Two things that you must remember when bending pipe this way.
1. The outer radius has to be stretched.
2. The inner radius has to be tucked in. (Slightly)
I have always used damp silica sand, heated the outside radius and limited bends to 60 deg. or less. 90 deg. bends have to be cut on the inner radius or tucked in and welded later.
If you were mandrel bending, you would be stretching the outside radius as you pulled the pipe in the bender. The inner radius would be supported from crushing.
Tubing, depending on size, can be bent using a spring inside to prevent crushing.