Mechanical Behavior Of Titanium Clad Steel Welded Joints
Ramirez 10-12 Layout 1 9/11/14 4:54 PM Page 369WELDING RESEARCHSUPPLEMENT TO THE WELDING JOURNAL, OCTOBER 2014Sponsored by the American Welding Society and the Welding Research CouncilMechanical Behavior of Titanium Clad SteelWelded JointsMechanical properties of Ti clad steel welded joints deposited with different interlayer ma terials were evaluated using microhardness, bend, and shear bond strength testing in theas welded, after PWHT, and in thermally cycled conditions.BY J. E. RAMIREZABSTRACTTi clad steel welded joints made with different interlayer material joining processcombinations were evaluated using microhardness, bend, and shear bond strength test ing. The effect of thermal cycling on the shear bond strength was evaluated as well. Ingeneral, all the welded joints present the highest hardness level at the interlayer Ti inter face and across the first Ti layer. The maximum hardness in welded joints made with theNi Ti, NiCu Ti, and NiCr Ti interlayer systems was 607, 568, and 554 HV0.5, respectively.In the V Ti and Ti V systems, the respective maximum hardnesses were 307 and 409HV0.5, respectively, at the Fe V interface. The maximum hardness observed in weldedjoints made with the Cu Ti interlayer ranged from 300 to 350 HV0.5. Different softeningresponses to either thermal cycles of additional Ti layers or PWHT were observed in dif ferent types of joints. Most of the joints failed the bend tests in the as welded andPWHTed conditions. The Ni Ti , NiCu Ti , and NiCr Ti welded joints failed along the inter layer Ti interface and through the Ti weld layers. The Cu Ti welded joints made with theCSC GMAW process failed along the Cu Ti interface. The bond shear strength of both Fe Cu and Cu Ti interfaces in Cu Ti welded joints made with a combination of CSC GMAWand GTAW P processes in the as welded, PWHTed, and thermally cycled conditionsranged from 204.5 to 259.8 MPa (29.6 to 37.6 ksi). The Fe Cu interface showed a largerdisplacement under maximum load as compared to that observed in the Cu Ti interface.KEYWORDS Cladding Titanium Ti Clad Steel Thermal Cycling Interlayer MaterialsIntroductionTitanium (Ti) clad steel is widelyused for large pressure vessels andother equipment in different industries to take advantage of the corrosion resistance of Ti, but at a lowercost than solid Ti construction. Titanium-clad steel is produced by rollbonding (usually with an interlayer),direct explosive bonding (usually without an interlayer) (Ref. 1), or by a combination of explosive bonding and rollbonding (Ref. 2). Interlayers are usedto improve the bond strength of theclad steel or to overcome metal plasticity compatibility restrictions encountered in roll bonding. Industrial-gradepure iron (Fe), ultralow-carbon steel,niobium (Nb) alloys, tantalum (Ta) alloys, copper (Cu) alloys, and nickel(Ni) alloys have been used as interlayers 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 successfully fusion welded directly to steelbecause it has limited solubility forFe. If the solubility limit is exceeded,as in fusion welding, brittle intermetallic 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 material is stripped back 15 to 20 mmfrom the weld joint, after which thesteel is welded and inspected. Next,the space where the cladding was removed 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 complexity of irregular geometry at nozzle penetrations and attachments,complexity of testing for joint integrity (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 develop reliable cost-effective methodsof joining or repairing Ti-clad steelJ. E. Ramirez (jose.ramirez@dnvgl.com) was a principal engineer with EWI and now is a principal engineer at DNV.GL in Columbus, Ohio.OCTOBER 2014 / WELDING JOURNAL 369-s
Ramirez 10-12 Layout 1 9/11/14 4:54 PM Page 370WELDING RESEARCHABFig. 1 — A — Titanium deposit layer that cracked and broke off a commercially pure Fe weld deposit during cooling; B — weld deposit thatcracked.BACplates that provide a continuous jointwith acceptable mechanical and corrosion properties with and withoutpostweld heat treatment (PWHT).Experimental ProceduresTi-clad steel welded joints weremade using different interlayer material-joining process combinations. Thewelded joints were tested in as-weldedand PWHT conditions.Some of the weldedjoints were tested afterexposure to thermal cycling. The mechanicalbehavior of the jointswas evaluated using microhardness, bend, andshear bond strengthtesting.Fig. 2 — A — General view of joint design; B — cross sec tion of joint after deposition of interlayer material; C —after completion of the joint.Materials and WeldingConditionsTable 1 — General Characteristics of the Welding Consumables Used as Interlayer Materials andTi Layer for Welding the Ti Clad Steel PlatesJoint DesignationWelding ProcessFiller Metal DesignationWire Size CSC-GMAWCSC-GMAWCSC-GMAWGTAWGTAWCSC-GMAW/GTAWCPNi (ERNi-1)NiCu (ERNiCu-7)NiCr (ERNiCr-4)CPCu (ERCu)CPCu (ERCu)CPTi 62/0.0450.062/0.035V-TiTi Layers370-s WELDING JOURNAL / OCTOBER, 2014 VOL. 93Titanium Clad Base Metal andInterlayer MaterialsThe deposition of the interlayermaterial and corresponding Ti layersof the welded joints was done in 150 200-mm (6- 8-in.) explosion Ticlad steel samples. The explosion Ticlad steel base metals consisted ofSA-516-70 carbon steel with a nominalthickness of 27.5 to 38.0 mm (1.1 to
Ramirez 10-12 Layout 1 9/11/14 4:54 PM Page 371WELDING RESEARCHABFig. 3 — A — General view of Ti clad steel welded joints made with the CSC GMAW process; B — joint made with a combination of theCSC GMAW and the GTAW P processes.ABFig. 4 — A — General view of Fe Cu interface bond shear strength samples; B — test setup.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 potential 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 DesignThe Ti-clad steel base metal samples have a widegroove prepared bythe strip-back method. The joint design of the wide-groove included a rootthat was between 19.0 to 25.0 mmTable 2 — Ranking in Decreasing Order of Suitability of Interlayer Welding Process Combination forMaking Full Size Ti Clad Steel Welded JointsRankingDescription of Interlayer SystemInterlayer Design (a)Welding ProcessComments1Fe-Cu-TiCSC-GMAW GTAW-P1. Poor wettability of Ti on Cu2Fe-Cu-TiCSC-GMAW1. Short contact tip life duringdeposition of Ti2. Poor wettability of Ti on Cu3Fe-Ni-TiCSC-GMAW1. Short contact tip life duringdeposition of Ti2. Cracking susceptibility4Fe-NiCu-TiCSC-GMAW1. Short contact tip life duringdeposition of Ti2. Cracking susceptibility5Ti-V-FeGTAW-P1. Cracking susceptibility6Fe-NiCr-TiCSC-GMAW1. Short contact tip lifeduring 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.OCTOBER 2014 / WELDING JOURNAL 371-s
Ramirez 10-12 Layout 1 9/11/14 4:54 PM Page 372WELDING RESEARCHFig. 6 — Microhardness profile of Ti V Fewelded joint with two carbon steel weldmetal layers, in the as welded condition(GTAW P process).Fig. 5 — Microhardness profile of NiCr Ti welded joint with one and three Tiweld metal layers (1Ti, 3Ti), in the as welded and PWHTed conditions (CSC GMAW process).(0.75 to 1.0 in.) wide and a 22-deg bevelangle. Additionally, the groove was machined to a depth of about 2.50 mm(0.10 in.) into the steel substrate, asshown in Fig. 2A and B. This joint design replicates the Ti portion of Ti-cladsteel butt joints, which is the more critical part of this type of joint.Welding ProcessDifferent joining processes were consid-ered based on the metallurgical characteristics of the selected interlayers, andon the typical dilution of the joiningprocesses. The latter is significant because low-dilution processes limit theamount of melting, as well as the thermal experience of the base metal attemperatures where intermetallic compounds may form. Considering thecommercial availability of consumables,ease of deployment in the field, and relatively low equipment investment, arcwelding processes were considered theprimary process of choice.A relatively new gas metal arc welding(GMAW) process variant called controlled 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 dilution with minimal spatter. Welding parameters 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 current sequence has three levels to set(start, pulse, and end). These three cur-Table 3 — Welding Conditions for Depositions of Different Layers of Weld Metal in the Weld Joints Using CSC GMAWArc Current SequenceWeld LayerNi on steelTi on NiNiCu on steelTi on NiCuNiCr on steelTi on NiCrCPCu on steelCPCu on steelTi on CpCuTi on TiShort-Circuit Current SequenceShielding GasStartCurrent(A)StartCurrentTime (ms)PulseCurrent(A)PulseCurrentTime (ms)EndCurrent(A)StartCurrent(A)StartCurrentTime (ms)PulseCurrent(A)PulseCurrentTime (ms)EndCurrent(A)100% He100% He100% He100% He50%Ar/50%He100% He100% He100% He100% He100% 60506050506060Wire Feed SpeedUp WFS(m/min)Down al ArcLength (mm)0.01.00.00.50.00.50.00.00.00.5372-s WELDING JOURNAL / OCTOBER 2014, VOL. 93Weaving ParametersOscillation ward TravelSpeed 1.8
Ramirez 10-12 Layout 1 9/11/14 4:54 PM Page 373WELDING RESEARCHFig. 7 — Comparison of microhardness profiles of Cu Ti weldedjoints with three Ti weld metal layers deposited with the CSC GMAW process and with a combination of CSC GMAW andGTAW P processes.rent levels are used to control the beadshape and size. The start and pulse levels 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 system senses the voltage drop and prevents 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 shortcircuiting 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 compared to other arc welding processes.Welding ConditionsDue to the complexity inherent todissimilar metal joining, CSC-GMAWwelding parameters and weaving pa-Fig. 8 — General view of side bend samples obtained fromNiCu Ti welded joints in the as welded and PWHT conditions(CSC GMAW process).rameters were developed and optimized(Ref. 18). Six interlayer-joining processcombinations were ranked based ontheir general wettability behavior, weldability, and the ability to achieve acceptable 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 interlayer system are listed in Tables 4–6.Figure 3 shows a general view ofsome of the welded joints made for mechanical evaluation. The welded joint inFig. 3A shows a stepwise configurationat the ends. The three levels of the stepwise configuration from the end towardthe center of the sample correspond tothe surface of the weld deposit of the interlayer material, the surface of the firstTi deposit layer, and the surface of twoadditional layers of Ti. This arrangement allowed the characterization ofdeposits of the interlayer material in theas-welded condition and an evaluationof the effects of thermal cycles inducedduring the deposition of one and threelayers of Ti on the properties of the interlayer materials and the welded jointas a whole. Figure 2C shows a cross section of a complete welded joint (interlayer material plus three Ti layers). Thewelded joints were subjected to radiographic examination to evaluate thesoundness of the joints and to determine the location of different specimens required for the mechanicalevaluation.Postweld Heat TreatmentThe PWHT of the welded joints wasconducted following the guidelines ofSection VIII of the ASME code for carbon 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)Table 4 — GTAW P Parameters Used for the Deposition of First Layer of Ti in the Cu Ti welded JointPeak current (A)250Back current (A)10Peak current time (s)0.1Back current time (s)0.5Wire entry angle (deg)15Wire typeERTi-1Electrode type2% CeElectrode preparation (deg)30, noflatWire feed peak (mm/s)Wire feed back (mm/s)Arc voltage (V)Travel speed (mm/s)Wire to electrode distance (mm)Wire diameter (mm)Electrode diameter (mm)Shielding gas type8.58.012.21.11.10.93.275% He25% ArOCTOBER 2014 / WELDING JOURNAL 373-s
Ramirez 10-12 Layout 1 9/11/14 4:54 PM Page 374WELDING RESEARCHAFig. 9 — Cracks observed in longitudinal side bend samplesobtained from Ti V Fe welded joints in the PWHT condition(GTAW P process).were controlled to be equal or lessthan 400 F/h/in. (8.7ºC/h/mm). Cooling rates above 800 F (427 C) wereequal or less than 500 F/h/in.(10.9 C/h/mm).Evaluation of MechanicalBehavior of Welded JointsThe Ti-clad steel welded joints wereevaluated before and after PWHT. Thejoints were evaluated using microhardness profiles, bend testing, and bondshear-strength testing. The effect ofthermal cycling after PWHT on theshear-bond strength of some jointswas evaluated as well.Microhardness TestingMicrohardness profiles were determined in the through-thickness direction of the deposited weld metalsstarting from the steel substrate toward the surface of the last layer of Tiweld deposit. The microhardness profiles of the welded joints were determined in deposits with one and threeBTi layers, respectively, and in the aswelded and PWHTconditions. Thehardness readingwas determinedusing hardness Vickers scale with a loadof 500 g (HV0.5).Bend TestingFig. 10 — A — Microcracking observed in the Fe V interface ofwelded joints deposited with the GTAW P process; B — crackarrested at the V Ti interface.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 requirements 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 interlayer system because crack-free jointsTable 5 — GTAW P Parameters Used for the Deposition of Second Layer of Ti in the Cu TiPeak current (A)Back current (A)Peak current time (s)Back current time (s)Wire entry angle (deg)Wire typeElectrode typeElectrode preparation (deg)160800.10.2515ERTi-12% Ce30, noflatWire feed peak (mm/s)Wire feed back (mm/s)Arc voltage (V)Travel speed (mm/s)Wire to electrode distance (mm)Wire diameter (mm)Electrode diameter (mm)Shielding gas type374-s WELDING JOURNAL / OCTOBER 2014, VOL. 936.46.49.21.11.10.93.275% He25% Arwere difficult to make. Additionally,welded joints from the Ti-V-Fe system were tested only in the PWHTconditions because the bend samplescracked during machining in the aswelded conditions. This may indicatethe buildup of a high level of residualstresses during the welding of thisdissimilar metal joint.Bond Shear Strength TestingIn order to measure the shear bondstrength of interfaces between dissimilar material layers in some of the weldedjoints, shear-strength testing was conducted 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
Ramirez 10-12 Layout 1 9/11/14 4:54 PM Page 375WELDING RESEARCHAFig. 11 — General view of side bend samples obtained fromCu Ti welded joints in the as welded and PWHT conditions(CSC GMAW process).Fig. 4B) to restrict the vertical displacement of the area of the sample corresponding to the interlayer materialand/or Ti weld layers. The rest of thesample can be displaced freely in thevertical direction. During testing, loading 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 evaluation. 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.the Fe-Cu interfaceand a shear bondstrength couponrepresenting theCu-Ti interface wasmachined andtested according tothe requirements ofASTM B898. Theshear bond strengthresults were compared to those obtained fromspecimens thatwere not exposed tothermal cycling.BFig. 12 — A — General view of bend test samples; B — cracksobs
general, all the welded joints present the highest hardness level at the interlayer Ti inter face and across the first Ti layer. The maximum hardness in welded joints made with the Ni Ti, NiCu Ti, and NiCr Ti interlayer systems was 607, 568, and 554 HV 0.5, respectively.
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