Induction heats the material from the inside, with eddy currents and no open flame. Heating is even around the whole circumference and repeatable, so it is easier to hold the interpass temperature within the required range. In practice that means a shorter time to temperature and less risk of local overheating than with an oxy-propane torch.
Preheating is required on steels with higher carbon content and on heavy sections. For plain carbon steel above 25 mm the usual starting point is 100 to 150 degrees Celsius, for quenched and tempered steels the temperature comes from the material data sheet and the welding procedure. A heater with control and a sensor holds the set value and most machines log the run for the record.
Power depends on the mass of the part and on the required temperature. A 3.5 to 5 kW unit on 230 V is enough for removing bearings, bolts, bushes and for heating small parts. For pipe and structures with a wall above 20 mm you need a machine from 15 kW upwards on 400 V, liquid cooled, with an induction cable or blanket wrapped around the part.
Pay attention to the accessories. Induction cables, heating blankets and pistol heads are matched to the shape of the part, and continuous work needs a cooler.