§1. Project framing: choosing a material for an application [00:02]
The purpose of the project is for you to present something you're interested in. Fiberglass is fine, but it's a little broad to just say "fiberglass" everywhere because it's been around for so long. You might pick an application and say, okay, people use fiberglass in small craft, in boat hulls — why don't they make them out of wood? Because they do make some out of wood. Why don't they make them out of another type of plastic? Hopefully you've learned some things about properties of plastics, why you want to use a composite in that application — but composites are more expensive than steel or aluminum. People make boats out of aluminum. Why would you make one out of steel? People make boats out of steel. There are lots of materials, and you could say, here's an application, someone has chosen fiberglass, why in that application?
Take small boats, or take tennis rackets — focus it a little bit more. "Fiberglass is a material, glass fiber in a plastic resin" — okay, I want a little more depth than that, a little more thought into why someone would choose fiberglass in this application.
On Thursday we've got a guest lecturer, Adam Powell, who's going to talk about Infinium, a new process that started out here at MIT. The professor went over to Boston University, and now they've started this company that's going through about ten or twenty million dollars of funding for a new environmentally clean way to make all kinds of metals — almost everything on the periodic table. On Friday, if I have time, I've been thinking about doing something like comparing soda bottles, or beverage containers. What are the materials of choice in beverage containers? There's aluminum. Plastic. Paper — actually most paper is a composite, even if it's just the plastic-coated paper of milk cartons. If you go to those drink boxes, that's a seven-layer composite, pretty complex. Glass. And one other: steel.
In Japan — anybody ever lived in Japan for a while, visited Japan? If you go to the vending machine and you get a soda, is that an aluminum can? Nowadays it depends. When I lived there thirty years ago it was always steel, and it was a political thing. Japan doesn't have an indigenous aluminum industry, but their government wanted to protect their steel industry and give them a bigger market. So the Japanese used steel — not because it was cheaper, not because it was better, but because the government was trying to give a market to the steel companies. Not that the steel companies really needed it in 1985, but they still had it. I haven't been back to Japan for ten years now, but I understand they are using more aluminum cans now, because they don't need to protect their steel industry anymore. It gets down to economics.
What do they make cars out of in Europe — steel or aluminum? In general, steel. But at the price of fuel in Europe, aluminum should be cheaper. So why do they make steel cars? Because everybody wants to sell into the United States, and at the price of fuel in the United States, steel is cheaper. If the Europeans were only selling in their own market, all their automobiles would be aluminum. But then they couldn't sell in the United States — they'd be priced out of the market.
One day this week I might ask you to start writing down what your topics are, so think about it. You've only got about ten minutes for the presentation — that means ten overheads max. Practice your presentation in front of a mirror to make sure you can do it in ten minutes. If you can do it in ten minutes with no audience, you'll probably do it in twelve minutes with an audience. Making eye contact, or if someone raises a question in the middle of it.
As a young assistant professor giving my first few talks at conferences, I would go over that talk fifteen times and literally stand in the hotel room in front of the mirror and give the talk over and over. Now I don't have to do that anymore — but maybe I should. If you haven't had a lot of experience giving presentations, you need to practice. You don't just walk in and do it.
§2. Carbon and alloy steels: hardness, hardenability, hardening ability [06:27]
We started talking about steels last time. They're made everywhere; it's one of the most technologically advanced materials, if not the most. We know more about steel than anything else, just because of the size of the market. There are many types of steels, and I gave a handout last time — if you didn't get it, see Jerry.
One of you asked, why are there so many steels? I pointed out last time that the carbon content of the steel determines its strength to a first approximation. A 1050 would be a half-percent carbon steel; here's a 1010, for a carburized part, that's a tenth of a percent. The last two digits are hundredths of a percent of carbon, and typically most carbon steels are less than one percent carbon. So here's a stabilizer bar which is a 1090 steel. Lawnmower blade of 1060. A spring at 1080. High strength, high carbon; low strength, low carbon.
Then we also have alloy steel parts that have typically different amounts of chrome, nickel, molybdenum, and vanadium, and there's a whole wide range of these. There's a difference between hardness, hardenability, and hardening ability. Hardness is just the resistance to indentation. It's a direct measure of the strength. The hardness measurement, if you measure it in kilograms per square millimeter or KSI, is about three times the tensile strength of the material, whether it's steel or copper or aluminum, for a ductile material.
Then there's hardenability, which is how deep you can harden it. I was asked once by a professor — would be now mechanical engineering, but at the time he was naval architecture — working on the America's Cup. He had a four-inch-thick keel beam. The keel beam is not just on a big ship; the keel is the thing you build everything up from. There's a keel beam on a 747, a big aluminum beam they build everything up from. But on a sailboat — that particular America's Cup — they had a big steel piece that created the resistance to being blown sideways. This was supposed to be four inches thick.
HY-130 was a very weldable steel — more weldable than most steels, still got problems, but the Navy had developed it in the 1950s and 60s for submarine hulls, and it needed to be weldable. It's got about two-tenths-percent carbon. Well, this was four inches thick. It had to have enough hardenability to transform to martensite four inches deep, so you get uniform strength through the thickness. With carbon steel, you might be able to get a good eighth of an inch or quarter of an inch thick before you lose your ability to quench it fast enough. But when you start putting these alloying elements in, it slows down the transformation, and you can heat something and quench it such that you get hardness four inches, eight inches, even deeper than that in some steels. For this keel beam, supposed to be four inches thick, they were actually bending it in some of the trials — it was plastically deforming. Better than snapping, but still. They also wanted weight down in the bottom of the ship — they want light weight, but if you're going to have weight you want it in the bottom. They used to put lead in these ships in the bottom — you're paying a huge penalty, but they needed the weight toward the bottom.
Hardenability is the depth and distribution of hardness that can be induced by quenching. Hardening ability is what the maximum hardness can be, and at about 0.6 carbon you get to the maximum hardness in most steels. HY-130 only had about two-tenths-percent carbon, and it only had 130 KSI yield. If you went to 4340, which is readily available in big heavy sections — we use it to make all kinds of high-strength components in thicknesses up to ten inches thick — it's got 0.4 carbon. The 40 is four-tenths-percent carbon, and you can get much higher hardness, much higher strength: 180 KSI rather than 130. So they wanted to use 4340 but didn't know how to weld it in that thickness. That's why he called me up one morning. I showed them how to weld 4340. It's not easy, not cheap, but who cares if you're building America's Cup — it's only one of a kind. You may only be sailing one ship, but they actually have multiple hulls on these things. Very interesting business.
§3. Learning to weld: oxyacetylene and arc [13:06]
Student: Can you weld really well?
Not particularly — I don't do it every day. I can weld. I was taught when I was an engineer at Bethlehem Steel by the technicians. I said I want to learn how to weld, and they took me out and I did a lousy job.
I always remember one time though. I was walking through my lab — not like it looks like now, but they used to have a bunch of boxy welding setups. Just like they have the blacksmith shop in the basement, they used to have a guy — it started when I was a student. My thesis advisor became the head of this little thing. It was to help a technician — the guy who had the welding lab before me left in 1968 when I came as a freshman, and he had a technician, Tony Zona, and Tony didn't want to go to Wisconsin with Professor Adams, so he stayed at MIT. He didn't really have anything to do, so he started teaching an art sculpture class, and they would do oxyacetylene welding.
So I was walking through my lab twenty years later, and I had four or five of my students all standing around one of these tables with the oxyacetylene torch. I said, what are you doing? They were trying to teach themselves to oxyacetylene weld. They looked terrible, like turds on a plate. I said, that's not how you do it — give me that. I took the torch and I put down an oxyacetylene bead, and it was a very nice looking weld bead. I told them how to do it and walked away. What they didn't know is that was the first time in my life I had ever oxyacetylene welded. But I did understand heat flow and welding, and I understood the thermal time constant for oxyacetylene welding as opposed to arc welding. I knew you had to go slow, and I actually knew how slow you had to go. Most people try to rush it.
I also do a thing about reaction times. If you go to Wentworth Institute of Technology, they will probably teach you welding by starting you with oxyacetylene. The reason is the thermal time constant is on the order of three to five seconds of that weld pool, so you've got to go really slow — three to five seconds in one spot. If you do that you get a good weld. For an arc weld, the thermal time constant is on the order of a tenth to three-tenths of a second. Ten times faster, because the heat intensity is ten times greater. This makes sense if you take the heat transfer part of my course.
It's because of reaction time. You don't start out on the highest speed of a video game — you have to build up your skill and your reflexes, and a welder has to do the same thing. There are many places where they give people one week of instruction in welding, start them with arc welds, they make terrible welds for several days, and then they send them out to make terrible welds in production. But if you go through a four-month class, they'll start you with oxyacetylene for several weeks and then move you to arc welding. It's hand-eye coordination — let's face it, if you're manually welding. So yes, I do know how to weld. Am I the best welder in the world? No. If I welded a pressure vessel, not only would it not pass inspection most likely, but I wouldn't want to stand next to it when it's pressurized. You really need to do it on a regular basis. The codes — depends on which code — require that if you have not welded in the past year, you must be requalified, because you haven't kept up your skills. There's skill, but I don't practice sufficiently.
All the rest of you could weld if you wanted to learn. Many students come to me and say, I'd like to learn to weld, so I send them off with one of the technicians, and usually after about one or two hours of making lousy welds they say okay. Very few students over the years have ever gone for more than a few hours' worth of practice.
§4. Why there are so many steels: alloying economics [17:44]
Getting back to steels: there's hardness, hardenability, and hardening ability. Hardness is merely how strong it is. Hardenability is how deep you can get a hardness. Hardening ability is what the maximum strength is that you can get under the best conditions.
We have many, many steels because they're used in such high volume that people will literally demand, I want a tenth of a percent nickel more, or a tenth of a percent nickel less. That doesn't sound like much, but a tenth of a percent nickel is two pounds in a ton. At $10 a pound, that's $20 a ton, so it makes a difference, particularly if you're buying 100,000 tons — you'd be talking $2 million. So there are many steels.
Does that answer the question? I think it was your question several weeks ago — I said I would cover why we have so many steels. I also talked about the Achilles heel of steel being corrosion, and we'll get to that. The easiest way is to jump into stainless steels as the next class of material, separate from the carbon and low alloy steels.
§5. Stainless steel genealogy and 304 [19:16]
The stainless steels — this comes out of a book by John Sedriks called Corrosion of Stainless Steels, and he has a genealogy of stainless steels. The first stainless steel was basically what we call 304 stainless steel, discovered around 1907 in Sweden. Back then they were just learning how to make steels of different compositions and adding different things. You may have heard of 18-8 stainless steel — basically, instead of 19 chrome 10 nickel, 18 chrome 8 nickel — it's all within the range of 304 stainless. It's either 40% or 60% of all stainless steel. I think it's 40% of all stainless steel and 60% of all the austenitics.
There are different types of stainless steel: austenitic, ferritic, martensitic, duplex, precipitation hardening. The austenitics have face-centered cubic crystal structure, they're non-magnetic, and 304 is the most common of all. You'd like to improve on that alloy that's been around for 100 years. One thing they do is add sulfur or selenium for machinability. We also add sulfur to plain old carbon steels for machinability. You end up with sulfur inclusions, and when the chips are coming off the lathe or milling machine, those inclusions — which tend to form stringers — cause the chips to break up. You don't get this long pigtail corkscrew that'll whip around and cut people's arms off.
§6. Manganese, sulfur, and the early steel industry [21:51]
Student: Why is manganese in steel?
It's not stainless steels per se — steels in general have to have manganese for sulfur control. Steel cannot be made without manganese. Although we do make some now that have extremely low manganese.
To make steel in the old days — actually even today — you take iron ore, coal, and limestone, put them in a blast furnace, you make cast iron. The problem is the coal may have two or three percent sulfur in it, and if it does, the sulfur ends up getting in the steel. Iron sulfide is a very low melting alloy, and if you take that steel without any manganese and try to roll it — when you heat it up to the hot working temperature of 1,800 to 2,000 degrees Fahrenheit and try to roll it, it will just shatter, because it has liquid grain boundaries of iron sulfide. That's what happened in the 1880s, and they didn't understand it was sulfur until they were using different ores in different parts of the world and some people had this problem and some didn't. They started analyzing it and they found manganese — the steels with three-tenths of a percent or more manganese would not shatter.
Then people started doing metallography — they had learned about metallography in the 1880s — to look at where the sulfur was, and they found you formed manganese sulfides. If you look at the periodic table, the only two elements that have higher melting sulfides than iron are manganese and molybdenum. When you add molybdenum to iron, you get a low melting sulfide. When you add manganese, you get a high melting sulfide that melts above the melting temperature of the iron. The only element in the periodic table that does that. There is no other element that will tie up the sulfur impurity. So you have to either get rid of your sulfur impurity, or you have to put manganese in the steel.
If you didn't have manganese, the steel would not be hot-workable. It's like if you were trying to hot-work a Slurpee — a liquid on top of solid ice particles. The steel would have that kind of consistency; when you compress it, it would actually crack apart. Not exactly like a Slurpee, but you basically have liquids along the grain boundaries.
A steel officially is an iron-carbon alloy, but it always has some manganese — usually about three-tenths of a percent up to two percent — and it has some silicon, quite often. The silicon is there to get some of the oxygen out; manganese will take some of the oxygen out too. I always talk too much about steel. Historically they would say the manganese had to be 8 to 10 times the sulfur content because you have to tie up the sulfur. Nowadays we've learned to get the sulfur out of the steel while we make it, using the right types of fluxes. One of the guys who taught the world how to do this using the principles of physical chemistry was John Chipman of the Chipman room. The world learned how to make steel from John Chipman. He and Nick Grant, who was a professor starting here in 1946 — the two of them published some of the first articles on control of sulfur in steel in the 1930s. People knew you put manganese in, but they didn't know how much, and they were wasting a lot of money on manganese. Virtually all the manganese in the world goes into alloying for steel.
Next thing they did was teach people how to get rid of the phosphorus, how to control it. A lot of the iron ores in Europe are high in phosphorus, and phosphorus can be good in steel in small quantities, but bad in large quantities. There's a lot about the alloying elements of steel. People teach whole courses at some universities on this — not much anymore, but there's still a few.
§7. 303, 304, and the Sub-Zero refrigerator hinge [27:20]
You start with 304, and you can add sulfur — in stainless steels you add selenium because you can afford it. You can't afford it in most carbon steels. Some carbon steels have selenium, but only if you can't have sulfur for some other reason. Selenium is beneath sulfur on the periodic table. So all you've done is go down the periodic table, and you get these inclusions in the steel that help break up the chips. When we machine brass, we used to put lead in because it would break up the chips. Free-machining brass is a leaded brass — it forms lead inclusions rather than sulfur inclusions. That's 303 stainless. What do we lose when we do that? It's not weldable anymore. You can't weld 303 stainless. Same composition as 304, just has high sulfur.
I meant to bring in a hinge from a Sub-Zero freezer. Anybody know what a Sub-Zero refrigerator is? It's an $11,000 home refrigerator. Very high-end. Obviously none of you have millionaires for parents. I have a son who just bought a house and it has a Sub-Zero refrigerator, and a Wolf stove — goes for only five or six thousand. Very high-end kitchen appliances. I have a Sub-Zero hinge because someone made a mistake. They had a little pin that was supposed to be made out of carbon steel and welded to a carbon steel plate to make the hinge, and someone used 303. When they machined these little pins, and when they went to weld them, they had cracks. They didn't see the cracks. They plated them, put them on the refrigerator, and people bought $11,000 refrigerators and the doors started falling off. They were not happy. And these are the type of people who have enough money to sue you. So if you're going to shaft somebody, do it to poor people — they can't afford attorneys. That actually cost about $7 million worth of repairs.
§8. Duplex, precipitation hardening, and high-manganese stainlesses [29:46]
Duplex stainless steels: they increase the chromium and lower the nickel to get higher strength. You can double or triple the strength by going to a duplex. A duplex stainless is merely a mixture of about 50/50, or 30/70, or 70/30, of austenite and ferrite — ferrite being a body-centered cubic magnetic phase. [Tom holds up a piece of duplex stainless steel.] This came off a centrifuge that was used to separate starch from other things. They started selling a lot of these centrifuges when they started making methanol from corn, because they had to separate the starch from all the other liquid. The centrifuge weighs about a ton, spins around like 3,000 RPM, about four feet in diameter. Made out of cast duplex stainless steel because they needed high strength.
If you're not careful with duplex stainless steels — you have higher chromium and lower nickel, and the ferrite can get hydrogen in it. But the real problem is you can get iron-chrome intermetallic phases that embrittle it. You want to see a brittle steel — here it is. [Tom indicates the fracture edge.] It did have a little bit of ductility. If you look right on this edge, you can see the curvature of the centrifuge, and you can actually see a slight change in the curvature, where the large grains of this casting started to stretch — what we call orange peel, because it looks like a surface of an orange. It can be extremely brittle if you don't do it properly, but you can get 100 KSI strength rather than 30 or 40 KSI for the 304. So you can triple the strength.
They use duplex now — it's become very popular in the last 15 or 20 years — for downhole sour gas applications where you need high strength in deep wells in the oil business. Before that it was very specialized; they used it on the catapults of Navy aircraft carriers. High strength corrosion resistant alloy. Better chloride stress corrosion resistance than 304, so if you had a chloride environment and you wanted high strength, you could go to duplex stainless. Hard to weld, but high strength and some interesting mechanical properties. You pay a price in some other things — can be toughness, can be a welding price.
Precipitation hardening stainlesses have been around for a long time. [Tom produces a pair of medical-grade scissors.] Here's a $300 pair of scissors — actually probably $600 now. Medical grade. They actually have a nickel-base hard alloy insert brazed in them. Because they're medical, instead of being a $40 pair of scissors, 25 years ago when I got them they were $300, I bet they're $600 now. Precipitation hardened stainless. You wouldn't do that for a regular pair of scissors — what do you care about it on the handle — but the doctors want to know that if they're paying $600 for a pair of scissors, they're getting the best quality. Why it's precipitation hardened — if you're going to put inserts in the cutting edge, I don't know why you need to precipitation harden the handle, but anyway. The Navy used to use these on hydrofoils, on the foils that go through the water. They make the base ship out of aluminum for lightweight, but the foils need corrosion resistance — aluminum would just erode away. The America's Cup boats now are basically hydrofoils. I have no idea what they use — they might be using a precipitation hardened stainless. You can get 180 KSI out of these steels. You have to go through a fancy heat treatment, harder than regular carbon or alloy steels, but you can get tremendous strength.
201 and 202 add manganese and nitrogen, and lower the nickel, to get higher strength — 80 to 100 KSI, and it's cheaper. Manganese instead of chrome. Manganese is a lot cheaper than chrome. I don't actually see these that often. If you go really high manganese — like 12 to 13% — you get what they call Hadfield's manganese steel, which is not officially a stainless steel, but it's an alloy that work-hardens so rapidly that you literally cannot cut it with a saw. It's easily formable in its annealed condition, but you go to saw it and as you're cutting, those chips harden — about triples its strength — and it will dull saw blades. One of the uses of Hadfield's manganese steel was prison bars. You got a file? Yeah, you want to dull your file. Try to file through Hadfield's manganese steel, because it transforms as you're work-hardening it to this hard martensitic steel.
§9. Martensitics, crystal structures, and hydrogen embrittlement [35:25]
The martensitic steel — you take the nickel out, lower the chrome down to around 10 or 12%, and you get the 403, 410, and 420 steels. Many of the scissors and medical instruments are actually made out of the martensitics. They're magnetic. Martensite is basically the same as ferrite, except you've trapped carbon in the lattice and it becomes body-centered tetragonal — but very barely body-centered tetragonal rather than body-centered cubic. Austenite is face-centered cubic; ferrite is body-centered cubic; martensite is body-centered tetragonal. Duplex is FCC plus BCC, that's why you call it duplex. And precipitation hardening is basically FCC with precipitates, which do the hardening.
In general strength levels: austenitic about 40 KSI. Ferritic might be 40 to 80, depending on how work-hardened. Martensitics could be heading up toward 160 — if you take piano wire, deform it into wire, you can get up to 300 KSI; body-centered tetragonal you can be up to 180 KSI. But you really like to be less than 120 for hydrogen embrittlement reasons. Duplex typically about 100 KSI, good ductility, good chloride resistance. And precipitation hardening 180 KSI with good ductility and reasonable hydrogen embrittlement resistance.
The only ones that hydrogen embrittle are the ones that are BCC. Frankly it's still a matter of opinion among metallurgists what causes hydrogen embrittlement. There are four or five theories — strain in the lattice — I suspect when we get really good at quantum mechanics on a larger crystal, rather than just a few atoms, we might be able to figure out what hydrogen does in terms of embrittling the body-centered cubic structure. You can embrittle the austenitic, FCC structure, but it takes 50 to 100 times as much hydrogen to do it, so we don't usually talk about hydrogen embrittlement unless it's ferritic. The austenitics and the precipitation-hardened grades are much more resistant.
§10. Molybdenum, 316, and the Fenway Frank hot dog cooker [38:44]
If you have corrosion resistance in aqueous systems, particularly with chlorides, you add molybdenum for pitting resistance. You might have 2% molybdenum in a 316 stainless steel. The 304 stainless steel cost you $2,000 a ton, with 18% chrome and 8% nickel, rather than $400 a ton for carbon steel. So 304 stainless is just as expensive as aluminum. For 316, you add 2% molybdenum. Molybdenum is going for $20 or $30 a pound. 40 pounds for 2% per ton — you're adding $1,200 a ton. Big jump in price to go to 316. You can also go to 317, which they used to use for a lot of medical applications — that's 4% molybdenum, so you're talking $24,000 a ton.
To show you the difference — this is the most dramatic. [Tom holds up a cracked, rusted stainless steel component.] People use 316 all the time because it's weldable, it's got stress corrosion cracking resistance, but you have to be careful — these are all cracked. You can see the cracks in this thing, it's all rusty. This came out of the Fenway Frank hot dog cooker over here in Everett. The hot dog cooker is about the size of this room — it's a big steam oven. They chop up all the types of meat you'd never want to eat, make a slurry out of it, extrude it, put it inside the casing, with a lot of salt — that's why you need chloride resistance. The original hot dog cooker was all made out of 316 stainless steel, had been around for 10 or 20 years no problems. They had to modify some things, it was all specified to be 316, and they put the new stuff in, and within several weeks the new stuff was getting rusty. It was cracking. They called me in, and I actually got to see how you cook hot dogs. Lots of interesting things in this work. They asked me to figure out why. We brought it back, turns out it's 304. Someone used 304 rather than 316. Maybe a harmless mistake, because 40 to 60% of all austenitic stainless steel is 304, and 316 is a lot less in tonnage. Just a mix-up. But that 2% molybdenum went from something that would look stainless for 15 or 20 years or more in this environment, to something that could only last a couple of weeks. That's the most dramatic example I've seen. Plenty of failures where people use 304 rather than 316, but nothing this dramatic.
§11. Active vs. passive stainless: the New England Aquarium and Diet Coke [42:05]
People will often bring me a piece of steel and say it's supposed to be stainless and it can't be, because it's rusty. Well, I'm sorry — it's stainless. There are two types of stainless: rusty and unrusted. More specifically, if you look at the galvanic series — which I didn't bring with me — the galvanic series is what dissimilar metals will attack one another in a corrosion series.
The galvanic series has magnesium as the most electroactive and carbon as the least. Everything else in between. Platinum is close to carbon, zinc is close to magnesium, copper is somewhere in the middle, iron somewhere in the middle. Stainless steel comes in two varieties: active and passive. What gives stainless steel its corrosion resistance is a protective oxide — for our purposes call it Cr₂O₃·nH₂O — a complex oxide on the surface that has very good aqueous and high temperature corrosion resistance, except in the presence of chlorides. Active is when you take off that protective oxide, and now you're exposing a steel without one. Passive: no rust. Active: rust. There are lots of ways to take it off.
Student: What about fluorides?
Fluorides are terrible — worse than chlorides — but there aren't a lot of fluorides we expose things to usually. Chlorides are everywhere, fluorides are not. The best way to take this chrome oxide off is not hydrochloric acid, it's hydrofluoric [hydrochloric] acid. That's the most effective.
50 nanometers — very thin. Doesn't take much to take it off. Stick it in hydrochloric acid, you take a passive stainless and turn it into an active stainless within minutes. 316 with the molybdenum is resistant to three times the chloride concentration of 304. 317 is probably resistant to five times. Typical atmospheric corrosion resistance of 304 might be a thousand parts per million chlorine — if you were raining or spraying salty water. 316 would be 3,000 PPM. 317 might be even higher.
The reason you could use it in the body — anybody know what the salt content of the body is? Seawater is about 30,000 parts per million, and your blood is about 30,000 parts per million. When I was in fourth grade they taught me that's proof we evolved from the fishes, because our blood content was the same as saltwater — if you believe that. A little oversimplified, but basically inside our body is like seawater in terms of chlorides. One important difference: we don't have the same oxygen content as at the surface of the ocean. We might have an oxygen content similar to the bottom of the ocean. I guess that proves we started out not a surface fish but a deep sea fish. We were deep sea predators.
In any case, this is better and better chloride resistance. It really depends on not just the chlorine but the oxygen content, because the chlorine screws up the protective oxide layer when it forms. The nice thing is, usually even in 304, if you break off that protective oxide layer — which is very easy to do — it will reform just in the air as a protective layer, unless there are some chlorides around.
If you want to go over to the New England Aquarium — how many people have been to the aquarium? What's the outside made out of? Stainless steel. What was the specification? They wanted to make the outside look like fish scales, and they were going to use 304 stainless panels, and they were to be passivated. I read the spec. Passivation means in the mill they took the coils of steel before they had been stamped into fish scales, and they passed them through a bath of nitric acid — about a 5 or 10% solution, not concentrated. It's an oxidizing acid. You clean the stainless steel in the nitric, but you also oxidize it to form the protective oxide. It's now passivated. You can passivate 304 stainless in 15 minutes in nitric acid.
So they bought the passive stainless steel, cut it into panels the size of fish scales, then took a grinder and brushed on this texture. What happens when you brush on the texture? You're taking off the protective oxide. What happens when you then put it in a marine environment, Boston Harbor? You get oxidation right along those little scratch marks. If you go look carefully — very few people do, but I had to, I got paid for it — you will find rust. If you go around the side to the loading dock where the trucks come in to drop off the food, you'll see where they actually scraped up against the stainless steel with their bumpers, and you have big bands of rust, because they mechanically abraded away the protective scale. In a chloride environment, the protective scale did not reform like it would inland. But right there on the coast.
We had to figure out how to clean it. I tried citric acid — that will work like nitric acid. We had to have something that was not going to destroy Boston Harbor, because all the environmentalists. One of the things I tried was Diet Coke. It didn't work all that well, but Diet Coke is phosphoric acid, and it also is a cleaning solution. Who could complain about dropping Coke into Boston Harbor? Not that kind of coke — the drink.
I had a case like that once, in New York City — one of these 20-story buildings above the approach to the George Washington Bridge — and they had a weld crack. When the cab driver let me off, I didn't know the addresses in New York City, and he dropped me off and there were all these little empty capsules all over the sidewalk, which were basically crack cocaine capsules. They told the homeowners, the condo owners, at a meeting before I was there, that they had a crack in the basement, and after the meeting a bunch of the homeowners wanted to go down there and get some of the crack. True story. I got to see the crack — eight feet long. Had a piece of it in my lab. I didn't eat it.
§12. The L grades and the AOD revolution [51:15]
You have this interaction between oxygen and chlorine in stainless steels. Chlorine is not good for the steels. You can add molybdenum at a tremendous cost, but they do all the time. Then there's the L grade of these — all three of them — which means extra low carbon. That means you can weld it without getting chrome carbides that also destroy the corrosion resistance. I'll talk about that next time, but let me tell you that most of the stainless steel we make nowadays is made by a process that gives us very low carbon — less than 300 parts per million carbon. Which prevents these chrome carbides and loss of corrosion resistance that occurs during welding.
The process is called argon-oxygen decarbonization. It's now the process by which every bit of stainless steel and most nickel-based superalloys in the world are made. It was developed right down here in the basement of Building 8 by a guy named Krivsky, who was doing his doctoral thesis under John Chipman. They were doing basic research, bubbling argon through molten steel and looking at the carbon monoxide reaction — carbon and oxygen in the steel — finding out what would happen. They found if you bubbled argon rather than air, you would have a low enough oxygen potential in argon — the oxygen potential is very low — you would basically pull carbon and oxygen out. It used to cost a small fortune to make 304L stainless. Nowadays with AOD steelmaking, you get low carbon without any difficulty, such that we hardly ever make 304 stainless anymore.
Back in 1950 it was a super premium to get 304L, because it doubled your steelmaking time — you had to have multiple slags to get the carbon out. The process wasn't patented at MIT. Krivsky was doing this basic science research, then went to a company and realized this process could be used commercially to get the carbon out of stainless steel. Since about 1960, virtually every stainless steel in the world is made by argon-oxygen decarbonization. You just bubble the argon through and take the oxygen and carbon out. I once estimated that's worth about a billion dollars a year to the world steel industry. I'll see you tomorrow. I thought I was going to finish up steel — well.