Besides the compression of the iron from pounding they were also said to have absorbed carbon from the hoof.
I was of course simply a machinist, not a metallurgist. However it would seem to me that bringing the iron to a welding heat would relieve it of any compression that it may have obtained through pounding. Also for a piece of iron or steel to absorb carbon as in carburizing it is necessary for the mat'l to be brought up to its critical temperature. It is quite obvious this temp was not reached in a horse's hoof.
