As you know, the Roman army depended on quality metals for their armour and weapons, and so a very large number of Romans, whether slave or free, were needed to supply the sodiers with good equipment. Though it was slaves who did nearly all of Rome’s grunt work, there were quite a few poor people who needed to earn money for a living. Most did so by joining the army, but others took up work a potterers, weavers, jewelers, carpenters, smelters, or blacksmiths.
Surprisingly, metal techniques in the iron age did not change much from those in the bronze age. Casting (pouring molten metal in a mold), forging (smashing hot metal into shape with a hammer), smelting (seperating a metal from the ore), and alloying (combining two inferior metals to form a superior alloy, such as copper and tin to form bronze) were all developed in the bronze age. It was not the techniques, but more the variety of metals, notably iron.
Bronze is actually much harder than iron, and so keeps a sharper edge and holds it better than iron. However, the reason Iron pretty much replaced bronze is for several reasons. One, it is much more abundant than either of the metals (copper and tin) that are alloyed to make bronze. Two, carbon can be added or subtracted from the iron to make it harder or softer, (which the Romans actually figured out pretty early in their history) which means that they could have the edge hard so it can stay sharp, and the spine soft so it will not crack on impact, unlike bronze, which could shatter with a hard blow. And, because Iron was softer, it was easier to forge, though it was not cast.
The reason Iron was not used before bronze is the fact that Iron melts at a much higher temperature than either copper or tin, and technology was not advanced to that high temperature. It was with the use of coal and charcoal that people could get the temperatures high enough to smelt Iron, and even then, it was not enough to melt Iron, as smelting does not require actual melting of the Iron, just enough heat to get the Iron and Carbon to separate. Because of this, the Romans did not do any casting with Iron, whereas much, if not most, of bronze weaponry was cast.
So, the main reason bronze became replaced with iron is mainly because iron was cheaper and more common. Because of this, it was customary for Roman legionary Centurions and standard-bearers to wear bronze helmets, to show their rank.