When you heated the stud did you get it red hot and did you heat the cast iron iron around it? If you did and let it cool very slowly by covering it should have taken the temper out of the stud and allowed you to drill it. If it cooled fast all it did was make it harder. I would try heating again to red hot and warming the cast iron around the stud to expand slightly. This will tend to break the bond between the threads and the casting. Cover it and allow it to cool very slowly and let it cool down to room temperature before you do any drilling. Use a center punch in the center of the stud and see it you can mark the the stud. If you can it is soft enough to drill but make sure the drill is shark and is removing material. If you allow the drill to rub without cutting it will harden the stud again. Many times when you can't get a fastener to unserew, try tighten it first just a little and then try removing it again. This works many times for me.