mechanical behavior of titaniumclad steel welded...

Introduction

Titanium (Ti) clad steel is widelyused for large pressure vessels andother equipment in different indus-tries to take advantage of the corro-sion resistance of Ti, but at a lowercost than solid Ti construction. Tita-nium-clad steel is produced by rollbonding (usually with an interlayer),direct explosive bonding (usually with-out an interlayer) (Ref. 1), or by a com-bination of explosive bonding and rollbonding (Ref. 2). Interlayers are used

to improve the bond strength of theclad steel or to overcome metal plastic-ity compatibility restrictions encoun-tered in roll bonding. Industrial-gradepure iron (Fe), ultralow-carbon steel,niobium (Nb) alloys, tantalum (Ta) al-loys, copper (Cu) alloys, and nickel(Ni) alloys have been used as interlay-ers in the cladding process (Refs. 3–6).Typical thickness of Ti-clad rangesfrom 2.0 to 19.0 mm (0.08 to 0.75 in.)depending on the application.

Titanium has not been success-fully fusion welded directly to steelbecause it has limited solubility for

Fe. If the solubility limit is exceeded,as in fusion welding, brittle inter-metallic compounds and carbidesform (Refs. 7, 8). Cracks form inthese phases due to the thermalstresses induced during cooling andcomplete separation along the Fe-Tiinterface may happen in the weldedjoint, as shown in Fig. 1. To avoidwelding Ti directly to steel, the mostcommon method of joining cladplates is the Batten Strip technique(Refs. 1, 9–11). The Ti cladding mate-rial is stripped back 15 to 20 mmfrom the weld joint, after which thesteel is welded and inspected. Next,the space where the cladding was re-moved is filled with Cu, Ti, or steelfiller strips. Finally, a Ti cover stripor Batten Strip about 50mm wide iswelded over the joint using filletwelds and gas tungsten arc welding(GTAW) techniques. This method hasseveral disadvantages including com-plexity of irregular geometry at noz-zle penetrations and attachments,complexity of testing for joint in-tegrity (including no reliable methodto inspect for root side purge failure),open root joint configuration subjectto widespread corrosion damage ofthe steel in the event of local failuresof the fillet welds on the battenstrips, potential for service failuresdue to low-cycle fatigue, difficulty ofrepair, and relatively high fabricationand testing costs.

Therefore, there is the need to de-velop reliable cost-effective methodsof joining or repairing Ti-clad steel

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OCTOBER 2014 / WELDING JOURNAL 369-s

SUPPLEMENT TO THE WELDING JOURNAL, OCTOBER 2014Sponsored by the American Welding Society and the Welding Research Council

Mechanical Behavior of TitaniumClad SteelWelded Joints

Mechanical properties of Ticlad steel welded joints deposited with different interlayer materials were evaluated using microhardness, bend, and shearbond strength testing in the

aswelded, after PWHT, and in thermally cycled conditions.

BY J. E. RAMIREZ

ABSTRACTTiclad steel welded joints made with different interlayer materialjoining process

combinations were evaluated using microhardness, bend, and shearbond strength testing. The effect of thermal cycling on the shearbond strength was evaluated as well. Ingeneral, all the welded joints present the highest hardness level at the interlayerTi interface and across the first Ti layer. The maximum hardness in welded joints made with theNiTi, NiCuTi, and NiCrTi interlayer systems was 607, 568, and 554 HV0.5, respectively.In the VTi and TiV systems, the respective maximum hardnesses were 307 and 409HV0.5, respectively, at the FeV interface. The maximum hardness observed in weldedjoints made with the CuTi interlayer ranged from 300 to 350 HV0.5. Different softeningresponses to either thermal cycles of additional Ti layers or PWHT were observed in different types of joints. Most of the joints failed the bend tests in the aswelded andPWHTed conditions. The NiTi, NiCuTi, and NiCrTiwelded joints failed along the interlayerTi interface and through the Ti weld layers. The CuTi welded joints made with theCSCGMAW process failed along the CuTi interface. The bondshear strength of both FeCu and CuTi interfaces in CuTi welded joints made with a combination of CSCGMAWand GTAWP processes in the aswelded, PWHTed, and thermally cycled conditionsranged from 204.5 to 259.8 MPa (29.6 to 37.6 ksi). The FeCu interface showed a largerdisplacement under maximum load as compared to that observed in the CuTi interface.

KEYWORDS • Cladding • Titanium • TiClad Steel • Thermal Cycling • Interlayer Materials

J. E. Ramirez ([email protected]) was a principal engineer with EWI and now is a principal engineer at DNV.GL in Columbus, Ohio.

Ramirez 10-12_Layout 1 9/11/14 4:54 PM Page 369

plates that provide a continuous jointwith acceptable mechanical and cor-rosion properties with and withoutpostweld heat treatment (PWHT).

Experimental Procedures Ti-clad steel welded joints were

made using different interlayer mate-rial-joining process combinations. Thewelded joints were tested in as-welded

and PWHT conditions.Some of the weldedjoints were tested afterexposure to thermal cy-cling. The mechanicalbehavior of the jointswas evaluated using mi-crohardness, bend, andshear bond strengthtesting.

Materials and WeldingConditions

TitaniumClad Base Metal andInterlayer Materials

The deposition of the interlayermaterial and corresponding Ti layersof the welded joints was done in 150-¥ 200-mm (6- ¥ 8-in.) explosion Ti-clad steel samples. The explosion Ti-clad steel base metals consisted ofSA-516-70 carbon steel with a nominalthickness of 27.5 to 38.0 mm (1.1 to

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WELDING JOURNAL / OCTOBER, 2014 VOL. 93370-s

Fig. 1 — A — Titanium deposit layer that cracked and broke off a commercially pure Fe weld deposit during cooling; B — weld deposit thatcracked.

Table 1 — General Characteristics of the Welding Consumables Used as Interlayer Materials andTi Layer for Welding the TiClad Steel Plates

Joint Designation Welding Process Filler Metal Designation Wire Size (in.)

Ni-Ti CSC-GMAW CPNi (ERNi-1) 0.062NiCu-Ti CSC-GMAW NiCu (ERNiCu-7) 0.062NiCr-Ti CSC-GMAW NiCr (ERNiCr-4) 0.062Cu-Ti CSC-GMAW CPCu (ERCu) 0.062Cu-Ti CSC-GMAW CPCu (ERCu) 0.062

GTAW CPTi (ERTi-1) 0.035V-Ti GTAW — 0.062/0.045

Ti Layers CSC-GMAW/GTAW ERTi-1 0.062/0.035

Fig. 2 — A — General view of joint design; B — cross section of joint after deposition of interlayer material; C —after completion of the joint.

A B

A B

C


1.5 in.) and SB-265-1 Ti clad with anominal thickness between 4.8 to 8.0mm (0.188 to 0.313 in.). Based onmetallurgical characteristics and po-tential compatibility with the Fe-Tisystem, and availability as commercialwelding wires, the interlayer materialsthat were used for joining Ti-clad steelinclude commercially pure (CP) nickel(Ni), nickel-copper alloy (NiCu),nickel-chromium alloy (NiCr), CPvanadium (V), and CP copper (Cu)(Ref. 12). The general description ofthe welding consumables used forwelding of the Ti-clad steel plates islisted in Table 1.

Joint Design

The Ti-clad steel base metal sam-ples have a widegroove prepared bythe strip-back method. The joint de-sign of the wide-groove included a rootthat was between 19.0 to 25.0 mm

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Fig. 3 — A — General view of Ticlad steel welded joints made with the CSCGMAW process; B — joint made with a combination of theCSCGMAW and the GTAWP processes.

Fig. 4 — A — General view of FeCu interface bond shearstrength samples; B — test setup.

A

A

B

B

Table 2 — Ranking in Decreasing Order of Suitability of InterlayerWelding Process Combination forMaking FullSize TiClad Steel Welded Joints

Description of Interlayer SystemRanking Interlayer Design (a) Welding Process Comments

1 Fe-Cu-Ti CSC-GMAW + GTAW-P 1. Poor wettability of Ti on Cu

2 Fe-Cu-Ti CSC-GMAW 1. Short contact tip life duringdeposition of Ti2. Poor wettability of Ti on Cu

3 Fe-Ni-Ti CSC-GMAW 1. Short contact tip life duringdeposition of Ti2. Cracking susceptibility

4 Fe-NiCu-Ti CSC-GMAW 1. Short contact tip life duringdeposition of Ti2. Cracking susceptibility

5 Ti-V-Fe GTAW-P 1. Cracking susceptibility

6 Fe-NiCr-Ti CSC-GMAW 1. Short contact tip life during deposition of Ti2. Cracking susceptibility

(a) The designation of the interlayer system indicates the sequence of deposition of the different interlayer materials andTi layers in the joint.

Ramirez 10-12_Layout 1 9/11/14 4:54 PM 71

(0.75 to 1.0 in.) wide and a 22-deg bevelangle. Additionally, the groove was ma-chined to a depth of about 2.50 mm(0.10 in.) into the steel substrate, as

shown in Fig. 2A and B. This joint de-sign replicates the Ti portion of Ti-cladsteel butt joints, which is the more crit-ical part of this type of joint.

Welding Process

Different joining processes were consid-

ered based on the metallurgical charac-teristics of the selected interlayers, andon the typical dilution of the joiningprocesses. The latter is significant be-cause low-dilution processes limit theamount of melting, as well as the ther-mal experience of the base metal attemperatures where intermetallic com-pounds may form. Considering thecommercial availability of consumables,ease of deployment in the field, and rel-atively low equipment investment, arcwelding processes were considered theprimary process of choice.

A relatively new gas metal arc welding(GMAW) process variant called con-trolled short circuit (CSC)-GMAW waschosen to deposit the selected interlayersand Ti layers in the welded joints. TheCSC-GMAW process involves “pulsing”the wire feed in conjunction with thewelding current to achieve improvedcontrol of welding heat input and dilu-tion with minimal spatter. Welding pa-rameters of the CSC-GMAW processinclude up-wire feed speed (Up WFS)(m/min), down-WFS (m/min), initial arclength (mm), arc current sequence, andshort-circuit current sequence. Each cur-rent sequence has three levels to set(start, pulse, and end). These three cur-

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Fig. 6 — Microhardness profile of TiVFewelded joint with two carbonsteel weldmetal layers, in the aswelded condition(GTAWP process).

Table 3 — Welding Conditions for Depositions of Different Layers of Weld Metal in the Weld Joints Using CSCGMAW

Arc Current Sequence Short-Circuit Current Sequence

Start Start Pulse Pulse End Start Start Pulse Pulse EndWeld Layer Shielding Gas Current Current Current Current Current Current Current Current Current Current

(A) Time (ms) (A) Time (ms) (A) (A) Time (ms) (A) Time (ms) (A)

Ni on steel 100% He 100 NA 100 NA 100 50 NA 50 NA 50Ti on Ni 100% He 80 5 60 5 40 40 2.5 60 NA 60

NiCu on steel 100% He 100 NA 100 NA 100 50 NA 50 NA 50Ti on NiCu 100% He 80 5 60 5 40 40 2.5 60 NA 60NiCr on steel 50%Ar/50%He 100 NA 100 NA 100 50 NA 50 NA 50Ti on NiCr 100% He 80 5 60 5 40 40 2.5 60 NA 60CPCu on steel 100% He 130 NA 130 NA 130 50 NA 50 NA 50CPCu on steel 100% He 150 NA 150 NA 150 50 NA 50 NA 50Ti on CpCu 100% He 120 5 100 5 80 40 2.5 60 NA 60

Ti on Ti 100% He 80 5 60 5 40 40 2.5 60 NA 60

Wire Feed Speed Initial Arc Weaving ParametersLength (mm)

Up WFS Down WFS Oscillation Speed Dwell Oscillation Forward Travel(m/min) (m/min) (mm/s) Time(s) Amplitude Speed (mm/s)

(mm)

10 15 0.0 7.4 0.2 19.8 27.68 10 1.0 12.0 0.3 22.9 11.8

10 15 0.0 9.4 0.2 20.3 27.68 10 0.5 12.0 0.3 23.6 11.8

15 15 0.0 7.4 0.2 21.1 31.58 10 0.5 12.0 0.3 22.3 11.8

15 15 0.0 14.6 0.3 17.8 11.010 10 0.0 19.8 0.3 16.5 11.08 10 0.0 12.0 0.3 21.1 11.88 10 0.5 12.0 0.3 23.6 11.8

Fig. 5 — Microhardness profile of NiCrTi welded joint with one and three Tiweld metal layers (1Ti, 3Ti), in the aswelded and PWHTed conditions (CSCGMAW process).


rent levels are used to control the beadshape and size. The start and pulse lev-els have a time associated with them.For the end current level, the current ismaintained until the next sequence isinitiated.

During the arc phase, the end of theelectrode is melted and a droplet isformed. At the same time, the electrodeis feeding forward toward the weld pool.The forward wire feeding speed is sethigher than the melt-off rate so that thearc will short out. Upon shorting, thedroplet at the end of the electrode ispulled into the weld pool by the liquidpool’s surface tension. The control sys-tem senses the voltage drop and pre-vents the current from spiking severely.A current sequence is implemented toallow resistive heating. The heat allowsfor a smooth arc ignition. At the sametime, the wire feeders reverse directionso that the electrode is being pulledaway from the weld pool. This makesthe short circuit break mechanically.This differs from any other short-circuiting process, which relies on theelectrode exploding to reestablish thearc. The process represents an advancein short-circuit metal transfer of theGMAW process (Refs. 13–17) and offersreduced heat input and dilution com-pared to other arc welding processes.

Welding Conditions

Due to the complexity inherent todissimilar metal joining, CSC-GMAWwelding parameters and weaving pa-

rameters were developed and optimized(Ref. 18). Six interlayer-joining processcombinations were ranked based ontheir general wettability behavior, weld-ability, and the ability to achieve accept-able welding conditions, as listed inTable 2. Table 3 lists the CSC-GMAWwelding parameters developed and usedfor depositing each interlayer materialand the subsequent Ti layers in thewelded joints. The GTAW-P parametersused to deposit the different Ti layers inwelded joints made with the Cu-Ti in-terlayer system are listed in Tables 4–6.

Figure 3 shows a general view ofsome of the welded joints made for me-chanical evaluation. The welded joint inFig. 3A shows a stepwise configurationat the ends. The three levels of the step-wise configuration from the end towardthe center of the sample correspond tothe surface of the weld deposit of the in-terlayer material, the surface of the firstTi deposit layer, and the surface of twoadditional layers of Ti. This arrange-ment allowed the characterization ofdeposits of the interlayer material in the

as-welded condition and an evaluationof the effects of thermal cycles inducedduring the deposition of one and threelayers of Ti on the properties of the in-terlayer materials and the welded jointas a whole. Figure 2C shows a cross sec-tion of a complete welded joint (inter-layer material plus three Ti layers). Thewelded joints were subjected to radi-ographic examination to evaluate thesoundness of the joints and to deter-mine the location of different speci-mens required for the mechanicalevaluation.

Postweld Heat Treatment

The PWHT of the welded joints wasconducted following the guidelines ofSection VIII of the ASME code for car-bon steel welded constructions. Theholding temperature was between1125° and 1150°F (607° and 620°C)and the holding time ranged from 1 h,15 min. to 1 h, 52 min. depending onthe thickness of the full-size joint.Heating rates above 800°F (427°C)

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Fig. 7 — Comparison of microhardness profiles of CuTi weldedjoints with three Ti weld metal layers deposited with the CSCGMAW process and with a combination of CSCGMAW andGTAWP processes.

Fig. 8 — General view of side bend samples obtained fromNiCuTi welded joints in the aswelded and PWHT conditions(CSCGMAW process).

Table 4 — GTAWP Parameters Used for the Deposition of First Layer of Ti in the CuTi welded Joint

Peak current (A) 250 Wire feed peak (mm/s) 8.5Back current (A) 10 Wire feed back (mm/s) 8.0Peak current time (s) 0.1 Arc voltage (V) 12.2Back current time (s) 0.5 Travel speed (mm/s) 1.1Wire entry angle (deg) 15 Wire to electrode distance (mm) 1.1Wire type ERTi-1 Wire diameter (mm) 0.9Electrode type 2% Ce Electrode diameter (mm) 3.2Electrode preparation (deg) 30, no Shielding gas type 75% He

flat 25% Ar


were controlled to be equal or lessthan 400°F/h/in. (8.7ºC/h/mm). Cool-ing rates above 800°F (427°C) wereequal or less than 500°F/h/in.(10.9°C/h/mm).

Evaluation of MechanicalBehavior of Welded Joints

The Ti-clad steel welded joints wereevaluated before and after PWHT. Thejoints were evaluated using microhard-ness profiles, bend testing, and bondshear-strength testing. The effect ofthermal cycling after PWHT on theshear-bond strength of some jointswas evaluated as well.

Microhardness Testing

Microhardness profiles were deter-mined in the through-thickness direc-tion of the deposited weld metalsstarting from the steel substrate to-ward the surface of the last layer of Tiweld deposit. The microhardness pro-files of the welded joints were deter-mined in deposits with one and three

Ti layers, respec-tively, and in the as-welded and PWHTconditions. Thehardness readingwas determinedusing hardness Vick-ers scale with a loadof 500 g (HV0.5).

Bend Testing

The ductility ofthe welded joints wasevaluated usingtransverse and longitudinal side-bendtests of samples in the as-welded andPWHT conditions. Two samples in theas-welded condition and two samples inthe PWHT condition from each systemwere tested for a total of four specimensper welded joint. According to the re-quirements of the ASME code SectionIX, the bend tests were run using an 8Tdiameter mandrel or die, where T is thethickness of the bend sample.

Bend testing was not conducted inwelds made with the Fe-V-Ti inter-layer system because crack-free joints

were difficult to make. Additionally,welded joints from the Ti-V-Fe sys-tem were tested only in the PWHTconditions because the bend samplescracked during machining in the as-welded conditions. This may indicatethe buildup of a high level of residualstresses during the welding of thisdissimilar metal joint.

Bond ShearStrength Testing

In order to measure the shear bondstrength of interfaces between dissimi-lar material layers in some of the weldedjoints, shear-strength testing was con-ducted according to the requirements ofASTM B898 (Ref. 19). Figure 4 shows aview of some Fe-Cu interface shearbond strength samples, and test setup.As shown in Fig. 4B, the sample is setbetween two alignment bars to controllateral displacement of the sample andforce the sample to move only in thevertical direction. One of the alignmentbars also acts as support (left-side bar in

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Fig. 9 — Cracks observed in longitudinal side bend samplesobtained from TiVFe welded joints in the PWHT condition(GTAWP process).

Fig. 10 — A — Microcracking observed in the FeV interface ofwelded joints deposited with the GTAWP process; B — crackarrested at the VTi interface.

Table 5 — GTAWP Parameters Used for the Deposition of Second Layer of Ti in the CuTi

Peak current (A) 160 Wire feed peak (mm/s) 6.4Back current (A) 80 Wire feed back (mm/s) 6.4Peak current time (s) 0.1 Arc voltage (V) 9.2Back current time (s) 0.25 Travel speed (mm/s) 1.1Wire entry angle (deg) 15 Wire to electrode distance (mm) 1.1Wire type ERTi-1 Wire diameter (mm) 0.9Electrode type 2% Ce Electrode diameter (mm) 3.2Electrode preparation (deg) 30, no Shielding gas type 75% He

flat 25% Ar

A

B


Fig. 4B) to restrict the vertical displace-ment of the area of the sample corre-sponding to the interlayer materialand/or Ti weld layers. The rest of thesample can be displaced freely in thevertical direction. During testing, load-ing was applied to the sample in thevertical-down direction through aplunger, as shown in Fig. 4B. As a resultof the plunger force and restraint of thealignment/support bar, a shear forcewas induced at the interface under eval-uation. Only welded joints made withthe Cu-Ti interlayer system using acombination of CSC-GMAW andGTAW-P processes were tested. Theshear bond strength of the Fe-Cu andCu-Ti interfaces was determined in theas-welded and in the PWHT condition.

Effect of Thermal Cycling

A section from a welded jointmade with the Ti-Cu/(CSC-GMAW +GTAW-P) combination and in thePWHT condition was subjected to 12thermal cycles. During each thermalcycle, the sample was heated to atemperature of 496ºC ±14º (925ºF±25º) and held at that temperaturefor one hour. The sample was then al-lowed to cool to a temperature lessthan 38ºC (100°F). The sample wasvisually inspected and evaluated withdye penetrant before and after the 12thermal cycles were completed to de-termine the presence of cracks. Nocracking was observed in the sample.Based on these results, one shearbond strength coupon representing

the Fe-Cu interfaceand a shear bondstrength couponrepresenting theCu-Ti interface wasmachined andtested according tothe requirements ofASTM B898. Theshear bond strengthresults were com-pared to those ob-tained fromspecimens thatwere not exposed tothermal cycling.

Experimental Observationsand Discussions

Microhardness Profiles

Microhardness profiles obtainedfrom some of the welded joints areshown in Figs. 5–7. In general, all thewelded joints present the highesthardness level at the interlayer-Ti in-terface and across the first Ti layer, as

can be observed in Fig. 5. This is con-sistent with the results of the light-and electron-microscopy characteriza-tions of the weld metal deposits thatindicated the presence of secondphases in those regions of the weldedjoints, as reported in a previous paper(Ref. 12). The maximum hardnesses inthe welded joints made with the Ni-Ti,NiCu-Ti, and NiCr-Ti interlayer sys-tems were 607, 568, and 554 HV0.5, re-spectively. In the V-Ti and Ti-Vsystems, the maximum hardness read-ings obtained from the weld deposits

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Fig. 11 — General view of side bend samples obtained fromCuTi welded joints in the aswelded and PWHT conditions(CSCGMAW process).

Fig. 12 — A — General view of bend test samples; B — cracksobserved in longitudinal side bend. Samples obtained fromCuTi welded joints in the aswelded and PWHT conditions(combination of CSCGMAW and GTAWP processes).

Table 6 — GTAWP Parameters Used for the Deposition of Filling Layers of Ti in the CuTi Welded Joint

Peak current (A) 140 Wire feed peak (mm/s) 10.6Back current (A) 140 Wire feed back (mm/s) 10.6Peak current time (s) 0.1 Arc voltage (V) 9.5Back current time (s) 0.25 Travel speed (mm/s) 1.5Wire entry angle (deg) 15 Wire to electrode distance (mm) 1.1Wire type ERTi-1 Wire diameter (mm) 0.9Electrode type 2% Ce Electrode Diameter (mm) 3.2Electrode preparation (deg) 30, no Shielding gas type 75% He

flat 25% Ar

A

B


were 307 and 409 HV0.5, respectively,at the Fe-V interface. The maximumhardnesses observed in the weldedjoints made with the Cu-Ti interlayerranged from 300 to 350 HV0.5.

Different softening responses tothermal cycles induced by either weld-ing of additional layers of Ti weld metalor PWHT were observed in differentwelded joints. In the Ni-Ti weldedjoints, more softening was induced inthe interface and first layer of Ti welddeposit by deposition of additional lay-

ers of Ti weld metalthan that induced bythe PWHT. The hard-ness of the NiCr-Tiwelded joint withthree Ti-layers showsa high value near theinterface between thesecond and third Tilayers, as shown inFig. 5, but the inter-face softened as a re-sult of the PWHT. Ingeneral, the NiCu-Tiand NiCr-Ti weldedjoints did not experi-ence major softeningas a result of the ther-mal experience in-

duced during either welding or PWHT,as shown in Fig. 5.

In the Fe-V-Ti system, extremelyhigh hardness was not observed acrossthe weld deposit, in spite of the ob-served presence of second phases at theFe-V interface (Ref. 12). However, thepresence of microcracks at the Fe-V in-terface may have influenced the resultsof the hardness readings. On the otherhand, in the Ti-V-Fe system, a highhardness peak was observed at the V-Fe

interface, as shown in Fig. 6. This mayhave resulted from a potential combina-tion of Fe and Ti at that interface. Thehigh degree of solid solubility betweenV and Ti could have induced a relativelyhigh concentration of Ti in the V welddeposit, making it available for reactionwith Fe at the V-Fe interface. Electronprobe microanalyzer (EPMA) analysiswas not conducted in weld metal de-posits made with either the Fe-V-Ti sys-tem or the Ti-V-Fe system to confirmthis statement. The weld metal of theCu-Ti welded joints shows the softestdeposits, especially in the PWHT condi-tion. This system experiences moresoftening during PWHT than during ad-ditional welding thermal cycles. In theCu-Ti system, as a result of the PWHT,the hardness level through the weld de-posit dropped to around 200 HV0.5. Dueto a wider Cu-Ti interface in weld metaldeposited with a combination of CSC-GMAW and GTAW-P processes (as com-pared to that deposited with only theCSC-GMAW process), a wider hard re-gion at the Cu-Ti interface and a lowerdegree of softening induced by thePWHT were observed, as shown in Fig. 7.

Side Bend Tests

Most of the bend samples obtainedfrom the welded joints failed the bendtest in the as-welded and PWHT con-ditions. Bend samples from the Ni-Ti,NiCu-Ti, and NiCr-Ti welded jointsfailed primarily along the interlayer-Tiinterface and through the Ti weld de-posits, as shown in Fig. 8. This may bedue to the formation of a wide andcontinuous hard interlayer-Ti interfaceconsisting of second phases, as well as

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Fig. 13 — A — General view of bond shear strength samplesof the FeCu and CuTi interfaces; B — ductile characteristicsof the shear fracture of the FeCu interface; C — brittle characteristics of the shear fracture of the CuTi interface.

Table 7 — Bond Shear Strength of the FeCu and CuTi Interfaces in TiClad Steel Welded Joint Deposited with a Combination of CSCGMAW and GTAWP Processes, in the AsWelded, PWHTed,and Thermal Cycled Conditions

Interface Condition Bond Shear Strength MPa (ksi)

Fe-Cu As-welded 204.5 (29.6)PWHTed 222.6 (32.2)

Thermal cycled 230.0 (33.3)Cu-Ti As-welded 259.8 (37.6)

PWHTed 227.2 (32.9)Thermal cycled 231.9 (33.6)

ASTM B898-99 Requirements 137.9 (20.0)

A

C

B


the presence of second phases throughthe first Ti layer (Ref. 12). In the NiCr-Ti interlayer system, second phaseswere observed even in the third layerof Ti weld deposits (Ref. 12). The dis-tribution of these hard, low-ductilityphases through the weld deposits maydetermine the paths of the cracks ob-served in the tested specimens.

As shown in Fig. 9, a bend sampleobtained from a Ti-V-Fe welded jointin the PWHT condition showed thepresence of small cracks in the sample;however, the cracks were perpendicu-lar to the Ti-V and V-Fe interfaces andwere confined to the V layer. This be-havior may be due to the microstruc-tural characteristics and mechanicalproperties of these two interfaces. TheTi-V interface was mainly free of sec-ond phase precipitation, and was nar-row and well defined (Ref. 12). Thepresence of microcracking in the V-Feinterface after welding, as shown inFig. 10A, may have acted as the nucle-ation sites for the cracks induced dur-ing the bend test. The good ductility ofthe steel weld metal deposits and ofthe Ti-V interfaces may have arrestedthe cracks, as shown in Fig. 10B.

The criteria established in theASME code Section IX for the evalua-tion of a bend test as part of qualifica-tion of a welding procedure indicatethat no crack larger than 3.0 mm (1⁄8in.) in any direction is allowed. How-ever, in this specific case, the thick-ness of the V layer was less than 3.0mm (1⁄8 in.). As a result, all the cracksobserved in the bend sample wereshorter than 3.0 mm (1⁄8 in.). Therefore,this bend sample met the require-ments established by the ASME code.Additionally, the amount of strain im-

posed during the bend test is largerthan that imposed by most metal-working processes used during themanufacturing of a vessel or duringservice. However, questions arise re-garding the unknown behavior ofthese microcracks that may be presentat the Fe-V interface during serviceand their potential effect on the in-tegrity of a vessel.

Most of the bend samples obtainedfrom Cu-Ti welded joints made with theCSC-GMAW process failed along the Cu-Ti interface, as shown in Fig. 11. Thisbehavior may be due to the narrow Cu-Ti interface, and the low degree of alloy-ing of Cu in the Ti weld metal depositsobserved in these welded joints (Ref.12). Cracking was not observed in anyone of the three layers of Ti weld de-posits. Low dilution of the Ti weld metalmay be responsible for the good ductil-ity of the Ti weld deposit observed dur-ing the bend test.

Conversely, the transverse and lon-gitudinal bend samples obtained fromCu-Ti welded joints made with a com-bination of CSC-GMAW and GTAW-Pprocesses showed the presence ofcracking along the Cu-Ti interface andcracks in the first Ti layer that wereperpendicular to this interface, asshown in Fig. 12. This different behav-ior (compared to the welds made onlywith the CSC-GMAW process) mayhave resulted from a wider hard Cu-Tiinterface observed in the welded jointsmade with a combination of weldingprocesses. The cracking did not propa-gate through the complete thicknessof the Ti-weld deposits, which indi-cates a good ductility of the last twolayers of Ti weld deposited in thewelded joints.

Bond Shear Strength and Effectof Thermal Cycling

The results of the bond shearstrength testing of the Fe-Cu and Cu-Ti interfaces in Cu-Ti welded jointsmade with a combination of CSC-GMAW and GTAW-P processes in theas-welded, PWHT, and thermal-cycledconditions are listed in Table 7. Thebond shear strength of both interfacesin all conditions ranged from 204.5 to259.8 MPa (29.6–37.6 ksi) and ishigher than the requirement of 137.9MPa (20.0 ksi) established by standardASTM B898-99. The shear bondstrength of both interfaces was not af-fected by thermal cycling.

As shown in Fig. 13A and B, the Fe-Cu interface did not experience com-plete separation during testing. Theshear path was located only along theCu weld metal deposit. This may haveresulted from the large difference instrength between the carbon steel sub-strate and the Cu deposit as indicatedby the microhardness profile shown inFig. 7. The small tolerances allowed inthe testing setup may have also con-tributed to this behavior. Figure 13Bshows the ductile characteristics of theshear fracture of the Fe-Cu interface.

Conversely, the Cu-Ti interface in theas-welded and PWHT conditions sepa-rated completely during the test, asshown in Fig. 13A and C. The shear pathwas located along the Cu-Ti interface.Although there is also a large differencein strength between the Cu deposit andthe first Ti layer as shown in Fig. 7, theconfiguration of the test setup forcedthe fracture to take place along the in-terface or along the first Ti layer. Figure13C shows the brittle characteristics of

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Fig. 14 — A — Stressdisplacement curve obtained during the bond shear strength testing of the FeCu interface; B — CuTi interface fromwelded joint in the aswelded conditions (combination of CSCGMAW and GTAWP processes).

A B


the shear fracture of the Cu-Ti interface;however, in the thermal-cycled condi-tion, the Cu-Ti interface did not experi-ence complete separation.

The strength-displacement curvesobtained during the bond shear testsalso indicate the more ductile behaviorof the Fe-Cu interface as compared tothe behavior observed in the Ti-Cu in-terface. The Fe-Cu interface shows alarger displacement under maximumload as compared to that observed inthe Cu-Ti interface, as shown in Fig. 14.

Based on the experimental observa-tions, the multilayer approach used inthe Ti-clad steel welded joints resultedin great improvement in mechanical be-havior of the welded joints. Eventhough none of the weld joints passedthe bend test, the multilayer approachresulted in an improved ductility ascompare to the high degree of embrit-tlement normally observed in weldsjoining Ti directly to steel, as shown inFig. 1. Therefore, it is recommendedthat the multilayer approach be furtherexplored as a way to develop reliablecost-effective methods of joining or re-pairing Ti-clad steel plates.

Conclusions• The interlayer-Ti interface and first

Ti layer show the highest hardness lev-els in most of the different weldedjoints. The maximum hardnesses in thewelded joints made with the Ni-Ti,NiCu-Ti, and NiCr-Ti interlayers were607, 568, and 554 HV0.5, respectively.Some degree of softening was inducedin the Ni-Ti welded joint by the thermalcycles of additional Ti layers. The PWHTdid not induce a major softening in theweld metal deposited in these threewelded joints.

• In the V-Ti and Ti-V systems, themaximum hardnesses of the weld de-posits at the Fe-V interface were 307and 409 HV0.5, respectively; however,the presence of microcracking at thatinterface may have affected the hard-ness reading obtained from the V-Ti in-terlayer weld deposit.

• The maximum hardnesses ob-served in the welded joints made withthe Cu-Ti interlayer ranged from300–350 HV0.5. The high hardness re-gion observed in the welded jointsmade with a combination of CSC-GMAW and GTAW-P processes waswider than the region observed in the

welded joint made with the CSC-GMAW process only. As a result ofPWHT, the hardness level through theweld deposit dropped to approxi-mately 200 HV0.5.

• Most of the samples obtained fromthe welded joints in the as-welded andPWHTed conditions failed the side bendtest. The samples from the Ni-Ti, NiCu-Ti, and NiCr-Ti welded joints failedalong the interlayer-Ti interface andthrough the Ti weld deposits. In thesamples obtained from the Cu-Ti/(CSC-GMAW + GTAW-P) welded joint, crack-ing was observed only along the Cu-Tiinterface and in the first Ti layer. Mostof the samples obtained from Cu-Ti/CSC-GMAW welded joint failed alongCu-Ti interface.

• A bend sample obtained from theTi-V-Fe system passed the bend test inthe PWHTed condition. The observedcracks were confined to the V interlayerand were shorter than 1⁄8 in. The effect ofthese microcracks on the integrity ofthe welded joint during service is notyet known.

• The shear-bond strength samplesrepresenting the Fe-Cu and the Cu-Tiinterfaces of the welded joints madewith the Cu-Ti interlayer and with acombination of CSC-GMAW andGTAW-P processes passed the test inthe as-welded, PWHT, and thermal-cy-cled conditions. The shear-bondstrength of these interfaces ranges be-tween 204.4 and 259.8 MPa (29.6 and37.6 ksi) and was higher than the137.9 MPa (20.0 ksi) shear-bondstrength required for reactive and re-fractory metal clad plate by ASTMstandards. The Fe-Cu interface showeda more ductile behavior than that ofthe Cu-Ti interface.

This paper was prepared based ondevelopment work supported by DMCClad Metal, Materials Technology In-stitute, and Eastman Chemical, as partof a group-sponsored project at EWI,Columbus, Ohio.

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