Well by sanding the 2 tubes to be welded and applying flux, it should not give you difficulty, whenever you make these joints, put the iron tube inside the copper tube, not the other way around and the flame of the torch that is blue and separated from the 2 tubes to be welded, you load more heat, but before heating, the rod heats it on the tip and when it falls, you put it in the flux so that it sticks on the tip and immediately heat the 2 tubes, you load it more to the iron and when you see it red you approach the flux that gets into the groove of the tubes and pass the rod that falls and alone will travel in the unions of the 2 tubes, remove the torch and that's it, it's a matter of practice, It does not have much science, it happened to me, if you have to expand the copper tube so that the expansion is as deep as possible, never leave it short, because the tube is covered with solder and if it is not necessary to expand, try to enter the iron tube as much as possible in the copper tube, the Welding gas always uses the black map tank, it is the one with the highest temperature.