Swords made of iron (as opposed to bronze) appear from the Early Iron Age (c. 12th century BC), but do not become widespread before the 8th century BC.
Early Iron Age swords were significantly different from later steel swords. They were work-hardened, rather than quench-hardened, which made them about the same or only slightly better in terms of strength and hardness to earlier bronze swords. This meant that they could still be bent out of shape during use. The easier production, however, and the greater availability of the raw material allowed for much larger scale production.
Eventually smiths learned that by adding an amount of carbon (added during smelting in the form of charcoal) to the iron, they could produce an improved alloy (now known as steel). By quenching (making the steel hard and brittle) and tempering (removing the brittleness), swords could be made that would suffer much less damage, and would spring back into shape if bent. It took a long time, however, before this was done consistently, and even until the end of the early medieval period, many swords were still unhardened iron. Several different methods of swordmaking existed in ancient times, including, most famously, pattern welding. Over time, different methods developed all over the world.
The Proto-Celtic Hallstatt culture -8th century BC- figured among the early users of iron swords. During the Hallstatt period, the same swords were made both in bronze and in iron. At the end of the Hallstatt period, around 600-500BC, swords were replaced with short daggers. The La Tene culture reintroduced the sword, which was very different from the traditional shape and construction of the Bronze Age and early Iron Age, and much more like the later swords that developed from them.
The iron version of the Scythian/Persian Acinaces appears from ca. the 6th century BC. In Classical Antiquity and the Parthian and Sassanid Empires in Iran, iron swords were common. The Greek xiphos and the Roman gladius are typical examples of the type, measuring some 60 to 70 cm. The late Roman Empire introduced the longer spatha (the term for its wielder, spatharius, became a court rank in Constantinople).