It depends upon the size of the part needing the overlay, and which cobalt alloy (21<6<12<1).  It could be as simple as torch heat and as complex as custom electrical heating or furnaces.

If you are using St 1, you will probably need to heat the part to several hundred degrees, begin welding immediately, and placing it back in the furnace between layers to ensure the heat is maintained.  Then you will need to place it in fire-retardant blankets or a furnace to ensure slooow cooling.