Can fire toivel a metal object or must it be water
It depends on what has to be done. If the metal vessel was bought from a non Jew, it must be toveled in water, and it must be in a kosher mikvah. However if the the metal was used with something non kosher, then the absorbed non kosher taste can be purged with fire. If someone buys a second hand pot from a non Jew he will have to purge the non kosher taste in the pot, and then tovel the pot in a mikvah.
Y:D 120-1, 121-1.