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Attractive Materials Process

Use of a magnetic field can increase the strength and fracture resistance of steel and other ferromagnetic alloys.

ORNL researchers have discovered that applying a magnetic field to steel and other ferromagnetic materials could dramatically improve their strength, wear resistance, and lifetime and make the materials easier to shape into parts, using less energy than conventional methods. Their initial research results are attracting the attention of industry, including manufacturers of steel, superconducting magnets, automotive parts, and knee and hip implants.

The promising discovery occurred in ORNL's Metals and Ceramics (M&C) Division and in experiments at the National High Magnetic Field Laboratory at Florida State University.

Steel Worker.
Steel Worker.

The insight comes at an opportune time for the U.S. steel industry because magnet companies now manufacture cost-effective superconducting magnets that generate increasingly stronger magnetic fields with low consumption of energy.

"This finding could revolutionize materials processing as it provides a new approach to developing novel materials with enhanced performance that would be unachievable through other processing avenues," says Gerard M. Ludtka, a materials scientist in ORNL's M&C Division who is leading this investigation. "The steel, heat treating, forging, welding, casting, chemical, and cast iron industries can benefit from re-search on materials processing using powerful magnets."

Steel and other ferromagnetic materials are usually modified so they can be more easily shaped into long-lasting components by changing their recipe—the ingredients in their chemical composition, the temperature to which they are heated, and the speed at which they are cooled. Turning a magnetic field on and off could dramatically strengthen a material by, for example, increasing the solubility of desired alloy additions to form a microstructure never seen before, possibly generating improved properties. Changing a material can be accomplished using magnetic processing because this method has been shown to alter the fundamental phase stability and microstructure evolution kinetics exhibited by ferromagnetic materials.

"A magnetic field can influence the energy levels of electrons within an atom, affecting the chemical bonding and crystal structure and behavior of the material," Ludtka says.

Ludtka found that magnetic fields can soften steel, making it easier to machine the material to form desired component shapes. "After a steel part is machined, the usual heat treatment can be given to the material so the desired properties return," he says.

In a seed-money project, Ludtka in 2000 demonstrated that residual stresses in a steel sample at room temperature can be reduced 80% by exposing the steel to a low magnetic field with strengths ranging from 1 to 6 Teslas. Residual stresses can lead to cracks, fracture, and failure of the material during exposure to altered temperatures or applied stress.

"We envision using magnetic fields from a 2-Tesla magnet, like the magnet in an MRI medical imaging device, to treat steel components of a diesel engine that has operated a long time," he says. "These parts develop residual stresses over the life of the engine because of cyclic exposure to hot and cold temperatures. We would zap each component with a magnetic field and, based on literature results, conceivably increase life expectancy by 30%."

In a 2001 project supported by internal funding from ORNL's Laboratory Directed Research and Development Program, Ludtka and his colleagues—Roger Kisner, Gail Mackiewicz Ludtka, John Wilgen, Roger Jaramillo, and Don Nicholson—showed they could double the desired mechanical properties of certain microstructures using magnetic field processing.

Magnetic field processing might lead to high-strength steel so strong that the thinnest of sheets would form the body of a car, greatly reducing weight and increasing efficiency, Ludtka says. The technique might even produce a "dream steel" for national security—a structural alloy for skyscrapers that becomes stronger and more resistant to catastrophic failure under the heat after an impact, such as being struck by an airplane.

Magnetic processing could be used by the orthopedic implant industry to make improved artificial knees and hips. "By stabilizing more of the preferred hexagonal phase microstructure, magnetic fields could confer increased wear resistance and longevity on cobalt alloy implants," says Ludtka, who received the Department of Energy's E. O. Lawrence Award in 1994.

Research Horizons

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