Even as turbomolecular vacuum pumps have displaced most small laboratory sized oil diffusion pumps these days because of perceived ease of use and cleanliness, most high vacuum heat treating furnaces still rely on a large oil diffusion pumps to generate the pressures below about 10-3 Torr needed for many metal conditioning processes.
The main reason for this is that turbomolecular vacuum pumps have a physical size limit due to the high rotational speed of the rotor. That size limit is around 320 mm or 13 inches inlet diameter and may vary a small amount from manufacturer to manufacturer. In many cases the pumping speed may not be high enough as it is directly related to the inlet size of the pump. Metal can disintegrate at very high speed, so the tip speed of the rotor blades has to be within the safe limit. Turbomolecular pump rotors have to move faster than the speed of the gas molecules they are pumping in order that the rotor blades can deflect the gas molecules downwards in the pump mechanism. The second reason that turbomolecular pumps are not used in many metal treating systems is they cannot tolerate any particulate matter entering them. They must only be used on clean vacuum systems.