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Welding Journal | January 2014

amount of time required to set up and tear down was also recorded. Resistance The resistance heater uses resistance pads made up of a resistant element woven through ceramic tiles. This construction results in a heating pad with enough flexibility to allow for contouring the pad around or inside components with varying profiles. The element consists of a conductor having high resistance, so when electrically energized, the element heats up. The ceramic tiles both conduct this heat to the component as well as electrically insulate the heating element from the component. The heated tiles only transfer heat to the valve through radiant heat and conductive heat where the pads are in contact with WELDING JOURNAL 53 the valve. The resistance heating pads were first fastened to each other with wire and to the valve to keep them in place — Fig. 2. Next, the whole assembly was covered with an insulating ceramic fiber blanket. Two preheating zones (with separate control) were used with each zone using two resistance heating pads. The pads were arranged such that each of the two zones was on opposite sides of the valve. The resistance heating controller uses one thermocouple per zone to monitor the temperature and control the output to that zone. Each zone had a thermocouple resistance spot welded onto the outside of the valve. The control thermocouple was connected to the valve within ¼ in. (6 mm) of the location for the measurement thermocouple. The second thermocouple was on the other side, on the outside of the valve. A thermocouple placed on the inside of the valve ¼ in. (6 mm) away from the one used for the data recorder was plugged into the machine for reference only. The resistance machine was programmed to preheat the valve to 550°F as quickly as possible, and then maintain the temperature for one hour. A previous test showed that when setting the machine to preheat to 500°F, it required more than six hours for the inside to reach 500°F after the outside had attained this temperature, so programming the controller to reach the higher temperature on the outside was used as a means to through-heat the part more rapidly. This is believed to have happened because there was not a large enough temperature differential between the inside surface and the outside surface. Because the pads were touching the valve so close to the thermocouple, the temperature did not rise high enough above 500°F to create that differential. The data recorder was then turned on, the power meter was set to record, and the resistance heating power source was set to preheat. Both the data Fig. 1 — Position of induction heating coil, and measurement and control thermocouple cables. Fig. 2 — Placement of resistance heating pads and thermocouple cables. Fig. 3 — Direct flame preheating setup. recorder and the power meter record time


Welding Journal | January 2014
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