iron
鐵 centers on the metal iron, both as a raw material and in its characteristic dark, strong form.
鐵 is the traditional form of 鉄. It combines 金 (metal) with a phonetic component that historically indicated the sound. The modern simplified form 鉄 replaced the right side with 失.
The left side 金 means metal. The right side looks like a dense, heavy block of raw iron with many parts. Together they show iron as a strong, dark metal.
For テツ, imagine a blacksmith hammering iron on an anvil with a sharp 'tets' sound: tets -> テツ, and the iron takes shape.