Stress Relief Goes Green

If someone says they know how to eliminate 98 percent of energy consumption without sacrificing quality in the area of stress relieving, people are naturally skeptical. But when the U.S. Department of Energy reports that the American metalworking industry can save up to 98 percent of its energy consumption by using a certain stress relief process, the only question is "how?"

Most Americans agree that we need to reduce our oil imports. So does the U.S. Department of Energy. For over 30 years the U.S. DOE has had a special department, Energy Efficiency and Renewable Energy (EERE), sponsor and help promote highly effective energy saving processes and inventions ? but only if they work. One research study was conducted on a sub-harmonic vibratory stress relief process called Meta-Lax® (metal relaxation).

Following this study, the U.S. DOE wrote a Tech Brief.1 According to their findings, they stated that sub-harmonic vibratory stress relief can "reduce energy consumption by up to 98 percent compared with natural-gas-fired heat treatments." Furthermore, they added that "this process is a proven substitute for 80 to 90 percent of the heat treatment stress relief applications."

THE PROCESS

Sub-harmonic energy is used to vibrate metal structures at or near the foot of the workpiece's harmonic curve. This energy level is maintained for a period of time which varies based on the material being treated. As the workpiece becomes stress relieved, the harmonic curve will shift to a new frequency location and stabilize at that new frequency.

The new stabilized harmonic frequency is the workpiece's true natural frequency. Taking periodic scans of the part or controlling the process with a computer and special software will verify that the workpiece has reached its stress relieved state.

The whole process takes one-half to two hours for most workpieces using a system that runs on 110 volt electricity. For example, a 1500 lb autobody carrier would take about 45 minutes to become completely stress relieved.

Contrast this to the energy consumption of a full heat treat (thermal) stress relief. Thermal stress relief requires a large furnace that must be elevated in temperature, often up to 1200 deg F, ramping up 100 deg F per hour. This high temperature must be maintained for one hour per inch of thickness, followed by gradually decreasing the temperature 100 deg F per hour so no stress is reintroduced.

This full thermal stress relief process requires at least 24 hours in the furnace. Because the process is so time consuming and expensive, most heat treaters promote an abbreviated form of thermal stress relieving. Commercial heat treat stress relief typically takes 6-8 hours.

Impressing the Army

At the U.S Army Watervliet Arsenal (Watervliet, NY) thermal stress relieving was the standard for years and was a mandatory part in the production of 120 mm mortar barrels. A 50 percent scrap rate had been the norm on the finished mortars due to the extremely tight tolerance specifications.

The metal relaxation process was used first as an additional stress relief process applied before finish boring. Scrap was eliminated. Then the sub-harmonic process was applied in place of the thermal stress relief, before any machining.

The results have continued to be impressive, even without using heat treat stress relief at all. According to Tim O'Connor, a mechanical engineer for Benet Laboratories at Watervliet Arsenal, "We achieved a $230,000 annual savings in energy costs for that one weapons system alone." Tim also mentioned that the sub-harmonic process reduced the stress relieving step from several hours to 30 minutes.

Saving 98 percent of the energy used in stress relieving at the Watervliet Arsenal was only the beginning of the overall energy savings. They eliminated the costly transportation of their parts to and from the furnace, thus saving fuel energy along with the huge savings of natural resources (i.e. iron ore and extra energy) that would have been needed to replace the parts from the 50 percent scrap rate.

Other benefits to "going green" are also realized by using the sub-harmonic stress relief process. For example, natural resources are also being saved because of the manufacturer's ability to use thinner plates in production due to less distortion, and eliminating secondary steps such as extra machining steps and straightening, both of which require heavy 440 volt electricity to operate the machinery.

Still other companies have reported even more eco-friendly benefits when they use sub-harmonic stress relieving during welding.

Some of these "going green" benefits include reducing high pre-heat temperatures which are frequently used during welding, reducing the rework needed from welding weld defects such as cracking and porosity, and in some cases eliminating the need for post weld stress relief completely since the weldment would be stress relieved throughout the welding.

In June 2006, the U.S. Department of Energy calculated the amount of energy saved and air pollutants reduced from the collective use of sub harmonic processing in the U.S. Using their formulas to recalculate the collective savings that using the new process has made through this March, the energy savings and air pollutant reductions are:
? Saved 50.9 million barrels of oil, and
? Reduced air pollutants equivalent to the output of 6.1 million automobiles.

Upon the completion of their 1999 Tech Brief, the U.S. Department of Energy discovered that this sub-harmonic stress relief process was the second highest energy savings invention of the over 500 projects that they had sponsored.

References
1. Barnett, Lisa, Inventions and Innovations Online, U.S. DOE, August 2002.
2. Reicher, Dan W., Impacts, US DOE, January 1999, pages 8-9.

Thomas Hebel is the vice president of Bonal Technologies, Inc., 1300 North Campbell Road, Royal Oak, MI 48067, 248-582-0900 ext. 236, www.meta-lax.com.