So today we're gonna be talking about iron and steel. So as you know, the original age was the bronze age and it was followed by the iron age. And the iron age was ushered in by a couple of developments. Number one was there was a growing scarcity of copper and tin.

And so it was becoming more difficult to make bronze and there was an increase in demand. There was also the ability to increase the processing temperatures a little bit, and both of these led to the development and utilization of iron and steel. So what is the difference between iron and steel?

So if you want to understand iron and steel, you have to start with bronze and say okay, bronze was an alloy, an alloy of copper mixed with either arsenic or tin. And those materials were actually dissolved into the material, so it became a solid solution hardened of material.

Steel on the other hand, is an alloy also, but steel is an alloy of iron mixed with carbon most commonly, all right? And in this case, the carbon actually rather than setting substitution, it's gonna wind up being interstitial or between the iron atoms. And that gives it some really interesting properties.

So it's been called probably the most useful combination in the history of society. In order to make progress with steel, you have to understand a couple of terms. Number one is the term forging. Forging means that you're beating on the material, or hitting with a hammer, or a rock, and that shapes the material.

And then, casting is another way of shaping the material, but in this case, you're pouring it into a mold, all right? Both of these are useful in iron and steel formation. So when we look at the forms of iron, there are several different forms, all right? You can have a low carbon iron.

This is called wrought iron, and that's where you have something between zero and 0.2% carbon, a very small amount of carbon. And this iron form is actually very ductile. It's typically pure, it has a very high melting point. So it's very hard to melt wrought iron. The next phase of iron is typically where you're adding something between this 0.2% and 2.1% carbon.

So it's only a couple percent carbon. Still a very small amount of carbon, but that's what we call steel. And steel is extremely hard. It can be 1,000 times harder than pure iron. And steel has a very high melting point. So steel is actually very hard to make.

And then finally, you have high-carbon steel. High carbon steel or high-carbon iron, where you're adding between 2.3 and 4.3% carbon to the iron, you get something we call cast iron or pig iron. And this actually has a lower melting point and was actually meltable in ancient times. But it was not forgeable because this material is very brittle.

It has so much carbon in it, that it tends to deteriorate the properties of it. So it can only be cast, thus the name cast iron. So in order to understand why there are these three different regimes, you have to look at the iron carbon phase diagram. And in this phase diagram what you see is, is that way over on the left near pure iron, you have a very, very small amount of carbon that would be soluble inside the iron.

And that's where you get the raw iron phase. And you'll notice that, according to phase diagram, the melting point is extremely high for that material. When you add something between 0.2 and 2.3% carbon, you get in this material that where if you heat it up to high temperatures, like austenite, then you can actually dissolve the carbon into the material.

Cuz austenite is a cubic material. So it can accommodate the carbon more easily. And so that's the region we call steel. And then once you get above that, that's where you enter your cast iron material, where you have so much carbon that it can't actually dissolve inside the iron anymore.

So it's always precipitating out in various phases, and that tends to make it very brittle. So in order to understand this further, you have to move into why in the world, or understand why steel is so strong. You have to understand how these phases react with carbon. So how much carbon do you add to the iron is the critical component.

When you add just a small amount of carbon, the material is very ductile. So for example, I have here a bobby pin and I have a paper clip. The paper clip has a very small amount of carbon in it, okay? And so when I heat this material up, so when I can take this material and I can bend it around and it's very malleable, right?

And so that's a low carbon iron alloy, all right? It has about 0.2%. So it's down in that wrought iron phase. Now what's interesting is if you heat this material up, and you quench it, so I'm gonna heat it up very hot. I'm gonna turn it into austenite, right, and then I quench it.

When I do that, I actually quench it so fast that the carbon doesn't have a chance to move around. Now there's only a little tiny bit of carbon in here, so it's not critical. But the iron actually no longer has that phasing or cubic phasing actually has a body centered tetragonal phase, and it makes it harder and so you can see when I go to bend it, this part is very hard and so it doesn't want to bend, but the stuff that wasn't quenched is still soft, and so it bends.

So I can make the material very hard, but it's not terribly brittle. And so this material can be very useful for example, if you wanted to make a sharp in edge on a scythe when you're harvesting wheat. All right, but this is very low carbon and it doesn't get as strong as it possibly could.

Now, if I take a bobby pin, it has higher carbon, it has about 0.7% carbon. And that carbon moves it all the way into the steel regime. So this is actually a form of steel, all right? Now what's interesting about this is that this is still ductile. It's harder to bend, all right, than your paper clip, but it is still ductile, all right?

And when I take this now and I quench it, all right, I heat it up. I made my austenite, I quench it. I now made a base calles Martensite and that Martensite is that body center tetragonal phase I was talking about before. This is also very hard. Now the difference is, is that I have a lot more carbon and that carbon didn't know where to go.

It got stuck in there and it induces a lot more strain. So this material becomes brittle and will break, all right? And so that was the challenge, right, is this that I've varied the amount of carbon. I could take it from something that would be hard, but still have some ductility to something that becomes even harder, but is now very brittle, all right?

So when they were trying to figure this out early on, they didn't understand what was going on with the phases, but they knew they could manipulate the properties of this material. And that's what made iron and steel so valuable. So, what we've been talking about is the heat treatment, and that's where the trade aspect of working with iron and steel came into play.

And so, understanding how this worked was critical. If you do a heated up, for example, if I take this same piece of steel. And I now heat it up, all right? And I slow cool it, okay? Then I no longer form the Martensite. I allow the carbon to form around where it wants to do.

And the material actually maintains its ductility. So the idea was I can make it harder if I quenched it. So if I did this process and I heated it up really hot, and then I quenched it. I could make it very hard but we know now this is very brittle.

Then what they would do is they would make it very hard and brittle, but then they would do a process called tempering. Tempering meant I come back again and I heat it up gently and I let it slow cool again. Now I've retained some of that hardness, but I've restored the ductility of the material, it's no longer brittle.

All right, so that ability to manipulate iron was critical in its development as a material. So, how do you make iron? All right, if you wanna make this material, to begin with, what you have to do is start with red dirt. Iron Oxide, all right? And, it can be either Fe2O3, or Fe3O4.

Fe2O3 is magnetite. It's black, actually, and Fe3O4 is the red stuff. You take some red dirt and you take some limestone. And that could be in the form of oyster shells or whatever, and you mix that with it, and you add some carbon. Now, you layer these together into a furnace that you build around it, usually out of clay or some refractory material.

And then you would light the fire, the carbon on fire and you'd blow bellows through it. You'd blow air through it with a bellows. And what that did was, that actually heats up the carbon really hot and makes carbon monoxide. That carbon monoxide is a very good reducer, and so it takes the iron oxide, and it reduces it back to iron by turning carbon monoxide into carbon dioxide.

So that's a great way to make iron. The problem is that mixed in with the iron is typically a lot of sand. And you gotta get that out of there. And that's where the limestone comes into play. So when you add the calcium carbonate, then what happens is the calcium carbonate will form a calcium oxide, lime, when you heat it up by giving off CO2, and then that lime reacts with the silica to form a lower melting form of sand, a calcium silicate, that forms a glass slag.

And so when you're done cooking this thing, you'll have iron and you'll have this calcium silicate slag, all mixed together, in what we call a bloom, all right? Now the early forms of iron making were done in what was called a bloomery. And you make the bloom iron by doing this exactly what we just said, heating it up for a long period of time until you're left with wrought iron and this slag.

Now you have to separate that. And the way you did that was you actually forged it, or you beat on it with a hammer for a long period of time. And you literally squeezed the iron out of the slag. So the slag would be sloughed off to the side.

And the more you beat on it, the more slag you got rid of, the more pure iron you had left behind. This formed a pure wrought iron. So this is a very low carbon iron, all right? It was not particularly strong. It's not that much stronger, actually, than bronze, to be honest with you.

So how did you then turn around and make steel? Well, you had to add the carbon back again. So now they get into how do you do that? The best way to do that, the melting point is too high so you can't take it up, melt it, and then just add some carbon.

So what you had to do is actually take the iron, and you heated it up inside a carbon-rich furnace, all right? It's called carborization. And what that did was diffuse the carbon into the surface of the steel and slowly convert that iron surface, that wrought iron into a steel, all right?

And so again, you had to be a very skilled tradesman to figure out exactly how long and what the right atmosphere was to do that. Now there was a second way of making iron, and that's with what's called a blast furnace. In this case, you take the same ingredients, you heat it a little bit hotter, and what happens is, is you get to the point where the cast iron melts, and it pours off the bottom of the furnace, and you collect it, it's also called pig iron.

The Chinese actually skipped the entire bloomery phase and just went straight to making cast iron. Now the challenge with cast iron is that you have too much carbon. Now the material is too brittle, much like this material was brittle when I had the carbon in the wrong form.

Now if I have the carbon all through the material, I can't make it into something that I can beat into a device. The only thing I can do with it is to cast it, all right? So in order to get that carbon out, then you had to use a refinery for it.

So what you did was you actually took your cast iron and you remelted it under an oxidizing atmosphere. And that oxidizing atmosphere pulled out the carbon and the silicon, you made CO2 and you made silica, and you skimmed that off and you were left again now with wrought iron, and then you are back to adding the carbon back to the pure wrought iron bar.

So there is all this manipulation that you had to do. Here is an example for what a blast furnace might look like, where you can see how you use the brick lining, and then you layer the iron oxide with the limestone, with the charcoal, and then you use bellows to heat it up.

Now in terms of the history of iron and steel, you have to go back, you can go back to 3,500 BC. And there were beads in ancient Egypt that were made from iron. But this was not iron that they actually made, this was iron from a meteor. So, the reason they understand that is because of the nickel content.

And so for the early forms of Iron, it was all extraterrestrial. Then they started, around 3,000 BC, creating the first iron production in Syria and Mesopotamia. And the Hittites actually had a mass production of iron by 1500, 1200 BC, and it was all bloom iron done in small batches, and then they would use water power to beat the blooms and to drive out the wrought iron.

As I said earlier, China starts the iron age much later. They actually don't start until about 700 BC, but they went straight to this blast furnace form of making cast iron and then using a finery forge to make the wrought iron, and then adding the carbon back again.

So if you skip forward to about the 1400s, you have an increasing need for iron. They want it for church bells and cannons, and so they start developing the blast furnace in a much bigger way, so making larger batches of cast iron for all these applications. By the 1500s, Japan actually is making the strongest steel in the world.

And what they did, which was really interesting because this was the steel that was used in the Samurai sword, was they used magnetite, which is a black sand found on a beach, and they used that magnetite with charcoal. So they didn't have to add the limestone because they didn't have the sand in there.

And the magnetite formed when they cooked it for a very long period of time and broke it apart. What they would have is carbon in very different concentrations throughout the whole melt of the iron that they made. And so what they would do is actually literally hit it with hammers to listen to the sound, and they could judge the carbon content by the sound of the iron.

And then they would take the material that had low carbon, and they would beat that into a sword shape, and then they would layer on top higher carbon steels. And so you had a ductile center to the blade, but you had a very high carbon brittle steel on the outside that gave it a very sharp edge.

And that's why that steel was, that composite steel was some of the best steel made for hundreds of years literally. Now all of this iron making started using up an awful lot of charcoal, which meant it was using a lot of timber. In fact, it's been claimed that the deforestation of most Northern Europe was due to the fact that they were trying to cut down trees to make charcoal for making iron.

And so by 1588, Queen Elizabeth limits the use of timber, and by the 1700s we have literally a timber famine. There's just not enough charcoal to go around. Now the obvious question to ask is, all right, you need carbon. Why not use coal, right? Coal was a great form of carbon.

The reason you couldnt use coal was coal has too much sulfur in it, and sulfur weakens steel dramatically. So they couldn't use it. They didn't understand why, but they knew that it just didn't work. It didn't make very good steel and iron. However, thanks to beer, they developed a way of improving the coal by coking it.

And by coking it means they basically cooked the coal to drive off these sulphur impurities, and in doing so, they create a sulphur-deficient, or pure, form of charcoal. And that charcoal then actually could be used, this low sulphur coke, could be used to create cast iron cheaply. And so by the 1700s, they start to create lots of low cost cast iron, and that's really basically when you see the start of the industrial revolution.

And so over the next 200 years from the early 1700s, you see things like the income increases by 10x, the population increased 6x, you became a machine based economy, everything that went with the industrial revolution. Much of that was driven by the fact that you now had cheap forms of carbon.

By the late 1700s, you actually invent a process called puddling. So I mentioned earlier that you had to take cast iron and get the carbon out. The way to do that originally was is they literally drove all the carbon out, and then they add more carbon back again.

Puddling was sort of an innovation, where they would actually take the molten pig iron, or cast iron, and then they would put it in an oxidizing atmosphere, and they had these specialists called puddlers, and they would watch it until it came to nature, is what they called it.

And it meant that they could actually observe when that material suddenly lost just the right amount of carbon, and then they would gather it up in a puddling ball, and they would make steel that way. So the steel was made literally ball by ball, if you will, pulling it out of the furnace.

A very laborious process, but a very specialized process for the puddlers. So the puddling process was replaced by the Bessemer furnace. It was invented by Henry Bessemer in 1855, and his idea was very simple. We now had a much hotter furnace so I could melt the cast iron blow air up through the melt.

And that would rapidly remove all the carbon and the silicon from the melt, and create wrought iron. Now initially he wanted to create steel with it. So he wanted to remove just the right amount of carbon that could change it from being cast iron to being steel. And it turns out that was very difficult, and it didn't work very well.

And so he initially sold this to everybody as a great way to make cheap steel, and it turned out that the steel reproducability was all over the place and people did not like that. So eventually he came back and they worked on it for a while and they figured out it would probably be better just to remove all of the carbon and silicon, and then go back and add the carbon again, and because you got too much hotter melts, you can actually melt the wrought iron now.

And you could add the carbon into the wrought iron, and that way you could actually form steel perfectly. The great thing about this was it not only made steel very reproducible, but it also was very fast. You could do a batch in 20 minutes. And so it was extremely efficient.

And Andrew Carnegie thought this was a marvelous process, brought it over, started creating steel rails for the Pennsylvania railroad. And you'll learn more about how that bloomed into the entire process of fabricating steel for US Steel, and made him, effectively, the richest man in the world. Today we actually still use something similar to that.

We call that the basic oxygen process, where we're now, instead of injecting air up through the melt, we actually inject pure oxygen, and that gives us, you don't bring in the impurities that are associated with air, the nitrogen, etc into the melt. And so that's how you make iron today, and it was the birth of steel that resulted in a tremendous impact on society, and as ubiquitous in everything we see today from buildings, to ships, to automobiles, etc.