§1. Hardness and hardenability [00:02]
If I took my Silly Putty and stuck my thumb in it, you get an indentation. If I calibrate my thumb with a certain force and measure the depth, I could get a hardness of the Silly Putty, okay. You do the same thing with steel, only you use a diamond. You might use 150 kilograms as the load on the diamond and you'll get a penetration about half a millimeter deep, depending on the hardness of the steel. Then you change it from a diamond to an eighth-inch hardened steel ball, or you change the load to 60 kilograms, and you get a different hardness scale. So we have hardness scales going all the way down to rubber and plastics. PEX actually is reasonably hard because it's cross-linked.
Hardenability, on the other hand, is how fast can you quench it to get this martensite, which is very strong and has hardness. But how deep will that penetrate into the steel? If you just have plain old carbon steel with nothing else in it except some manganese, you're not going to get very great depth of hardenability. It might be an eighth of an inch or a quarter of an inch, depending on how thick the piece is and how well the fluid can quench that chunk of steel.
But if you add enough alloy content — if I start making it out of a surgical stainless steel like this — [Tom holds up a pair of surgical scissors.] — twenty-five years ago this was a $300 pair of scissors, probably worth about $50 if it wasn't being sold to a doctor in a hospital. It has to have an extra couple hundred dollars worth of insurance because they sue doctors all the time. Anyway, that's a stainless steel. When we get to stainless steels I'll tell you which type, but it's actually a martensitic stainless steel. You can heat that up and harden it, take it out of the furnace and let it cool in the air, and it will harden. It's called an air-hardening steel. In fact, the only way to get it annealed if you heat it up is to leave it in the furnace and bring the furnace down very slowly — over ten hours or five hours or something. It would take an hour to transform that, typically, or to not transform it. So you can air harden it, take it out of the furnace, it cools down two or three minutes and you get full hardness, okay.
So you have a tremendous range. But that thing's only about 80 percent iron, it's got 20 percent alloying elements, and it's going to cost you five times as much as a piece of carbon steel per pound. So you can add hardenability, you add cost.
§2. Welding metallurgy textbooks and the Alexander Kielland disaster [03:00]
Any questions on that? If you just showed up, there are some other big handouts out there. The big handouts are for the fusion welding module that you're supposed to watch online. It is online. I've got a student who's loading it onto YouTube, which will be easier for you to access. If you have an MIT connection, for each hour-and-a-half lecture it might take two minutes for it to download — it has to download the whole hour and a half. If you're at home with a slow connection it could take half an hour. So the students last year decided I should put my lectures on YouTube because it'll keep downloading as you play, so you don't have to wait. So we're doing that — we're putting it on YouTube now.
There are a number of problems, and what I'm handing out is something from a welding metallurgy textbook. In fact, I brought with me a number of welding metallurgy textbooks. [Tom produces a stack of books.] I just went to the shelf and pulled off six books on welding metallurgy. Here's one on welding metallurgy just of stainless steels. Here's one just on nickel-base alloys. Here's a Metallurgy of Welding, sixth edition, by this guy in South Africa — he doesn't have a whole lot else to do in South Africa except write extra editions of his book for the last 40 years. He actually has sort of a physics background, but there's interesting stuff in here. Here's one on welding metallurgy written by Sindo Kou, professor at University of Wisconsin, graduate of this department, worked with Professor Flemings — a lot of solidification in there because he did his thesis on solidification. I've already showed you Stout and Doty, which is fourth edition back in 1980, and this is actually where that chart came out of Kou, or it came out of Easterling. Easterling was Swedish — he's passed away now — but he wrote this book on the physical metallurgy of welding.
And I suspect in one of the two modules you'll read, I do a case study right out of Easterling on the Alexander Kielland disaster. Anybody know what the Alexander Kielland was? About 1980, in the North Sea, they were drilling for oil — this is sort of the beginning days of drilling for oil in the North Sea. They had a production platform that was supposed to be a drilling platform, but they converted it to a hotel. It was a 250-bed hotel in the middle of the North Sea. And one night in a North Sea storm — which they're famous for — it had five legs, each about 20 feet in diameter. Think of a mini-submarine leg — not the same thickness because it didn't have to go to the same depth, but nonetheless. They had a fatigue crack that started at a weld defect. It was all made in the shipyard in France.
And the story is, for all their production welding of the legs and everything, they had very tight welding procedures and process control to have low carbon and get low hardness. But then they had their maintenance people go and mount hangers. The maintenance people didn't have the same type of welding procedure requirements and supervisors, so they just went out and got a piece of pipe — they were going to put a hydrophone on it for some of their drilling — and they welded on this six- or eight-inch pipe. It turns out to be a medium-carbon steel, had very good hardenability and hardness. And it ended up getting a hydrogen crack that was originally about an inch long, and that thing grew and ran for about 50 feet, all the way around one of the five legs. You lose one of your five legs, and in a storm the whole thing came down. A couple hundred people lost their lives. There are all kinds of studies by the Norwegian government about safety on jack-up rigs.
The hydrogen crack is going to grow within the first couple of days after welding, because remember the CO2, the Sprite example. But for the fatigue crack to grow, it was probably a year and a half — it's in the book. For a fatigue crack to grow 50 feet, it's growing with the waves. Every wave that comes by, so the number of cycles is millions and millions. But for the growth rate of a fatigue crack, there's a very rough rule of thumb. If you look at time versus crack length, we know from fracture mechanics that the crack growth rate goes with the number of stress cycles to the 3.5 power. Isn't that great — we can quantify things like that. That's what people at universities do. That's called the Paris law. Paul Paris is a retired professor at Ohio State University and he is one of the most arrogant people you'll ever meet. He's not the most arrogant I've ever met, but he's in the top five. Other than that, he's a nice guy.
So the fatigue-crack rule of thumb is, the crack grows 10 percent of its length in the first 90 percent of its life, then it really starts going fast. It grows the last 90 percent in the last 10 percent of its life. Maybe it's 80/20 rather than 90/10, but you've got to find the fatigue cracks when they're still small. On the Kielland, they probably could have found it when it was only a foot or two long. If they had found it before it grew to 50 feet, they would have been okay. When it gets to be 50 feet long, going nearly all the way around a 20-foot-diameter leg, it's not hard to understand why the leg falls down.
But for all you divers out there — they had just done a diving survey to look for cracks two weeks before. So do you think they missed it? Yeah. That was one of the problems. A lot of times these are commercial divers, not Navy divers — you guys are responsible. But some commercial divers, hey, it's cold in the North Sea, it's much easier to just go down there and come back up. Why bother to look for cracks while you're down there.
Student: [question about depth/conditions, inaudible]
The North Sea is only about 200 feet deep, so it wasn't that deep. But when you've got 50-foot waves, it's hard to survive. They actually had the right type of lifeboats — I'm not sure if they had enough. These were lifeboats with tops on them so they could be completely swamped, except they didn't have a good enough communication system. It happened in the middle of the night, people were asleep. Some of the lifeboats got launched too early. Sort of like the Titanic — they had enough lifeboats, they just didn't have them in the right place at the right time.
You can read about it. I don't remember which module gave that as a case study, but they had all kinds of safety rules that came out of that about oil rigs and tipping over. I think it also had to do with flooding of one of the legs — you lost one leg, and then another leg got flooded because they left a hatch open. When it tipped over you had all this water on the inside that wasn't supposed to be bearing weight. So they had all kinds of rules about closing the hatches.
Four years later, off Brazil, Petrobras had a similar failure. There are nice pictures from three days of sunny weather of this thing listing at about 45 degrees — a half-billion-dollar oil rig listing at 45 degrees. There was nothing they could do. Everybody got off, no one died in that one. But they left a hatch open. So there's a reason you're supposed to yell at people if they leave hatches open.
Which is actually also the reason the Titanic went down. It was unsinkable and had all these compartments, except the gash went for six compartments along the side and it could tolerate four. Who would ever expect a gash the length of six compartments? Sort of similar to this guy in Italy who went close to the rocks. The failures just keep coming back.
§3. The welding-defect taxonomy [13:17]
There's a philosopher at Harvard named George Santayana who is famous for the quote that those who do not remember the past are condemned to repeat it. So here are typical welding problems as listed by one of these welding handbooks for steel. Porosity — you folks have probably seen porosity before in welds. I'll pass this around — this is something from the British Welding Institute, put together 30 years ago, showing different types of cracking. [Tom passes around a defect reference card.] Here's some surface porosity. Here's an x-ray with restart porosity — they restarted a weld and you get porosity. Here's a crater pipe, then you've got microporosity. There are other cracks in here. This is something to train inspectors on the types of defects you can expect in welds.
Porosity is not usually a problem. The structural welding code allows like six eighth-inch pores for every foot of weld. So every two inches you can have a pore. Porosity doesn't have sharp edges — it's not sharp like tearing a piece of paper. It's like the hole you punch in paper, and it doesn't weaken it as much. It does weaken it, but not as much. You usually have enough toughness in steel, and so you have to control the deoxidizers.
Hydrogen cracking we're going to spend a fair amount of time on. Lamellar tearing — something that occurred in steels back in the days when we had higher sulfur, like when I started. Typical sulfur content of a steel made in the United States in 1970 was 0.025. The specs might say it has to be less than 0.040, which means that spec was probably written in the 1940s. Some ASTM specs still say it has to be less than 0.050, which means that spec was probably written in the 1920s and hasn't changed for almost 100 years.
When I was an engineer at Bethlehem Steel, the Japanese were selling A360 — just simple I-beams — with 0.005 sulfur. Bethlehem Steel was trying to get an extra twenty dollars a ton, trying to charge an extra eight percent to give you low sulfur. The Japanese, everything they made was low sulfur — they couldn't give you 0.025. So today we have in general 0.005 sulfur in our steel, but 30, 40 years ago we didn't in the United States. They did in Japan. If you went back 50 years ago, they didn't have it anywhere in general unless they went to very expensive processing.
Lamellar tearing is basically: you get sulfide-oxide inclusions and they get rolled out in the plate, and it's like having a piece of steel plate that's like a sheaf of paper — it's laminated. You get hydrogen in there and it separates just like a sheet of paper would. I can take a pad of paper and fan through the pages. With lamellar tearing, you get the stresses of welding, you get hydrogen in there, and you'll get nice tears. There are pictures in that book of lamellar tearing.
Reheat cracking — this is if you're stress-relieving a pressure vessel. Particularly if you have vanadium in your alloy steel, you can get carbides precipitating at grain boundaries. You need to be careful of the composition of the steel. We learned 40 years ago how to correct most of this. My second boss when I worked in industry had done his doctoral thesis on this, and they sort of learned how to solve the problem in the 1960s.
Solidification cracking — it says keep the proper manganese-to-sulfur ratio. Well, this was a problem back in 1920, when we had high sulfur like 0.05 sulfur. The proper ratio, if you go back and read a metallurgy text from 1920 or 1930, is that the manganese-to-sulfur ratio has to be greater than six. If I have 0.05 sulfur, six times that is 0.3 manganese. Most steels have 0.3 manganese; many of them today have one percent manganese. So it's not really a problem, unless you're doing something like the Mass General subway stop. When they call me up, I say, what's the composition? If it's a 1900 piece of steel, I would also start worrying about the manganese-to-sulfur ratio. It wasn't just hardenability and hardness — I would start worrying about some of this in the really old steels.
But in general it's not a problem, although I did work on this problem. Bethlehem Steel owned shipyards when I was in the research lab, and they wanted to weld over primer. Anybody been on a shipyard where they wanted to weld over primer? This comes up about every three to five years, because ordinarily in a shipyard you get pre-painted plates. They bring in the plates from the steel mill, put them in a yard, it rains on them, they get all rusty. They take them into the panel shop and they shot-blast them to get the rust off. Then they paint them. You can't weld over paint because the paint will cause solidification cracking — in part because the paint has a lot of phosphorus in it for corrosion protection, and phosphorus causes solidification cracking.
So one of my first projects was: "Tom, how can we come up with a weld metal that can weld over paint?" People had been trying to do this for 50 years, and I didn't solve it either. What people have done is gone to different composition paints, but it still doesn't work very well. When I was in Japan in the 1980s visiting shipyards, they were trying to do it. About eight years ago up here at Bath Iron Works, they were trying to do it. In general the best thing is to grind the paint off for three inches on either side, weld it, then repaint it. But sometimes those paint jobs aren't good, and that's why we see the corrosion picture you folks have been showing us — a lot of times it starts in that repainted area because it's not as good a paint job.
Low heat-affected-zone toughness — use low heat input. If you develop a proper welding procedure to begin with, it's not usually a continuing problem. Low fusion-zone toughness due to coarse columnar grains — again, choose the right steel chemistry to begin with, get the right welding procedure, and you shouldn't run into these things. So if I look at all the things some welding engineer put together, really the only one you have to worry about every day is hydrogen cracking — someone fooling around with things.
§4. Soviet underwater welding at the Paton Institute [20:45]
And so, yesterday Tom was talking about welding underwater. If you're wet welding, you are generating hydrogen. That arc is about 60 percent hydrogen, because you're breaking down the water into hydrogen and oxygen, so you have carbon, hydrogen, and oxygen. I probably ought to give you an hour lecture, since we have so many divers and former divers, on underwater welding. But one of the things I was thinking of — you said there's one company, Broco or something.
In 1980 I was over at the Paton Welding Institute in Kiev, which was the world's largest welding institute. It still probably is. Has five or ten thousand people. Paton's father had been a hero of the Soviet Union under Stalin in World War II because he repaired the armor on tanks and got them back to the front to kill those Germans. He was a welding engineer in Kiev, and after the war they named it the Paton Welding Institute.
The way they gave out money under the Soviet system was through the Soviet Academy of Sciences. We have a U.S. National Academy of Sciences, but they're a quasi-governmental agency — Abraham Lincoln formed them. They're right near the Lincoln Memorial on the Mall in Washington, but they're really a private organization. They get a lot of research support out of Congress. The Defense Department, the Materials for 21st Century Defense Needs study, that was a National Research Council, National Academies study. If Congress has some concern about an environmental problem or climate change, the National Academies will do a study. They get people to volunteer — they don't pay anybody on the committee. They have a full-time staff, but they ask people at universities, in government, and in industry to come sit on these studies. They do it for free because it's prestigious to be on a National Academies panel. Ultimately you can never conclude anything because it all has to go through a peer review that sanitizes everything. That's why it's quasi-governmental.
In the Soviet Union, they had seven or ten members of the Soviet Academy of Sciences, and they gave out all the research money. It's sort of like taking the Department of Defense 6.1 budget plus the Department of Energy — these guys had billions of dollars to give out. They gave a certain amount to each of the republics, the Soviet Socialist Republics. Paton's son, who became the head of the Paton Institute, was on this panel of seven or ten people. He was from the Ukraine, which was the richest of all the Soviet republics — had 50 million people, productivity was much better. So he controlled all the money for the top level that came to the Ukraine, and then he was president of the Ukrainian Academy of Sciences, which divvied up the money once it got to the Ukraine. So his institute was a very big institute.
When I went there in 1980 — we may talk about some of this when we get to titanium welding, I got to talk to their titanium welding expert, who was really the guy who taught them how to weld the Alpha submarine — they also gave me demonstrations. One of them was underwater welding. In the Soviet economic system the money didn't get divvied up on economic bases, it got divvied up on political bases. Someone with a fair amount of clout at the Paton Institute was really interested in underwater welding, and they showed me a wet weld. I saw the weld made, and they took it out of the water and it didn't look like some dog had just left this on the sidewalk. It actually looked like a regular weld, and it was a beautiful weld. We didn't have anything like that. Now, it's still going to be full of hydrogen, the weld chemistry is going to be different, but it was a beautiful weld.
When peace broke out in the early 90s, the former Soviets were looking for capital. The American Welding Society went over to Kiev to talk to the people at Paton, and one of the big things they were trying to sell was their technology on underwater welding. I'll bet you money that Broco was the one who eventually licensed the Soviet technology. They were light-years ahead of anyone else in the world on making welding electrodes for underwater welding.
And the same type of thing on titanium. For whatever reasons — it might have been a political-military reason — but they had this guy Gurevich who had all kinds of funding to weld titanium. In the meantime, David Taylor Naval Lab at Annapolis had a little welding lab and they were welding titanium. I can tell you the story when we get to it of building the Sea Cliff, which was like the Alvin submarine — this is about 1980 and how they couldn't do it. With all the technology that David Taylor had produced, they really couldn't do it. They had to go to another process.
§5. The iron-hydrogen phase diagram [26:37]
So now — yesterday I handed out this. [Tom locates a phase diagram handout.] I think this particular phase diagram. So this is a phase diagram of the iron-hydrogen system. We want to talk about hydrogen cracking. This is temperature, this is 100 percent iron, and this is weight percent hydrogen. Hydrogen's very light, so down here they also talk about cubic centimeters of hydrogen per 100 grams of iron. They use that unit because you basically take a sample of iron that's got hydrogen in it just after you weld, you weigh it, put it in a machine that's going to drive off the hydrogen by heating it up, and you measure the hydrogen. They normalize it to 100 grams of metal you put in there. You saw how fast that stuff comes off.
I had a student who actually timed how long it took from making the weld to getting it into the analyzer. You can lose half your hydrogen in the first two minutes, and it's very difficult to actually measure it. So no one's really measured it — you can back-calculate how much hydrogen was there originally, but you can't really measure it because it comes out so quickly. Cubic centimeters of hydrogen per 100 grams converts to parts per million with about a 1.02 factor — almost exactly the same. So even though this is cubic centimeters of hydrogen per 100 grams, which is what the analyst comes up with in the chem lab, it's almost the same as parts per million. I prefer to talk about parts per million — it's a little more scientific. This is parts per million by weight. So this is 0.28, yeah, okay.
So this is about 30 parts per million right here. This is atomic percent, this is weight percent, so this is parts per million by weight. When we talk about parts per million we're talking about by weight. So this is 30 parts per million roughly — that's the solubility of hydrogen in liquid steel. When you solidify, it drops to about 10. You want to lose two-thirds of it — you're oversaturated by a factor of two, everything's at equilibrium. But things aren't at equilibrium when you're underwater welding and your atmosphere is 60 percent hydrogen.
In theory, if you look at equilibrium thermodynamics of 60 percent hydrogen, you should only get about 70 or 80 percent of the amount for one atmosphere. This is for one atmosphere of hydrogen in your shielding gas. If you're down 300 feet you can get even more hydrogen, because you have a higher pressure. So hydrogen gets worse as you dive deeper. But the reason it really gets worse is the arc — it's not diatomic hydrogen, which is what this is, it's monatomic hydrogen, because the heat of the arc dissociates the diatomic hydrogen into monatomic hydrogen.
There is a plot — maybe I can find it in Kou's book — of the difference between the solubility of hydrogen as monatomic versus diatomic. [Tom searches the book.] No, I won't find it that way. He gives me the one in Texas. I won't find it right now — maybe I'll find it at the break.
It turns out you can dissolve 50 times roughly, depending on the temperature, the amount of monatomic hydrogen as diatomic hydrogen. The bottom line of all this is, you can dissolve from a welding arc lots of hydrogen into the weld metal — [A device beeps in Tom's pocket.] let me turn that off again, a little button I get every now and then — even more than 30 parts per million. That's not a good thing.
§6. Distortion versus residual stress [32:00]
It turns out that if I start looking at the steels — I put this up earlier — if I look at full restraint. What do I mean by full restraint? When you weld steel or aluminum — this was actually done for aluminum, comes out of Kou's book I think — you have a large angle, you get lots of distortion. Aluminum shrinks six percent on solidification; steel shrinks like 3.2 percent. So aluminum distorts more than steel because it shrinks more. If you put in a V-shape weld, which you need for access to get the heat down to the bottom, you'll end up shrinking more at the top and you will end up with something that distorts.
You can change the angle a little bit. If you have no angle and you do an electron beam weld, you can get almost no distortion. But you can't do electron beam welding all the time, so you have a distortion problem. You can do all kinds of things. If you have a double-groove weld and you weld everything from one side and then go back to the top, you'll end up with lots of distortion. If you balance your weld — one side, then the other side, then fill up that side and then fill up the other side — you can balance it and get rid of your distortion.
In fact, I had a student — my very first student, a bachelor student who graduated in 1977 or 78. He went to work for a company called Electric Boat. One of John's things in the 1980s was the submarine torpedo tubes — these long tubes, like 30 feet long. They have to be very straight. They'd weld these things together, they'd have distortion, and they'd have to go in there and bore it out, machine it. John taught them: they could weld the circumferential welds, and as they're going around in circles, rotating the whole pipe, you have a little dial indicator at the far end and you can see when it's going out of the centerline, distorting. He would monitor that while they're welding, and he'd change where he would start the next weld on the next pass — this is fairly heavy wall — to try to bring it back to the centerline. If he couldn't do it, he would actually go in there and grind out part of the weld and re-weld it to bring it back. He could weld those things by being careful, monitoring as you go, to control the distortion, so all they had to do was a skin pass in welding to clean the thing on the inside. So you can do things to control the distortion.
But if you have a very highly restrained system — big heavy plate and egg-crate type construction, like you have in all your tanks and stuff you build inside, whether it's a surface ship or a sub — you have a lot of restraint and the material can't just distort. You put that last panel — you put the top on a six-sided box, that last panel has nowhere to go because the other five panels are holding it in place. So the distortion is not distortion so much as it is residual stress. You can trade distortion versus residual stress.
I don't have it on this thing, but it turns out if you're welding sheet metal less than three-eighths of an inch thick, you don't get a lot of residual stress. If you have an eighth of an inch thick, it's almost impossible to get residual stress, because something is so flimsy it can't support a lot of stress. It's a beam, and the beam is so thin it basically relieves its own stress just by allowing a little bit of bending in the sheet metal. The sheet metal may not be perfectly flat, but it's good enough.
If you're an inch or an inch and a half thick, the plate is so rigid, you don't have too much distortion most of the time — you have lots of residual stress. You can have yield-level residual stresses, the maximum stresses you can get in the weldment, when you're a thick plate like one or one and a half or two inches thick, which is something you do in submarines. The absolutely worst situation for distortion is somewhere in between. No distortion when you're thin, because the plate or sheet is flexible. No distortion when you're very heavy. So distortion can be a problem, but if you weld-sequence properly on submarines you can control it. The absolutely worst situation —