In doing argon replacement of the atmosphere in the furnace, the lower the
flow rate, the better off you will be. The problem here is that you have to
heat the gas to the temp of the oven in order to allow the gas into the
chamber. Failure to do so makes the surfaces you are heating cool. Note
that the sealing doesn't need to be inside the furnace but can also be
outside the furnace. Extra insulation followed by even just a styrofoam
sealing/insulation layer can help things immensely.
Excellent would be a nice sealing door for the furnace with a pressure
limiter so the oven doesn't blow out and you loosen a valve during purge and
then shut it and keep the pressure to some value after that.

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Why do penguins walk so far to get to their nesting grounds?