Industry’s relentless pursuit of product performance improvements is now challenging the capability of available/existing thermal-processing technologies (i.e. heat treating). In fact, the EPA-mandated requirement for lightweighting vehicles underscores the urgent U.S. need for achieving product strength improvements.
Current, traditional thermally activated processes for material performance improvements and strengthening have evolved empirically and are largely isothermally based, stabilized reactions. Using this approach, the heat input, flow and its redistribution are driven only by an applied, elevated-temperature differential. Therefore, any thermal acceleration requires elevated surface temperatures, which can result in several undesirable effects and other process-capability limitations.
Advanced Induction Heating Capabilities
Induction heating has continued to improve its ability to selectively focus and provide a highly controlled, internal thermal profile of a part, enabling specifically programmed, thermally driven metallurgical reactions to be optimized for maximum strengthening results. Notably, the latest innovations in equipment versatility and FEA-modeled hardware tooling provide unmatched, precise capability in accelerating processing speeds.
Truly, there is no limit on obtainable temperature. Induction heating generates heat within the subsurface of the part, and the depth level of this heat distribution can be programmed, as well as dynamically profiled, to achieve desired metallurgical results. These results are accomplished by the efficient utilization of environmentally clean, electrical energy. The resultant internal heat profile of the part can be tweaked using a wide range of in-situ computerized process-control parameters. Therefore, the processing can be fashioned to provide optimal thermal-driven reactions for improved part performance.
Induction heating thermal dynamics are vastly different from those applied in traditional isothermal systems. In fact, in cases of short heat times (1-2 seconds or less) or complex dynamic thermal geometric profiles, these dynamics cannot be accurately recorded nor graphically display the material’s in-situ thermal response. This, in turn, inhibits a more complete understanding and evaluation of the multiple reaction issues involved, which limits the establishment of an optimum process.
Utilizing the latest advances in computer modeling (including the use of fully coupled, electromagnetic/thermal and FEA/FED modeling), however, these challenges/limitations are addressed, and the actual dynamics can be simulated. This approach enables the complex thermal logistics and subsequent inter-reactive reactions to be graphically displayed and understood and preferential adjustments implemented and/or reactions/process parameters/dynamics identified to improve part performance. This coupling of material science and simulation process engineering provides an advanced understanding and review of viable alternative solutions.
Despite the significant progress achieved in induction heating, it is still limited by its very nature of being a primarily thermal-driven process. The increasing demands for improved industrial part performance cannot be solved solely with traditional heat-treating processes.
High Magnetic Processing Benefits and Impacts
To exceed the materials performance limits solely achievable with traditional induction heating approaches, a totally new, innovative and disruptive technology and approach has been developed – High Magnetic Field Processing (HMFP). HMFP coupled with induction heating processing overcomes the energy barriers limited by thermal-energy processes alone. In fact, HMFP has been shown to impact all materials.
Strategically coupling HMFP with induction heating processing has been shown to significantly improve the performance of a broad array of parts. And it does it in far less time than thermal processing alone. This occurs because magnetic fields totally alter the conventional phase-transformation diagrams (Fig. 1). Specifically, a magnetic field shifts all the phase equilibria. In fact, relative to the traditional phase-transformation diagrams (zero magnetic field – dashed black lines), all the phase-transition temperatures and phase solubilities continually increase to higher temperatures and higher solubilities as the applied magnetic field is increased.
Two examples are presented here that demonstrate the dramatic and superior materials’ properties and performance that can be achieved in thermal/heat-treating and processing operations by strategically coupling induction heating and HMFP technologies. The first example demonstrates the unprecedented superior strength and ductility in steels achieved utilizing this technology approach. The second example demonstrates the dramatic materials performance improvement achieved by tuning the electromagnetic acoustic/ultrasonic transducer (EMAT) effect that is simultaneously present during HMFP.
Specifically, a thermomagnetic field processing (TMFP) environment (EMAT effect superimposed) changes the dimensionality of phase diagrams, resulting in a 3-D continuum of temperature-versus-composition equilibrium phase diagrams. As noted, Figure 1 shows an example of the phase equilibria shift achieved with the TMFP environment where a 9-Tesla (T) magnetic field has:
• Shifted transformation temperatures on the order of 27K (nominally 3K/T)
• Increased carbon solute solubility
• Increased the A1 (eutectoid) temperature
• Shifted the eutectoid chemistry to higher carbon levels than for the 0-field condition (blue lines are “magnetic” phase diagram)
In addition, phase-transformation kinetics in ferromagnetic systems are accelerated significantly upon cooling (or tempering) under the influence of a high magnetic field. Consequently, TMFP processing promises to be a major energy-saving technology for many industrial-process efficiency applications, and it is a perfect fit for induction heating applications that typically involve short processing times.
Superior Strength and Ductility
An example of great technological relevance is the fact that induction heat treatment of a new high-strength Carpenter Technologies’ bainitic alloy (Table 1) in a high magnetic field appears to have two major benefits as a result of the phase equilibria and kinetics impacts of thermomagnetic processing. These are shown in Figure 2 and summarized in Table 2, where thermomagnetic processing of substitutional solute alloys appears to mitigate normally occurring chemical inhomogeneity banding and simultaneously improve both strength and toughness in this alloy.
Traditionally, strength and toughness have an inverse relationship. Although not shown, microstructural evaluation concluded that the TMFP material had finer lath microstructure and finer, more copious carbide dispersion. Furthermore, these property improvements show performance superiority of this alloy to much more expensive 250-grade maraging steels (Fig. 3).
An example of the simultaneous improvements in both strength and all measures of ductility that can be achieved by coupling HMFP and induction heating processing is shown in Table 2. This unprecedented property/performance improvement is unachievable by any other existing processing technology.
Electromagnetic Acoustic/Ultrasonic Transducer (EMAT) Processing
In a TMFP environment, the processed material always has an electromagnetic acoustical transducer (EMAT) effect superimposed. This EMAT technology presents a game-changing breakthrough for achieving performance improvements in materials in both liquid and solid phases.
What is EMAT Technology?
EMAT is a noncontact acoustic processing technology that is superior to conventional acoustic processing technology methods.
How Does it Work?
As shown in Figure 4, when induction heating is applied to a workpiece through a coil in the presence of a high (static) magnetic field, a high-intensity acoustic signal is superimposed. The resulting noncontact EMAT dramatically affects nucleation and growth by fundamentally altering the solid/liquid interfaces. The ORNL research team has examined the microstructure of a variety of materials consisting of aluminum, pure Mg and a commercial cast iron.
Solidification experiments were conducted at the National High Magnetic Field Laboratory (NHMFL) using a custom sample envi-ronment and instrumentation developed by the magnetic processing team at ORNL. Each material sample was induction melted by conventional means, and parallel samples were solidified under EMAT near 10 kHz driven by static magnetic fields up to 20 T under a controlled atmosphere in custom cylindrical crucibles. A comparison to the conventionally solidified material reveals that the high-intensity acoustic-frequency EMAT promotes a finer-scale microstructure and uniquely interesting morphological features.
ORNL researchers have demonstrated that EMAT technology can significantly improve materials performance by:
• Accelerating phase-transformation processes
• Enhancing nucleation and growth during phase transformations
• Reducing grain size and elemental segregation during solidification
• Yielding more homogeneous cast microstructures and properties
• Generating nondendritic structures for increased plasticity and strength
• Reducing porosity and gas concentration for improved mechanical properties
• Minimizing residual stress
• Enhancing diffusive processes by enhancing the mobility of diffusing species
• Enhancing processes that have threshold activation energy
EMAT processing has significant potential for heat-treating applications, as well as during casting, deformation processing, etc. Some potential applications include enhanced catalytic reactions; refinement of inclusion size; enhanced fatigue life; enhanced ma-chinability; enhanced activation of carbon nanotubes; melt degassing; grain refinement; enhanced nucleation and growth; reduced residual stresses; reduced grain-refining alloy additions; and reduced macro- and micro-segregation.
The goal of the following EMAT processing was to achieve wrought-like microstructures and properties in an as-cast product. Figure 5 shows the significant microstructural impact of EMAT processing on a cast iron. In Fig. 5a, the as-received cast iron consists of graphite nodules (dark, round features) surrounded by a dominantly ferritic matrix (white regions) with some evidence of interspersed pearlite (irregular gray-shaped islands).
The average hardness value is 278 VHN. Fig. 5b shows how solidifying the same cast iron under 4.5 T enhances the dissolution of graphite nodules into the surrounding regions. This results in a much smaller ferritic zone (white areas) around the nodules and more pearlite in the remaining microstructure, which would enhance apparent strength. The average hardness value (385 VHN) is 38% higher than in the as-received, non-EMAT condition.
Solidifying the as-received cast iron under 9 T results in further dissolution enhancement of some of the graphite nodules. The average hardness value in this condition (421 VHN) is 51% higher than the as-received condition. Since hardness typically correlates with strength, one would anticipate that the resulting strength level would also increase with increasing magnetic field strength during casting of this cast iron.
These are just two examples of how these two technologies can be strategically coupled to meet current and future significant material property and performance challenges. IH
Acknowledgements and Copyright Notice: This research sponsored and utilized ORNL’s unique and world-class Thermomagnetic Processing Facilities supported by the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, Advanced Manufacturing Office, under contract DE-AC05-00OR22725 with UT-Battelle, LLC.
Notice: This manuscript has been authored by UT-Battelle, LLC, under Contract No. DE-AC05-00OR22725 with the U.S. Department of Energy. The U.S. Government retains and the publisher, by accepting the article for publication, acknowledges that the U. S. Government retains a nonexclusive, paid-up, irrevocable, worldwide license to publish or reproduce the published form of this manuscript, or allow others to do so, for the U.S. Government purposes.
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3. “Magnetic Processing of Steel Strip and Next Generation Alloys”, Ludtka, G. M., et al, ORNL Tech. memo, ORNL/TM-2012/608, April 26, 2013, www.osti.gov/servlets/purl/1080248/
4. “Non-Contact Ultrasonic Treatment of Metals in a Magnetic Field”, Wilgen, et al, in Materials Processing Under the Influence of External Fields, Han, Ludtka, and Zhai, editors, Symposium Proceedings from the TMS 2007 Annual Meeting, Orlando, FL, February 25-March 1, 2007, TMS Publishers, Warrendale, PA, 2007.
ORNL HMFP Industrial Prototype Facilities and Capabilities
ORNL is home to a world-class HMFP and TMFP R&D processing facility and has four distinct superconducting magnet processing cells. This includes the world’s first industrial-scale HMFP magnet facility (Fig. 6) equipped with computer-controlled, integrated induction heating and quenching capability. This 9-T-vertical, 8-inch-diameter bore magnet represents a technology that targets scalability and energy efficiency and has the following capabilities:
• 9-inch-long uniform field zone
• 0 to 9 T magnetic-field capability
• Inert-gas induction furnace: 0-1800˚C
• 200-KW dual-frequency induction heating
• Liquid polymer-quench system
• Gas-quenching system