I encounter this a lot of this but usually the trap arm is tubular brass or DWV copper with a solder threaded solder ring that is screwed into the cast iron tee. The steel thread have deteriorated and may have also destroyed any threads left in the santee. I would take a sawzall and cut through the inside of the pipe into the threads about a 1/4 inch apart and the peel out the old nipple. Then I use a 1-1/2 pipe tap and re-tap the threads in the santee. With steel nipples I think you are going to have to open the wall and replace the santee. I have had some success re-tapping after removing a steel nipple but not much. Sometimes there is no easy fix. The threads left in the santee that were under the brass solder ring are okay. The brass threaded solder ring was only about 2 or 3 threads deep. You will notice after I re-tapped it the other threads are pretty much gone.

Here is a video of another one I recently repaired less than 3 Min https://www.youtube.com/watch?v=8YL_Z8JqPbY&x-yt-cl=85114404&feature=player_embedded&x-yt-ts=1422579428

http://s3.supplyhouse.com/images/pro...wd126-24-3.jpghttp://ts1.mm.bing.net/th?&id=HN.608...d=1.9&rs=0&p=0 I have always called it a Marvel ring..I use them on urinal drains and lav drains on block walls. . and i call these Desankoshttp://ts1.mm.bing.net/th?&id=HN.607...d=1.9&rs=0&p=0 nice tutorial David,