Heat treating, aging system also permits core sand removal.
A heat treating and aging system designed for aluminum castings in continuous and batch configurations combines heat treat, quench system and aging oven with core sand removal capability.
As the U.S. government adopts more stringent requirements for improved miles per gallon ratings, U.S. automobile manufacturers have no choice but to go to smaller, lighter, higher compression engines. This requirement calls for the automotive companies to make many engine parts out of aluminum alloys, the aluminum part weighing approximately 33% of the same sized part in cast iron.
To further improve the efficiency and power of the cylinder heads, automotive companies are experimenting with and perfecting smaller, all aluminum, high compression engines. If these engines are to be successful, they will need improved metallurgical strength properties and will have to maintain dimensional stability during normal automobile operating temperatures.
By working closely with automotive suppliers, Consolidated Engineering Company has designed a continuous roller hearth solution heat treating and age hardening system for aluminum automotive castings. The system, along with a batch type design for lower volume applications, is designed such that when a casting emerges from the end of the process, all core sand is fully removed and the casting is, for example, T6 solution heat treated and age hardened.
The continuous roller hearth line, with a 6000 lb per hour or 800,000-1.2 million cylinder heads per year capacity, was engineered and installed as a turnkey system by the manufacturer. Aluminum alloy automotive cylinder heads cast in a permanent steel mold with sand cores currently are being processed in this system.
Continuous System Design
The system is comprised of an eight zone solution heat treat furnace operating at 980F with a temperature uniformity of [+ or -] 4 [degrees] F throughout the entire furnace chamber.
A preheat chamber using exhaust flue gases from the other seven zones of the furnace brings the work up to a temperature of 380F before it enters the furnace. This provides a savings in fuel consumption, as well as shortened cycle time.
The furnace removes 600 lb of foundry sand per hour with 18 integral stainless steel sand troughs and screw conveyors located on the bottom of the furnace.
The water quenching system is fully automatic and is designed to fully submerge a load 15 seconds after it exits the furnace. The quench tank is provided with water heating at 140F and agitation via a twin propeller agitator and direction vanes. Cooling plates inside the tank restore the 140F prequenching temperature after each cycle.
The work basket is transported from the quench tank unload conveyor to the charge conveyor of the aging oven by a mechanical transfer system. The work basket is processed through the four zone roller hearth aging oven operating at 450F with [+ or -] 5 [degrees] F uniformity. Upon exiting the age oven, the basket is positioned in an air cooling chamber for work cool-down.
Some of the features of the system include the following.
Minimum Handling--Castings may be loaded into a heat treating basket that is not touched again until the castings emerge from the system, ready for machining.
Material Handling--The heat treatment system is easily interfaced with automatic storage and retrieval systems for furnace loading and aging oven loading and unloading. Various types of castings can readily be grouped for specific treatment cycles.
In the present application, the automotive company is able to set up its machining center for a certain type of cylinder head and to run those cylinder heads through the machining process in "closed groups."
Sand Removal--Sand removal is a key feature of this system, because it eliminates the need for shaking or chiseling the casting. Sand shakeout systems are not preferred in many aluminum casting applications because of their potential to scar or dent the casting surface.
By carefully controlling the furnace atmosphere the binders are burned out of the sand so that the sand becomes clean, dry and loose. It does not fall out in chunks. Various heat cured phenolic shell cores and coldbox phenolic urethane cores are successfully treated in the current application.
In addition, the furnace acts as an incinerator in destroying products of binder decomposition. The system monitors and controls oxygen levels and internal furnace pressures with a series of mechanically operated flue/exhaust ducts equipped with oxygen probes. With the medium velocity tempered air gas burners used in the furnace, the system reduces binder burn-out emissions to within acceptable limits, eliminating the need for further scrubbing or incineration of emissions.
The exhaust system delivers the waste gas to a collection chamber preceding the furnace for preheating the castings prior to their entrance into the furnace. At the same time, large, heavy duty, stainless steel hoppers beneath the roller bed gather all the sand from the furnace and quenching system and take the sand to one gathering point by conveyor.
Much of the sand is removed from the castings during the quenching process. Therefore, removal of the sand from the quenching water required a unique design solution. The system employs augering systems, pumps and cyclonevibratory sand dryers.
Another important consideration is the orientation of the part as it travels through the furnace. Sand should be allowed to fall freely from the casting during travel through the furnace. Thus, the basket or tray design becomes very important. Parts typically are angled so that easier sand removal is possible.
If this is not possible, or the part is more sophisticated, the sand can be blown out after heat treating. This problem also can be overcome by high pressure adjustable water spray heads in the quench tank.
This process can help eliminate problems associated with waste sand disposal. Scott Crafton, vice president/engineering, says, "Not only is the sand reclaimable, but the binders are fully burned and are no longer an environmental hazard."
Consideration must also be given to alloy type and end use of the castings, so that the proper heat treatment cycle is used. By changing line speeds and temperatures, the parts can be solution heat treated or annealed and/or aged. In by-passing the furnace, the casting can be loaded directly into the oven for aging only.
Batch Furnace Design
The smaller production furnaces are batch type with bottom opening doors above a quench tank, or smaller, scaled down roller hearths. The proprietary furnace atmosphere and pressure control system achieves sand removal in the same manner as the continuous roller hearth system. Small batch systems can range from as small as 100 lb per hour to 8000 lb per hour, with sand removal rates as high as 1200 lb per hour.
The two zone gas fired furnace has an alternating air flow system which enables the high velocity air to be impinged on the castings from all directions during the treatment cycle. The clean, granular sand is collected by hoppers, which are above the main furnace door for collection of waste sand into sand bins located on a mobile transfer car below the furnace.
Once the furnace load reaches the end of its cycle, the bottom door is moved to an open position and the load is lowered into the quench tank. Any sand not previously removed is collected in the quench tank by alternating intake pipes. These deliver the wet sand slurry to a cyclone vibratory sand dryer, which delivers the dry sand to collection bins. If annealing is the required heat treating process, the load is placed on the transfer car and the quenching process is by-passed.
"The economies of smaller batch type systems are just beginning to be realized," says Crafton. "The economics of reduced labor input and a high quality product will continue to increase the profitability of moderate to medium size casting companies."
PHOTO : Shown is a view of the entire roller hearth continuous heat treating system. To the right
PHOTO : is the solution furnace. At the left is the age oven and cooling zone.
PHOTO : The quenching system and tank are shown.
PHOTO : This closeup view of the solution furnace shows the preheating chamber at the far right
PHOTO : and the furnace controls at the bottom.
PHOTO : Design of the batch drop bottom heat treatment furnace and quench system is shown.
Paul M. Crafton, Jr. Consolidated Engineering Co Kennesaw, GA
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|Author:||Crafton, Paul M. Jr.|
|Date:||Sep 1, 1989|
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