Reducing Cycle Times and Improving Uniformity & Metallurgical Results with High Efficiency Convection Heating

System installed at Coline equipped with high efficiency convection heating Click here for case study

The Aluminum Group at SECO/WARWICK filed a patent application in June 2004 for a "Method for uniform flow distribution of recirculated process gas in heat processing equipment" that has been cleared for final approval in 2005.

Research and development of high efficiency convection heating technology is an ongoing continuous improvement project for SECO/WARWICK.  Our efforts are directed at improving the heating rate of aluminum castings, forgings, and extrusions while improving the metallurgical results and reducing the cycle time of the heat treatment process.

Over the last 50 years, SECO/WARWICK has designed and manufactured hundreds of furnaces for various aluminum heat treating processes including coil annealing, foil annealing, log/billet homogenizing, ingot heating, and solution heat treating. Heat transfer to the work has been, for the most part, accomplished by the mass flow system. The fans used in these furnaces are typically axial flow, where the fan can produce a large volume of air flow with a static pressure of 3 to 4 inches water column. Load spacing and distances between surrounding fixtures needs to be closely monitored to control uniformity.

In solution heat treating, the goal is to heat the work to a temperature just below its melting point, known to metallurgists as the "solvus." The solvus range allows the hardening elements in the alloy to evenly flow throughout the complete cross section of the work piece. During the  upstream processes, such as casting, rolling, and extruding, these hardening elements have settled unevenly throughout the work. Although not visible to the naked eye, as the aluminum is heated and the solvus range is reached, these elements will disperse evenly throughout the part.

As load sizes and production demands grew, the load density had to be factored into the heating time. It would take longer to complete the heating process for a densely packed load than for a more open load arrangement. After heating, the load is subjected to several hours at what is called "soak" or "hold", where the work temperature is maintained without interruption, at a uniformity level of approximately 10 to 15 degrees F. After the soak duration is complete, the load is quickly removed from the furnace and quenched in water or a water/glycol mixture, to lock the hardening elements of the alloy into their now evenly dispersed position.

What can high efficiency convection achieve? 

Improved Uniformity:   Metallurgists and customers alike are demanding closer temperature tolerances than ever before, with more specifications written that includes uniformities of 2 to 3 degrees 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 plus or minus 3 degrees F using an air temperature that may be only 2 or 3 degrees 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 at which the heating air passes across the load ultimately determines the time it takes to heat the work. In the typical mass flow system, the fan is positioned some distance away, and a series of baffles, zone dividers, and turning vanes/etc. are used to direct the air through the load. 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:  Recent test results have shown that aluminum castings, can in some cases, achieve the desired tensile and hardness qualities in less time. By closely controlling the heating rate, the soak time can be reduced. Normal operating procedures that typically require 1 hour to heat and 4 hours to soak, have been reduced to 20 minutes to heat and 1 hour to soak, while achieving acceptable metal properties. These findings are very encouraging and we anticipate additional process time reductions, as we move ahead.

SECO/WARWICK has pioneered many innovations in both convection heating and cooling technologies for the aluminum industry. Customer requirements for more efficient equipment along with reduced cycle times that produce improved metallurgical results has prompted SECO/WARWICK to develop new technologies. Still in the testing phase, the test results for this technology will be published in upcoming issues of Metal Minutes. What is certain is that an affordable high efficiency convection heating design has the potential to become the process of choice in solution heat treating and aluminum alloys.

Case Study: Co-Line Welding, Inc. Achieves ±1°F Uniformity in Solution Heat Treatment System with Jet Impingement Technology Click Here

Feature Article January 2005, Jack Mahoney Author