Can a Microwave be used to Kasher other dishes? For example, to Kasher a metal bowl it could be filled with water and placed on a platter in the microwave for an extended period of time. The water in the bowl would come to a boil, and then something could be dipped inside to make the boiling water overflow. In which case, the bowl could now be considered Kashered?
The Shulchan Aruch O:CH 452- 4 says that we can not do haga’la with water from Chamei Teveria, because it isn’t heated by fire, because taste that was absorbed by heat of a fire can only be purged by the heat of a fire. The poskim say that the same would apply to a microwave, that we can only do hagala with the heat of a microwave if the vessel became trief with the heat of a microwave, but not to kasher taste absorbed with the heat of a fire.
As a side point, you question is not really too applicable... because we can not heat water in a metal utensil, as it will cause the microwave to go on fire, plastic is questionable if it can be kashered in the first place, and pottery ceramics can also not be kashered..
Shvus Yitzchok 6-9 ( 18) in the name of R’ Eliyashiv zt”l, Kovetz Halachos 11-17, Ohel Yackov 452-5 (4) in the name of Knei Bosem and R’ M. Deutsch shlit”a, Also see Halachos of Pesach pg. 156. Also see Even Yisroel 9 pg. 102, and Halichos Shlomo 3 ftnt. 3 regarding water heated by solar heating.
Rav Eider's Halachos of Pesach (XV:B:10) writes:
The water may be heated by gas, oil, electricity, or any similar fuel. Water heated in a solar boiler can only be used to kasher utensils which absorbed issur with a solar utensil. Water heated in a microwave oven can be used for kashering a microwave oven or utensils used exclusively in microwave ovens. However, it is questionable whether water heated by this method can be used to kasher utensils used on a stove or in a conventional oven.
He explains in footnote 75 that boiling water in a microwave would be similar to the hots springs of Tiveria which wouldn't work, but says Rav Moshe Feinstein considered water boiled in a microwave as good enough for standard hagala.
Really not a good idea to put metal in the microwave though.