Thermocouple junctions are not "soldered," theyre resistance welded, or just twisted together...
You'll have to compensate for the cold junction also. Then you still have to figure out where to put the thing! You wont be able to solder with it since it will be in your way. One thermocouple will probably cost more than your lousy soldering iron anyway. And the AD595 cold junction compensation chip aint cheap either ($17-$25)!
Current control, or a new soldering pencil, is way more practical.