Never heard of sweet iron, perhaps you meant Pig Iron, those hard to cut chains wold have been made with a manganese alloy.Old timers used to talk about what they called "Sweet Iron", most of what I saw described as such was in chains,as in log chains.Hard as heck,couldn,t cut it with a hacksaw. I don,t know exactly what was in it,but thought I,d throw it in there. looked a lot like your material.
I climbed up the hill where they lowered wagons down it was steep and dangerous to climb up with a detector in oregon near mount hood on luarerl hill they tied chains to wagons and lowered them down lots of wagons crashed and people diedWhat makes you think this is from a wagon axel ?
OkMIne looks way older than those