Luby,
Applying cold/heat to a mechanical fastener is really not the best process for separating the two. Delta temperatures are the best way to assemble them by shrinking one and expanding the other in a close tolerance application. Are you just trying to remove rusted bolts? if you are, are you sure they are Cres steel? (stainless) if they are you must have galvanic corrosion from dissimilar materials. Write this down..
Aerokroil! without a doubt the best penetrating oil out there. Spray it on and wait awhile..if it doesn't release it, nothing will. It will be time to remove the nut with a nut splitter. The physical properties and linear expansion of materials is academic in this situation because there is no way to shield the heat from the adjacent part. A rosebud tip on an acetylene torch will work to shock the assemble like Sammy said. I have done this many times but have zero control on the thermal isolation of parts. Everything gets hot...Aerokroil... then heat and some moderate tapping to break the rust free. If that doesn't work...Nut splitter or grind it off.
Bob