Iron loves to incorporate stuff. Problem is that it tends to make small-ish crystals of iron surrounded by a coating of stuff.

Imagine the iron crystal is used chewing gum, and the stuff is sand, then your material is made of dirty used chewing gum balls sticking one another with sand reducing the stickiness.

The cheapest form of iron ally is cast iron, with a lot of carbon. Works like your chewing gum balls with a hard coating on each grain. It’s strong but brittle as you can break apart the grains by breaking the hard brittle coating between the grains. It’s still good for casting, but can’t be bent into a shape, or forged into a shape.

If you reduce the carbon amount you can thin the brittle coating of grains. Needs more processing. More cost but better iron. Still brittle-ish, a further process is to add some material that softens both the grain and the coating. Medium strength, but ductile enough to be shaped. It’s the iron you see for general low cost purposes.

If you process more you can remove the carbon, now you have the good steel.

The good steel then can be tweaked. One way is to heat it up until the coating melts and is dissolved in the grain, then quench (fast cooling) and you have “frozen” the structure into a grain with thin/nonexistent coating and the dirty stuff dissolved in a harmless way int he grain. Do this with carbon steel and you have a good sword or tool. Some processes allow to add carbon to the surface only, so when you quench you get a strong flexible core and a harder surface that resist abrasion, good for blades, gears, tools.

Another process is to add very strong things to the coating or the grains or both. Chromium, molibdenum, vanadium, and so on. This way you get “super steel”. It’s quite expensive but according to the mix, you can get something very strong and flexible to make springs. You can get something just strong, hyper strong, usually to make spanners, tools, and expensive mechanical parts. There’s special mix that can give the grains a “skeleton” and an armor as a coating. These last category can be used to make drill bits and similar cutting tool, able to cut almost as Diamond.

The fascinating part of iron alloys is that it’s a material that loves to mix with a lot of stuff and loves to change grain size and shape and grain coating thickness, or even having no grain coating, all of this for each mix and each temperature, and iron also can be “frozen” in its structure by quick cooling. So it’s one of the few metals that can be mixed, heated to a point it gets a special grain, then “frozen” in that state.

I think I spent 3 moths at school to learn the most common (but not all) iron alloys, while it took just a month to discuss ALL the other metals. That’s how much you can play with iron alloys.