I believe this to be Nodular Iron. I could be wrong, but this is what I was under the impression that it was.
I have welded with 7018 using a pre/post heat method, and have had success, and I have tig welded them with Stainless filler, and I have used my millermatic mig.
The mig required quite a bit of pre heat. I had to really get it hot, and make a few passes, but this worked fine and I have had no problems with the tubes. The tubes are sort of a weak link on the 14B, but some welding usually makes easy work of this. Peening the welded area is probaly as important as the pre heat and post heat. Be sure to relieve the stress as the stresses will be different between the two types of metals.
Depending on which axle you have, the perches might be cast iron, so high nickel cant hurt there too.