|A typical basket containing aluminum blocks for conveyance through a conventional roller hearth heat treatment system. |
The thermal processing of high volume cast aluminum cast engine blocks and cylinder head components is a key concern of automotive engineers today. Engineers are faced with challenges as a result of the current energy crises, reduced demand, and purchaser’s preferences for smaller, more compact automobiles, which consume less fuel and produce reduced emissions.
Engine development has undergone a significant transformation. Designs have evolved, as have their weights, overall size, and mechanical performance. Engine blocks and cylinder heads have seen the introduction of several new casting technologies and modified alloy compositions to handle the higher compression levels and elevated operating temperatures. With the introduction of these new technologies, greater demands have been put upon the thermal treatment methods used to optimize the products mechanical properties while also minimizing the residual stress levels.
Due to the long heat treat cycle requirements for cast aluminum engine blocks and cylinder heads, furnace engineers were left with limited design options and were forced to design long linear conveyorized heat treatment systems. These systems involved the conveying of densely loaded baskets loaded with castings on rollers through a roller hearth heat treatment system. They are large, and consume significant amounts of floor space for staging of baskets.
By nature of the repetitive heating and cooling cycles, conveying baskets over time deteriorate and as a result require replacement and maintenance. When not replaced and maintained, heating and quenching profiles are inconsistent, resulting in inconsistent metallurgical properties. Basket misalignment and quench delays result in lost production time and equipment damage, and ineffective baskets cannot be loaded optimally.
Though collaboration with automotive designers and development testing, Can-Eng engineers have designed and commissioned a new heat treatment technology used by manufactures of cast aluminum engine blocks and cylinder heads. These systems combine two significant improvements over other designs.
First the system eliminates the need to load and unload castings into dense baskets and secondly, the systems integrate a piece-by-piece handling strategy. This novel technology first referred to as Can-Eng’s Basketless Heat Treatment Technology (BHTS) integrates a multiple level, modernized rotary hearth configuration.
The basic system design is cylindrical: the product is loaded and unloaded through common openings. It is positioned upon an internal carousel that indexes through a heating and soaking cycle. The carousel can be arranged to handle a wide family of engine block and cylinder head products, and is driven via a single high accuracy drive unit. The modernized rotary hearth furnace integrates multiple levels for increased system capacity.
The BHTS technology provides for the efficient use of a forced recirculation system that utilizes a single recirculating fan versus other basketless designs which integrate multiple recirculation fans and combustion zones. The furnaces engineers utilized computational fluid dynamic (CFD) modeled air distribution plenums and anti-stratification baffles to ensure hot gases maintain a uniform flow and velocity throughout the entire heating and soaking phases of the cycle. Improved temperature monitoring and controls strategies have been integrated for precise product temperature control.
Following commissioning of this system, it was realized via Datapaq monitoring that production castings processed in the BHTS were capable of achieving improved product temperature uniformity of ±2°C over conventional technology, and a 50% reduction in the time required to heat-up the product. As a result of improved process temperature control users are taking advantage these features and are further optimizing their time and temperature and production capacity.
Furnace designers have struggled with the development of uniform quenching systems for integration with batch or roller hearth systems, due to the dense loading practices, variations in loading, basket misalignment, and limitations in delivering the quench media in a uniform and reliable method. Engineers understood that quenching of engine castings is one of the most critical steps as well as a dominant area for the creation of residual stresses. This lead to the development of new and improved quench methods integrated with the BHTS that provided improved quenching uniformity and flexibility for processing various components. Development of the improved quench design ensued rapidly through the use of computational fluid dynamic (CFD) modeling and bench testing.
These new methods offer a combination of benefits over traditional technologies. The primary improvements include the individual part quenching of castings for both conventional basket applications and basketless processing methods. Individual casting quenching provides significant quenching uniformity across the castings various sections. This is achieved through independently controlled and monitored quench distribution nozzles and integrated in-situ process monitoring and closed loop feedback.
An additional benefit of the system relates to the reduction of system size, through the piece-by-piece handling of castings through a more efficiently sized system. These systems can be floor mounted and do not require costly pits and added infrastructure. The recirculating, heating and cooling systems are sized more efficiently for continuous steady state operation versus the slug loading basket type quench systems. The control of the quench media temperature can be controlled within a tighter control band, improving the part to part uniformity and should eventually contribute towards improvements in product quality CpK values.
The integration of these new quench developments has created a number of new opportunities for further improvements in the quenching system design. These new concepts are opening new doors for the development of hybrid quenching technologies that provide gradient quenching, using a combination of quench medias. Can-Eng Furnaces International has been successful in commissioning a flexible heat treatment system that provides significant benefits over conventional and other Basketless technologies. By utilizing this new technology, manufactures of cast aluminum engine blocks and cylinder heads can take advantage of the following quality and cost improvements: improved temperature profile and fan life; individual casting handling; 40% minimum reduction in fuel consumption; a 30% reduction in floor space requirements;reduction in mechanical requirements; flexibility to process engine blocks and cylinder heads with the integration of both water, precision air and hybrid quenching; reduced residual stresses levels; reduced part to part variation; and improved quenching system efficiencies.
Manufactures of aluminum engine castings now have an alternative when evaluating new capacity requirements. BHTS can greatly assist manufactures overcome industrial competition and ensure profitability.
Timothy D. Donofrio is the Aluminum Equipment product manager, and Phil Romanin is the proposal co-ordinator, Aluminum Product Group, both with Can-Eng Furnaces Ltd., Niagara Falls, ON. Visit www.can-eng.com