Introduction Modern steel coil processing lines (such as pickling and galvanizing) benefit greatly from a continuous feed of steel strip, a process that requires coil end join- ing (Ref. 1). As is shown in Fig. 1, contin- uous processing is achieved through the combined use of an accumulator (Fig. 1B) (Ref. 2) and a coil end welding machine (Fig. 1C). Due to the finite capacity of the accumulator, one main constraint on weld- ing process selection is welding speed. Ad- ditional constraints, such as material thickness, result in a variety of welding processes being used for coil end joining, including gas tungsten arc welding (GTAW), gas metal arc welding (GMAW), flash welding (FW), resistance (mash) seam welding (RSEW-MS), and laser beam welding (LBW) (Ref. 3). Note that the last two are mostly used on tin coating and recoiling lines, where the sheet thick- ness is less than 1 mm. Joining of advanced high-strength steel (AHSS) coil ends brings additional chal- lenges to coil end joining due to inherent high strength, prompting difficulties in end shearing, and faying surface alignment, and a propensity for localized hardening in welding microstructures. Indeed, exces- sive hardness in the fusion and heat- affected zones has been reported in the case of laser beam welding of dual-phase (DP) and transformation-induced plastic- ity (TRIP) steels (Ref. 4). The presence of such excessive hardness can provide a met- allurgical notch, precipitating joint failure during subsequent mill operations. To eliminate potential problems stemming from the as-cast microstructures found in fusion welding, a solid-state welding process is desired for joining AHSS coil ends. While resistance welding (RW) and flash welding (both solid-state processes) are currently widely employed in coil join- ing, RW is slow and is limited to small thicknesses (Ref. 1) while FW can be dif- ficult to control because it is susceptible to irregular arcing and incomplete fusion on the strip edges. Even resultant strip breaks of 0.2% are not acceptable, as equipment is damaged and production lost. There- fore, an improved solid-state joining process is desired for joining AHSS coil ends. Previous work has demonstrated that a coupled high-frequency induction heat- ing/pressure welding (termed hyper-inter- facial bonding) operation can produce faying surface coalescence in butt joint con- figurations and minimize thermally induced changes in grain size of ultrafine-grained steel (Ref. 5). Heating times for 5 × 5 × 30- mm specimens were shown to be very rapid (0.2 s to 1600°C at 1 MHz and 50–59 kW) (Ref. 5), indicating that high-frequency welding can satisfy the time constraints as- sociated with coil end joining. Additionally, previous research demon- strated that high-frequency welding could produce good welds in AHSS specimens (Fig. 2), as evidenced by successful limited dome height formability testing (Ref. 6). The long history of successful high- frequency induction welding (HFIW) of joints in tubular products and structural shapes (Ref. 7) also suggests the usefulness of high-frequency welding for coil end join- ing. This present work builds on this foun- dation by developing numerical and ex- perimental techniques for simulating high-frequency welding of dual-phase steel coil ends, thereby 1) providing insight into fundamental high-frequency heat- ing/material interactions, 2) establishing operating parameters, and 3) demonstrat- ing the feasibility of joining DP steel coil ends with high-frequency welding. Physical Simulation Overview High-frequency induction heating/ pressure welding of dual-phase steels was performed at small scale through induction heating using a solid-state-controlled, state- of-the-art 100-kW variable-frequency SUPPLEMENT TO THE WELDING JOURNAL, OCTOBER 2009 Sponsored by the American Welding Society and the Welding Research Council Transient High-Frequency Welding Simulations of Dual-Phase Steels Numerical and experimental simulations were used to investigate high-frequency welding of advanced high-strength steels BY R. BAUMER AND Y. ADONYI KEYWORDS High-Frequency Welding Dual-Phase Steels FEA Modeling Heating Rates R. BAUMER is a Graduate Student at Massa- chusetts Institute of Technology, Cambridge, Mass. Y. ADONYI is the Omer Blodgett Professor of Welding and Materials Joining Engineering at LeTourneau University, Longview, Tex. ABSTRACT Continued development of ad- vanced high-strength steel (AHSS) re- quires a corresponding improvement in joining technology. One promising joining method is high-frequency butt joint welding. Seeking to validate the utility of this process for joining AHSS flat sheet specimens for steel mill pro- cessing lines, high-frequency butt joint welding of flat sheet steel was investi- gated through a combined numerical and experimental simulation method- ology. Simulated welds were produced and pre-Curie and post-Curie temper- ature heating rate differences were ob- served with infrared radiation (IR) imaging. Good correlations were found between numerical predictions and actual heating rates. Final metal- lographic analysis revealed complete coalescence of faying surface, with only minor hardening at the weld in- terface. It was concluded that high-fre- quency welding shows good potential for coil joining in steel processing lines. 193-s WELDING JOURNAL WELDING RESEARCH