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Bringing the Heat

April 14, 2005
New developments in heat-treating processes are on the way, to help metalcasters improve their performance with this value-adding technology.

Heat treating can add value to some finished castings, and some customers’ orders include heat-treating as a design element. But, heat treating is not casting, so metalcasters may not be entirely up to date on advances in the technology. Never mind: there are people for that, and they have ideas:

Mass heating

Last June, Seco/Warwick Corp.’s Aluminum Group made a patent application for its “Method for uniform flow distribution of recirculated process gas in heat processing equipment.” It is the product of an ongoing, continuous-improvement project directed at improving the heating rate of aluminum castings (as well as forgings, and extrusions), and improving the metallurgical results, while lowering the process cycle times.

Most heat-treating systems for castings involve mass-flow heating or solution heat treating, and each presents some inefficiencies. In mass-flow heating, load spacing and distances need to be managed carefully in order to achieve uniformity. In solution heat treating, load sizes and production demands mean that load densities must be factored into heating times. Also, after heating, the load is subjected to several hours of holding at a uniform temperature, followed by quenching in water or a water/glycol mixture, to lock in the effects of hardening.

Customer requirements for more efficient equipment and reduced cycle times, along with better metallurgical prompted Seco to seek a new approach based on convection heating. Convection involves creating a furnace atmosphere in which circulating currents transfer heat from comparatively high-temperature regions to comparatively low-temperature areas.

Seco already has a number of earlier innovations with convection heating and cooling, and while this effort is still in the testing phase, the Aluminum Group’s project engineer Jack Mahoney recently listed several advantages convection offers for mass heating.

Improved Uniformity. “Metallurgists and customers alike are demanding closer temperature tolerances than ever before,” according to Mahoney, “with more specifications written that include uniformities of 2°-3°F. Many aircraft suppliers have even more stringent requirements regarding maximum air temperatures used for heating. For example, often times we are given specifications that state a metal temperature tolerance of ±3°F using an air temperature that may be only 2°-3°F higher than the maximum metal temperature. How the air is directed to the work becomes even more critical as things like baskets, internal fixtures, and even the load itself can affect the path the heating air takes.”

Reduced Heating Time. The speed of heating air passing across the load determines the time it takes to heat the work, according to Mahoney. Typically, mass-flow systems have a fan positioned at a distance and a series of baffles, zone dividers, and vanes direct air through the load. Referring to the convection process, he writes: “The jet-heated system positions the discharge of heating air close to the load with direct impingement of air onto the work via the jets. The speed in which the air contacts the work is increased, which causes the heating coefficient to rise, thereby speeding up the heating time.”

Metallurgical Benefits. Mahoney cites test showing that aluminum castings can, in some cases, achieve the desired tensile and hardness qualities in less time using the new approach. “By closely controlling the heating rate, the soak time can be reduced. Normal operating procedures that typically require one hour to heat and four hours to soak have been reduced to 20 minutes to heat and one hour to soak, while achieving acceptable metal properties. These findings are very encouraging and we anticipate additional process time reductions, as we move ahead.”

According to Seco, the patent-approval process is on track and more details on the development will be reported later this year.

Hot, and fast

Last fall Dana Corp. introduced a new heat-treating technology, AtmoPlas atmospheric plasma microwave technology. By using microwave-absorbing plasma, this process heats metal parts up to 1,200°C in seconds and controls arcing. Traditionally, microwave technology requires creating a vacuum, but AtmoPlas operates at atmospheric pressure.

Another big plus for AtmoPlas is the evidence that it reduces energy (95% of the energy created is absorbed) and maintenance costs. Tests show equivalent or better metallurgical properties than those produced by blast furnaces. (Dana indicates it may be used with advanced ceramics and hard metal coatings as well as metal parts.) “While we expect the AtmoPlas technology to boost operating efficiencies in our metal-treating operations, we’re equally excited to demonstrate its value to other manufacturers and research organizations,” said Dana chairman/CEO Michael J. Burns. “The potential of this proprietary process is far-reaching in industry.”

The plasma-generating process may be adapted to other metal processes, too, like brazing, sintering, carburizing, annealing, tempering, and nitriding.

Dana is refining AtmoPlas microwave plasma technology and looking for partner companies to develop more commercial applications.

New flowmeter

Finally, those seeking a more immediate solution, Fluid Components Intl. has introduced a mass flowmeter it says sets a new price-performance standard. The ST75 Series flowmeter measures and monitors gas flows in line sizes from 0.25 to 2.0 in. (6 to 51mm) with a single instrument and provides three outputs of the mass flow rate, totalized flow, and gas temperature. Combining a non-clogging, small-size thermal-dispersion sensing element with microprocessor-based electronics and precision calibration, the ST75 delivers fast, direct mass flow measurement without routine maintenance scheduling. It’s available in nine different line size configurations (0.25 and 2 in.; 6 to 51 mm) in standard T-fittings, either NPT or tube, for easy in-line installation. It services a flow range from 0.008 to 839 SCFM (0.013 to 1425 NCMH) depending on line size, making it suitable for low-flow and high-flow applications in industrial furnaces, ovens, and heat-treating systems.