As I understand , the 60xx implies 60000 psi in tensile strength and 70xx is 70000 psi. I used 7014 and 7018 and once I had gotten the second strike on the 7018 rod down, it lays a beautiful bead with out lots of effort. Check some of the electrode charts online to give a better clarity and a bunch of guys with infinitely better understanding will likely chime in.
By second strike I mean that there appears to be a glass like insulating bead that forms on a rod after you stop welding so getting current flowing means I had to strike off the little insulator.
Been practising with 7018-1, it's a little bit harder than 6013 but not so bad. Restarting is quite hard, even with filing the rod before restarts, rod keeps sticking a lot. But that might be because I'm using AC.
Would you recommend leaving a small gap on the corner joints for better penetration?
The very small sized rods a bit harder to work with in my experience. 5/32" and up will not be glowing red when you lift your hood. So the restart is easier.
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How bad are your gaps? On my fire boxes I always run the whole thing out with 6011 (6013 in your case) use a wire wheel to clean all he welds and then I'm going back over with 7018 for a good finish that I can use a flap disk to round off the edges.
They are pretty bad at the worst spots, somewhere in the region of 1/2". I wish I had oxyacetylene so I could heat it up and shape the plate for better fit. Big hammer did something to the gap, but not enough to make it fit properly...
Here are some pics of the worst gap:
I tried to do the root with 7018, have to try with 6013. But I think it'll be pretty hard to fill the gaps. Maybe I should try to weld some angle solid to the inside where the gaps are? Btw, you can see where I tried to fill the gap with 7018 in the pic, didn't work out that well
With those gaps, do you think the welds will be strong enough? The gaps are only on two sides, so most of the box can be welded pretty well. I read about gap filling techniques and every one of them had a disclaimer that it's not going to be very strong weld...