Think of steel as a mud pie. The iron is the dirt and the carbon is the water. Too much dirt and it won’t clump together, too much water and it will fall apart but if you can get the mixture just right it will be able hold its shape much much better than either of the other two scenarios

Edit: it will be easier to fall apart on either side without the precise ratio

Cast iron is even less "iron" than steel, it's just called that way. Iron is a base metal. Steel is an iron-carbon alloy with the right proportion of carbon, where the carbon makes it harder. "Cast iron" is not iron but it's another iron-carbon alloy with too much carbon, so the carbon makes it brittle.

Steel is made from iron. Iron ore is heated in a blast furnace to remove it’s naturally occurring oxygen, coal is added to supply the carbon.

Iron - oxygen + carbon = steel

if you put them on a line of increasing carbon content, they actually overlap. you'd have elemental iron on the left, then wrought iron, and then partway up wrought iron you'd also have steel. and then just steel for a while, and then overlaps with cast iron.

wrought iron is distinct from low carbon steel due to the inclusion of slag in wrought iron. wrought iron is a very crude alloy. that gives it some properties different from low carbon steel, but its real virtue is how comparatively cheap it is to make. not having to worry as much about the impurities means less processing.

the difference between very high carbon steels and cast iron is a bit harder to explain, but has to do with how the carbon is tied up with the iron. cast iron has free carbon in it where steel doesn't.

as for "why is the naming convention so screwed up?" we discovered them out of order. cast iron is just barely iron in a meaningful sense, wrought iron is early man's best attempt at purifying iron completely. landing on the sweet spot in between is really hard, in no small part because while you're trying to make the steel behave, it's trying to rust and also set everything on fire.

Let's start with a die (as in dice). It's a cube with four corners and 6 faces with dots. Even if we made a blank die, we still have the 4 corners and 6 faces.

Iron, or ferrite, has a crystalline structure just like this blank die, with one atom at each corner forming a cube. Since it's also hollow inside that cube, there's one more packed in atom. This is called a body centered cube.

Now I take that die and I heat it up until it's white hot in a fire and the energy moves things around and the little atom in the middle pops out when the cube expands and vibrates. Now it settles back on the face of the cube making a die with just one dot on one face. This is austenite, and the structure is face centric now.

We can pack one atom on each face making dice with all one faces, which is awesome! With all of those atoms, the inside of the cube is too small for another iron so it's empty for the moment. When I toss that dice back into the fire though, a smaller carbon atom is able to slip in past the one dot faces and fill the center. Now we have an alloy of iron and carbon.

If threw a dense box of iron dice all neatly packed into the fire, the whole thing can heat up and change from ferrite (body centric) to austenite (face centric) but the carbon can only penetrate so far into the box because it has to move from one die center to another. This is basically "carburizing" a steel billet. If we fill every single die with carbon, the box will be too full and break open, so there's a sweet spot of how much carbon we want.

The way we made steel for a long time was to blow hot air into "the box" of molten dice and introduce lots of carbon into the iron, making "pig iron." Then the pigs were remelted without carbon present, ejecting lots of carbon as the cubes opened up aiming for some percentage of carbon between 1 and 3%.

There's is absolutely a difference between all of the percentages. I think of them as low carbon (mild) steel, high carbon, and cast; but this is a huge oversimplification. Cast alone has ductile, white, and grey varieties each with different properties. High carbon could be 0, A, W, D, white/blue/hitachi varieties again with different properties. Low carbon steel could be wrought or mild. Wrought iron has lots of silica in it as a flux agent and is no longer really made, but is desirable for it's properties to the right person.

As for ductility, no pressure on knowing that term specifically. I know you understand the concepts. In my mind, you could think of ductility as the ability to stretch without tearing like pizza dough, you want a really glutenous dough to make a big pizza. Malleability is a quality referring to bendyness, spaghetti is not malleable until it's cooked and then it's very malleable. Also, machinability is the ability to cut it down cleanly, room temp kerrygold butter has supreme machinability, ice has no machinability. Weldability is another quality, a gummy bear can have it's head reattached with a lick, but you can't put a sliced apple together again.

As an aspiring metalworker who forges, smelts, machines, and welds... I think about this stuff a lot.

It actually has to do with crystalline structures of iron/carbon elements that form within the steel, IIRC.

I think it's this video that explains it well, if it's not this one, it's #2 or #4 in the same series: https://youtu.be/r1z6-h7XACg

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.

Normal Iron has no carbon, steel is less-than 2% carbon, and cast iron is more than 2% carbon

Low Carbon Steel or Mild Steel. It have between 0.05 and 0.25% of carbon. It have lower strength than other form of steel, but it's also more ductile which mean it's easier to machine or work with. This make that steel ideal for structure like building, bridge, etc. Because even if they have a lower strength, they are easy to shape into different shapes. The fact that it's ductile make is also safer in building, basically the building will bend which give notice that something is weird and it won't just collapse on the people right away.

High Carbon Steel have between 0.61 and 1.5% carbon. This type of steel is very strong, very hard and harder to machine or form into shape. This make it ideal for item that will have to endure a lot of wear like tools.

Medium Carbon Steel have between 0.26 and 0.6% carbon. It's an happy medium between lower and high carbon steel. Things like gears, bolts, automotive components, etc are usually made of that steel. They are parts that will suffer some regular wear so mild steel would be too damage, but it doesn't suffer enough wear to justify the higher cost and difficulty of working with high carbon steel.

Cast Iron have between 1.7 and 3% carbon. It's much harder than Steel, but also very brittle. Mild Steel usually around 130HB of hardness, medium carbon steel is around 200HB, but cast iron is closer to 400HB.

Stainless Steel have a high content of chromium (between 10 and 30% depending the type), but it can have different level of carbon up to 1.2%. That said, just like normal steel, the most used type of stainless steel will have lower amount of carbon than that.

Alloy steel well those can be anything, different type of metal added to the steel (usually in small percentage) will have different effect. Molybdenum increase the toughness so some cutting tools, turbine blade, rocket motors' parts, etc. Tungsten increase the melting point. Boron increase the hardness. Bismuth make it easier to machine. And many more.

Wrought Iron have a very low carbon, less than 0.08%. As you can see from 0.05-0.08% it could be both wrought iron or low carbon steel. That's because wrought iron isn't define by the carbon content, but by how it was produced. See in the past we were not able to measure the amount of carbon, so we didn't categorized iron by their content like we do today, but rather by the way we produced it. Wrought Iron (low carbon) was made from iron ore in a bloomery, but it take a long time. Pig Iron (high carbon) was produce by using a blast furnace and it was faster to produce than Wrought Iron. What you could do after is take your molten pig iron and add stuff like coke, limestone, you can also burn the impurity and you end up with cast iron. Or you can take your pig iron into a finery forge or puddling furnace to make wrought iron, this was faster than the earlier bloomery. Other things we used to do was beat the hell out of cast iron to slowly remove the carbon out of it or mix cast iron with wrought iron to reach a better level of carbon. Both medium were relatively expensive at the time.

It's only later when we develop steel metallurgy that steel took over the traditional iron, and now we categorize steel by their content, rather than the method of production. But now since both way of categorizing exist, it can be a bit confusing.

Iron costs 2 ore and has a 50% chance to smelt. Steel costs 1 iron ore and 2 coal and has a 100% chance to smelt. Silly question.