AUSTIN- frt axle is cast iron. In the WHOLE U S of A there's only about eight (8) foundries that sand cast carbon steel alloys. I worked for one of them. There's many more that COULD sand cast carbon steel castings but don't. Foundry I worked for had three 10 ton carbon arc melting furnaces, the 10 ton is nominal, they could pour 22,000 to 24,000# of metal when needed. We did a LOT of work for General Dynamics, M1 tank parts, CAT was another big customer. I found out several years after I left there that an open die forge shop also made a forged and machined shock absorber body that we cast in steel. It was almost a foot in diameter and just over 4 feet long.
Not near as many cast steel parts as you would think in the world. IH had lots and lots of foundries, and not a single one cast steel or aluminum, and only one in Waukesha, Wi cast Ductile iron, also known as nodular iron. Everything was gray iron. Their Louisville foundry was their biggest, but also had big iron foundries in Memphis, and Indianapolis. Also had a steel forge shop in LVL, and they had a row of seldom used STEAM HAMMERS that also closed die forged parts in the most violent manner I ever saw at the Canton, Illinois plant! I saw a couple steam hammers working at Interstate South-West in Navasota, Texas, now known as Ellwood Forge -Navasota, owned by Ellwood City Forge in Ellwood City PA, a big open die forge shop.
As for welding your gray iron frt axle, it can be done, but I think finding a used axle with no grooves would be best. The carbon in the gray iron forms a hard layer next to the weld bead which makes the casting brittle.