From Gamer Heroes: “Forged Iron can be found on Rhom. Continue following the main objective on Earth, eventually you will reach the Labyrinth. Once you are at the Labyrinth there is a single portal you are able to use, the rest are blocked or inaccessible. This portal leads to the 2nd biome in the game, Rhom. Iron Forged also drops in other areas as the actual physical drop rate is tied to the current level of your weapons. Once you have +5 weapons, Forged Iron can drop anywhere, we just found it more common on Rhom.”
Source: N4G PC Where To Find Forged Iron In Remnant: From the Ashes