Y-12 researcher Ed Ripley pours salt into the microwave salt bath vessel.

Microwaves and Salt

December 2007 / January 2008 By: Bill Wilburn Volume 5 Number 6

Print this Article

E-mail this Article

A microwave salt bath developed at the Y-12 National Security Complex in Oak Ridge, Tenn., is a major technological step in heat-treating metal that can save significant amounts of money, energy and time. Metals are heat-treated to harden them or, conversely, to keep them from becoming too brittle—€”a process known as annealing, which makes metal softer and more ductile. Here's an example: the small soft wire inside a twist tie on a loaf of bread and a steel guitar string are essentially the same metal, except that the bread tie wire has been annealed and the guitar string has been hardened.

Heat-treating is often done using a bath of molten carbonate salt. The salt has to be heated until it changes from a solid to a liquid—€”the point called phase change—€”and the metal immersed into a vat of the molten salt. Temperatures are high, usually above 850 degrees centigrade.

Melting the salt is where Y-12 comes in. Researchers have developed a microwave-heated salt bath. For more than 20 years, the facility has been a leader in microwave technology, conducting ground-breaking research and development in metal melting and casting using microwave energy. Y-12 has been awarded 39 patents for microwave-technology-related research and development.

"The microwave salt bath is another example of where Y-12 is setting the standard in industrial applications of microwave heating and metal melting technology," said Ed Ripley of Technology Development, who developed the new microwave molten salt bath technology.

Current systems for heat-treating of metal or other manufactured components are typically conducted on a large scale for reasons of economy. Submerged electrodes, for example, one of the traditional methods of heat-treating metal, are expensive because of the energy costs. This method also has to be done on a fairly large scale and run 24 hours a day. Gas-fired salt baths are less expensive but are also less efficient because they don't get as hot as the electrically heated salt bath.

"Submerged electrodes and other traditional methods don't lend themselves to starting and stopping because when the molten salts cool, they solidify to rock-like hardness. It has to basically be jack-hammered out of the vessel," Ripley said.

"Because the electrodes, salt pan and liners of traditional salt baths degrade, there can be down time and significant maintenance costs. Changing from one type of heat-treating salt to another requires an outage and complete change-out of the bath."

Microwave molten salt bath processing uses much less energy. It can be started and stopped easily, which eliminates the need for energy intensive and expensive continuous running. The size of the salt bath vessels also can be varied so only the amount of salt bath needed for the job is heated, which in itself can mean a significant energy savings. The microwave method quickly and easily converts to a variety of industrially accepted heat-treating salt mixtures without loss of valuable production time.

"Reliability of the microwave units is very good as well. Industrial microwave units are very robust. They are capable of thousands of hours of trouble-free, sustained operation," Ripley said.

"Studies have shown that the products produced by this method are indistinguishable from traditionally processed components. This technology could mean a huge change in the way metals are heat treated; it takes the best features of conventional methods and improves them."

Bill Wilburn is public affairs representative for the Y-12 Security complex.

We neglected to include the two R&D 100 awards won by Y-12 in our report in the October-November issue. Here they are.
—€”The editors

Rapid Deployment Shelter System (RDSS)
What It Is: A mobile surgical suite.
What It Does: The 400-square-foot shelter can be easily set up in less than two minutes and offers substantial protection against small arms, as well as nuclear, biological and chemical contamination.
Potential Applications: With a few modifications it could be turned into anything from a command and control center to a logistics or operations center.

Personal Annunciation Device (PAD)
What It Is: A self-arming, multiple-use accident notification device.
What It Does: The device goes beyond existing technology by including radio frequency identification (RFID) technology for accountability.
Potential Applications: A commercial version of the PAD could be the size of a car key fob and easily worn by anyone over an extended time.