Abstract: The invention relates to a method for heat-treating metal work pieces, whereby a cooling gas flow is produced in a vacuum furnace by means of a fan, driven by a three-phase AC motor, in order to quench the work pieces. The aim of the invention is to improve the heat treatment of metal work pieces in such a manner as to improve the quenching in a simple and inexpensive manner. For this purpose, the three-phase AC motor comprises first three-phase windings designed for a lower supply voltage and second three-phase windings designed for a higher supply voltage. The three-phase AC motor is operated with the first three-phase windings until a minimum pressure defined in terms of the motor output is reached in the vacuum furnace and with the second three-phase windings until a minimum pressure defined in terms of the motor output is reached or exceeded in the vacuum furnace.