correlation with their ultimatetensile strength and cavitation resistance. Thehardness values measured on the surfaces ex­posed to cavitation also correspond quite well tovalues equivalent to their ultimate strength. It ap­pears to be not so much the initial hardness or thestrain energy (area under the stress-strain curve)that controls cavitation resistance, but rather thestrain-hardening capability under cavitation ex­posure (Ref 18). Figure I3(b) shows that strainhardening is restricted to a very thin surface layer« 50 um), which is even thinner for the cobalt­containing alloys.

Cobalt-containing austenitic stainless steelsare about ten times more resistant to cavitationerosion than the standard 300-series stainlesssteels (Fig. 14). Although cobalt-containingstainless steels may become less ductile be­cause of their high work-hardening coefficient,their ductility is good enough to be welded orcast without cracking. The as-welded hardnessis around 25 HRC, with work-hardened materi­als reaching 50 HRC. With a tensile elongationbetween 10 and 55%, the annealed yieldstrength is around 350 MPa, and the ultimatestrength can exceed 1000 MPa (145 ksi). Thecorrosion resistance is fair, comparable to thatof type 301 stainless steel, being somewhat lim­ited by the higher carbon content. Nevertheless,

301

30B

1020

Fe-1BCr-l0CoA_

II-- Stellite-21

Fe-18Cr-l0CoA

Stellite-21d d

50 100 150 200 250Cavitation time, min

Cavitation-induced surface (a) and cross­section (b) hardening in various materials.

{--30B

o(ID- - - - 1020 (ferrite)

o100'----_-'-_---'--__'----_-'-_---'--_---'

-50

400

500

'0~ 400Ol

~>I~300

c"E«ls:§ 200~

~.Q

::: 300

>I

~ 0

s ~""'--:«i 200s:e -----l:l.-__--==-_oo

~ ""-------XJo---_:>----~~'""t

(a)

Fig.13

(b)

Source: Ref 18

116/ Introduction to Stainless Steels

Table 9 Properties of copper-clad stainless steel brazing alloys

Layerthickness Tensile 0.2%yield strength Elongation in

Material system ratio MP. ksi MP. ksi 50mm(2ln.),%

Two-layer systems

C12200/304LSS 6/94 590 86 255 37 5513.5/86.5 650 94 300 43 55

CI2200/409SS 15/85 400 58 215 31 36

Three-layer systems

CI2200/304LSS/ 10/80/10 600 87 310 45 55CI2200 13n4/13 575 83 290 42 53

32/34/32 380 55 170 25 48CI2200/409SS/ 10/80/10 385 56 205 30 37C12200 15/80/5 385 56 205 30 37

Source:Ref 25

rosion resistance. Clad metals provide a means ofdesigning into a composite material specific prop­erties that cannot be obtained in a single material.

Self-brazing materials, such as copper-cladstainless steel (Cu/SS or Cu/SS/Cu), provide anexample of the unique properties designed into aclad material. Clad brazing materials are pro­duced as strips, using the cold roll bonding tech­nique. The strips comprise a base metal that is cladwith a brazing filler metal on either one or bothsides. These products are used primarily in high­volume manufacturing operations, such as theproduction of heat exchangers, brazed bellows,and honeycomb structures. The use of a self-braz­ing sheet reduces the total part count, simplifiesthe assembly operation (because the brazing fillermetal is always present on the core material), andreduces assembly time and, therefore, cost. In ad­dition, there is no need for the application of fluxor for its subsequent removal. This not only savesthe initial purchase cost of the flux, but also thewaste-management cost associated with the dis­posal of the spent material.

Figure 15 depicts an automotive transmissionfluid cooler that was assembled using clad braz­ing materials. A turbulator is brazed to a copper­clad stainless steel base and cover. The base andcover are formed from a stainless steel strip con­taining copper braze on one side. After brazing,the dimensional changes in this part are minimal,

Stainlesssteel

Aluminum

Fig. 17 Photomicrographs of cross sections of type304 stainless-steel-clad carbon steel. (a)As­

polished. 300x. (b) Polished and etched. SOOx

(a)

Fig. 18 Stainless-steel-clad aluminum truck bumpermaterial that combines the corrosion resis­

tance of stainless steel with lightweight aluminum

ing, power, and pollution control industries. Spe­cific uses include heat exchangers, reaction andpressure vessels, furnace tubes, and tubes and

(b)

Designing Clad Metals for CorrosionControl (Ref 6)

which is important when making a hermeticallysealed heat exchanger. Figure 16 shows a typicalclad brazing strip of copper-clad stainless steel.Properties of two-layer and three-layer brazingstrips are listed in Table 9. Additional material onclad brazing alloys can be found in Ref 24 and 25.

Clad metals designed for corrosion controlcan be categorized as follows:

• Noble metal clad systems• Corrosion barrier systems.. Sacrificial metal systems• Transition metal systems.. Complex multilayer systems

Proper design is essential for providing maximumcorrosion resistance with clad metals. This sectionwill discuss the basis for designing clad metals forcorrosion resistance.

Noble metal clad systems are materials hav­ing a relatively inexpensive base metal coveredwith a corrosion-resistant metal. Selection of thesubstrate metal is based on the properties requiredfor a particular application. For example, whenstrength is required, steel is frequently chosen asthe substrate. The cladding metal is chosen for itscorrosion resistance in a particular environment,such as seawater, sour gas, high temperature, andmotor vehicles.

A wide range of corrosion-resistant alloysclad to steel substrates have been used in indus­trial applications. One example is type 304 stain­less steel on steel. Figure 17 shows cross sectionsof this material. The uniformity of the bond inter­face is apparent in Fig. 17(a), and in the polished­and-etched condition (Fig. 17b), themetallographic structure of the stainless steel isclearly visible. The grain structure is analogous tothat of annealed stainless steel strip.

Clad metals of this type are typically used inthe form of strip, plate, and tubing. The noble met­al cladding ranges from commonly used stainlesssteels, such as type 304, to high-nickel alloys,such as Inconel625. These clad metals find vari­ous applications in the marine, chemical process-

Photomicrograph of typical clad brazingmaterial, C12200 copper clad to 3041.

Fig. 16stainless steel

Stainless Steel Cladding and Weld Overlays / 117

Low-carbon steel

(a)

Low-carbon steel

Stainless steel(b)

Fig. 19 Illustrations of the corrosion barrier princi-ple. (a)Solid carbon steel. (b)Carbon-steel­

clad stainless steel

tube elements for boilers, scrubbers, and othersystems involved in the production of chemicals.

Another group of commonly used noble metalclad metals uses aluminum as a substrate. For ex­ample, in stainless-steel-clad aluminum truckbumpers (Fig. 18), the type 302 stainless steelcladding provides a bright corrosion-resistantsurface that also resists the mechanical damage(stone impingement) encountered in service. Thealuminum provides a substrate with a highstrength-to-weight ratio.

Corrosion Barrier Systems. The combina­tion of two or more metals to form a corrosion bar­rier system is most widely used where perforationcaused by corrosion must be avoided (Fig. 19).Low-carbon steel and stainless steel are suscepti­ble to localized corrosion in chloride-containingenvironments and may perforate rapidly. Whensteel is clad over the stainless steel layer, the cor­rosion barrier mechanism prevents perforation.Localized corrosion of the stainless steel is pre­vented: The stainless steel is protected galvani­cally by the sacrificial corrosion of the steel in themetal laminate. Therefore, only a thin pore-freelayer is required.

The example shown in Fig. 20 of carbon steelclad to type 304 stainless steel demonstrates howthis combination prevents perforation in seawa­ter, while solid type 304 stainless steel does not.This material can be used for tubing and for wirein applications requiring strength and corrosionresistance.

(a)

Carbon steel cannot be used when increasedgeneral corrosion resistance of the outer claddingis required. A low-grade stainless steel with goodresistance to uniform corrosion but poor resis­tance to localized corrosion can be selected. illseawater service, type 304 stainless steel that isclad to a thin layer of Hastelloy C-276 provides asubstitute for solid Hastelloy C-276. ill this corro­sion barrier system, localized corrosion of thetype 304 stainless steel is arrested at the C-276 al­loy interface.

The most widely used clad metal corrosionbarrier material is copper-clad stainless steel(Cu/430 SS/Cu) for telephone and fiber optic ca­ble shielding. In environments in which the corro­sion rate of copper is high, such as acidic orsulfide-containing soils, the stainless steel acts asa corrosion barrier and thus prevents perforation,while the inner copper layer maintains high elec­trical conductivity of the shield.

Sacrificial metals, such as magnesium, zinc,and aluminum, are in the active region of the gal­vanic series and are extensively used for corrosionprotection. The location of the sacrificial metal inthe galvanic couple is an important considerationin the design of a system. By cladding, the sacrifi­cial metal may be located precisely for efficientcathode protection, as described for the stainless­steel-clad aluminum automotive trim shown inFig. 2.

Transitional Metal Systems. A clad transi­tional metal system provides an interface betweentwo incompatible metals. It not only reduces gal­vanic corrosion where dissimilar metals arejoined, but also allows welding techniques to beused when direct joining is not possible.

Complex Multilayer Systems. ill manycases, materials are exposed to dual environ­ments; that is, one side is exposed to one corrosivemedium, and the other side is exposed to a differ­ent one. A single material may not be able to meetthis requirement, or a critical material may be re­quired in large quantity.

In small battery cans and caps, copper-clad,stainless-steel-clad nickel (Cu/SS/Ni) is usedwhere the external nickel layer provides atmos­pheric-corrosion resistance and low contact resis­tance. The copper layer on the inside provides theelectrode contact surface as well as compatible

(b)

cell chemistry. The stainless steel layer providesstrength and resistance to perforation corrosion.

Welding Austenitic-Stainless-CladCarbon or Low-Alloy Steels (Ref 26)

To preserve its desirable properties, stainless­clad plate can be welded by either of the two fol­lowing methods, depending on plate thicknessand service conditions:

• The unclad sides of the plate sections are bev­eled and welded with carbon or low-alloy steelfiller metal. A portion of the stainless steel clad­ding is removed from the back of the joint, andstainless steel filler metal is deposited.

CD The entire thickness of the stainless-clad plateis welded with stainless steel filler metal.

When the nonstainless portion of the plate is com­paratively thick, as in most pressure vessel applica­tions, it is more economical to use the first method.When the nonstainless portion of the plate is thin,the second method is often preferred. When weld­ing components for applications involving elevatedor cyclic temperatures, the differences in the coeffi­cientsof thermalexpansion of thebase plate and theweld should be taken into consideration.

All stainless steel deposits on carbon steelshould be made with filler metal of sufficientlyhigh alloy content to ensure that normal amountsof dilution by carbon steel will not result in a brit­tle weld. In general, filler metals of type 308, 316,or 347 should not be deposited directly on carbonor low-alloy steel. Deposits of type 309, 309L,309Cb, 309Mo, 310, or 312 are usually accept­able, although type 310 is fully austenitic and issusceptible to hot cracking when there is high re­straint in a welded joint. Thus, welds made withtype 310 filler metal should be carefully in­spected. Welds made with types 309 and 312 fillermetals are partially ferritic and therefore arehighly resistant to hot cracking.

The procedure most commonly used for mak­ing welded joints in stainless-clad carbon or low­alloy steel plate is shown in Fig. 21. Stainless steelfiller metal is deposited only in that portion of theweld where the stainless steel cladding has beenremoved, and carbon or low-alloy steel filler met­al is used for the remainder. The backgouged por-

(e)

Fig. 20 Photomicrographs of cross sections of materials after 18 months of immersion in seawater at Duxbury, MA. (a) Low-carbon steel. (b) Type 304 stainless steel. (c) Carbon­steel-clad type 304 stainless steel

118/ Introduction to Stainless Steels

(b) Fitted up

(b) Fitted up

(d) Surfaced fromside B

(d) Inlaid and welded

(c) Welded from side A (d) Welded from side B

Butt joint-------~

(a) Faces beveled

(c) Welded from side A (d) Welded from side B

Corner joint -

(a) Faces beveled

Fig.23 Procedures for welding V-groove butt andcorner joints in stainless-clad carbon or low­

alloy steel plate, using stainless steel filler metal exclusively,The clad plates are beveled and fitted up (a and b, butt andcorner joints), The root of the weld is cleaned and gouged, ifnecessary, before depositing stainless weld metal from thestainless steel side (d, butt and corner joints),

(b) Fitted up

(b) Filled up

q.--e- ..3 '<, .. ­a(min)

( a) Faces beveledand claddingstripped

(c) Welded from side A,weld ground flush an side B

~----------------MethadA------ ---------

(c) Welded fromside A,weld ground flush on side B

~----------------Method B---------------~

Fig. 22 Alternative procedures for joining stainless-clad carbon and low-alloy steel plate involving different tech-niques for replacing portions of the stainless steel cladding removed before welding the carbon or low-alloy

steel side. The joint is prepared by beveling side A and removing a portion of the stainless steel cladding from side B to aminimum width of 9.5 mm WB in.) from each side of the joint, and the joint is fitted up in position for welding. Use of a rootgap (not shown) is permissible (a and b, methods A and B).Carbon steel filler metal is deposited, and the root of the weld isground flush with the underside of the carbon steel plate (c, methods A and B).The area from which cladding was removedis surfaced with at least two layers of stainless steel weld metal (d, method A), or an inlay of wrought stainless steel can bewelded in place (d, method B).

filler metal is limited to replacement of the clad­ding that was removed prior to making the carbonor low-alloy steel weld. This method is more ex­pensive than the method described in Fig. 21 be­cause of the cost of removing a larger portion ofthe cladding and depositing more stainless steelfiller metal. Because there is no danger of alloycontamination from the cladding layer, method Ain Fig. 22 permits the use of faster welding proc­esses, such as submerged arc welding, in deposit­ing the carbon steel weld.

In depositing the stainless steel weld metal, thefirst layer must be sufficiently high in alloy con­tent to avoid cracking as a result of normal dilu­tion by the carbon steel base metal. A stringerbead technique should be employed; penetrationmust be held to a minimum. If the proper weldmetal composition is not achieved after the sec­ond layer has been deposited, a portion of the sec­ond layer should be ground off and additionalfiller metal should be deposited to obtain the de­sired composition. Figure 22(d) of method Bshows an alternative procedure in which the ex­posed carbon steel weld on side B is covered bywelding an inlay of wrought stainless steel to theedges of the cladding.

The most common method of joining stain­less-steel-clad carbon or low-alloy steel platewith a weld that consists entirely of stainless steelis shown in Fig. 23. This method is most fre­quently used for joining thin sections of stainless­clad plate. The same basic welding procedure isfollowed for both the butt and comer joints shownin Fig. 23. After the plate has been beveled and fit­ted up for welding, a stainless steel weld is depos­ited from the carbon steel side, using a filler metalsufficiently high in alloy content to minimize dif­ficulties (such as cracking) resulting from welddilution and joint restraint. Types 309 and 312filler metals are suitable for this application.

SIDE A

(d) Gouged from side B

( f ) Protective plate weldedan

Weld metal (carbon steel)

~D?i;~3'11fj~~-Cladding SluE B

(b) Fitted up

(e) Welded from side B

Fig.21 Procedure for welding stainless-clad carbonand low-alloy steel, using stainless steel filler

metal only in portion of joint from which cladding was re­moved. (a) and (b) The clad plates are machined for a tightfitup, with the bottom of the weld groove not less than 1.6mm (1/16 in.) above the stainless steel cladding. (c) Carbonsteel filler metal is deposited from side A (a low-hydrogenfiller metal is used for the first pass), taking care not to pene­trate closer than 1.6 mm (1/16 in.) to the cladding. (d) Stain­lesssteel cladding on side Bis backgouged until sound carb­on steel weld metal is reached. (e) The backgouged grooveis filled with stainless steel weld metal in a minimum of twolayers. (f) When required for severely corrosive service, aprotective strip of stainless steel plate may be fillet welded tothe cladding to cover the weld zone.

tion of the stainless steel cladding should be filledwith a minimum of two layers of stainless steelfiller metal (Fig. 2Ie); an additional layer is rec­ommended if a high weld reinforcement at thecladding surface can be tolerated.

If the cladding is of type 304 stainless steel, thefirst layer of stainless steel weld metal should beof type 309 or 312. Subsequent layers of weldmetal can be oftype 308. If the cladding is of type316, the first layer is deposited with type 309 Mofiller metal and the subsequent layers with type316. When the cladding is of type 304L or 347, thewelding procedure must be carefully controlled toobtain the desired weld metal composition in theouter layers of the weld. Chemical analysis ofsample welds should be made before joining cladplates intended for use under severely corrosiveconditions.

In some applications, anarrow protective plateof wrought stainless steel of the same composi­tion as the cladding is welded over the completedweld (Fig. 21f) to ensure uniformity of corrosiveresistance. The fillet welds joining the protectiveplate to the cladding should be carefully inspectedafter deposition. These welds, of course, are madewith stainless steel filler metal.

Figure 22 illustrates an alternative method(method A) of welding clad plate, in which a carb­on or low-alloy steel weld joins the carbon steelportion of the plate, and the use of stainless steel

After the stainless steel weld has been depos­ited from the carbon steel side (Fig. 23c), the rootof the weld is cleaned by brushing, chipping, orgrinding, as required, and one or more layers ofstainless steel filler metal are deposited (Fig. 23d).The filler metal composition should correspond tothat normally employed to weld the type of stain­less steel used for cladding. If the cladding is type304, the final layer of weld metal should be type308. If the cladding is type 316, it may be neces­sary to backgouge before deposition of the finalweld metal layers to ensure that the proper weldmetal composition is obtained at the surface of theweld.

ACKNOWLEDGMENTS

The editor thanks Howard Ocken, ProjectManager, Electric Power Research Institute(EPRI) and Raynald Simoneau, Vice-PresidenceTechnologie, Institut de Recherche d'Hydro­Quebec (IREQ), for their significant contribu­tions to this article. Mr. Ocken supplied materialon cobalt-free NOREM alloys developed atEPRI. Mr. Simoneau contributed material on co­balt-containing IRECA alloys that he developedatIREQ.

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