Using material of an unknown specification on a safety critical part like an axle is not really the best idea.
That said half-shafts and their like are going to be made of a tough steel but from first hand experience I can state that manufacturers may and do fuse/weld steels with different properties together depending on the duty required, they may also vary the heat treatment over the components length for the same reason. All of which means you don’t know what you’ve got.
On balance I’d say buy material of a known provenance.
