Heat Treatment for Nonferrous Metals

The evolution of heat-treatment processes for aluminum and copper
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Various Heat-Treating Applications for the Aluminum Industry

Heat treating is a critical thermal process for not only aluminum parts and products but also for the coating of aluminum sheets. Heat treating of aluminum is a key production process for a wide variety of manufactured goods and consumer products. Whether it is the aging of cast parts or curing of coated sheets, oven design is a critical aspect of the overall quality of the finished aluminum product.

By Brian Wendt

Industrial Heating

Aluminum manufactured products of various sizes have been steadily replacing steel in industrial designs such as automobiles, airplanes, appliances and many more applications due to the light weight and flexibility of aluminum. Today, we are also seeing an increasing demand for affordable, lightweight, stamped metal parts, which is driving the development of specialty laminated products in coil-coating aluminum-sheet lines.

Multiple Batch Aging Ovens with Guillotine Doors

This article highlights two different heat-treating (annealing) systems designed for the aluminum industry. The first project features a batch-type aging oven designed for the manufacture of automotive parts. The second featured system is a curing oven with an integrated air-pollution-control system designed for coil coating laminated aluminum rolls.

These systems illustrate the challenges in designing specialized industrial ovens for the aluminum industry and the material benefits to precision and uniformity in the heat-treating process that allow for the proliferation of aluminum products in our everyday lives.

Heat-Treating Batch Ovens for Precision Aging of Aluminum Parts

Several alloys are heat treated to increase the ease of forming or the strength of the finished product. Unlike steel or iron, aluminum requires rigid heat control to achieve optimal results, so specialized industrial ovens and equipment are required.

With great attention to detail in the oven design, proper heat-treating methods result in aluminum alloys that are easy to work during the forming process. Following that, a hardening process helps the material resist wear and corrosion for a lasting aluminum product.

The following project case study outlines the workings of multiple custom-engineered, gas-fired, batch-type aging ovens with cooling capabilities for aluminum structural components for a Tier 1 automotive supplier (Fig. 1). While some alloys can be processed at near-ambient temperatures, the more specialized parts often use custom alloys that require a heat-treating process called aging to fully precipitate the dissolved elements and reach their maximum hardness. This process is also called precipitation hardening. While each alloy has an optimal aging temperature, precipitation hardening typically happens between 240-460°F. Similarly designed ovens are also often used by aerospace parts manufacturers.

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Fig. 1. Batch-type aging ovens with cooling capabilities for aluminum structural components

These ovens are used to control the heating and cooling of the aluminum products loaded into the batch chamber. Featuring a multiple-tiered rack system that holds stackable wire heat-treat bins, the unit has sufficient capability to handle multiple bins per batch.

At maximum loading capacity, the ovens can heat 6,500 pounds of aluminum from 70-400°F in under two hours. In addition, the system was designed to meet stringent temperature-uniformity requirements of +/-5°F (+/-3°C) at setpoints of 320°F (160°C), 375°F (190°C) and 401°F (205°C) with a guarantee to meet all CQI-9 requirements.

The recirculation system utilizes combination airflow to maximize heating rates and temperature uniformity of the product. This is achieved by adding two recirculation fans inside the oven, each at 10,000 CFM capacity. Each motor is equipped with variable-frequency drive (VFD) to regulate the airflow.

The oven chamber itself is constructed from 18-gauge aluminized-steel interior, and 18-gauge carbon-steel exterior with the oven-wall panel in a tongue-and-groove-type assembly and insulated with 5-inch-thick mineral-wool insulation. In addition, each oven has an exhaust/cooling blower with approximately 3,000 CFM capacity and PLC controls. The blower can accept exhaust air up to 400°F. The exhaust fan operates at lower speeds during the heat-up, but it operates at speeds up to 100% during the quench to remove heat from the parts.

The oven burner has a specialized nozzle design that allows the combustion air and fuel to be mixed directly inside the burner nozzle. These highly reliable burners, with a turndown ratio of 20:1, are designed with link valves to control the supply of combustion air as well as natural gas (fuel). The fuel flow is automatically adjusted, which allows for the precise control of temperature by modulating the fuel and combustion-air supply.

Continuous airflow is also a key element, so the oven design includes supply plenums installed on both sides of the work chamber with the return plenum mounted on the ceiling of the work chamber. This allows the recirculation air to return through the ceiling-mounted plenum back into combustion/recirculation chamber (Fig. 2).

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Fig. 2. Oven design includes supply plenums installed on both sides of the work chamber with the return plenum mounted on the ceiling of the work chamber

Creating uniform temperature throughout the oven is a challenge because of its high velocity with very high heat-transfer coefficient. The oven work or “batch” chamber is also used as a cooling zone. The cooling in the oven chamber is achieved using a modulating fresh-air cooling damper and exhaust fan.

After the heating soak cycle is complete, the burners shut off. At the same time, the fresh-air VFD cooling-damper control regulates the exhaust-fan speeds to the desired cool-down tolerances, and fresh air is drawn back into the oven via the combustion chamber and supply-plenum nozzles for effective cooling. The cooling fresh-air pneumatic damper is installed on the oven combustion chamber, and the cooling fan is on top of the working chamber to ensure proper airflow.

The guillotine door system is an integral design element for these batch ovens. These heavy-duty guillotine doors are equipped with pneumatic latches on both ends of the door to prevent leakage and heat dissipation. To ensure operational ease, the door latch mechanism is operated through a touch-screen button configured on an HMI screen (Fig. 3) with a mechanical safety latch and a rod lock assembly to hold the door open for personnel safety.

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Fig. 3. Batch oven operating controls

Coil-Coating Oven and Thermal Oxidizer

The increasing demand for affordable, lightweight, stamped metal parts has driven the development of specialty laminated-aluminum products. These engineered materials combine the attributes of different gauges and grades of metal sheet with those of high-density polymer-mastic dampening materials. Together, the completed laminated product provides outstanding strength, weight and vibration-dampening performance at a lower total overall cost.

Producing these materials at affordable prices requires large-scale coil-laminating production lines. Ensuring a consistent and uniform cure of the polymer mastic is critical to product performance.

Cure Uniformity Critical To Laminated Aluminum Sheet Performance

A leading developer of laminated-aluminum sheet required a continuous-cure oven system for their new line. The system not only cures the applied coatings, it also heat treats the aluminum in the process. It required heat-treating expertise to develop a custom and innovative continuous-oven system to deliver a solution that achieves peak metal temperature (PMT) uniformity within +/-0.5% of target across the 5-foot-wide multilayered laminated sheet traveling at speeds of approximately 270 feet per minute (Fig. 4).

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Fig. 4. Coil-coating continuous-oven system

Utilizing an energy-efficient zone control, highly specialized temperature-control modules were key to delivering the high velocity and high-flow heat required to achieve the outstanding PMT uniformity seen in production. Each energy-efficiency zone’s control module includes tandem VFD controlled-recirculation fans directed to dedicated top and bottom recirculation plenums that incorporate a custom, proprietary nozzle design specifically for this application (Fig. 5).

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Fig. 5. Coil-coating oven combustion chamber

A major advantage to coil coating is the energy savings. Not only does the process utilize high-speed material processing, but it also allows for thermal recycling of the majority of the painted coil’s VOCs back into the curing ovens, where they are reused as fuel.

This particular project required that the oven systems for coil coating large aluminum sheets had VOC emissions controls integrated into the overall design. These integrated air-pollution-control systems can often be optimized with the heat-treating system via primary and secondary heat-recovery methods to ensure maximum fuel efficiency (Fig. 6).

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Fig. 6. Integrated air-pollution-control system control panel

For example, the three-zone continuous-oven system recovers heat from the custom-integrated air-pollution-control system, which is engineered to direct hot, clean air from the oxidizer exhaust to the various zone controllers. This approach facilitates the highest energy recovery with the smallest overall footprint.

Conclusion

Heat treating of aluminum is a key production process for a wide variety of manufactured goods and consumer products. Whether it is the aging of cast parts or curing of coated sheets, oven design is a critical aspect of the overall quality of the finished aluminum product.

Before selecting a specialized oven type and design for any aluminum process, it is important to first consult process engineers that can optimize your operations and integrate all necessary air-pollution-control technology into the thermal-system design.

Brian Wendt is with Epcon Industrial Systems, LP; The Woodlands, Texas.

 

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