7018 is a low hydrogen rod. If you do not keep it in a rod oven after opening the can, it will absorb water and cause cracks in high strength steel. But can be used on low strength steel like any rod with hydrogen.
My understanding is 8918 and 9018 are also low hydrogen rods. Can only be used on high strength steel if kept water vaper free, but can be used on low strength steel regardless.
The test we learned for that was to make a butt weld between two
pieces of scrap, grind the beads mostly flat and then bend the seam
over double along the weld line. 1/8" steel was bent with a hammer in
the shop vise and they had a 50 ton press for heavier pieces.
Try it with a single unwelded piece of steel first to see how or if it
cracks. We were practicing on mixed scrap from a fab shop, mostly
plasma torch cutouts.
It depends on what you mean by "sound", and what the geometry of your
weld is. Your 8018 and 9018 welds will definitely fail at lower
deformation than a 7018 weld, and cracking will be more likely, but not
necessarily a problem. In all cases I have seen where 9018 was required,
preheat to 400-500F was also required, but these were multi-pass welds on
~2% chrome-molly steel. Why not try a few welds and give them the hammer
If there is a 3-letter code after the xx18 designation your rods are
probably special purpose, such as for chrome-molly, and a preheat
requirement (or a brittle weld) is more likely.
While the rods might work for your purposes, for any critical weld on
structural steel, where failure would cost a lot or possibly hurt
someone, I would use 7018 even if I had to go buy it.