A U.S. aluminum casting foundry needed a furnace to help improve process quality and efficiency while satisfying growing production demands. The foundry was
using five pit furnaces, an overhead crane and a single stationary quench tank to solution heat treat aluminum castings. The equipment, which had been in service more than 30 years, was cumbersome and no longer efficient. To meet quality demands of the end users, including the AMS-2750D heat treat specification, improvements were needed in furnace temperature uniformity and in meeting required quench times.
A case study illustrates how revamping the system satisfied those goals. Among the points covered are the initial scope of work, project goals and how these goals were met in the two-phase project. Details on the performance and operating characteristics of the new drop-bottom furnaces also will be provided.
The original chamber size for the first furnace held a load of castings in a basket volume approximately 6 by 5 by 6'. The load to be processed per batch consisted of 3 tons of aluminum castings of all shapes, sizes and arrangements as well as approximately 8,000 lb of steel baskets, racks and load hoist bars. Because meeting the AMS-2750D specification was a goal, the heat treat process was designed to meet a ±10°F uniformity at a furnace temperature setting of 1,000°F (538°C). To meet this, the furnace’s direct-gas-fired burner and air circulation system needed to be sized properly. In this case, the air volume calculated to meet this demand was sized at 30 air changes per minute. Calculations were made that compared the velocity of air delivered to the product load vs. the number of air changes. The air change calculation was determined to be the higher volume of airflow, which was the best design for the process. A direct-fired gas burner system was sized to heat the load from ambient to operating setpoint within 1.5 hours.
In addition to meeting the production volume requirements, the furnace needed to have the ability to record and document the process information. To meet this, the electrical control system and PLC program were developed to operate by using preprogrammed heat cycle recipes that could be selected based on the type of casting being processed. Variables designed into the recipe include heatup ramp rates, total process time after the parts reach temperature, maintained operating temperature and the quenching rate.
Temperature datalogging during the process is critical to meeting the quality verification of each run of castings. It was accomplished by the use of a 12-point paperless temperature data recorder. As part of the goals of the control system, temperature data was to be recorded during each heating cycle. The information was to be stored in the recorder itself and transmitted via an Ethernet connection to the factory data-acquisition system for permanent recordkeeping. The Ethernet connection also gives the manufacturer the capability of controlling the furnace and monitoring operations from a remote office or workstation.
After proper heating, the second most critical feature of the process is the time the product load needs to be completely quenched. In this process, 15 sec were allowed to open the doors and drop the elevated load approximately 8' into the water quench tank positioned below the furnace door opening. Due to limited overhead height in the work area, the combination quench tank and load platform rail car were located in a pit in the factory floor. The quenchant fluids used as well as the rate of circulation and quenchant temperature varies depending on whether castings or sheet metal parts are being processed. In this case, fresh water circulated in the tank at a rate of five volume exchanges per hour was found to be the correct quenching rate. City water was used for the aluminum castings being processed.
In addition to meeting the increased production goals, other manufacturing improvements included decreasing the amount of material handling by heat treating larger loads at one time. This reduced the amount of fork truck traffic in the area required to operate the equipment. Other process improvements included:
• An automated process cycling once every 8 hours.
• Improved quality by designing tighter furnace temperature uniformity.
• Improved control of the quench rate.
• Faster quenching times.
System control improvements included:
• Modern data acquisition and documentation of the process parameters.
• Accurate control of the heating cycle (i.e., the process temperature).
Better part quality was a result of quenching at higher fluid circulation rates and heat dispersion from the parts. By using ceramic insulation and more efficient burners with higher turndown ratios and lower emissions, heat loss was reduced.
Within weeks of completing the installation, the original goals for increased production, improved quality, reduced energy consumption and reduced labor were being met and exceeded. Quality of the heat treated castings had reached a level that met the AMS-2750D requirements.
Late in 2011, as the economy gained strength, the foundry sought out a solution that would double production volume while keeping the manufacturing space to a minimum. Because the heat treat area was located in a pit, the additional equipment needed to be kept in the same area. A decision was made to extend the existing pit. After further analysis of the layout, it was determined that a second furnace having the same capabilities as the first furnace could be placed in line with the existing furnace, thus minimizing the size of the pit extension. The existing load cart and quench tank would be used for loading and quenching both furnace product loads.
Once it was determined that the furnaces could be located physically in one area, the next issue to resolve was combining the controls and human-machine
Once it was determined that the furnaces could be located physically in one area, the next issue to resolve was combining the controls and human-machine interfaces (HMIs) so each system could monitor and view process times for each furnace.
interfaces (HMIs) so each system could monitor and view process times for each furnace. In addition, both HMIs could be used to control either process, and data could be sent to the in-house data acquisition system from either multi-point chart recorder.
With the controls figured out, manufacturing labor was reduced from a full-time operator loading and transferring loads of castings to an area operator that loaded baskets of castings once every four hours. Because the process is designed to be automatic, loading takes place at one location and is controlled by an operator directing the movement of the bridge crane.
Energy consumption of the new furnaces has reduced the overall gas consumed in comparison to the original five furnaces. Excess heat in the work area is significantly less due to better insulating qualities of the new ceramic and the elevated location of the furnace away from the operators. Heat is collected and taken away from the area by roof ventilation and an exhaust fan.
After the completion of the second phase of the expansion, the goals at the onset of the project, including higher production volumes, less material handling, reduced manpower, higher energy efficiency, and the effective use of manufacturing floor space, were all accomplished.
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