To put Ferenir's dragonforging in very simple terms, he can create items whose durability match adamantine, without actually needing any. He can do it with simple steel.
Now, iron is trickier, because it's softer, and items need to be of a certain endurance to withstand the magic infusion.
I'm sure most of you are familiar with adamantine and its properties from fiction, be it from dnd, marvel(wolverine's bones are made of adamantium), or others. It is a commonly used fantasy metal alongside mithral.
But for those that don't, it's an extremely rare metal, in some verses only found on meteorites even, that is immensely durable, nearly, if not, indestructible.










