§1. Ultrasonics, weld metal, and the TMI insurance fight [00:11]
We've seen a crack, in fact, that we've never seen before — that's what happened at Seabrook. It was scary to find cracks in the weld metal. The ultrasonics couldn't pick up the cracks because the weld metal has these big coarse grains and the ultrasonics is too noisy to look at. The base material has a better signal-to-noise ratio than the weld metal. It's not just that it's reflecting off the crack — the sound is also reflecting off lots of other things adjacent to it, like the grain structure.
Student: So anyway, just to ask, would they have to replace a couple, the top of the pressure vessel, or would they put like a big chunk of steel in there as well?
They'll appreciate a replaceable piece. They have to go in there for some changes. And eventually you're going to replace water, so you just put a patch in. I remember what happened at TMI [Three Mile Island] — on the first day and a half they called it stress corrosion cracking. And it was stress corrosion cracking. But then some attorney for GPU [General Public Utilities] read their insurance contract, which excluded corrosion. So after a day and a half they called it SAC, stress-assisted cracking. What type of cracking is not stress-assisted? You don't get a crack unless you have stress. But they needed to get that "corrosion" word out of the press releases. And then we got into the debate of, corrosion is a slow gradual process — if something occurs in thirty minutes, is it a slow process? You'd say no.
The second one went to arbitration, in Ohio. It was a graphite pressure vessel, and the question was, did it explode or did it collapse — because explosions were excluded from the policy, collapses were covered. So I went to the arbitration. It turned out the chief arbiter was the former vice president of American Nuclear Insurers, the ones who insured Three Mile Island. So this guy knew me from that. The big-shot attorney is asking questions: what's the definition of corrosion, what's the definition of an explosion? By the way, the word "explosion" comes from the Latin which means to clap or make a loud noise. So it turns out explosions make loud noises, okay. Collapses can make a loud noise too if you have something three stories tall and it collapses. So he's asking me these questions about how to define these words, and finally he gets frustrated and says, "Are we just going around in circles?" I said, "You may be, sir. I'm just here for the ride." Everybody cracked up.
§2. Introducing nickel-based alloys [04:28]
Okay, today we're going to a whole new one. Nickel-based alloys. This is basically straight out of Lippold's other book — Jonathan Lippold's workbook on welding metallurgy and weldability, the most recent book. The first book he wrote was on weldability of stainless steels, in 2005. The other book on weldability of nickel-based alloys, John Connelly had one. This should be on Stellar. We still don't know what's happening with Stellar — if you try to link by course number to 10.3 or 3.371 you can't get anything, but if you go to 3s171 you're authorized.
People like to think of MIT as a technologically sophisticated school. We only got manual registration about ten years ago. All the other universities in the country were doing computerized registration, but MIT was still doing optional carbon paper.
The problem with nickel alloys is they're expensive. You're talking around — I don't remember my numbers — about $6.50 a pound for nickel as a metal. We use commercially pure nickel for some situations, but like most pure metals it doesn't have very high strength. So we strengthen it by adding copper, molybdenum, iron, or chrome and iron. Nickel-chrome-iron is like saying stainless steel, except instead of eighteen percent chrome, eight percent nickel, and seventy-two percent iron, it's got seventy-two percent nickel and eight percent iron. We just switched the iron-nickel ratios. Then nickel-chrome-moly, nickel-iron-chrome-moly, and the cobalt alloys.
These things have been around for a hundred years. There was a guy, Elwood Haynes, from Indiana, who was interested in bicycles. He started making some of these alloys, and they were better than stainless steel. They were a little pricey, but they had better corrosion resistance. So we use these in all kinds of applications. The alloys haven't changed a whole lot in the last forty or sixty years, because they're not used in huge items. There's not a huge market to improve these things — until people started learning how to weld zirconium. We had been welding titanium. If you look at the periodic table, titanium and zirconium are right below each other. Zirconium is the cladding on your reactors because boron is a poison; zirconium has a lower neutron cross-section. Hafnium follows that — hafnium is a poison for neutrons just like boron, so when you make zirconium really pure you take the hafnium out. Anyway, we did a lot more welding on titanium from the 1950s on, and it wasn't really until the 1980s or 1990s that, from the titanium, we learned how to process zirconium.
People always used a nickel-molybdenum alloy for the reactor vessels for making acetic acid. Acetic acid is the highest-volume organic acid. They make acetate films, all kinds of plastics, pharmaceuticals — it's just a raw material like wood or steel. The largest inorganic acid is sulfuric acid; almost all the gasoline has to go through a sulfuric acid catalyst. About the only thing corrosion-resistant to acetic acid in its manufacture was this nickel-moly alloy. There's nothing special about the Monels — they were just nickel-copper alloys. They changed a little bit but about the same as they were sixty years ago. There's not a lot of sophistication, the kind there is in aluminum, certainly not the kind there is in steel.
§3. Precipitation-strengthened alloys and intermetallics [10:13]
But then you get over here to the precipitation-strengthened. These alloys have titanium or aluminum, which form intermetallics — very high-melting intermetallics. Nickel-tri-aluminum melts at about 2003 centigrade. Nickel melts at about fifteen or sixteen hundred centigrade. The alloy can have tremendous high tensile strength. These are the workhorse alloys for generations of jet turbines. Lots of sophisticated precipitation-strengthened alloys — not very weldable, but in general we don't weld them in place. We can weld them, but there's not very much you can do. Some repairs, yes. There are some special ones where you don't have to worry about intermetallics — that's sort of a research project over at Oak Ridge National Lab, and they get tens of millions of dollars for turbine compressor stuff. And then ODS, oxide-dispersion-strengthened, where you have very fine oxide inclusions for strengthening.
If you go through Lippold's book, he'll tell you which alloys go with what. Gamma prime is the nickel-aluminum-titanium intermetallic phase, and if you get that as a precipitate you can get very good high creep strength. This stuff doesn't dissolve until about 1,200 degrees centigrade in an alloy that melts at around 1,350 centigrade. So it maintains its strength to very high temperatures. Nickel-three-niobium is gamma double prime — similar high-melting phase, but if you've got gamma double prime with niobium rather than gamma prime with titanium, you're much more weldable. People prefer Inconel 718 because it stays away from gamma prime, which is very difficult to weld without cracking. Then they throw in all these other things as carbide formers. You can get higher creep strength from these superalloys for jet turbines by creating some of these different phases — the sigma phase, the mu phase, Laves phase. They call them topologically close-packed phases. Some of these can be very brittle. In stainless steel the sigma phase can embrittle the steel. And then chromium and aluminum are oxide-forming, increase your strength.
So nickel alloys are used for corrosion resistance — primarily solid solution — and high-temperature strength. These are our workhorse materials for very high-temperature processes, above where stainless steel will go. Stainless steel, let's say 600 degrees C is the size you want to use it to; you can get up to a thousand on some of these, even eleven hundred in some cases.
§4. Solidification cracking and the Bettis specification [13:00]
One of the problems with these alloys — like the Monels the Navy uses — is they are the model system in the laboratory for developing solidification cracking. If you want to study solidification cracking, you won't find a better system than nickel-copper alloys. And it has to do with the grain structure during welding. If you make a very deep weld and you have something with an elliptical shape, your last part to solidify will be up in here. You get rejection of solute. You have to make sure the field welds are the right shape. If you have a concave shape with a steep slope, you'll form a centerline crack.
If you look at the microstructure, this is what happens. As it shrinks, you end up with these solid dendrites. Think of the solid dendrites as ice crystals in syrup. Anything, as it shrinks on solidification, you end up with these nice little cracks. Sometimes you can actually squeeze some of that liquid into the cracks, and they call this self-healing. But it's still a weak spot in the material. Maybe it's not a real crack, but it's still a weakness.
The only spec I've ever seen — and I had to try to qualify this for a company — I told you, for five years I did hundreds of failure analyses for a consulting engineering firm down south. There is a company in eastern Massachusetts that was making some pipe welds for Navy reactors, in these nickel-based alloys, and they're so prone to cracking that Bettis — Westinghouse Bettis, one of the Navy's two labs for reactor materials and design, along with General Electric KAPL [Knolls Atomic Power Lab] — they've been downsized a lot since they designed the reactors in the 50s and 60s, but they still exist. It's one of the two areas of the government where you still have competing labs, which is an expensive way to go.
They could make this electron beam weld. This is about a half-inch-thick material; I don't know what part of the reactor it was, all I got was test samples. There are two electron beam welders in eastern Massachusetts — a lot of electron beam welding shops in New England compared to other parts of the country. The first electron beam welder was at Pratt & Whitney in Connecticut, a General Electric plant, for the aerospace industry. A bunch of European companies came over, those experts in electron beam came over into Connecticut and Massachusetts, and they're still here. One of the earliest ones was started by — one of my welding professors at MIT, Koichi Masubuchi knew him — back in the 60s, started electron beam on a job-shop basis as a consulting business.
They make these welds, and they had to magnetically spin the beam to get a wider weld, then overlap it by thirty percent and make another weld right next to it, and they had to do this without any cracks more than about a millimeter in size. This is the first spec — and the only spec — I've ever seen in welding where cracks are allowed. But they had to be smaller than about a millimeter because they needed the corrosion resistance. They found they couldn't weld it without cracks, so they went back, did the fracture mechanics, and said okay, the cracks are no bigger than what we can live with for the life of this thing.
Those guys tried to qualify those welds. I was getting samples for six months. They kept sending samples, they spent a fortune, they couldn't get their beams operating precisely. They kept failing the spec. They wouldn't talk to me — I didn't have security clearance, so they couldn't tell me what they were doing or how they were doing it. You get this kind of solidification cracking all the time.
§5. The Savage school and the metallurgy bias [19:07]
Lippold did his thesis at RPI, and the big name there was Warren Savage — in welding he was the top guy in the 1970s in the country. He developed lots of cracking tests. This is one called the transverse strength test. You take a gas tungsten arc torch, lay a weld bead on your material, and then while you're welding you deform it in place. So you're putting a big strain on it while it's solidifying. Then you go along and look for cracks — you find them, you find how they form, you find the microstructure with this testing process.
So we know how to design the alloys. They were designed in the 50s and 60s, and Savage was doing his research in the 60s and 70s, for the companies making these alloys, trying to improve them. If you get cracks — if you make a big enough weld, you're going to have some cracks. And the question was, how do you get enough cracks so you can find out which alloys are most resistant?
I think I told you, when I started in this business, most of the welders thought the only way you solved a welding problem was welding metallurgy. If the only tool you have is a hammer, every problem is a nail. I remember sitting in my first office at 137 First Arrows here saying, how am I going to compete with those half-dozen, a dozen Warren Savage students who have all been brought up on welding metallurgy? If I'm going to be in welding, I'm going to do something else. So I chose welding processes, the physical chemistry of the process. And you still see in Lippold's three books a good example of everything being welding metallurgy.
NAVSEA tells you guys, take a welding metallurgy course. I don't tell you take a welding course — they tell you take a welding metallurgy course. It's not where we solve the problem. We didn't learn to solve any of these problems. There's sort of no problems anymore. After fifty years we know how to control the metallurgy and the composition so we don't have these problems we had when they were developing the alloys. The problems today are more process problems, like keeping the moisture out of your shielding gas, or keeping the lube off the wires. We're having the same problems we had in the 1930s with hydrogen cracking, that still shows up in various forms.
§6. The artist's-conception microstructure [22:06]
The other thing about the nickel-based alloys — this entity is out of one of those books, but he stole it from somebody else. Anselm wrote a book on nickel-based superalloys before him. This is an artist's conception of microstructure. This direction is increasing aluminum or titanium. Here you have nearly pure nickel, and over here you have a nickel-based superalloy that's going to go on a jet engine turbine blade. You start out with something that doesn't have anything except a few M23C6 carbides along the grain boundaries. These are similar to the carbides that cause chromium carbide sensitization in stainless steels. You get grain boundary cracking when you form these carbides — in nickel-based superalloys too, doesn't have to be iron-based.
As you move over and put in more chrome and a little more titanium, you'll start getting MC carbides — one metal atom for every carbon atom. These MC carbides form at higher temperatures; they're sort of blocky. As you're adding titanium, you get some gamma prime — nickel-three-titanium — you get a few of these. As you keep increasing the concentration, your grain boundaries get more carbides, and you get more and more gamma prime precipitates. You get over here to even higher concentrations and all of a sudden these gamma prime precipitates become kind of square rather than round. There's good crystallographic reasons for that. You get volume fractions of gamma prime to the nickel matrix that can be eighty-five percent intermetallic surrounded by some solid solution, and that's what gives you the really good strength at high temperatures.
You can also get modules — and you have to forge them if they're not cast. So the turbine disks, you have to forge in the right way to break up some of these original solidification structures. Or if you don't want to start with a casting, you can do what the Air Force started: spray these alloys into a fine powder, solidify it that way so you don't get these big coarse blocky carbides and intermetallics, and you don't have to break them down by forging — you take those powders, already fine-grained, and consolidate them. You get a more uniform start without some of these big blocky ugly things. You still want the blocky gamma prime precipitates. But you don't want segregation of different alloying elements during solidification. You don't really like to start with a casting because it gives you big blocky regions — kind of like raisins in plum pudding or raisin bread. You want those raisins fine and uniform, small. With powder metallurgy you can get more uniform, more homogeneous, and even higher properties.
§7. The three circles and the submarine hull [25:33]
Student: That's a weakness, I mean — these alloys, the way you always push your, you explain how a submarine really has a, not high-strength but better-treated steel, right? So we need to reconcile that — but the idea is, the hull of a submarine is rated for a certain period of time, right? And so the implication is that — you make sure it's stronger, it alleviates a lot of stress on it. Going deep gets rid of —
I've never forgotten my three interacting circles — the metallurgy circle, which is microstructure, is one, and there's only so much I can do after I've made my material. Then there's the stress circle. If I relieve the residual stresses or shrink the strain, if I keep it nice and clean — my environment, don't let hydrogen get in, don't let boric acid get up in there — if I keep the system clean by my process or by putting hydrazine in on shutdown to get the oxygen out of the water, I shrink that circle. You can't solve all the problems in the metallurgy circle. Some you have to solve in the mechanical behavior circle, which is residual stresses. Some you have to solve in the corrosion circle, which is the environment. You want to think of it that way. When General Electric had their big stress corrosion cracking problem, they worked on all three circles. One of my problems with most of the philosophy of welding in this country right now is that everything's in the metallurgist's circle. You don't solve it just looking in one area. You attack every area. So that's your question.
Student: I don't — not exactly, I guess. My question was, submarines, the strength of their hull — but also, over time their hull doesn't fail, right? So what is it — like brittle fracture — that we're afraid of, and what is it now?
Submarines aren't like a surface ship. You might have corrosion and thinning. If it were just the hull, that hull would last a lot longer. It's all those other components inside that drive the lifetime. The Los Angeles class — they wanted to extend the lifespan. When you look at the hull, that's all good. After you pressure-harden it, it's going to last you as long as you're preventing corrosion — it'll last forever. So that's really not the limiting component.
Well, that's assuming you never cut through that hull in the previous thirty years. I remember seeing a picture of the [Patrick Henry] back in 1988, and it looked like a patchwork quilt, with all the penetrations. If you just built your hull and never touched it for the next thirty years, it'll last another thirty years, or a hundred years, as long as you keep your zincs on, your cathodic protection. But when you start welding other things on it, it no longer has that 30-foot-diameter circle or 20-foot-diameter circle accurate to whatever millimeters. Not very many, right? It's a pretty perfect circle. Once it's thirty years old, I've done so many penetrations, that thing won't have the same depth capability because it's no longer a cylinder — it's sort of a funky cylinder. I bet when they were evaluating it, they were measuring diameters at different positions and saying, is this hull worth saving? But it's not from corrosion or metallurgy of the hull — it's from the structure, the geometry of the hull.
If it failed in the hull — most ships, Sealift ships, Coast Guard ships, they're thrown away to nothing. They're held together by paint at the end. But on Navy ships, it's not really the hull, it's all the components inside. They're advancing the technology so rapidly that they want to build a new ship. Who wants the old ship? You don't want to be assigned to a fifty-year-old. The Coast Guard, they'll do that — you go to the Coast Guard. Okay, we've got to stop.