The successful completion of a project at the BAE Systems facility in Brough, East Yorkshire, has demonstrated how automation and modern engineering design can be applied to a traditional heat treatment process.
The new installation is a Salt Bath Solution Treatment plant supplied by Mannings Thermal & Environmental Engineers Ltd of Southport, Merseyside to meet the strict criteria and safety requirements of BAE Systems. The complete process is carried out in stages and is designed for the treatment of aluminium alloy components, which are used in the manufacture of airframes. The treatment improves malleability of the material so that the parts can be pressed into shape during later stages of the manufacture. The components are placed in a specially designed workbasket and immersed in a salt bath where they are uniformaly heated at a temperature of 500 C. After a period of 30 minutes in the salt bath the basket is automatically transferred to an air agitated polymer quench tank where the final metal properties are obtained. The workbasket is then moved to a rinse tank before being finally set on a loading/unloading station.
The operation is carried out inside an enclosed force-ventilated room and the entire process can be viewed at all times through observation windows in the side walls. Control and monitoring takes place from a panel outside the room while the treatment is in process – and this is achieved in a simple but ingenious way. The room door can be locked from the outside by the same key, which is used to activate the control panel. While the room is occupied the key is held captive in the door lock and the control panel cannot be activated. The key can only be released after the door is locked. George Wright, who is Project Manager at Mannings said, “it was essential that the equipment design and layout minimised hazards and provided a clean working environment. We worked closely with BAE Systems to ensure that the designs and equipment met their requirements.”
The salt bath itself has a wall thickness of 20mm and contains 2.5 tonnes of molten chemical salt. Heating is supplied by coiled wire electrical resistance elements on the outside of the bath and these provide radiant heat. To ensure uniform heating of the bath contents, the temperature is controlled to within 50C by burst firing thyristors in two vertical zones and an independent over-temperature interlock has been installed for additional safety. Because the melt out time for the salt is twelve hours there is also a facility to automatically maintain the bath at a reduced temperature overnight. The bath is enclosed in the heating chamber and this is lined with low thermal mass insulation at the sides and refractory brickwork at the base.
Once the components have been treated for the required time period, the pneumatically operated lid of the salt bath opens automatically and a robotic basket transfer machine swings into action. Rapid transfer of the workbasket from the salt bath to the quenching tank is essential to ensure that the desired mechanical properties are produced in the components and this transfer can be completed within 7 seconds. The Transfer machine has two counter balanced parallel arms which are positioned on each side of the two tanks. Steel pins, on the end of each arm, locate on a “flight bar” on the basket as the arm rotates through 360 degrees, picking up the basket and moving it in a semi-circular arc to set it down in the quench tank. The design ensures that the transfer is completed smoothly and that the basket remains vertical at all times.
It is essential that the temperature in the quench tank is maintained between 10 and 40 degrees C and this is achieved by an electric immersion heater, which increases the temperature when required and a chiller unit, which is brought into operation when the temperature needs to be reduced. Air agitation is also provided in the tank to promote uniform quenching.
The final stage in the process takes place in the water rinse tank, which is also air, agitated. An added feature of this tank is the spray rinse nozzles on each side of the outlet to the tank. These come into operation automatically once rinsing is complete and the basket is being withdrawn from the tank. Tony Chester who is Process Facilitator at BAE Systems said, “Mannings were able to meet our specification for the new plant and the end result has proved to be a first class installation. We have been able to ensure the safety of personnel and improve the production process at the same time.”
Mannings apply their designs to a wide range of heating problems and they have been able to revolutionise many traditional processes by the application of modern engineering techniques, materials and technologies. Many industries can benefit from similar improvements to those experienced by BAE Systems and new applications are being continually discovered.
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