Induction Weld Seam Characterization of Continuously Roll Formed TRIP690 Tubes

metals

Article Induction Weld Seam Characterization of Continuously Roll Formed TRIP690 Tubes

Alexander Bardelcik * and Bharathwaj Thirumalai Ananthapillai School of Engineering, College of Engineering and Physical Sciences, University of Guelph, Guelph, ON N1G 2W1, Canada; [email protected] * Correspondence: [email protected]; Tel.: +1-519-824-4120

Received: 5 March 2020; Accepted: 20 March 2020; Published: 25 March 2020

Abstract: The weld seam characteristics of continuously roll formed and induction seam welded TRIP690 tubes were examined in this work. These tube are subsequently used in automotive hydroforming applications, where the weld seam characteristics are critical. The induction seam welds are created through a solid-state welding process and it was shown that by increasing the induction frequency by 26%, the weld seam width within the heat aﬀected zone (HAZ) reduced due to a plateau in the hardness distribution which was a result of a delay in the transformation of martensite. 2D hardness distribution contours were also created to show that some of the weld conditions examined in this work resulted in a strong asymmetric hardness distribution throughout the weld, which may be undesirable from a performance perspective. An increase in the pressure roll force was also examined and revealed that a wider total weld seam width was produced likely due to an increase in temperature which resulted in more austenitization of the sheet edge prior to welding. The ring hoop tension test (RHTT) was applied to the tube sections created in this work. A Tensile and Notch style ring specimen were tested and revealed excellent performance for these welds due to high peak loads (~17.2 kN) for the Notch specimens (force deformation within weld) and lower peak loads (~15.2 kN) for the Tensile specimens for which fracture occurred in the base metal.

Keywords: induction welding; roll forming; TRIP690; metallography; microhardness testing; ring hoop tension test

1. Introduction In order to combat the effects of vehicle greenhouse gas emissions on global warming, the Corporate Average Fuel Economy (CAFÉ) standards [1] were enacted by the United States administration and research activity into vehicle lightweighting of body-in-weight (BIW) structures has been targeted by automakers. One of the methods used to reduce BIW weight has been the development and implementation of Advanced High Strength Steel (AHSS), which allows for down gauging of sheet metal thickness while maintaining high crash performance standards [2]. Some of the candidate AHSS steels include Dual-Phase (DP), Complex Phase (CP), Transformation Induced Plasticity (TRIP) and Quench and Partitioned (Q&P) steels. Another approach to BIW lightweighting is through the implementation of innovative metal forming manufacturing processes such as: (1) press hardening of steel [3] and high strength aluminum alloys [4]; (2) warm forming of aluminum [5] and magnesium [6]; and (3) hydroforming of tubular steel [7]. Hydroforming of AHSS takes advantage of both lightweighting methods, making it an attractive process for OEMs. Therefore, the focus of this work will be on the seam weld characterization of TRIP690 straight tubular stock that has been manufactured for hydroforming applications. The proceeding sections will highlight the significant deficit within the literature on the topic of thin gauge roll formed tubes that have been induction seam welded. In particular, the effect of the induction weld frequency on the resultant weld microstructure

Metals 2020, 10, 425; doi:10.3390/met10040425 www.mdpi.com/journal/metals Metals 2020, 10, 425 2 of 16 Metals 2020, 10, x FOR PEER REVIEW 2 of 16 willmicrostructure be explored, will along be with explored, the development along with of the amechanical development test of to a evaluate mechanical the fracture test to performanceevaluate the offracture these complexperformance TRIP690 of these weld complex seams. TRIP690 weld seams. 1.1. Hydroforming 1.1. Hydroforming Tubular hydroforming is now a mature metal forming technology in which a continuous tubular Tubular hydroforming is now a mature metal forming technology in which a continuous tubular section is pressurized with a liquid media and formed to the cross-section of a die as shown in Figure1 section is pressurized with a liquid media and formed to the cross-section of a die as shown in Figure for the high-pressure [8] and low-pressure [9] hydroforming processes. Continuously roll formed 1 for the high-pressure [8] and low-pressure [9] hydroforming processes. Continuously roll formed and seam welded tube stock is the most commonly used in automotive hydroforming applications. and seam welded tube stock is the most commonly used in automotive hydroforming applications. Tube pre-bending operations, such as precision mandrel rotary-draw, are used to first pre-bend the Tube pre-bending operations, such as precision mandrel rotary-draw, are used to first pre-bend the straight tubes prior to the hydroforming process as shown in Figure1a. Significant amounts of strain straight tubes prior to the hydroforming process as shown in Figure 1a. Significant amounts of strain hardening that are imposed on the tube during pre-bending must be considered when designing the hardening that are imposed on the tube during pre-bending must be considered when designing the subsequent hydroforming process [10], as the non-linear strain path of the deformation may lead to subsequent hydroforming process [10], as the non-linear strain path of the deformation may lead to premature fracture [11]. This is especially true for the weld seam, which in some situations is located premature fracture [11]. This is especially true for the weld seam, which in some situations is located along the tubular cross-section where pre-bending and hydroforming may impose significant strains along the tubular cross-section where pre-bending and hydroforming may impose significant strains that can lead to premature failure (or burst) due to the strength and ductility differential between the that can lead to premature failure (or burst) due to the strength and ductility differential between the weld and base metal. weld and base metal. (a) (b) High-Pressure Hydroforming Low-Pressure Hydroforming

Figure 1. (a) High [8] and (b) low pressure hydroforming [9]. Figure 1. (a) High [8] and (b) low pressure hydroforming [9]. 1.2. Tubular Roll Forming 1.2. Tubular Roll Forming The tube stock commonly used in hydroforming is manufactured using the continuous tubular roll formingThe process. tube stock This commonly is a continuous used in process hydroforming in which is a manufactured sheet metal strip using is deformed the continuous into a circulartubular profileroll forming by passing process. it through This is a seriescontinuous of roll process forming in stages which in a successionsheet metal as strip shown is deformed in the Figure into2 a schematic.circular profile The roll by formingpassing processit through is followed a series byof aroll seam forming welding stages station, in insuccession which the as tubular shown section in the isFigure closed 2 usingschematic. high-frequency The roll forming induction, process laser oris tungstenfollowed inertby a gasseam (TIG) welding welding station, processed. in which A final the calibrationtubular section of the is cross-sectional closed using high geometry-frequency is then induction, conducted, laser followed or tungsten by cutting inert ofgas the (TIG) tube welding [12,13]. Inprocessed. addition A to final the consistencycalibration of of the the cross wall-sectional thickness geometry in the finished is then tube conducted, product, followed the quality by cutting of the weldof the seam tube is[12,13] of critical. In addition importance to the for consistency the subsequent of the pre-bending wall thickness and hydroformingin the finished process tube product, which thesethe quality tubes will of undergo.the weld Althoughseam is theyof critical do not specifyimportance the weldingfor the processsubsequent used, pre Groche-bending et al. [and12] showedhydroforming that continuous process which roll forming these resultedtubes will in aundergo. weld seam Although thickness they that do is greaternot specify than the the welding average cross-sectionalprocess used, thicknessGroche et of al. the [12 tube] showed due to thethat force continuous applied toroll the forming edges of resulted the sheet in during a weld welding. seam Thisthickness results that in baseis greater metal than failure the whenaverage the cross tubes-sectional were subject thickness to free-expansion of the tube due bulge to the tests. force Through applied finiteto the element edges of simulation, the sheet Grocheduring welding. and Breitenbach This results P [13 ]in showed base metal that thefailure vertical when leveling the tubes strategy were (W)subject roll to forming free-expansion process couldbulge betests. used Through to enhance finite the element thickness simulation, of the tube Groche near theand seam Breitenbach weld for P enhanced[13] showed performance that the vertical during leveling hydroforming. strategy (W) roll forming process could be used to enhance the thickness of the tube near the seam weld for enhanced performance during hydroforming.

MetalsMetals 20202020,, 1010,, x 425 FOR PEER REVIEW 33 of of 16 16

Roll Forming Velocity ( )

푣 Multi-Stage Seam Tube Cutting Roll Forming Welding Calibration

TRIP690 Sheet Weld Metal Coil (Slit) Parameters , , Figure 2. A schematic of the퐻푧 tubular푊 푁 roll forming process. Figure 2. A schematic of the tubular roll forming process. 1.3. Induction Seam Welding in Roll Forming 1.3. Induction Seam Welding in Roll Forming Induction welding (or high frequency induction, HFI) is the most common seam welding process usedInduction in the roll welding forming process(or high due frequency to the high induction, production HFI) rates is andthe robustmost common production seam characteristic welding processof this process used in [14 the,15 roll]. During forming the process induction due welding to the process,high production a tube passes rates through and robust an induction production coil characteristicwith a gap between of this theprocess sheet [ edges14,15]. that During is then the closed induction at the welding weld rolls process, (or pressure a tube passes rolls),which through apply an inductiona squeeze coil pressure with toa gap the tubebetween edge the as shownsheet edges in Figure that3 .is This then welding closed processat the weld is a formrolls of(or resistance pressure rolls),welding which due apply to the a sheet squeeze edges pressu heatingre to (duethe tube to skin edge eff asect shown and proximity in Figure e3ff. ect)This as welding a result process of a high is afrequency form of resistance current induced welding on due the to work the sheet piece. edges The skinheating effect (due is ato phenomenon skin effect and that proximity causes the effect) high asfrequency a result currentof a high to frequency stay close current to the surface induced of theon the tube work prior piece. to closing The skin the gap,effect while is a phenomenon the proximity thateffect causes causes the the high current frequency to intensify current as theto stay gap close between to the the surface tube edges of the decreases, tube prior thus to closing resulting the in gap, the whilehighest the temperature proximity effect as the causes tube edge the current is forced to togetherintensify atas thethe pressuregap between rolls the [14 –tube18]. edges The temperature decreases, thusof the resulting tube edge in doesthe highest not exceed tempe therature liquidus as the temperature tube edge is of forced the metal, together therefore at the induction pressure welding rolls [14 is– 18]also. T referredhe temperature to as a forgeof the weldingtube edge process, does not which exceed can the be liquidus classified temperature as a solid-state of the welding metal, therefore process. inductionIn order to welding complete is this also type referred of a solid-state to as a forge weld, welding a squeeze process, force mustwhich be can applied be class to theified two as edgesa solid of- statethe tube welding to form process. a joint In [14 order]. to complete this type of a solid-state weld, a squeeze force must be appliedConsidering to the two these edges aspects of the oftube the to induction form a joint seam [14 welding]. process, the induction coil power (kW) frequencyConsidering (kHz) these and other aspects parameters of the induction as shown seam in welding Figure3, process, must also the be induction carefully coil set power in order (kW) to frequencyachieve a sound(kHz) weld.and other In terms parameters of thin gauge as shown automotive in Figure tube, 3, must there isalso little be tocarefully no published set in literatureorder to achieveon the topic,a sound but weld. there In has term beens of somethin gauge activity automotive in roll forming tube, there of heavy is little gauge to no pipes. published In roll literature formed onpipeline the topic, steel but Kim there and Kimhas been [19] showedsome activity that increased in roll forming weld power of heavy resulted gauge in higher pipes. temperaturesIn roll formed at pipelinethe weld steel seam. Kim This and had Kim the [19 eff]ect showed of changing that increased the gap atweld the power point of resulted contact. in Gha highffarpourer temperatures et al. [15] atmodified the weld the seam. edge This condition had the of aeffect 5 mm of thick changing sheet metalthe gap in orderat the to point reduce of contact. the temperature Ghaffarpour gradient et al. at [the15] seammodified weld the and edge improve condition the final of welda 5 mm microstructure. thick sheet metal Numerical in order work to toreduce simulate the thetemperature induction gradientwelding processat the seam has ledweld to theand development improve the offinal complex weld microstructure. finite element (FE) Numerical models ofwork the rollto simulate forming theinduction induction welding welding process process in which has led the to coupled the development thermal and of electromagnetic complex finite processeselement (FE) are modeledmodels of to thepredict roll theforming thermal induction condition welding at the tube process edge [15in ,18which]. Okabe the etcoupled al. [20] enhancedthermal and these electromagnetic models by also processcouplinges theare elastic-plasticmodeled to predict behavior the of thermal the metal condition within the at FE the models tube edge to predict [15,18] wall. Okabe thickening et al. for[20 a] enhancedcontinuously these roll models formed by pipeline also coupling steel. Paulthe elastic [16] simulated-plastic behavior the roll formingof the metal and within induction the FE welding models of toa thin-walledpredict wall 1.27 thickening mm thick for low a continuously carbon steel roll tube formed and predicted pipeline thatsteel. a Paul 400 kHz [16] inductionsimulated weldingthe roll formingfrequency and resulted induction in a superiorwelding of weld a thin when-walled compared 1.27 mm to a thick tube weldedlow carbon at 200 steel kHz, tube which and resultedpredicted in thata wider a 400 and kHz less induction desirable welding HAZ. Although frequency not resulted directly in relateda superior to induction weld when welding compared of roll to formeda tube weldedtubes, recently at 200 kHz, developed which res analyticalulted in modelsa wider used and less to predict desirable the HAZ. temperature Although distribution not directly generated related todue induction to planar welding induction of heatingroll formed can betubes, a very recent usefully develop and effiedcient analytical tool in understanding models used to the predict effect thatthe temperatureinduction heating distribution parameters generated have due on thin to planar sheet metalinduction [21,22 heating]. can be a very useful and efficient tool in understanding the effect that induction heating parameters have on thin sheet metal [21,22].

Metals 2020, 10, 425 4 of 16 Metals 2020, 10, x FOR PEER REVIEW 4 of 16

Vee Angle

Edge Condition & Geometry Induction Coil Power (kW) and Frequency (kHz)

FigureFigure 3. 3.A A schematic schematic ofof thethe process parameters involved involved in in induction induction seam seam welding. welding. 1.4. Welding of TRIP Steels 1.4. Welding of TRIP Steels TheThe Transformation Transformation Induced Induced PlasticityPlasticity (TRIP) effe effectct in in steels steels was was first first observed observed in inthe the work work of of ZakayZakay et et al. al. [23 [23]] and and the the commercialized commercialized by by Nippon Nippon Steel Steel [24 [24]] as aas class a class of steelsof steels which which contained contained some volumesome fractionvolume fraction of retained of retained austenite austenite (γ) within (γ) awithin multi-phase a multi-phase microstructure microstructure that includes that includes ferrite (α) andferrite may ( includeα) and may martensite include ( αmartensite’) and bainite (α’) (andαB). bainite The excellent (αB). The strength excellent and strength ductility and properties ductility of theseproperties AHSS steelsof these are AHSS a result steels of the are TRIP a result effect of the during TRIP plastic effect deformationduring plastic (or deformation loading), which (or loading), results in thewhich transformation results in ofthe the transformationγ into α’, thus of delaying the γ into the onsetα’, thus of neckingdelaying due the to onset enhanced of necking strain hardening.due to Theseenhanced properties strain makehardening. TRIP These steels properties very desirable make for TRIP vehicle steels BIW very energy desirable absorbing for vehicle applications BIW energy and whenabsorbing coupled applications with the hydroformingand when coupled process, with the an hydroforming excellent opportunity process, an for excellent lightweighting opportunity can be realized.for lightweighting This is in fact can the be motivation realized. This for is the in current fact the research, motivation where for the the current development research, of continuouslywhere the rolldevelopment formed and of induction continuously seam roll welded formed TRIP690 and induction tube stock seam was welded investigated. TRIP690 Thetube main stock focus was of thisinvestigated. work will The be themain eff ectfocus of of induction this work welding will be frequencythe effect of and induction pressure welding roll force frequency on the and seam weldpressure characteristics roll force ofon rollthe formedseam weld tubes, characteristics which to the of authors’roll formed knowledge tubes, which has not to the been authors’ explored academicallyknowledge has in the not literature. been explored In addition, academically a mechanical in the literature. test to assess In addition, the performance a mechanical of thetest seamto weldsassess of the the performance roll formed tubeof the stock seam was welds developed of the ro toll testformed the weldstube stock created was for developed this work. to test the welds created for this work. Although there is no literature available on the topic of induction seam welded TRIP steel, there Although there is no literature available on the topic of induction seam welded TRIP steel, there has been considerable research conducted on the fusion (Resistance Spot Welding-RSW, Gas Metal Arc has been considerable research conducted on the fusion (Resistance Spot Welding-RSW, Gas Metal Welding-GMAW and laser welding) and solid-state (friction stir welding FSW and ultrasonic) welding Arc Welding-GMAW and laser welding) and solid-state (friction stir welding FSW and ultrasonic) of TRIP alloys. RSW of a high strength TRIP steel was shown to be susceptible to weld defects due to welding of TRIP alloys. RSW of a high strength TRIP steel was shown to be susceptible to weld defects thedue rich to chemistry the rich chemistry and thermophysical and thermophysical properties properti of thees material of the material [25]. Sajjadi-Nikoo [25]. Sajjadi-Nikoo et al. [ et26 al.] showed [26] thatshowed a fully that martensitic a fully martensitic RSW nugget RSW for nugget a TRIP for alloy a TR resultedIP alloy in resulted poor cross-tension in poor cross-tension test performance test dueperformance to interfacial due failure,to interfacial which failure, was remediedwhich was through remedied post through weld post heat weld treatments heat treatments that tempered that thetempered fusion zone the fusion nugget. zone Spena nugget. et Spena al. [27 et] spotal. [27] welded spot welded (using (using optimal optimal parameters) parameters) TRIP TRIP and and Q&P AHSSQ&P alloysAHSS andalloys showed and showed that thethat HAZthe HAZ for for these these alloys alloys were were void void of the initial initial retained retained austenite austenite ofof the the base base metal. metal.Zhang Zhang etet al.al.[ [28]28] usedused GMAWGMAW to weld a a TRIP600 TRIP600 (ferrite, (ferrite, retained retained austenite austenite and and bainitebainite BM) BM) and and showed showed that the weld weld exhibited exhibited ex excellentcellent mechanical mechanical and and impact impact properties properties due to due tothe the microstructural microstructural make-up make-up of the of fine the grained fine grained HAZ, HAZ,which whichconsisted consisted of ferrite of and ferrite granular and bainite. granular bainite.LopezLopez Cortez Cortez et al. [29] et al. also[29 ]found also found that the that HAZ the HAZ of a ofGMAW a GMAW welded welded TRIP800 TRIP800 automotive automotive steel steel consistedconsisted of aof mixed a mixed ferrite ferrite (polygonal) (polygonal) and bainiteand bainit (lower),e (lower), while thewhile fusion the zonefusion consisted zone consisted of martensite of andmartensite some ferrite and (Widmanstattensome ferrite (Widmanstatten and allotriophic) and allotriophic) and bainite (upper).and bainite Grachar (upper). et al.Grachar [30] laser et al. welded [30] anlaser automotive welded TRIP an (ferriteautomotive with bainite-austeniteTRIP (ferrite with BM) bainite-austenite and revealed that BM) the and microstructure revealed that within the the inter-criticalmicrostructure HAZ within (ICHAZ) the inter-crit consistedical of HAZ ferrite (ICHAZ) and areas consisted consisting of ferrite of lath and bainite-martensite-austenite. areas consisting of lath Thebainite-martensite-austenite. austenite was also shown toThe have austenite transformed was al intoso shown martensite-austenite to have transformed (M-A) into islands martensite- due to the austenite (M-A) islands due to the inter-critical heating within this ICHAZ. The microstructure within inter-critical heating within this ICHAZ. The microstructure within the fine-grained (FGHAZ) and the fine-grained (FGHAZ) and coarse-grained (CGHAZ) predominantly consisted of a martensitic- coarse-grained (CGHAZ) predominantly consisted of a martensitic-bainitic microstructure. bainitic microstructure. Like induction seam welding, friction stir welding (FSW) is a solid-state welding process as Like induction seam welding, friction stir welding (FSW) is a solid-state welding process as well. well. Medina et al. [31] conducted FSW trials on a TRIP780 (ferrite-bainite-retained austenite BM) Medina et al. [31] conducted FSW trials on a TRIP780 (ferrite-bainite-retained austenite BM) alloy and alloycharacterized and characterized the weld microstructure the weld microstructure to show a pr toedominantly show a predominantly martensitic stir martensitic zone (SZ) and stir zonea HAZ (SZ) and a HAZ which nearly matched the BM microstructure. They also found an increase in retained

MetalsMetals 20202020, ,1010, ,x 425 FOR PEER REVIEW 55 of of 16 16 which nearly matched the BM microstructure. They also found an increase in retained austenite austenite content within the SZ and HAZ when compared to the BM. Mironov et al. [32] joined a high content within the SZ and HAZ when compared to the BM. Mironov et al. [32] joined a high strength strength 1.2 GPa automotive TRIP alloy using FSW. The microstructure within the HAZ of this material 1.2 GPa automotive TRIP alloy using FSW. The microstructure within the HAZ of this material revealed a reduction in retained austenite content, which was suspected to have transformed into revealed a reduction in retained austenite content, which was suspected to have transformed into bainite (resulting in softening), while the SZ was predominantly martensitic. Mazzaferro et al. [33] bainite (resulting in softening), while the SZ was predominantly martensitic. Mazzaferro et al. [33] applied friction stir spot welding (FSSW) to TRIP800 sheets and observed the transformation of the applied friction stir spot welding (FSSW) to TRIP800 sheets and observed the transformation of the retained austenite into coalesced bainite within the thermo-mechanically aﬀected zone, which is within retained austenite into coalesced bainite within the thermo-mechanically affected zone, which is the intercritical temperature range. within the intercritical temperature range. 2. Materials and Methods 2. Materials and Methods 2.1. TRIP690 Material Properties 2.1. TRIP690 Material Properties In this work, a grade of TRIP690 sheet metal with a nominal thickness of 1.6 mm was roll formed intoIn 2” this (~50.8 work, mm) a ODgrade tubes. of TRIP690 The base sheet metal metal (BM) with microstructure a nominal thickness for this material of 1.6 mm is shown was roll in Figure formed4a intoand 2” consists (~50.8 ofmm) a ferritic OD tubes. matrix The with base retained metal (BM) austenite microstructure as shown byfor thethis elevated material and is shown featureless in Figure hard 4a and consists of a ferritic matrix with retained austenite as shown by the elevated and featureless phase in the SEM micrograph. The nominal hardness of the BM is 250 HV300. There also exists a small hardvolume phase fraction in the of SEM martensite micrograph. and bainite, The nominal but these hardness are nearly of the negligible. BM is 250 Uniaxial HV300. There tensile also specimens exists a small(2” gauge volume length, fraction ASTM of E8martensite type) were and extracted bainite, frombut these the roll are formed nearly tubesnegligible. along Uniaxial the longitudinal tensile specimenstube direction (2” (seegauge Figure length,4 inset) ASTM and E8 the type) nominal were engineering extracted from stress the vs. roll strain formed curve tubes for this along material the longitudinalis shown in Figuretube direction4b. The (see nominal Figure yield 4 inset) strength, and the tensile nominal strength engineering and total stress elongation vs. strain from curve repeat for thistensile material tests isis 570shown MPa, in 758Figure MPa 4b. and The 0.24, nominal respectively. yield strength, tensile strength and total elongation from repeat tensile tests is 570 MPa, 758 MPa and 0.24, respectively.

FigureFigure 4. 4. (a()a TRIP690) TRIP690 base base metal metal microstructure microstructure (α-Ferrite, (α-Ferrite, γ- Retainedγ- Retained Austenite, Austenite, α’-Martensite,α’-Martensite, αB- Bainite);αB-Bainite); (b) A (b representative) A representative engineering engineering stress stress versus versus strain strain curve curve from from a single a single uniaxial uniaxial tensile tensile test ontest a TRIP690 on a TRIP690 tubular tubular specimen. specimen. 2.2. Tubular Roll Forming Parameters 2.2. Tubular Roll Forming Parameters An industrial tubular roll forming apparatus was used to produce the tubular specimens that are An industrial tubular roll forming apparatus was used to produce the tubular specimens that examined in this work. During a single continuous run of the TRIP690 material the tube was roll formed are examined in this work. During a single continuous run of the TRIP690 material the tube was roll using a constant roll forming velocity. The vee angle, edge condition, weld power ( 1% variation) formed using a constant roll forming velocity. The vee angle, edge condition, weld± power (± 1% and pressure roll force ( 2% variation) was held constant as well (see Figure3). Based on all of the variation) and pressure roll± force (± 2% variation) was held constant as well (see Figure 3). Based on induction welding parameters used, the industrial roll forming welding controller computed the weld all of the induction welding parameters used, the industrial roll forming welding controller power for this experimental setup. The eﬀect of induction weld frequency was examined in this computed the weld power for this experimental setup. The effect of induction weld frequency was work and three separate specimens were manufactured and labeled LOW, MED and HIGH, which examined in this work and three separate specimens were manufactured and labeled LOW, MED represented a relative weld frequency of 1.0, 1.17 and 1.26, respectively as shown in Table1. The LOW and HIGH, which represented a relative weld frequency of 1.0, 1.17 and 1.26, respectively as shown weld frequency was less than 300 kHz, as reported by the tube manufacturer. In addition to these three in Table 1. The LOW weld frequency was less than 300 kHz, as reported by the tube manufacturer. specimens, a HIGH_PRF specimen was also produced and represents a condition where the pressure In addition to these three specimens, a HIGH_PRF specimen was also produced and represents a roll force (PRF) was increased by 14%, while the weld power was decreased by ~7% and weld frequency condition where the pressure roll force (PRF) was increased by 14%, while the weld power was was increased by 9% in order to observe the eﬀect that pressure roll force (plus lesser contributions decreased by ~7% and weld frequency was increased by 9% in order to observe the effect that from power and frequency) has on the weld seam characteristics. A non-contact temperature probe pressure roll force (plus lesser contributions from power and frequency) has on the weld seam is used to monitor the weld seam temperature during the roll forming process. The relatively crude characteristics. A non-contact temperature probe is used to monitor the weld seam temperature during the roll forming process. The relatively crude measurement was used to confirm that the weld

Metals 2020, 10, 425 6 of 16 Metals 2020, 10, x FOR PEER REVIEW 6 of 16 temperaturemeasurement was was slightly used to above conﬁrm the that Ac3the temperature weld temperature for all ofwas the weld slightly frequency above the cases. Ac3 temperatureNo accurate measurementfor all of the weldof the frequency weld seam cases. temperature No accurate was available measurement for the of roll the forming weld seam apparatus temperature used in wasthis work.available After for the the weld roll forming was created apparatus, the exterior used in of this the work. tube is After flushed the weldwith wascoolant created, as a part the exteriorof the roll of formingthe tube process is ﬂushed. with coolant as a part of the roll forming process.

Table 1. TubularTubular roll roll forming forming weld weld frequency frequency present presenteded on a on relative a relative scale scalewith withrespect respect to the toLOW the sampleLOW sample..

Sample IDID Weld Weld Frequency Frequency (Hz) (Hz) LOW 1.0 1.0 MED 1.17 1.17 HIGH 1.26 1.26 HIGH_PRF 1.35 1.35

2.3. Specimen Specimen Preparation Preparation Approximately 300300 mmmm longlong tubulartubular sections sections were were provided provided for for each each of of the the samples samples examined examined in inthis this work. work. These These sections sections were were then then cut intocut ~5into mm ~5 thickmm thick rings usingrings using a wet cuta wet saw cut in ordersaw in to order prevent to preventheating. heating. The rings The were rings then were cut then into ~50cut into mm ~50 arc section,mm arc withsection, the with weld the seam weld centered. seam centered. These sections These weresections then were hot mountedthen hot usingmounted a conductive using a conductive phenolic resin. phenolic The mountedresin. The specimens mounted werespecimens ﬁrst ground were firstusing ground a 220 grit using resin a bonded220 grit discresin and bonded then polished disc and using then clothpolished discs using and diamond cloth discs suspensions and diamond with suspensionsthe following with particle the following sizes: 9, 3 particle and 1 µ m.sizes: Ultrasonic 9, 3 and bath1 µm. cleaning Ultrasonic was bath employed cleaning between was employed grinding betweenand polishing grinding stages. and Inpolishing the as-polished stages. In state, the microhardnessas-polished state measurements, microhardness were measurements made, which were was madefollowed, which by etchingwas followed with a by 2% etching Nital solution with a to2% reveal Nital thesolution microstructure to reveal the during microstructure scanning electron during scanningmicroscopy electron (SEM) microscopy characterization. (SEM) characterization.

2.4. Microhardness Microhardness Testing A Clemex CMT microhardness apparatus was used to conduct all of the hardness testing for this work. For all of the hardness measurements, an indent load of 300 g was selected. Figure 55 showsshows aa schematic of the two different diﬀerent hardness profiles proﬁles (Centerline and 2D Contour) that were created for this work.

Centerline Distribution 2D Contour Distribution

Figure 5. A schematic showing the centerline and 2D contour hardness distributions. Figure 5. A schematic showing the centerline and 2D contour hardness distributions. The centerline distribution measurements were made at the mid-plane thickness of the specimens usingThe an indentcenterline spacing distribution of 150 µm measurements in order the prevent were false made measurement at the mid due-plane to plastic thickness deformation of the specimenscreated by using the neighboring an indent spacing indents. of After 150 µm the in first order centerline the prevent distribution false measurement measurement due was to pla made,stic deformationthe mounted created specimens by the were neighboring ground (~1 indents. mm depth),After the re-polished first centerline and distribution the hardness measurement distribution was measuredmade, the again.mounted This specimens was repeated were one ground more time(~1 inmm order depth to), provide re-polished three separateand the hardness distributiondistributions was that measured were then again. used toThis create was arepeated single average one more hardness time in distributionorder to provide profile. three separate hardnessThe automaticdistributions x-y that stage were of thethen microhardness used to create testera single was average then usedhardness to create distribution a regular profile. gridof hardnessThe automatic indents across x-y thestage weld of the seam microhardness as shown in the tester Figure was5 schematic.then used Thisto create grid patterna regular hardness grid of hardnessdistribution indents was then acro usedss the as weld input seam for aas MATLAB shown in script the Figure that was 5 schematic. developed This to generate grid pattern a 2D hardness hardness distribution contourwas then across used the as weld.input for a MATLAB script that was developed to generate a 2D hardness distribution contour across the weld. 2.5. Scanning Electron Microscopy 2.5. Scanning Electron Microscopy A FEI Quanta 250 field emission scanning electron microscope (SEM) was used to generate the micrographsA FEI Quanta of the 250 various field microstructures emission scanning along electron the weld microscope seam. These (SEM) micrographs was used wereto generate created the at micrographs of the various microstructures along the weld seam. These micrographs were created at

Metals 2020, 10, x FOR PEER REVIEW 7 of 16

known centerline distribution indents for reference. An accelerating voltage of 10 kV was used and the working distance varied from 9.1–9.4 mm for all of the SEM imaging conducted in this work.

2.6. Ring Hoop Tension Test (RHTT) Dick and Korkolis [34] utilized a Ring Hoop Tensile Test (RHTT) apparatus to measure the hoop flow stress properties of tubular specimens. They implemented 3D digital image correlation (DIC) methods to measure and verify the stress-state within the gauge length of the specimens. They were able to show that because the ring curvature does not change during the test, bending moments and shear forces are zero, allowing for a uniaxial tensile stress-state within the gauge length. This testing Metalsapproach2020, 10 ,was 425 adopted to test the weld seam properties for the current work. Figure 6a shows7 of 16 an image of the tubular specimens and the D-block tools that were displaced vertically to deform the knownring specimen. centerline distributionA 50 kN electro indents-mechanical for reference. tensile An frame accelerating was used voltage with of a 10cross kV- washead used velocity and the0.038 workingmm/s during distance the varied test. The from cross 9.1–9.4-head mm displaceme for all of thent and SEM axial imaging load were conducted measured in this for work. each test up to the point of failure. Petroleum jelly and a thin sheet of Teflon film were used to reduce the friction 2.6.between Ring Hoop the TensionD-blocks Test and (RHTT) specimen as shown in Figure 6b. Two different specimens were utilized in the study as shown in Figure 6c. The “Tensile” ring Dick and Korkolis [34] utilized a Ring Hoop Tensile Test (RHTT) apparatus to measure the hoop specimen was designed to replicate and ASTM E8 sub-size flat specimen with a gauge length of 25 flow stress properties of tubular specimens. They implemented 3D digital image correlation (DIC) mm. This specimen was machined with the weld seam located at the middle of the gauge length as methods to measure and verify the stress-state within the gauge length of the specimens. They were indicated in Figure 6c. These specimens were all orientated at 45° to the horizon as shown in Figure able to show that because the ring curvature does not change during the test, bending moments and 6a,b. In order to force the RHTT deformation within the weld seam region and not the base metal, a shear forces are zero, allowing for a uniaxial tensile stress-state within the gauge length. This testing “Notched” ring specimen was machined as shown in Figure 6c. The orientation of the weld seam for approach was adopted to test the weld seam properties for the current work. Figure6a shows an all of these tests was at 90° to the horizontal. A Correlated Solutions 3D DIC system was used to image of the tubular specimens and the D-block tools that were displaced vertically to deform the ring measure the strain distribution within the Tensile ring specimens only as shown in Figure 6b. It specimen. A 50 kN electro-mechanical tensile frame was used with a cross-head velocity 0.038 mm/s should be noted that all of the welds contained a small weld bead at the inner diameter (ID) of the during the test. The cross-head displacement and axial load were measured for each test up to the tubes, after scarfing (roll forming process used to remove majority of ID weld bead). In order to point of failure. Petroleum jelly and a thin sheet of Teflon film were used to reduce the friction between prevent stress concentrations at the weld seam during testing, the weld seam bead was carefully the D-blocks and specimen as shown in Figure6b. removed using a precision grinder and then polished smooth using 600 grit polishing paper.

Tensile Notched Specimen Specimen Specimen (c) (a) Weld (b) Weld Seam

45°

45° Weld Weld D-Blocks

Figure 6. (a) A solid model image of the ring hoop tension test (RHTT) tools and specimen used in

thisFigure work. 6. (b ()a) A A photograph solid model of image a speckled of the tensile ring hoop ring specimentension test with (RHTT the weld) tools seam and shown specimen at the used 45◦ in orientationthis work within. (b) A thephotograph RHTT. (c of) The a speckled Tensile andtensile Notched ring specimen ring specimen with the geometry weld seam used shown in this at work. the 45° Dimensionsorientation are within in mm. the RHTT. (c) The Tensile and Notched ring specimen geometry used in this work. Dimensions are in mm. Two different specimens were utilized in the study as shown in Figure6c. The “Tensile” ring specimen3. Results was and designed Discussion to replicate and ASTM E8 sub-size flat specimen with a gauge length of 25 mm. This specimen was machined with the weld seam located at the middle of the gauge length as indicated The results of the hardness distribution measurements (centerline and 2D contour), in Figure6c. These specimens were all orientated at 45 to the horizon as shown in Figure6a,b. In metallographic characterization and the RHTT will ◦be presented and discussed in the following order to force the RHTT deformation within the weld seam region and not the base metal, a “Notched” sections. ring specimen was machined as shown in Figure6c. The orientation of the weld seam for all of these tests was at 90 to the horizontal. A Correlated Solutions 3D DIC system was used to measure 3.1. Centerline Hardness◦ Distributions the strain distribution within the Tensile ring specimens only as shown in Figure6b. It should be noted that all of the welds contained a small weld bead at the inner diameter (ID) of the tubes, after scarfing (roll forming process used to remove majority of ID weld bead). In order to prevent stress concentrations at the weld seam during testing, the weld seam bead was carefully removed using a precision grinder and then polished smooth using 600 grit polishing paper.

3. Results and Discussion The results of the hardness distribution measurements (centerline and 2D contour), metallographic characterization and the RHTT will be presented and discussed in the following sections. Metals 2020, 10, 425 8 of 16

3.1. Centerline Hardness Distributions For each of the four induction seam welded specimens presented in Table1, three repeat centerline hardness measurements were made as shown in Figure7a–d. By linearly interpolating each of the three repeat distribution measurements at small and equivalent increments, an average hardness distribution curve was generated as shown by the bold and continuous curve in Figure7a–d. The repeatability of the individual measurements was excellent for the LOW, MED and HIGH_PRF specimens, while the HIGH weld condition had only good repeatability. Figure7b shows the three distinct zones for these welds, which are the base metal (BM), heat aﬀected zone (HAZ) and the weld zone (WZ). It should be noted that the weld zone terminology used here is synonymous with the fusion zone of a fusion welding process.

3.1.1. Effect of Weld Frequency on Centerline Hardness Distribution Figure7e compares the average centerline hardness distributions for the LOW, MED and HIGH weld frequency specimens. Based on this comparison, the LOW and MED weld frequency specimens presented a similar hardness distribution, with some variation at the WZ as will be discussed in a subsequent section. The total weld seam width (HAZ + WZ + HAZ) for the HIGH sample is more narrow than that of the LOW/MED samples for a hardness greater than ~325 HV300. This observation is highlighted in Figure7f, where the total weld seam width from the average hardness distribution curves in Figure7e is plotted with respect to hardness (from 300 to 450 HV 300 only) for the three weld frequency conditions. A simple regression analysis was conducted on the data and a logarithmic fit was applied to the data points in Figure7f, which confirms the similarity of the LOW /MED weld seam widths and the narrower weld seam width for the HIGH case. The difference in the total weld seam width is due to a narrower WZ for the HIGH sample and a distinct plateau of the hardness distribution at approximately ~325 HV300 for the HIGH sample. Qualitatively, Figure8 shows the narrower HIGH weld seam profile for an etched specimen where the brighter microstructure represents the approximate width of the WZ. It will be shown that the WZ consists of a predominantly martensitic microstructure for all of the welding cases, therefore it is likely that the HIGH weld frequency resulted in a lower welding temperature, which reduced the amount of material at the sheet edge that fully austenitized prior to quenching. Again, it should be repeated that this is a solid-state (or forge) welding process, where the sheet edge does not melt.

3.1.2. Microstructural Analysis of the Weld Seam The difference in the centerline hardness distribution for the LOW and HIGH case (Figure7e) is due to the microstructural composition within the HAZ for these two cases. A separate (from Figure7 data) centerline hardness distribution was measured for these two cases and shown in Figure9. The mounted samples were then etched and SEM micrographs were created adjacent to the indents which correspond to a common position as denoted as (1) to (4) in Figure9. At location (1), a similar mixed ferrite-martensite microstructure exists for both weld cases. Although it must be confirmed using an appropriate quantitative technique (i.e., Electron Backscatter Diffraction), there appears to be some of the original BM retained austenite that is observed as the blocky grains of the featureless constituent. Location (2) represents the part of the HAZ where the HIGH weld case exhibited a plateau in hardness (Figure7e), which can also be observed in the Figure9 centerline hardness distribution. A plateau was not observed in the LOW case, but for this particular location, the hardness was equivalent for both weld cases at 334 HV300. Upon observation of the microstructure at (2), the HIGH weld case microstructure consisted of ferrite-martensite, while the LOW weld case consisted of ferrite, martensite and a third phase which is defined by a ferritic matrix with a fine distribution of carbon rich precipitates that are likely carbides. This phase is labeled (αC) in Figure9 and is contained within grains of martensite. Without knowing the temperature-time profile for this complex seam Metals 2020, 10, x FOR PEER REVIEW 9 of 16

(combined with martensite) to be auto-tempered martensite within AHSS welds [36,37] and martensitic steels [38,39]. Both coalesced bainite and auto-tempered martensite are formed near the Metalsmartensite2020, 10 start, 425 temperature. It is expected that the strength of the phase is less than martensite,9 of 16 therefore the introduction of and the lower ferrite content in the LOW microstructure allows the 𝛼𝛼𝐶𝐶 hardness of these two microstructures to be equivalent. welding process, it is diﬃcult𝛼𝛼 to𝐶𝐶 hypothesis the precise phases which transform for these solid-state welds.A large Having difference said that, in similar hardness phases is observed which share in location the morphologies (3), where the observed narrowing in this of workthe HIGH have beenweld reportedcase was asdiscussed coalesced in bainite Section (combined 3.1.1 and withshown martensite) in Figure in 7e weld–f. The microstructures HIGH weld case for TRIPnow [has33] andsome high volume strength fraction steels of [35 ].introduced Additionally, into researchers the ferrite-martensite have commonly microstructure, reported thiswhereas type the of phaseLOW weld case is nearly void of ferrite and consists of a predominantly martensitic and microstructure (combined with martensite)𝛼𝛼𝐶𝐶 to be auto-tempered martensite within AHSS welds [36,37] and martensitic which results in the elevated hardness measurement for this weld frequency case. steels [38,39]. Both coalesced bainite and auto-tempered martensite are formed near𝛼𝛼𝐶𝐶 the martensite Location (4) is at the HAZ-WZ transition for the two cases and consists of a similar martensite start temperature. It is expected that the strength of the αC phase is less than martensite, therefore the and microstructure for both cases as evident by the similar hardness. There is a small volume introduction of αC and the lower ferrite content in the LOW microstructure allows the hardness of fraction of ferrite that can be observed (none shown for the LOW case in Figure 9) for both weld cases these𝛼𝛼 two𝐶𝐶 microstructures to be equivalent. at this location.

550 550 (a) Meas 1 (b) 500 Meas 2 500 LOW Meas 3 MED 450 450 ) ) Avg. 300 300 Base 400 400 Weld Zone HAZ Metal (WZ) (BM) 350 350

Hardness (HV Hardness (HV 300 300

250 250

200 200 -2000 -1000 0 1000 2000 -2000 -1000 0 1000 2000 Position (µm) Position (µm) 550 550 (c) (d) 500 500 HIGH HIGH_PRF 450 450 ) ) 300 300 400 400

350 350

Hardness (HV Hardness (HV 300 300

250 250

200 200 -2000 -1000 0 1000 2000 -2000 -1000 0 1000 2000 Position (µm) Position (µm) 550 2500 (e) HIGH (f) HIGH 500 MED MED 2000 LOW LOW 450

) 1500

300 400

350 1000

Hardness (HV 300

Total Total WeldSeam Width(µm) 500 250

200 0 -2000 -1000 0 1000 2000 250 300 350 400 450 500 Position (µm) Hardness (HV300)

Figure 7. TheThe repeat repeat and and average average centerline centerline hardness distribution for the ( a) LOW LOW,, (b) MED MED,, ( c) HIGH and (d) HIGH_PRF weld specimens.specimens. ( e) A comparison between the average hardness distribution for the diﬀerent weld frequencies. (f) A plot of the average HAZ width versus hardness range for the diﬀerent weld frequency samples. Metals 2020, 10, x FOR PEER REVIEW 10 of 16 Metals 2020, 10, x FOR PEER REVIEW 10 of 16 the different weld frequencies. (f) A plot of the average HAZ width versus hardness range for the differentthe different weld weld frequency frequencies samples. (f.) A plot of the average HAZ width versus hardness range for the Metalsdifferent2020, 10, 425weld frequency samples. 10 of 16

LOW HIGH LOW HIGH

1000 µm 1000 µm Figure 8. A low magnification SEM micrograph of an etched LOW and HIGH weld frequency case Figure 8. A low magniﬁcation SEM micrograph of an etched LOW and HIGH weld frequency case weld. weld.Figure 8. A low magnification SEM micrograph of an etched LOW and HIGH weld frequency case weld. 550 (4) 550 500 (4) 500 450 (3) (3)

) 450

300 400 )

300 400 (2) 350 (2) 350 (1)

Hardness (HV 300 (1) HIGH

Hardness (HV 300 250 HIGH LOW 250 200 LOW -1500 -1000 -500 0 200 Position (µm) -1500 -1000 -500 0 306 HV γ 334 HV Position362 (µm) HV 514 HV α αC 306 HV α γ 334 HV 362 HV α 514 HV α’ C α’ αC α’ α’ αC

HIGH α α’

HIGH α α α 3 µm α’ α’ α α 3 µm α’ 297 HV α γ 334 HV 431 HV α 497 HV 297 HV α γ 334 HV α 431 HV α 497 HV α’ α’ C α α α’

LOW ’ α’ α C α’ α α αC

LOW ’ α α’ C αC αC

(4) (1) (2) (3) (1) (2) (3) (4) Figure 9. The SEM micrographs at a common position ((1) to (4)) along the centerline hardness Figure 9. The SEM micrographs at a common position ((1) to (4)) along the centerline hardness distribution for the LOW and HIGH weld frequency cases (α-Ferrite, γ-Retained Austenite, distributionFigure 9. The for SEM the micrographsLOW and HIGH at a commonweld frequency position cases ((1) to(α -(4)Ferrite,) along γ- Retainedthe centerline Austenite, hardness α’- α’-Martensite, αC-carbide-rich phase). The scale marker is common for all of the micrographs shown. Martensite,distribution α forC-carbide the LOW-rich phaseand HIGH). The scaleweld marker frequency is common cases ( αfor-Ferrite, all of the γ-Retained micrographs Austenite, shown. α’- Martensite,A large diﬀ αerenceC-carbide in- hardnessrich phase is). The observed scale marker in location is common (3), where for all the of the narrowing micrographs of the shown. HIGH weld 3.1.3.case wasWeld discussed Zone Ferrite in Section-Martensite 3.1.1 Bandand shown in Figure7e–f. The HIGH weld case now has some 3.1.3. Weld Zone Ferrite-Martensite Band volumeWith fraction the exception of αC introduced of the MED into frequency the ferrite-martensite case, the remaining microstructure, cases all exhibited whereas thea local LOW drop weld in hardnesscase isWith nearly (down the voidexception to of ~400 ferrite of HV the and300 MED) consistswithin frequency the of aWZ predominantly case, for somethe remaining of martensiticthe repeat cases andmeasurements all exhibitedαC microstructure a localas shown drop which in Figurehardnessresults 7a in- thed.(down This elevated wasto ~400 unusual hardness HV300 because) measurementwithin the the WZ WZ hardness for for this some weld for ofall frequency the of the repeat cases case. measurements was ~500 HV300 as as shownexpected in forFigure a Locationpredominantly 7a-d. This (4) was is at unusualmartensitic the HAZ-WZ because microstructure. transition the WZ hardness for Upon the two for metallographic casesall of the and cases consists examination, was of~500 a similar HV 300the as martensite local expected soft regionforand aα predominantlyC withinmicrostructure the WZ martensitic was for bothdue casesto microstructure. the aspresence evident of byUpon ferrite the metallographic similar within hardness. the martensitic examination, There ismicrostructure a smallthe local volume soft as shownregionfraction inwithin of Figure ferrite the 10 that WZ. The can was ferrite be due observed-martensite to the (none presence band shown wasof forferrite ~30 the µm LOWwithin in width, case the in martensitic therefore Figure9) forit microstructurewas both not weld detected cases as inshownat some this location.in instances Figure 10 due. The to theferrite spacing-martensite of the bandindents. was It ~30is possible µm in width, that the therefore edge of itthe was tube not where detected the in some instances due to the spacing of the indents. It is possible that the edge of the tube where the

Metals 2020, 10, 425 11 of 16

3.1.3. Weld Zone Ferrite-Martensite Band With the exception of the MED frequency case, the remaining cases all exhibited a local drop in hardness (down to ~400 HV300) within the WZ for some of the repeat measurements as shown in Figure7a–d. This was unusual because the WZ hardness for all of the cases was ~500 HV 300 as expected for a predominantly martensitic microstructure. Upon metallographic examination, the local soft region within the WZ was due to the presence of ferrite within the martensitic microstructure as shown in Figure 10. The ferrite-martensite band was ~30 µm in width, therefore it was not detected Metalsin some 2020 instances, 10, x FOR PEER due REVIEW to the spacing of the indents. It is possible that the edge of the tube where11 of 16 the ferrite-martensite band formed (at the weld interface) did not austenitized fully due to either the ferriteedge condition-martensite of theband sheet formed or the (at complex the weld magnetic interface) ﬁeld did creatednot austenitized through inductionfully due forto either this particular the edge weldingcondition setup. of the sheet or the complex magnetic field created through induction for this particular welding setup.

α α’

25 µm

Figure 10. CentralCentral f ferrite-martensiteerrite-martensite band within the weld zone ( (WZ).WZ). ( α‐-Ferrite,Ferrite, α’’-Martensite).‐Martensite). 3.2. 2D Hardness Distribution Contours 3.2. 2D Hardness Distribution Contours For each of the four induction seam welding specimens presented in Table1, the 2D hardness For each of the four induction seam welding specimens presented in Table 1, the 2D hardness contour maps of the weld seam region are shown in Figure 11. At ﬁrst glance, the variation in hardness contour maps of the weld seam region are shown in Figure 11. At first glance, the variation in distribution through the thickness of the sheet is evident among the four diﬀerent weld conditions hardnessMetals 2020 ,distribution 10, x FOR PEER through REVIEW the thickness of the sheet is evident among the four different12 ofweld 16 examined in this work as discussed in the following. conditions examined in this work as discussed in the following. With respect to the center of the weld, only the HIGH frequency case resulted in a relatively symmetric0.5 hardness distribution, while the remainder of the weld cases consist of an asymmetric 1.00Hz LOW MED 1.17Hz hardness distribution on either side of the 1.00weld.W The LOW and HIGH_PRF consisted0.99W of local hard 0.25 regions distributed throughout the WZ and1.00N HAZ, which can have an impact on1.02N the damage accumulation0 and fracture performance within these weld seams during deformation. It is also evident that using a single centerline hardness distribution measurement for these types of seam - welds (mm) Position 0.25 does not capture the complex and asymmetric variation in hardness and microstructural distribution for these welds. - 0.5 The0.5 presence of the narrow central martensite-ferrite band was also observed in Figure 11 for 1.35Hz HIGH 1.26Hz HIGH_PRF the LOW, HIGH and HIGH_PRF weld frequency0.98W cases as was shown in centerline0.92W hardness 1.16N distributions0.25 (Figure 7) and micrograph (Figure1.05N 10). The highest hardness was observed near the top of the weld for the MED, HIGH and HIGH_PRF welds at a value of ~550 HV300 which is likely due to 0 an uneven heating of the sheet edge prior to welding, or the higher cooling rate at this surface from plateau the -quench0.25 media applied during the roll forming process. Position (mm) Position The narrower centerline hardness distribution profile of the HIGH frequency (See Figure 7) was - 0.5 also captured-1.0 by the- 0.5 plateau 0and steep0.5 gradient1.0 of-1.0 hardness -contours0.5 shown0 in Figure0.5 11, 1.0which is a Position (mm) Position (mm) desirable characteristic of induction seam welds [16]. This narrowing of the WZ and HAZ is consistentFigure through 11. The the 2D thickness hardness contourof the weld. maps forIn alladdition of the weld to the conditional asymmetry examined (with inrespect this work. to vertical plane) ofFigure the LOW11. The and 2D hardnessHIGH_PRF, contour the maps variation for all inof thehardness weld conditional through examinedthe thickness in this of work the . welds is high Withand respectundesirable to the from center a rep of theeatability weld, onlyperspective the HIGH as frequencythe weld material case resulted properties in a relatively will be inconsistent.symmetric3.3. Ring Hoop hardness Although Tension distribution, Test the total(RHTT) weld whileResults seam the width remainder (HAZ + of WZ the + weldHAZ) cases of the consist HIGH_PRF of an case asymmetric (Figure 7d) appearThe resultss to be of consistent the RHTT forwith the the two LOW different and MEDspecimen cases, geometries the 2D hardness (Tensile andcontour Notch) indicates that were that theconsidered high pressure in this roll work force are during presented seam in welding the following resulted sections. in a noticeably The LOW, wider HIGH total andweld HIGH_PRF seam width throughoutcases were tested, the thickness while the of MEDthe weld welds for were this particular not considered case. dueIn add to itiontheir, similaritythe average to thehardness HIGH within case. the wider WZ is relatively high when compared to the other weld seam cases, indicating a wider and higher3.3.1. Tensile strength Specimen WZ for Resultsthis case. By comparing the HIGH_PRF and HIGH weld cases, it is evident that anTh increasee force-extension in pressur resultse roll forcefrom (andthe RHTT frequency tests inconducted this case) on dramatically the Tensile altersring specimens the seam weldare characteristics,shown in Figure which 12a. Theis a resultLOW ofand the HIGH variation results in aretemperature similar with at the a peak sheet load edge of (due ~15.2 to kN frequency) and an andaverage the effectextension that ofthe ~4.8 increased mm. The force HIGH_PRF has on thecondition variation resulted in phase in a transflower ormationpeak load kinetics and extension for this particularof 14.3 kN material. and 3.1 mm, respectively. A second HIGH_PRF repeat test was conducted and showed a similar response. Figure 12b shows the true strain distribution (just prior to fracture) in the y-axis direction ( ) as quantified using DIC. For all weld cases, fracture occurred within the base metal (BM), well outside of the seam weld HAZ. Upon further investigation, the lower peak load of the 𝜀𝜀𝑦𝑦𝑦𝑦 HIGH_PRF weld was due to a reduction in the sheet thickness on the ID of the tube near the weld (due to roll forming setup), which resulted in premature localization of the BM in that region as shown in Figure 12b. The LOW case also exhibited this sheet thinning and localization, but to a lesser degree. In order to assess the resistance to deformation of the seam welds, the strain distribution ( ) with respect to vertical position (along the vertical dashed line slice shown in Figure 12b, HIGH case) 𝜀𝜀𝑦𝑦𝑦𝑦 was extracted from the DIC contours for a common test load of 14 kN and is shown in Figure 12c. With the zero vertical position being aligned at the center of the weld seam, the WZ experiences a strain just less than 0.01 for all of the weld cases. Figure 12c also indicates that the microstructural profile of the HIGH_PRF case is less resistant to deformation than the HIGH/LOW cases. This is true for the vertical position range from −1 mm to +1 mm, which represent the total weld seam width (HAZ + WZ + HAZ) and is unaffected by the sheet thinning region discussed above.

Metals 2020, 10, 425 12 of 16 hardness distribution on either side of the weld. The LOW and HIGH_PRF consisted of local hard regions distributed throughout the WZ and HAZ, which can have an impact on the damage accumulation and fracture performance within these weld seams during deformation. It is also evident that using a single centerline hardness distribution measurement for these types of seam welds does not capture the complex and asymmetric variation in hardness and microstructural distribution for these welds. The presence of the narrow central martensite-ferrite band was also observed in Figure 11 for the LOW, HIGH and HIGH_PRF weld frequency cases as was shown in centerline hardness distributions (Figure7) and micrograph (Figure 10). The highest hardness was observed near the top of the weld for the MED, HIGH and HIGH_PRF welds at a value of ~550 HV300 which is likely due to an uneven heating of the sheet edge prior to welding, or the higher cooling rate at this surface from the quench media applied during the roll forming process. The narrower centerline hardness distribution proﬁle of the HIGH frequency (See Figure7) was also captured by the plateau and steep gradient of hardness contours shown in Figure 11, which is a desirable characteristic of induction seam welds [16]. This narrowing of the WZ and HAZ is consistent through the thickness of the weld. In addition to the asymmetry (with respect to vertical plane) of the LOW and HIGH_PRF, the variation in hardness through the thickness of the welds is high and undesirable from a repeatability perspective as the weld material properties will be inconsistent. Although the total weld seam width (HAZ + WZ + HAZ) of the HIGH_PRF case (Figure7d) appears to be consistent with the LOW and MED cases, the 2D hardness contour indicates that the high pressure roll force during seam welding resulted in a noticeably wider total weld seam width throughout the thickness of the weld for this particular case. In addition, the average hardness within the wider WZ is relatively high when compared to the other weld seam cases, indicating a wider and higher strength WZ for this case. By comparing the HIGH_PRF and HIGH weld cases, it is evident that an increase in pressure roll force (and frequency in this case) dramatically alters the seam weld characteristics, which is a result of the variation in temperature at the sheet edge (due to frequency) and the eﬀect that the increased force has on the variation in phase transformation kinetics for this particular material.

3.3. Ring Hoop Tension Test (RHTT) Results The results of the RHTT for the two diﬀerent specimen geometries (Tensile and Notch) that were considered in this work are presented in the following sections. The LOW, HIGH and HIGH_PRF cases were tested, while the MED welds were not considered due to their similarity to the HIGH case.

3.3.1. Tensile Specimen Results The force-extension results from the RHTT tests conducted on the Tensile ring specimens are shown in Figure 12a. The LOW and HIGH results are similar with a peak load of ~15.2 kN and an average extension of ~4.8 mm. The HIGH_PRF condition resulted in a lower peak load and extension of 14.3 kN and 3.1 mm, respectively. A second HIGH_PRF repeat test was conducted and showed a similar response. Figure 12b shows the true strain distribution (just prior to fracture) in the y-axis direction (εyy) as quantiﬁed using DIC. For all weld cases, fracture occurred within the base metal (BM), well outside of the seam weld HAZ. Upon further investigation, the lower peak load of the HIGH_PRF weld was due to a reduction in the sheet thickness on the ID of the tube near the weld (due to roll forming setup), which resulted in premature localization of the BM in that region as shown in Figure 12b. The LOW case also exhibited this sheet thinning and localization, but to a lesser degree. Metals 2020, 10, x FOR PEER REVIEW 13 of 16 weld to the other is ~2.5 mm, therefore the material condition at the notch is represented by approximately Point 1 in Figure 9. Three repeat test results are plotted with the zero extension point referring to the 0.2 kN load measurement from each test. As a result of the elevated hardness/strength of the seam welds, all of the peak loads exceeded the peak loads from the Tensile specimens, which failed in the BM. For all three weld conditions, fracture occurred at either end of the notch which is at the interface of the HAZ and BM as indicated by the etched section of a failed HIGH weld case notch in Figure 12d. The HIGH/LOW results were similar at an average peak load of 17.2 kN, while the HIGH_PRF average peak load was slightly lower at 16.3 kN. It is possible that the thinning of the sheet (discussed in Section 3.3.1) for the HIGH_PRF case may have contributed to the lower peak loads. Nonetheless, these three weld cases were sufficient in preventing seam weld fracture and the HIGH/LOWMetals 2020, 10, 425peak forces were 13% greater in these tests when compared to the BM failures from13 ofthe 16 Tensile specimen tests.

εyy 20 0.35 (a) (b) 0.30 16

0.25 12 0.20

Force (kN) Force 8 0.15

4 0.10 LOW HIGH HIGH_PRF 0.05 0 0.0 2.0 4.0 6.0 0 Extension (mm) HIGH_PRF HIGH LOW

0.10 20 (c) (d) 0.08 16

0.06 12

(mm/mm) 2mm Notch yy ε

0.04 (kN) Force 8

True Strain, True 0.02 4

0.00 0 -4 -2 0 2 4 0.0 0.5 1.0 1.5 2.0 Vertical Position (mm) Extension (mm) FigureFigure 12. ((aa)) The The RHTT RHTT force force-extension-extension curves curves for for the Tensile Specimens.Specimens. ( (bb)) T Thehe digital image correlationcorrelation (DIC)(DIC) contourscontours of of true true strain strain in in the the y-axis y-axis (εyy ( ) just) just prior prior to the to pointthe point of fracture. of fracture (c) The. (c) weld The weldseam seam strain strain (εyy ) plotted( ) plotted versus versus vertical vertical position position at a test at force a test of force 14 kN. of (d14) ThekN. RHTT(d) The force-extension RHTT force- 𝜀𝜀𝑦𝑦𝑦𝑦 extensioncurves for curves the Notch for the Specimen Notch whereSpecimen three where repeat three tests repeat are shown tests forare each shown condition. for each The condition 2 mm notch. The 𝜀𝜀𝑦𝑦𝑦𝑦 2width mm notch is indicated width inis indicated the micrograph. in the micrograph.

4. ConclusionsIn order to assess the resistance to deformation of the seam welds, the strain distribution (εyy) with respect to vertical position (along the vertical dashed line slice shown in Figure 12b, HIGH case) wasAs extracted a result from of thethe DIC experimental contours for investigation a common testinto load the ofeffect 14 kN of and induction is shown seam in Figure welding 12c. parametersWith the zero in verticalroll form positioned TRIP690 being tubes, aligned a more at the thorough center of understanding the weld seam, has the been WZ gained experiences on the a impactstrain just that less induction than 0.01 forwelding all of thefrequency weld cases. has Figureon the 12 cresultant also indicates weld that seam the microstructuralcharacteristics, microstructureproﬁle of the HIGH_PRF and fracture case performance. is less resistant The to following deformation conclusions than the can HIGH be made:/LOW cases. This is true for the vertical position range from 1 mm to +1 mm, which represent the total weld seam width − (HAZ + WZ + HAZ) and is unaﬀected by the sheet thinning region discussed above.

3.3.2. Notched Specimen Results The force-extension results from the RHTT tests conducted on the Notched ring specimens are shown in Figure 12d. The notch width (see Figure6c) is 2 mm, therefore it encompasses the entire total weld seam width (HAZ + WZ + HAZ) and forces the deformation/fracture to occur within this region, rather than the BM. It should be noted that the distance from the BM-HAZ at one side of the weld to the other is ~2.5 mm, therefore the material condition at the notch is represented by approximately Point 1 in Figure9. Three repeat test results are plotted with the zero extension point referring to the 0.2 kN load measurement from each test. As a result of the elevated hardness/strength of the seam Metals 2020, 10, 425 14 of 16 welds, all of the peak loads exceeded the peak loads from the Tensile specimens, which failed in the BM. For all three weld conditions, fracture occurred at either end of the notch which is at the interface of the HAZ and BM as indicated by the etched section of a failed HIGH weld case notch in Figure 12d. The HIGH/LOW results were similar at an average peak load of 17.2 kN, while the HIGH_PRF average peak load was slightly lower at 16.3 kN. It is possible that the thinning of the sheet (discussed in Section 3.3.1) for the HIGH_PRF case may have contributed to the lower peak loads. Nonetheless, these three weld cases were suﬃcient in preventing seam weld fracture and the HIGH/LOW peak forces were 13% greater in these tests when compared to the BM failures from the Tensile specimen tests.

4. Conclusions As a result of the experimental investigation into the eﬀect of induction seam welding parameters in roll formed TRIP690 tubes, a more thorough understanding has been gained on the impact that induction welding frequency has on the resultant weld seam characteristics, microstructure and fracture performance. The following conclusions can be made:

Based on the measured centerline hardness distributions, the LOW and MED weld frequency • cases resulted in similar hardness distributions, while the HIGH weld frequency resulted in a narrower total weld width within the HAZ due to a plateau in hardness at approximately 325 HV300. The plateau in hardness was due to a delay in the transformation of martensite which indicates that the HIGH weld frequency resulted in a lower weld temperature. The HAZ was composed of increasing volume fractions of martensite and coalesced bainite • (or auto-tempered martensite) as the hardness increased from the BM towards the WZ. The presence of ferrite was observed within the predominantly martensitic weld interface which may have been due to insuﬃcient austenitization at the complex tube edge condition. 2D hardness distributions were shown to be important in identifying highly asymmetric weld • microstructure distributions, which are undesirable from a fracture performance perspective. An increased weld width was observed for a higher pressure roll force (with changes also to • frequency and power) and it is likely that the high weld force resulted in more of the sheet edge being austenitized, but also possibly aﬀected the phase transformation kinetics for this particular grade of TRIP690. The Ring Hoop Tension Test (RHTT) was used to evaluate the mechanical weld seam performance • of the tubes examined in this work and showed that peak loads were higher for the Notch specimen than the Tensile specimens by ~13% (for HIGH/LOW), indicating good weld fracture performance and the suitability of these tubes for hydroforming applications. From an experimental and numerical perspective, future work will focus on quantifying the • impact that induction welding parameters have on the complex temperature-time history of the seam welding process. This will allow for the optimization of the weld seam microstructure for this and future novel AHSS.

Author Contributions: Conceptualization, A.B.; methodology, A.B.; formal analysis, A.B.; data curation, B.T.A.; investigation, B.T.A.; writing—original draft, A.B. and B.T.A.; writing—review and editing, A.B.; supervision, A.B.; funding acquisition, A.B. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by the Natural Sciences and Engineering Research Council of Canada (NSERC). Conﬂicts of Interest: The authors declare no conﬂict of interest.

References

1. Corporate Average Fuel Economy | NHTSA. (n.d.). Available online: https://www.nhtsa.gov/laws-regulations/ corporate-average-fuel-economy#light-duty-stakeholders (accessed on 30 September 2019). 2. Bouaziz, O.; Zurob, H.; Huang, M. Driving force and logic of development of advanced high strength steels for automotive applications. Steel Res. Int. 2013, 84, 937–947. [CrossRef] Metals 2020, 10, 425 15 of 16

3. Karbasian, H.; Tekkaya, A.E. A review on hot stamping. J. Process. Technol. 2010, 210, 2103–2118. [CrossRef] 4. Bardelcik, A.; Bouhier, A.; Worswick, M.J. Three Point Bend Performance of Solutionized, Die Quenched and Heat Treated AA7075 Beam Members. Mater. Sci. Forum 2014, 794, 431–436. [CrossRef] 5. Naka, T.; Yoshida, F. Deep drawability of type 5083 aluminium-magnesium alloy sheet under various conditions of temperature and forming speed. J. Process. Technol. 2013, 89, 19–23. [CrossRef] 6. Tari, D.G.; Worswick, M.J.; Winkler, S. Experimental studies of deep drawing of AZ31B magnesium alloy sheet under various thermal conditions. J. Process. Technol. 2013, 213, 1337–1347. [CrossRef] 7. Dohmann, F.; Hartl, C. Tube hydroforming – research and practical application. J. Process. Technol. 1997, 71, 174–186. [CrossRef] 8. Bardelcik, A. Effect of Pre-Bending and Hydroforming Parameters on the Formability of Advanced High Strength Steel Tube. Master’s Thesis, University of Waterloo, Waterloo, ON, Canada, October 2006. 9. Oliveira, D.A. Interaction Between Forming and the Crash Response of Aluminium Alloy S-Rails. Ph.D. Thesis, University of Waterloo, Waterloo, ON, Canada, July 2007. 10. Bardelcik, A.; Worswick, M.J. Evaluation of load control end-feed in hydroforming of pre-bent DP600 steel tube using the extended stress-based forming limit curve failure criterion. J. Comput. Appl. Mech. 2006, 7, 133–154. 11. Simha, C.H.M.; Gholipour, J.; Bardelcik, A.; Worswick, M.J. Prediction of necking in tubular hydroforming using an extended stress-based forming limit curve. J. Eng. Mater. Technol. 2007, 129, 36–47. [CrossRef] 12. Groche, P.; Breitenbach, G.V.; Steinheimer, R. Properties of tubular semi-finished products for hydroforming. Steel Res. Int. 2005, 76, 181–186. [CrossRef] 13. Groche, P.; Breitenbach, G. Roll Forming Strategies for Welded Tubes. Steel Res. Int. 2008, 79, 40–46. [CrossRef] 14. The Tube and Pipe Journal-Choosing a Welding Process for Tubular Profiles. Available online: https://www.thefabricator.com/tubepipejournal/article/tubepipeproduction/choosing-a-welding- process-for-tubular-profiles (accessed on 15 November 2019). 15. Ghaffarpour, M.; Akbari, D.; Moslemi Naeeni, H.; Ghanbari, S. Improvement of the joint quality in the high-frequency induction welding of pipes by edge modification. Weld World 2019, 63, 1561–1572. [CrossRef] 16. The Tube and Pipe Journal-Selecting a welding frequency-Research on the optimal frequency for tube production. Available online: https://www.thefabricator.com/tubepipejournal/article/tubepipefabrication/ selecting-a-welding-frequency (accessed on 28 January 2020). 17. Weman, K. Welding processes handbook, 1st ed.; Woodhead Publishing Ltd.: Cambridge, UK, 2013; p. 89. 18. Nikanorov, A.; Baake, E.; Brauer, H.; Weil, C. Approaches for Numerical Simulation of High Frequency Tube Welding Process. In Proceedings of the International Conference on Heating by Electromagnetic Sources, Pandua, Italy, 22–24 May 2013; pp. 647–654. 19. Kim, C.M.; Kim, J.K. The effect of heat input on the defect phases in high frequency electric resistance welding. Met. Mater. Int. 2009, 15, 141–148. [CrossRef] 20. Okabe, T.; Yasuda, K.; Nakata, K. Dynamic observations of welding phenomena and finite element analysis in high-frequency electric resistance welding. Weld. Int. 2016, 30, 835–845. [CrossRef] 21. Li, F.; Ning, J.; Wang, T.; Liang, S.Y. Analytical modeling and sensitivity analysis of the temperature distribution in the planar scanning induction heating based on 2D moving heat source. J. Mech. Sci. Technol. 2019, 33, 5093–5102. [CrossRef] 22. Li, F.; Ning, J.; Liang, S.Y. Analytical Modeling of the Temperature Using Uniform Moving Heat Source in Planar Induction Heating Process. Appl. Sci. 2019, 9, 1445. [CrossRef] 23. Zackay, V.F.; Parker, E.R.; Fahr, D.; Busch, R. The enhancement of ductility in high-strength steels. ASM Trans. Q. 1967, 60, 252–259. 24. Matsumura, O.; Sakuma, Y.; Takechi, H. Enhancement of elongation by retained austenite in intercritical annealed 0.4C-1.5Si-0.8Mn steel. ISIJ Int. 1987, 27, 570–579. [CrossRef] 25. Ashiri, R.; Mostaan, H.; Park, Y. A Phenomenological Study of Weld Discontinuities and Defects in Resistance Spot Welding of Advanced High Strength TRIP Steel. Metall. Mater. Trans. A 2018, 49, 6161–6172. [CrossRef] 26. Sajjadi-Nikoo, S.; Pouranvari, M.; Abedi, A.; Ghaderi, A.A. In situ postweld heat treatment of transformation induced plasticity steel resistance spot welds. Sci. Tchnol. Weld. Joi. 2018, 23, 71–78. [CrossRef] 27. Spena, P.R.; Cortese, L.; De Maddis, M.; Lombardi, F. Effects of Process Parameters on Spot Welding of TRIP and Quenching and Partitioning Steels. Steel Res. Int. 2016, 87, 1592–1600. [CrossRef] Metals 2020, 10, 425 16 of 16

28. Zhang, M.; Li, L.; Fu, R.; Zhang, J.; Wan, Z. Weldability of Low Carbon Transformation Induced Plasticity Steel. J. Iron Steel Res. Int. 2008, 15, 61–65. [CrossRef] 29. López Cortéz, V.H.; Pérez Medina, G.Y.; Reyes Valdéz, F.A.; López, H.F. Effects of the heat input in the mechanical integrity of the welding joints welded by GMAW and LBW process in Transformation Induced Plasticity steel (TRIP) used in the automotive industry. Soldag. insp. 2010, 15, 234–241. [CrossRef] 30. Grajcar, A.; Rózański,M.; Stano, S.; Kowalski, A. Microstructure Characterization of Laser-Welded Mb-Microalloyed Silicon-Aluminum TRIP Steel. J. Mater. Eng. Perform. 2014, 23, 3400–3406. [CrossRef] 31. Medina, G.P.; Ferreira, H.L.; Robledo, P.Z.; Perez, A.M.; Valdes, F.E.R. Microstructural Development in a TRIP-780 Steel Joined by Friction StirWelding (FSW): Quantitative Evaluations and Comparisons with EBSD. Soldagem & Inspeção 2016, 21, 146–155. [CrossRef] 32. Mironova, S.; Satoa, Y.S.; Yoneyamab, S.; Kokawaa, H.; Fujiia, H.T.; Hiranoc, S. Microstructure and tensile behavior of friction-stir welded TRIP steel. Mat. Sci. Eng. A 2018, 717, 26–33. [CrossRef] 33. Mazzaferro, C.C.P.; Ramos, F.D.; Mazzaferro, J.A.E.; de Souza Rosendo, T.; Tier, M.A.D.; da Silva, A.M.; Reguly, A. Microstructure evaluation and mechanical properties of a friction stir spot welded TRIP 800 steel. Weld. Int. 2011, 25, 683–690. [CrossRef] 34. Dick, C.P.; Korkolis, Y.P. Mechanics and full-field deformation study of the Ring Hoop Tension Test. Int. J. Solids Struct. 2014, 51, 3042–3057. [CrossRef] 35. Keehan, E.; Karlsson, L.; Bhadeshia, H.K.D.H.; Thuvander, M. Electron backscattering diffraction study of coalesced bainite in high strength steel weld metals. J. Mater. Sci. Technol. 2013, 24, 1183–1188. [CrossRef] 36. Pouranvari, M.; Sobhani, S.; Goodarzi, F. Resistance spot welding of MS1200 martensitic advanced high strength steel: Microstructure-properties relationship. J. Manuf. Process. 2018, 31, 867–874. [CrossRef] 37. Saha, D.C.; Biro, E.; Gerlich, A.P.; Zhou, Y. Effects of tempering mode on the structural changes of martensite. Mat. Sci. Eng. A. 2016, 673, 467–475. [CrossRef] 38. Jarvinen, H.; Honkanen, M.; Jarvenpaa, M.; Peura, P. Effect of paint bake treatment on the properties of press hardened boron steels. J. Eng. Mater. Technol. 2018, 252, 90–104. [CrossRef] 39. Bourque, C.; Bardelcik, A.; Wells, M.; Chiriac, C. Short cycle tempering of an 1800 MPa grade of press hardening steel. Proceedings of Materials Science and Technology 2019, Portland, OH, USA, 29 September– 3 October 2019.

© 2020 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).