§1. Saugus Ironworks and the English energy crisis [00:02]
Cranes and hooks and shackles, or five or six muscular people, to move that sow bar around. The little pig iron things — I can take those and remelt them and make cast-iron cooking utensils, or I can take them and forge them into nails or horseshoes or whatever. They still call it pig iron. They don't talk about the sow bars anymore, but that's where the term pig iron came from. After they let it solidify, five or six hours after they cast it, they would take the metal out of the sand mold, take it over to the Ford [Forge] Shop, then just rake the sand back and dig more holes for the next casting.
After you get all the metal out, the next stuff that comes out is the slag. They would block it off, and they'd have another trough over here, because the slag is a molten glass, less dense. They would just form a bunch of slag, and the slag would go to the dump. So there's all kinds of pollution up there by Saugus Ironworks, for ten or fifteen years. If you take another one of my classes I talk about why they came to Saugus to make iron in the 1630s. England was having the energy crisis — they were deforesting, they had cut down all the trees. So what did we have over here? The natural resource: you have limestone and sand all over the world, you have iron ore all over the world. What they had in America in 1620 was trees to make charcoal. It all sort of fits together, it all sort of makes sense historically. This has nothing to do with welding though. Any other questions? It's okay, I like to tell the story.
§2. Hardenable alloys: aluminum, the Wright brothers, and beryllium copper [02:05]
Okay so I've got this iron. That actually started out as why is cast iron over here and why is steel over here. It's historical. But also cast iron melts at a lower temperature because of the eutectic. What you really like to have historically is something you can harden. I've got aluminum — I didn't have a piece of hardenable aluminum in my office, this is a piece of 5454 aluminum plate, it's non-hardenable. But many aluminum alloys — the aircraft alloys, the aluminum-copper that the Wright brothers used for their engine — was an aluminum-copper alloy. You heat it up, it has a phase diagram that looks something like this. You heat it up into a region over here, put the copper into solution, quench it, then after it's cooled to room temperature you heat it back up to an intermediate temperature and you form these copper precipitates.
Twenty years ago someone published a paper in Science. They had gone to the Smithsonian, gotten the little sample from the Wright brothers' aluminum engine, put it in the transmission electron microscope, and measured the size of the precipitates which had been aging for eighty or ninety years at the time. They showed the size was consistent with an aging time of ninety years — the precipitates had been coarsening over ninety years. They compared that with a three-hour aging, a four-week aging, a one-year aging, plotted them on the same log plot and showed it was consistent with diffusion of copper in aluminum. So aluminum can be hardened. I could have brought my baseball bat — the highest strength aluminum alloys are actually baseball bats, they're not aircraft alloys. They're precipitation hardened.
So you can harden aluminum by quenching and tempering. You can harden steel by quenching and tempering. There are some stainless steels you can harden, we call them precipitation hardenable — what a clever name we have. Copper alloys — this is my beryllium copper crescent wrench, cost about $130, you can buy one yourself. Anyone remember from last term why we use beryllium copper tools? You can buy a whole toolset, all kinds of tools, for a thousand dollars instead of $400 out of steel.
Student: Sparks.
They use them in spark-free applications like mines where you might have methane gas and you don't want a spark that goes kaboom. They're spark-free. Why are they spark-free? The thermal conductivity of copper alloys is much better. I can take two pieces of steel and bang them together and I can make sparks. That's how Boy Scouts or Girl Scouts start fires, right. With the right type of thing, I can generate enough frictional heat to get a thousand degrees and start a spark — that's one way to start a fire. You can't do it with beryllium copper. You can try all day, but the thermal conductivity is too high — you bang two pieces of beryllium copper together, you'll never get a spark.
So that's the precipitation hardenable alloy. That's got well over a gigapascal strength. 150, 180 ksi, something in that range. So it's just as hard as a hard normal steel, should have good wear resistance just like a steel crescent wrench, but instead of a ten-dollar crescent wrench it's a hundred-dollar crescent wrench. So that's a little bit of basic metallurgy. But we do need to understand some basic metallurgy.
§3. Hardness vs. hardenability [06:11]
Another piece of metallurgy we need to understand is the difference between the words hardness, hardenability, and hardening. Hardening is not that important. Here's something out of a materials engineering dictionary — I didn't intend Jerry to make twenty copies of this, but she did.
Let's do hardness first. Hardness is a measure of the resistance of a material to deformation — to indentation or abrasion. So play soft, not very hard: I can indent it with my thumb, I can even puncture it with my thumb. That's hardness. And it turns out, I'm going to show you in a little bit, that hardness is a function of the carbon content in steels.
Hardenability, on the other hand, is the depth to which I can harden something. Hardenability is a function of the alloy content. We have carbon steels, and it turns out I can only harden carbon steels to, let's say, less than half an inch deep. If I add a bunch of alloy to this, I can harden ten inches deep. All I'm doing with the alloy — if I add molybdenum to the steel, molybdenum likes to form carbides; chromium likes to form carbides; nickel doesn't, but nickel slows down the reaction other ways. The transformation between the FCC that I'm going to try to quench in, and then I might precipitate by tempering it, is slowed down by all these alloy elements. Some of them tie up the carbon so the carbon can't move around; some of them slow down the diffusion of the iron so the iron can't move around. So hardenability is a function of the alloy content. Hardness is a function of the carbon content.
If I put all this together: hardness is a function of carbon; hardenability is a function of the alloy content, and it's the depth of hardening. If I'm making automobiles out of sheet metal, I can use carbon steels because the sheet metal's no more than an inch thick. If I'm making some big pressure vessel, I'm going to need to use some alloy content. If I'm going to have a hard naval steel — if I'm building a nuclear submarine, here's a nuclear submarine steel, HY-80, inch thick, it's got to have a low carbon content, it's got about 3% alloy content in it. It's got nickel, chrome, molybdenum in this steel for hardenability. Because it turns out if I take that iron-carbon phase diagram and I quench it — I take it up here to the austenite and I quench it down here to the ferrite very quickly — I won't form a body-centered cubic crystal structure, I'll form a body-centered tetragonal crystal structure, which is known as martensite. Martensite is extremely hard. It's an athermal transformation, which means it occurs at near the speed of sound.
Student: [asks about the soft center after quench]
It'll be soft in the center after you quench and temper. The high hardness, the martensite that you're going to form, in this low carbon steel may only be an eighth of an inch deep — it's going to be less than half an inch. You can't get deep hardening in carbon steels, the reaction is too fast.
§4. Time-temperature transformation diagrams and the economics of alloying [11:34]
I can show you how fast the reaction is. I just happen to have this handy-dandy book — I had it at home, I didn't have a copy in my office because I never use it. This book is full of time-temperature transformation diagrams for irons and steels. You have to be a metallurgist to love these things. Here's a diagram showing steel I'm going to heat up to 1300 degrees — about 900 degrees C, which is 1350 Fahrenheit or so — and quench. This is called a time-temperature transformation diagram. This is time, this is temperature, and this is the C-curve behavior for transformation of the steel. If I quench it fast enough — well, how fast is fast enough? This is time in seconds. I have to do it in less than a second or two, in order to avoid it just transforming to ferrite. If I want to get martensite, to get really hard steel, I have to do it really fast. Unless I add alloying elements.
If I add molybdenum: this is a molybdenum-free steel. You can't see the plot down here — it's probably on the very bottom — but here it's one second, ten seconds, 100 seconds. This is just a series of steels: zero molybdenum, 0.15 molybdenum, 0.3, 0.38, and a half percent molybdenum. You can see how molybdenum is just changing the nose of that C-curve over from a second to thirty seconds, twenty seconds. I can slow the reaction down by a factor of twenty by adding half a percent moly to the steel. That does cost me a little bit. Presently, it's going to cost me about $100. It used to be $400 a ton. Steel is now $100-a-ton steel. So the question was, why are there so many steels? Well, if you're buying a hundred thousand tons, you want just the right amount of alloy. You don't want to pay for half a percent moly if you can get by with 0.4 moly for the thickness you're interested in. That's why there are so many steels. We use one and a half billion tons a year in the world, and at that quantity and these alloy prices, it's cost-effective.
We really will take twelve units on welding metallurgy if I don't even get to talk about welding from the first lecture. Excuse me, I need a drink. See, there's fewer bubbles, right? It's been an hour, there's still bubbles in there. If I shake it up I can get some — still CO2 in here, right. I had to get some pressure waves to help nucleate some. There's still more CO2 to come out, but it's evolving a lot slower than when I first poured it in. Believe it or not, this is the first time — I've told people about this before, but this is the first time I've ever bothered to go out and buy a carafe and waste three glasses of Sprite — not that Sprite's not a waste, this stuff shouldn't be on the market — to use three bottles of Sprite to show. It's not 10:00 yet, it's not even 10:05. So cool it.