what do you reckon 15 amps?10 amps is your problem, its too low, with low amps you are having to create too big a gap to maintain the arc which in turn pumps more heat into the job, scratch start is not ideal but it can be done with a bit of practice, make sure that your joint fit up is spot on, any gaps will add to the problem.
keep a very tight arc and use a thin filler rod, for 0.5 I would be using a piece of 0.8 mig wire as a filler, too big a rod means you have to move slower to melt it which again means more heat in the joint.
Its a piece of mild steel flue liner, i want to cut it and weld it to 135 degrees involving a huge butt joint.Chuck a block of brass behind it to take some of the heat away?
I've successfully laid down some beads on 0.5mm sheet steel and I'm a complete amateur. Was a challenge though.