 Thank you so much for coming out. What a fantastic start to the day. So right, my name is Jen, a hurricane rotor, I'm a subscale flight test engineer for Archer Aviation in Mountain View, California, where we build experimental planes and fly them for fun and profit. But I'm probably better known for the robots that I've built for the TV series Battlebots on Discovery Channel. And the most current of which is hijinks for which I am the team captain. And to support this, we also have team social media, and we have a little YouTube channel where I will go and talk about things like mechanics or material science, and their applications and combat robots. So in addition to telling you some really interesting facts about steel, at the end of this talk I'm also going to tell you which steel alloys we are currently using on the big robot and why. And right. Combat robots, obviously only one of the many, many applications for steel. The ubiquity of steel is really owed to our ability to alloy iron with other elements. So combining unique desirable properties to make new material. So first things first, before we talk about steel, we need to understand its primary constituent material and that is iron. If you have a PhD in material science, I'm very sorry, this may not be the talk for you unless you're here for the jokes. I do have jokes. But if you were very new to material science and new to material selection, then I've jammed a lot of information here that I hope is going to be useful for you. First all, we're going to have to talk about crystals. I'm literally going to show you my rock collection. This is happening. I'm being a little bit funny, but the truth is that everything that we desire about steel is predicated on its crystalline structure, the thing that it naturally makes out in the wild. And without that, it's just space lumps. So I moved one slide ahead of where I was supposed to. So right, first slide here is the calcite and selenite. These are just extremely common minerals that I just happen to have in my little collection. Calcite is a carbonate material. Selenite is also called gypsum. It is formed from calcium sulfate, dihydrate. It makes a wide variety of crystal shapes, as you can see here. Quartz is literally everywhere. It is silicon dioxide. It is the most common mineral on earth, often used in jewelry making because there's a huge variety of colors available as other elements have been mixed in during the crystallization process. So glass is also silicon dioxide, but when it's melted and reformed, it does not have a natural crystal structure, so it's not considered a mineral. In that state, we call it an amorphous solid. This little guy is carborundum, and it looks pretty cool. It has a lot of color in it, much like a bismuth. My sample will cut you if you hold it wrong. It is a very hard mineral, but it's also rather brittle. So pieces that would frequently break off. Diamond. It's kind of on the Mohs scale of hardness. I don't own any diamonds. I'm just not that fancy, but I do like them. They're very shiny. They're also a lot more common than the beers would have you believe. Anthracite. This one is the outlier because this is not a mineral. It's not a crystal. This is a coal. Although it follows the mineral naming convention and has ITE at the end, it's not a mineral at all. It is just a rock. It looks like a lustrous metal, but it's really compressed bio-matter, compressed under very high pressure over long periods of time. Iron however, iron is a lustrous ductile malleable silver gray metals in group eight on the periodic table and is known to exist in three or more allotropic forms. Allotropy, allotropism is the property that some chemical elements have that allows them to exist in two or more different forms in the same physical state known as allotropes of that element. Iron may have more than four, but the conditions that would be required for them to form are probably not going to happen on Earth, so we won't see them very often. That's right. Iron in space. The kind of crystal that iron will form depends on the conditions at the time of formation, which means that with heat and pressure, we can push iron through different phases. The first iron allotrope that we are interested in is called body-centered cubic. She's giving body. Temperatures below 912 degrees Celsius, that is 1674 Fahrenheit, F is for freedom. This is what you will commonly see in the wild. The body-centered cubic crystal structure of iron is also known as alpha iron, but we're going to call it ferrite for the remainder of this talk. On this diagram, each of the black dots represents an atom of iron, and they are held together in this shape by the bonds that are formed through the interaction of valence electrons. If you are a young person, please don't skip class. One critical aspect of the crystal structure is the slip plane, the direction in which the crystal will most easily deform. The rate of cooling from molten liquid to a solid will determine the crystal size. The faster a liquid cools or crystallizes, the smaller the grain size will be. The longer it has to form, the longer the cooling time, the larger the grain size will be. And in terms of hardness, material dependent, if you have a bunch of tiny grains that are all stuck together, rapidly forming with opposing crystal faces, that will be a stronger matrix because when you put force on it, the grains inside will push against each other in different directions and resist deformation. So small grains clustered together makes for a harder material than large crystal structures. Aha! There's one piece we have of visible iron lattice. So in this, the iron lattice can actually be seen with your naked eye. This is an iron meteorite that cooled very slowly over a long time out in space. It's also a lovely piece of jewelry. The process of production on Earth is going to happen a lot faster though. Okay. Right, the next alatrop we want to talk about at higher temperatures, iron also forms a crystal that is face centered cubic. She's giving face. This is sometimes called gamma iron, but for our talk we're going to call it austenite and this will be more important in a couple of minutes. On the Mohs scale of hardness, which we'll use for minerals, not for alloys generally, iron only comes in at about a four, four and a half, meanwhile a diamond is a ten, just like me. Meaning, if I have a diamond in one hand and a piece of iron in the other and I rub them together, that diamond will definitely cut the iron. The diamond is going to win the fight, at least in terms of a scratch test. In fact, I have a small collection of diamond discs in my shop that I use to cut and grind hardened steel. So we got that going for us. The diamond's crystal shape is called diamond cubic, how original. And as you can see, it's a much tighter arrangement of atoms. The reason why carbon is able to form so many molecular shapes is because each carbon atom has four valence electrons, so they can easily bond with other carbon atoms to form long chains or rings. Carbon atom can bond with another carbon atom two or three times to make double or triple covalent bonds between atoms. So it's a very friendly element, it has just free hugs for everybody. And because iron is soft, we can work it. We can heat it up, smash it into shapes, and it will generally hold those shapes when cooled. What it won't do in its pure form is hold a strong edge. And as you can imagine, it's going to get scratched or deformed when it comes in contact with a harder material. Diamonds, meanwhile, cannot be smashed into useful shapes. If you smack a diamond with a hammer, that diamond's going to shatter. So it's very hard, but it's very brittle. So it won't tolerate shocks or impacts. Many bonds make carbon strong, and carbon all on its own has some fantastic industrial applications, but it could never do for us what iron does. Okay, so if we have iron, which has the useful properties of thermal stability and malleability, but it's not very hard, and we have carbon, which has the useful property of hardness, but is brittle in its pure form, and I want both of those properties together. I will put my hands together and make an alloy. I think I'm only hoping I'm on the right slide now, so I can jump forward. Okay, no, this is good. I can pick up from here. We'll improvise. Iron atoms and iron molecules are larger than carbon, obviously. An atom of carbon has six protons, iron has 26. So when we heat up iron, we are also causing some expansion, and the carbon atoms can slip in between places where the iron is occupying in the matrix. So what we can do when we heat up iron, we can dissolve carbon into it. So when heating iron above 912 degrees Celsius, its crystalline structure changes from a body centered cubic to a face centered cubic, so now we have austenite again. Austenite can dissolve considerably more carbon than ferrite, as much as like two or so percent by mass at 1146C. On an industrial level, this is called an interstitial site when we had the carbon shoved in with the iron. Right, on an industrial level, if we're talking about scaled up steel production in order to get this kind of heat concentrated in one area, we need a blast furnace, we need iron ore, we need limestone, and we need coke. Not that coke. Not the other coke either. We're talking about the fuel, yes, the correct slide. Coke is a gray hard, porous fuel with a high carbon content. It's made by heating coal or oil in the absence of air for long periods of time, so if you thought that lump of anthracite I showed you earlier was a throwaway, you were incorrect, it is all connected. So anyway, coke, along with iron ore and limestone, we layered this into a blast furnace, add a lot of heat. The coke reacts with the blast air to produce carbon monoxide, which then reacts with the iron oxide, and we wind up with carbon dioxide and metallic iron. Blacksmiths a long time ago figured out how to do this manually on a small scale with hammers, which is very impressive. So by heating the iron ore, sprinkling in coal or charcoal dust, and then literally hammering it, heating it and folding it over and over again, that changes the physical structure of the material, it also just looks really cool. If you're interested, I think it's over there. You can do blacksmithing today if you're into that. It's so hot right now. But anyway, since we do not have time to talk about all of the gorgeous phases of iron, what I really want you to hold in your head is that as we add heat and dissolve the carbon into iron, we're going to get different kinds of steel that have different hardnesses and ductility. So we can also add carbon just to the outside of an already made piece of steel through a process called carburizing, and that gives us a condition called case hardening. So we have just the outer layer has a new flavor of hardness, but the inside stays soft and ductile. And once we have our high and low carbon steels, we're going to want to add other elements to get even more desirable properties. Those elements are going to be things like chromium, nickel, manganese. So in the broadest sense, we say there are four kinds of steel. The data I'm going to reference here comes from ASTM International, formerly known as the American Society for Testing and Materials America. In order for you, the gifted artist or engineer to make intelligent choices about which steel to use for your application, we have the central authority to grade and categorize. And we've kind of generally decided that the four kinds of steel should be carbon, alloy, stainless, and tool. And when we get the blend just right, we've got all the atoms in there vibing, then we need to harden it. Hardening gives us special properties. Remember when I said earlier that the crystal grain size depends on how fast you cool off? This is exactly what we're talking about. In steel hardening, we're still using heat to push the iron crystals through phases. And with the addition of varying amounts of carbon, we're going to have new crystals to play with. But it's not just the heat that matters, it's how fast you cool it. So there are a few processes in place here when it comes to hardening, namely heating, soaking, quenching, tempering, normalizing. Heh, very strange for us. And annealing. But first we have our carbon steel. We gradually heat that steel and let the heat soak all the way through, just like soaking a sponge. Not quite like soaking a sponge, but you get the idea. We want a uniform heat all the way through. Otherwise we might make weird inclusions when we go to quench it. An inclusion in mineralogy is literally just anything, any bit of a crystal that's formed inside of the crystal that's not part of the crystal. So if you have something that's trapped inside of your main crystal, in this case, a rutile that has formed inside of a ruby is an inclusion. In this case, it has made us a star ruby, which is quite nice. To be clear, inclusions are not always bad. Sometimes we can get very cool results, but we want to be able to control those results. So we want everything to be heated up uniformly, gradually over time. So now we have our hot, evenly heated steel, and we've heated it gradually. It has reached the alloys' critical temperature, and then we just drop it into a vat of liquid to rapidly cool it. That liquid is usually water or oil, although other delicious libations may also be used, such as brine or sodium hydroxide, delicious. Immediately after quenching the steel will be at that alloys' maximum hardness. It is also at the most brittle, which also kind of puts it at the moment of most danger, and I'm not actually joking this time. When we rapidly quench things, we are locking stress in between the crystals. And this stress gives the steel a little extra hardness, just like if you put an engineer under a little too much stress, you're going to get a little too much hardness. And what can happen is that steel will fail catastrophically, and this is a large release of energy. It can be like a small explosion. Also, like engineers, if you give us too much stress, we'll have small explosions. So the best thing to do is self-care, by which I do mean normalizing and annealing. But we want to get to the point where the steel is as strong as possible, but without being brittle or prone to failure. There are so many metaphors. So first we're going to talk about tempering. Tempering is how we get the useful balance of the hardness and the toughness. So the steel is gradually heated until the desired temper colors are drawn. It's a good visual indicator that you have reached the condition that you want. Generally, the temperature is going to be significantly lower than the always critical temperature, so you have some time to kind of ramp up and get there. And yes, the different colors reflect all of the balances, so get a good clue. And then the steel, after it's tempered, is re-quenched to fix or to set your new hardness. Normalization may be called for, normalizing involves heating the steel followed by a slow cooling to room temperature. So instead of dropping it into a vat of liquid and having a rapid cool, you're letting the source sit out and cool down slowly over time. If you have a very hard steel that you might need to work on, you may choose to anneal it. To anneal a material is to heat it above the recrystallization temperature, maintain that temperature, let it soak through, and then you are going to let it cool in still air. So for example, if you have a block of very hard steel and you want to cut just a little piece off of it and then use that to make something and then do a re-hardening, annealing will be part of that process. So you can make it less hard, work with it, and then make it hard again. OK, now back to crystals. So you know Faride and Austinite, may I introduce you to cementite. Cementite is an iron carbide formula, Fe3C, and it has an ortho-hombic crystal structure. If you learn all of these terms, you will be very cool, but only for a small group of people. Ortho-hombic lattices result from stretching the cubic lattice along two of its orthogonal pairs by different factors. So you wind up with a rectangular prism that has a rectangular base and height. Cementite contains 6.6-ish percent carbon by weight, which is pretty high. Cementite mixes with ferrite to form laminar structures called pearlite and bainite. Cementite has limited use all on its own. It's quite hard, but quite brittle. I've just been given the five-minute warning, so I think although we cut in late, what I might do is skip a couple of slides. How far ahead can I go? Pearlite, I'll do a video of this later. It'll be great. You know what? Let's talk about the steel on the robot. Otherwise we might not be able to. So abrasion-resistant steel. Right, this is a high-carbon manganese steel. The sauce is obviously not just in the alloy, but in the hardening or else I wouldn't be talking about hardening so much. Abrasion-resistant steel is pretty self-explanatory. It's widely used in industry for high-wear applications, like the teeth of an excavator or brought crushing construction equipment. It is also used by various militaries to armor vehicles, and I use it to armor hijinks. You have probably noticed from prior photos that hijinks has a monocoque frame. So for the most part, the frame and the armor are one and the same, and all of the frame supports the drive set up and the electronics components. So there is a danger in using AR steel for this application. AR is not meant to be a structural steel. There is some challenge that comes into play when you start welding it as well. I mentioned earlier, when you heat steel up, you are changing it. So as you are welding AR steel, you are changing the crystalline structure everywhere you weld it. So you're undoing some of the heat treating process that makes it good. And another danger is in weld contamination that can cause cracking, but I've seen a lot of folks do sort of field repairs on AR steel, big dirty stick welders out in the field. It seems like it's probably fine. I can tell you from experience that it works in combat, and it works a lot better than say like a 36 high carbon structural steel, which I use on the frame of battle royale cheese, which you see here getting destroyed by Hypershock. Back in 2018, funny story, not funny. I requested AR 400 for that robot, but it was not available. Blame the markets. So as far as I can tell to the best of my knowledge, when I say AR steel, you would be talking about Hard-Ox. They are one and the same. If you have Hard-Ox 400 or AR 400, they will have the same hardness. Hard-Ox is a brand name, like a Sharpie or Band-Aid. But yeah, same, same, but different. Meanwhile, our weapon bars are AR 500. I made a video about when we use AR 500 instead of tool steel, which is more common, but I'll give you the cliff notes on that. Tool steel shatters and I've had it shatter on me, so I don't use it anymore. It's not just me, either. Tombstone, Bloodsport, Icewave, all of these teams have used S7 for their weapon bars and had it shatter. I am using AR 500 and putting tool steel just on the teeth. But if we're gonna talk about hard steels, and I know I want to, we also need to talk about martensite, just very briefly. We have to. Martensite is kind of a special crystal. They're all special, but martensite is formed in carbon steels by rapid cooling of austenite. At a rate so high that carbon atoms cannot diffuse into the crystal structure in large enough quantities to make cementite. So the carbon becomes trapped in a distorted crystal matrix. This is done with fast cooling. What we have here is a body-centered tetragonal crystal. Stainless steel, the ones you've probably seen most often are ferritic or austenitic. So grades like 304, 316, none of these would get used in combat robots, but they do get used in medical and marine. The exact composition's gonna vary, but you're gonna have at least 15 to 17% chromium if you're gonna have a stainless. And I need to tell you about this because, right, the weapon stack. I'm gonna just squeeze it all in. I like to use 174PH stainless steel for weapon shafts on combat robots. 174PH is a precipitation hardened, martensitic stainless steel. The high chromium content gives it extra hardness, makes it great for machining. You get a buttery smooth surface finish, which is important in an application like a big old weapon shaft because we are inviting many challenges by choosing a live shaft for the weapon core. The entire shaft needs to be essentially a bearing surface as frictionless as possible. It has to move very, very freely. Oh, at the top of the weapon stack, there's a ball bearing. It's main job is to keep the assembly aligned down at the bottom are large thin bushings. They look like this. Very smooth. And the whole point is so that when you just walk up to the bar and touch it, it'll just spin freely without a lot of force involved. See if I can move on to the next slide. Oh, right. Meanwhile, sorry, I'm feeling anxiety coming in from the time running out. On the weapon shafts, we use 4140 steel. That's chromium, molybdenum, manganese, low alloy steel, known for its toughness, high fatigue strength. About half the cost of 17.4 pH works just as well. We use case hardened steel instead of through hardened. I like case hardened because it will tend to fail by bending instead of shearing. And I think that's better for a drive shaft. And lastly, I was just gonna show you the 1095 carbon steel top plate, which we lovingly refer to as the fan service armor. If you've ever unironically suggested using sword steel on a robot, this is sword steel, this is for you. A 1095 is mostly iron, a little bit of carbon, sulfur, phosphorus, sprinkle of manganese. It's widely popular in knife making. It has the property of work hardening so it becomes harder as you hammer it or as you drill or machine it, which makes it a pain in the butt sometimes. I've used this top armor to fend off the 500 pound hammer robot called Chomp. Mind you, this was installed as a sandwich, so there was AR underneath of it. Can you armor your whole robot in sword steel? That is a choice you can make, but I'm not going to. I'm pretty happy with the choices that I've made. Sorry this talk got cut short by technical difficulties. We'll do something for you later to make up for it, but in the meantime, thank you for coming out and talk to you soon.