While it's good to use correct rod and amperage, when welding things like rebar and high carbon it's good to preheat the stock a little if welding onto something like a big bearing, and always to post-heat the weldment. When sticking a handle to a billet, I bevel the rebar end a little so it's clean and there is room for full penetration, and when done welding, I brush the flux off and immediately put in in the forge if that's running already, or at least anneal the weld a bit with a torch. Otherwise the high carbon stuff and even parts of the rebar may air harden and micro-crack to some extent. You will have observed this if you've ever welded on a handle or billet ends, then had the welds crack at room temp with no impact.
You'll want to direct the rod a bit more at the thicker side of the weld (the billet end) making sure that both sides of the joint are wetted thoroughly by the weld pool while you go. When using a smooth arcing but lowish penetration rod like 6013, again, beveling the rebar well is your friend and will get you a more fully welded joint. Sometimes I'll do a root pass, then a cover pass too to increase the fillet strength. As far as erring on the side of judgement, it's better to weld hot and get deep penetration with a little undercutting than to weld cold and get a tall bead without much integrity at the core. Ideally you'll find the point of balance between the two, based on your machine (each machine will run just a bit differently in terms of actual performance at a given amperage setting.)
They still come off sometimes, but at that point I just clamp it in the post vise and re-weld while everything is still at a red heat, and keep going.
