It Sees What No Eye Can See
What a challenge: to “see” behind the walls of an electric arc furnace. Yet this is the problem tackled by Charles R. “Rigel” Woodside of the Energy Systems Dynamics Division of the National Energy Technology Laboratory. It began when NETL’s Paul King, Woodside’s advisor during his graduate studies at Oregon State University, interested him in the subject of vacuum arc remelting, known as VAR. VAR is a crucial step in creating metals and alloys with the advanced properties and performance needed for certain applications in the aerospace, power generation, defense, medical and nuclear industries.
The problem with VAR is that the metal ingots produced occasionally contain “freckles,” “white spots” or other defects, causing the metals to be unsuitable for their intended use. Since VAR is a time-consuming and expensive process, producing rejected ingots is an extremely undesirable outcome. But who can see within the enclosed crucible of the furnace—within the metallurgical vacuum inside—to know what causes these defects?
Because of Woodside’s work, the answer has arrived: It’s the new electric current locator (ECL) that can “see” where eyes cannot. Developed under a CRADA between NETL and the Specialty Metals Processing Consortium, the ECL tracks the positions of the electric arcs inside the VAR furnace in real time. Knowing where the arcs are shows how energy is being distributed to the molten metal during the remelting process. “Seeing” the arcs is a first step toward controlling them and thereby controlling the melting process, which is necessary for consistently defect-free materials. This new technology opens up materials processing possibilities that weren’t possible before.
To understand better how this works, let’s look at VAR.In a crucible—typically made of copper surrounded by a water jacket to cool the metal and control the rate at which it solidifies—an electrode composed of the metal to be remelted is inserted. Electric current starts an arc between this electrode and a very small amount of the same metal that is placed under the electrode. As the electric arcs melt the electrode and the metal chips underneath, the spacing between the electrode and the gradually solidifying ingot of remelted material must be carefully controlled so that the process is uninterrupted until complete. The control of this very complex heat transfer is critical to producing defect-free material. Any interruption can cause ingot defects.
BUT arc melting processes such as VAR have not been well understood because there is no way to directly view the arcs that are melting the material. Instead, process control has to rely on system current and voltage.
“It’s sort of like welding and watching the power meter on the power supply rather than the piece that is being welded,” says Woodside, who was instrumental in both conceptualizing and developing the ECL. “Sure, the job might get done, but it’s going to be difficult to consistently get a good weld.” In the same way, he points out, it is the lack of seeing arc positions that keeps scientists and engineers from devising ways to control the arcs.
The first major breakthrough for the ECL began when Woodside was working with NETL as part of his graduate studies in 2006. During that time, he says, “most of the concepts for the [technology were conceived but it was a much longer journey to realize these concepts and demonstrate the technology at an industrial site.” After graduation, when he was employed by NETL, he continued development of the capabilities of the ECL.
Woodside was helped along the way by experiments run at Sandia National Laboratories in the 1980s. They created a special version of a VAR furnace that allowed the arc region to be viewed for a short time with high-speed photography. These experiments conclusively proved that the arcs produced in the VAR could occasionally became constricted, confined to one region of the electrode for a significant amount of time, which led to defects in the produced ingot. Commercial furnaces, unfortunately, have no such camera.
Therefore, answering the question: “How can we know what the arcs are doing?” became Woodside’s goal. Tracking arc movement is a huge step for quality control because it opens the door for the next step, developing a way of actually controlling the arcs and preventing them from becoming constricted.
The ECL works by using a magnetic field measurement that can “see” through the furnace walls to locate the arcs. It traces segments of electric current—much as if they are line sources of electric current, like wires—by using magnetic flux density vector measurements combined with an overall system current measurement.
To operate the ECL, sensors about the size of a 1-centimeter cube are placed on the walls of the furnace, the number of sensors required depending on the number of current segments to be located. These sensors work with a suitcase-sized container of electronics and a computer that completes the system, all together weighing about 30 pounds.
Initially envisioned as monitoring device for processes such as VAR, the ECL technology can easily be retrofitted to existing furnaces. The technology has been successfully applied to an industrial VAR during commercial production of a titanium alloy, during which some previously unreported aspects of arc behavior were identified and described in publications. For instance, the VAR showed that different patterns of arc distribution can arise for different melts, even though the same alloy is being melted using the same control program. These differences were not detectable by previous furnace controls.
A patent application for this arc tracker, the ECL, has been filed, and NETL is now looking for commercial partners to further refine the technology and license it for commercial applications.
A paper in which Woodside describes the ECL technology won “best graduate student paper” at the 2010 IEEE (Institute of Electrical and Electronics Engineers) International Instrumentation and Me