There are at least 2 different chinese characters for iron, and the one in common use, the one you mention, has a different old chinese pronunciation (from the turko-mongolic word). The radical for the popular version clues one in to the original non barbarian pronunciation. (Maybe it referred to something else.) The radical for the other one means “barbarian”
https://en.wiktionary.org/wiki/銕#Chinese
It is not uncommon for chinese characters to be written the same but pronounced differently (and meaning something else) throughout the ages. And for the meaning and pronunciations of different characters to mix or merge
Indeed according to historian David Wagner, “ The evidence presented thus far suggests that iron-smelting techniques developed in the West had by the eighth century BC been brought by nomadic peoples of Central Asia all the way to the Pacific coast, and that these techniques diffused to the smiths of the Chinese states by way of various non-Chinese peoples of the northwest, in what is now Xinjiang. ”