The basic principal of kashering is that an item becomes kosher by the same process by which it became nonkosher. Thus, an item used directly on a flame (such as a rosting spit or oven grates) must be made kosher directly on a flame. Boiling water is this case will not work.
Items which are used to cook (hot water) are made kosher by boiling water. That is, one is able to use a less strict method. Of course, if one wishes to use libun on an item that does not require it (but will not harm the item) it will kasher it.
Bleach and detergent can be used to clean the pot or make the food crust inedible, but not to make the pot kosher. That is, they can clean the item so that it is ready to be kashered but cannot make the item kosher.
Kashrut.org - Jewish Law - kashering
The basic concept of Koshering these pots are to cook with them in the
same way to remove any possible edible flavors that may come out of
the wall of the pot. Each type of item has a specific process related
to its standard use. To determine which process is to be used, we
follow its most common use.
You take the item and drop it into boiling water and then remove it and
put it under cold water. Make sure the entire item gets to the boiling
water, even if you need to turn it around and immerse it again. If the
item to bekoshered is a pot, you may put enough water into that pot
and just steam it up with the cover on, and then put it under the cold
water. If there was extra caked on food that you cannot remove,
which can often build up in cracks by the handle, just put detergent
and/or bleach on it to render that food inedible.
Since the most commen time people will kasher items is for Pesach, many sites explain the kashering process with reference to it.
The Kashering Primer – Passover 2016 give details on the various methods of making something kosher and when they apply.