There are "easy outs" to fit pipe. If the break has rust it's probably steel. You need a penetrating oil that includes a mild acid. It eats the rust in steel and cast iron making the threads looser. Heat applied to the elbow may help, but if the elbow is cast iron too much heat can crack it.
An alternate method is to insert a metal cutting blade into the fitting and cut thru the full length of the pipe fitting just to where the threads show. Then with a punch and hammer, hit the edge of the pipe on both sides of the cut, collapsing it toward the center. Work your way down the cut line and the fitting eventually becomes loose. Easier if you take the elbow off to somewhere it's easy to work on. Or once the elbow is off, take it to a machine shop.