Iron is an important resource for a fortress, as an often-plentiful metal that can be used to make just about anything necessary, from weapons to furniture to trade goods. Iron is also a necessary ingredient in the production of steel, the best non-spoiler metal in the game for weapons and armor. If you are unable to produce enough steel in your fort, then the next best metal for military purposes is iron.
The three ores of iron, which are Hematite, Magnetite, and Limonite, can only be found in sedimentary layers, with the exception of hematite, which can occasionally be found in igneous extrusive layers.
Note that iron does not rust, so there's no harm in leaving iron objects out in the rain or submerging them in water.
- Melee weapons. Mostly useful for edged weapons. Good for bolts as well. You can also make crossbows out of iron but there are better metals.
- Armor. You will generally never face better armor than iron armor. You may face however creatures made of stronger materials.
- Construction, if you are so inclined.
- Good quality furniture. It is quite heavy, however.
- Magma-safe mechanisms.
- Anvils, which can only be made from iron or steel.
Iron comes from three ores: hematite (most common), limonite and magnetite (appears in large clusters).
Any of these iron ores can be smelted at a smelter to produce four iron bars. If you have a standard smelter, a bar of fuel will be necessary; magma smelters require no fuel to operate.
Iron will also be available through goblinite, as well as the dwarven and human caravans.
Iron may be combined with other materials at a smelter, mostly for the production of steel
Note that fuel is used as a reactant: even magma smelters need fuel for these reactions. A standard smelter will require two bars of fuel, one as a reactant and one to operate the furnace. This conforms with the process of carbonization of iron and pig iron in ferrous metallurgy in real life.
Dwarfoloid picked out a lot of the material numbers here to replace my placeholders.
Colors added by Bohandas.
[IMPACT_YIELD:542500] Was 1080000, but just using 3.5x tensile multiples for everything until better numbers are available, which might not be likely
[COMPRESSIVE_STRAIN_AT_YIELD:319] bulk modulus 170 GPa
[TENSILE_STRAIN_AT_YIELD:73] young's modulus 211 GPa
[SHEAR_STRAIN_AT_YIELD:189] shear modulus 82 GPa