As early as 1948, the US General Motor Company applied to the medium frequency as a fundamental wave, but there was no inductive, but there was no available power supply and the appropriate power density, so this method has practical applications. Until 1993, Germany, Norwegian inventions were registered in Europe. Synchronous dual-frequency method early production is applied to powder metallurgical engines timing sprocket. The characteristics of the synchronous dual-band gear quenching process are as follows:
1. Two different frequencies of current are simultaneously lost to an inductor.
2. The selected frequency and power are simulated by the computer.
3. The heating time is short, generally within 1s.