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A Process for Improved Induction Heating of Powder Metal Compacts

An unusual ``incubation'' period is encountered at the beginning stages of medium frequency (3 kHz) induction heating of unsintered, low alloy, steel powder metal compacts. This incubation period is characterized by poor load coupling to the induction coil and consequently, very slow and inefficient heating. An unusually high material resistivity has been isolated as the cause of this incubation period. A technique is described for eliminating the incubation problem by preheating the material contained within the reference depth to 4000 F by either radiant heating or radio frequency induction heating. This preheating results in more than an order of magnitude reduction in the resistivity of the preheated material and a considerable net energy savings during subsequent medium frequency heating to the sintering temperature. A mechanism is also postulated for the cause of the unusually high compact resistivity and its reduction via preheating.