Automotive suppliers are constantly faced with cost pressures from its customers and as result are seeking out new efficient methods of heat treating high volume aluminum components. Processing aluminum castings and formed components can benefit from a modernized rotary hearth system that has been developed to take advantage of lean manufacturing and can be fully integrated into the manufacturer's process providing improved efficiency, productivity and quality.
IntroductionAluminum is highly favored by automobile engineers due to its unique combination of strength, reduced weight, crash energy absorption, and corrosion resistance. These properties and others are typically enhanced by heat treatment. Aluminum is used in a wide range of automotive applications including engine blocks, engine cylinder heads and pistons, intake manifolds, wheels, transmission housings, suspension parts, brake drums, brake cylinders and pistons, radiators and heater cores, body panels, bumper beams, structural parts and various seat tracks, shells and headrests.
The use of aluminum automobile components is growing steadily from an average of 54 pounds (25 kg) per car in 1960 to 273 pounds (124 kg) in 2002, mainly to reduce weight and improve fuel economy. According to the Drucker Worldwide report, "Global Automotive Content Forecast Through 2010", it is projected that there will be 318 pounds (144 kg) of aluminum used in North American light vehicles by 2010 (Fig. 1).
Aluminum Heat TreatmentAluminum castings and formed products are typically thermally treated to achieve a desired set of mechanical properties, proper metallurgical structure and acceptable residual stress levels. Table 1 shows the typical heat treatment processes for selected aluminum alloy sand and permanent mold castings: T4 solution heat treated and naturally aged; T5 artificially aged only; T6 solution heat treated and then artificially aged; and T7 solution heat treated and artificially overaged.
Heat treatment processes typically require multiple heating and holding steps with intermediate cooling or quenching operations. The soak time at temperature required to achieve the desired properties is a function of alloy's ability to develop a homogeneous solid solution. Soak times can vary from less than a minute for thin sheet to as long as 20 hours for large sand castings.
Conventional Roller Hearth SystemsAs a result of the heat treating requirements, furnace engineers developed large batch and continuous furnaces that employed long continuous roller hearth conveying systems to satisfy the demand for high volume aluminum product production. In one such system (Fig. 2), loaded baskets of A356-T6 castings are positioned at the entry end for charging into the solution heat treatment furnace. Following the appropriate ramping and soaking (which can vary between 3 and 12 hours), baskets are rapidly discharged to a water quench system. Following solution treatment and quenching, the loaded baskets are charged into an inline artificial aging oven to complete the thermal processing.
Equipment Size LimitationsAs aluminum automobile product requirements increase, furnace design engineers are forced to build larger heat-treating systems. Presently it is not uncommon for roller hearth systems to span over two hundred feet in length. Expansions to satisfy new aluminum product capacities continue to increase equipment and facility capital costs. In some cases, excessive capital requirements resulted in forcing product engineers to investigate alternate materials resulting in the failure to secure potential business and growth.
Conventional Conveying Basket ConcernsIn addition to equipment and facility costs, aluminum product manufacturers are subjected to the perpetual costs and problems associated with maintaining and replacing consumable product conveying baskets (Fig. 3). Once loaded, both the product and the basket are continuously subjected to the thermal environment and repetitive self-destructing sequence of operations. This sequence involves repeated heating cycles to solution treatment temperatures where thermal oxidization occurs, resulting in a reduction of the baskets section modulus over time. Following solution treatment, baskets are rapidly discharged to a quenching media, subjecting the baskets to additional thermal stresses, which further reduce the basket's physical and dimensional stability. Baskets deteriorate further as a result of lift truck mishandling during the movement throughout the manufactures facility.
Basket deterioration presents challenges for high volume heat treatment systems. These challenges include:
1. Tracking inconsistencies leading to basket misalignment, jam-ups and excessive quench delays.
2. Inability to effectively load baskets, reducing the optimal output of the system.
3. Less than optimal heating and quenching profiles due to inconsistent product spacing, resulting in varied metallurgical properties.
To combat potential processing risks, manufacturers have adopted basket inspection and maintenance programs. These programs are costly and incur additional manpower and basket inventory for the operation of the system.
As a result of dense basket loading, it is widely understood that a temperature gradient within the performance requirements will exist during the ramping to set point temperature. This temperature gradient is most pronounced when comparing parts located in the center and the outside of the baskets. In some cases this temperature gradient contributes to a variation in product dimensions and mechanical properties achieved.
Another major area of concern is that, in the overall process, the energy consumed through the heating of baskets has no redeeming value. For example, heating a 2000 lb (900 kg) gross load containing 1000 pounds (450 kg) of A356 castings from ambient temperature to 1000°F (537°C) requires approximately 471,000 BTU (Fig. 4). This total energy consumption consists of 161,000 BTU delivered to the basket and 310,000 BTU to the aluminum product or 34% and 66% respectively.
New Efficient Heat Treatment SystemEquipment Origin
Roller hearth conveying systems are the most commonly used systems for the transporting of loaded baskets through a high volume aluminum heat-treating systems. NFK and Can-Eng engineers realized that there were opportunities to improve the design and as such, evaluated alternate equipment options available. The goal was to develop a system that satisfies increasing product demand while improving productivity and quality. Target areas of design improvement included; reduction in energy consumption, reduction in plant floor space requirements, improved thermal profiles, ease of operation and maintainability, and singular part flow. Of course the system cost had to be justifiable from a commercialization point of view.
As part of our research, developers analyzed existing technologies' benefits and drawbacks. Consideration was given to several system configurations and upon completion of the study, the researchers concluded that there was an existing system configuration, which if modified, could achieve all the targeted areas of improvement. The system identified is the rotary hearth furnace, a proven technology in use for over 50 years. The basic system design is cylindrical; product is loaded and unloaded through common openings. The product is positioned upon an internal carousel that rotates through a heating and soaking cycle. The carousel is driven from below via a single drive unit.
Modernized Rotary Hearth FurnaceThe modernized rotary hearth furnace is based on similar design characteristics as its predecessor but integrates multiple carousel levels as a means of increasing the system capacity (Fig. 5).
The patent-pending design can be orientated in a side-by-side layout to accomplish both the solution treatment and artificial aging processes (Fig. 6).
A major advance of this arrangement is that individual aluminum products can be placed into the rotary systems, thus eliminating any need for part conveying baskets. A robotic handling system transfers the product in a singular part flow manner from the charging table and into one of the various solution furnace carousel levels. The part is appropriately positioned within the carousel and placed into the carousel fixture. This fixture ensures each part is properly supported and receives uniform recirculating airflow during the ramping and soaking stages of the processes.
Furnace Design CharacteristicsThe rotary furnace is designed to incorporate one centrally located, axial flow fan to recirculate large volumes of heated process air to each carousel level. This forced convection air is delivered through a distribution baffle that directs high velocity air through the work chamber and back into a heating plenum where the process air is reheated prior to being reintroduced into the work chamber. Each vertical column is separated to ensure temperature profiles are maintained. The carousel is supported by a base that rotates within the cylindrical shell (Fig. 6).
The carousel is designed and manufactured to support product families of similar geometry. The product's unique features are carefully considered during the design of the furnace system to ensure products are properly supported to prevent damage or distortion. Opportunities to process different product families can easily be accomplished through the integration of a modified internal carousel. The carousel design integrates a structural frame capable of supporting the product's mass at processing temperature. The carousel base is fitted with a driven chain around the outer perimeter of the carousel base. The carousel rotates via one easily accessible drive motor. Each rotational position is carefully controlled via communications between the drive and carousel sensors. To prevent heat leakage, a fixed trough and rotating knife-edge sealing arrangement is incorporated on the stationary shell and rotating carousel.
Each hearth level is accessible via an independently operated door system. The airflow through the work chamber is vertical, providing a curtaining effect, minimizing heat leakage through the door openings. Door opening and closing, carousel rotation and robotic handling is all controlled and verified via a central programmable processor. Inspection of the system is provided via access doors positioned in the upper and lower shell sections. In cases where residual foundry sand remains within the casting prior to heat treating, a simple sand collection and discharge system can be easily integrated into the system. System sizing is determined through detailed calculations, incorporating the product's required thermal profile and the physical geometry.
Quenching SystemsGiven the singular part flow in the Basketless Heat Treatment System (BHTS), quenching is simplified. The quenching system, whether it is a hot water, polymer or controlled air media, is smaller and can be mounted at floor level. As a result, BHTS quench systems do not require foundation pits and ancillary facility requirements. They are sized smaller and are no longer burdened by the additional cooling requirements of a basket.
ConclusionsA new efficient heat treatment system is now commercially available to manufacturers of aluminum castings and formed products. Multiple users of the BHTS have exploited the numerous benefits over conventional roller hearth and other types of heat treatment systems. Energy consumption (Fig. 7) is greatly reduced as a result of eliminating baskets and reduced electrical usage.
The system is cylindrical and more compact, taking advantage of height and thereby reducing floor space expansion costs. In addition, the furnace volume is greatly reduced (Fig. 8).
Operator and maintenance involvement is reduced as a result of fewer burners, fans and drives to be monitored. Equipment spare inventory is reduced as a result of fewer components integrated into the system. Product quality is improved through single part processing, reducing the temperature profile spread over densely loaded basket systems and quench optimization. Lastly, work in process (WIP) inventories can be reduced enhancing lean manufacturing processing.
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