I'm kashering a number of non-kosher, metal utensil with a kosher pot. If heat is a mechanism through which things can become non-kosher, does kashering my utensils in the pot (via Hagalah) make my pot non-kosher? If not, why doesn't the treyf-ness transfer?
If it's past a day, I believe the kashering pot is fine without soap. If the treife utensils were used within a day, that is a different story.There are some caterers who have boiled treife tensils in a toxic solution before kashering them. However, Rav Blumenkranz zt"l writes in his pesach book that this should not be relied upon.
Your concern is that the non-kosher taste reenters the utensils or enteres the pot. If you are concerned about this (despite the valid comment above about a day-old taste being rotten), put soap in the water before you put the non-kosher utensils in the pot. That way, any non-kosher taste mixes immediately with the soapy water. That renders any such taste non-edible, which means it is not food, which means it is no longer non-kosher and can not make the pot (or anything else) non-kosher. This works also if the non-kosher utensils have been used within the past day. As I understand it, caterers use this approach often. (Perhaps there are caterers reading this who might chime in.)