§1. CCT diagrams and the martensite transformation [00:03]
We've been talking about steels. Now I want to get into the weldability of steels, but first to remind you what we learned. If we look at cooling of steels, depending on the cooling rate — A, B, C, or D — and the time in seconds versus temperature, you might form martensite, austenite, ferrite, pearlite, or bainite, which have these different time-temperature profiles. This is a continuous cooling transformation diagram, a CCT diagram.
The transformation from austenite to ferrite, from austenite to pearlite, from austenite to bainite — these are all diffusion-controlled transformations, which is what most of you know about phase transformation. Some atom has to diffuse through the crystal, and it takes some time to do that. The austenite-to-martensite transformation is somewhat unusual. It doesn't just occur in iron-carbon alloys; it occurs in nickel-titanium alloys, some copper-based systems, a number of different systems have martensitic phase transformations. The martensitic phase transformation is basically just a shearing or displacement of the crystal lattice, so it goes from one structure to another, and that means it occurs at something close to the speed of sound.
The speed of sound is something on the order of 5,000 meters per second in a solid, so the time for the austenite-to-martensite transformation to occur is something on the order of 10 to the minus 10 seconds. Doesn't take long. There's no diffusion of atoms that has to occur — it's just a shearing of the lattice — and that means you will always get martensite if you don't get one of these others. If you can beat the nose of the C-curve, like this A, you will get martensite. Martensite is extremely hard, brittle, unless it's been tempered to allow some of the carbon to diffuse around and relieve some of the strains imposed on the lattice. So you don't want to get untempered martensite. You do want to get something that's hard; you sometimes like to get bainite.
You're going to get a transformation when you weld, okay, in the steel, and that transformation can lead to excellent weld matching. You can get the same strength as the base material. You can't always get that in some of the aluminum alloys, but systems that have a change in crystal structure from one crystal structure to another — such as the iron-based systems or the titanium-based systems — you can usually get matching strength between the weld metal and the base material. That's one of the advantages the iron-based system has.
§2. Hydrogen cracking: residual stress, arc strikes, and the Venn diagram [03:10]
Now I've told you that the Achilles heel of steels in terms of welding is hydrogen, and what can happen is you'll get cracking. Here's a picture of a hydrogen-induced crack in a weld. Typically they start at the toe of the weld, or sometimes down towards the bottom and the root of the weld — basically at stress concentrations. And there is a residual stress pattern here. You don't get cracks unless you have stresses, but just from the fact that the fusion weld is made by having localized heating, you're going to get localized stresses. The localized heating produces, on cooling, due to the thermal expansion differences, the residual stresses, and those residual stresses can result in cracking.
Another example. Something as innocent as an arc strike — an arc strike is just where someone hits the arc against the steel somewhere they didn't intend. These are stray arcs. It's not in the weld zone, but the guy's got his helmet on, he can't see through it unless the arc is giving a lot of light, and if he accidentally hits the steel somewhere, you'll get a little arc strike. That's what an arc strike might look like at higher magnifications on some steel. Here's an arc strike with little cracks growing from the edge. You end up with tensile residual stresses right around the outside heat-affected zone. The fusion zone was molten and doesn't have as high stresses, but you get a very high concentration of thermal stresses during cooling, and you'll get cracks radiating from that little arc strike. So arc strikes may be cosmetically unappealing, but they can actually be harmful, and many of the codes will require that you grind off the arc strikes. You don't like welders who go around hitting the electrode where they're not supposed to when the lights aren't on — when the arc's not going, so far as that goes.
Hydrogen cracking has many names. It can be called hydrogen cracking. Sometimes people confuse hydrogen embrittlement with hydrogen cracking. Hydrogen embrittlement is the fact that hydrogen in the steel can cause the steel to act in a brittle manner. Hydrogen cracking is the actual production of a crack, due to the hydrogen embrittlement and an applied stress. One thing worthwhile thinking of — there's what we call a Venn diagram that relates to hydrogen embrittlement. If I think of stress as one of the components necessary to form a crack, and hardness — the higher the hardness of the steel, the greater the tendency to cracking — and finally the amount of hydrogen that's present, these three combine to give me cracking.
If I have enough of all three — the reason we like a Venn diagram is the circles can expand or contract — and if I have all three of them in sufficient quantity, I'll get cracking in the intersection zone. If I can reduce the stress, or reduce the hydrogen, or reduce the hardness, I can get rid of my overlap zone in the center where all three come together, and therefore I don't get cracking. So those are the three things I can do to prevent hydrogen cracking: reduce stress, reduce hydrogen, or reduce the hardness.
§3. Why hydrogen migrates to the crack tip [07:13]
I was looking online this morning — there are a couple of nice videos that my secretary will send you a link to. I didn't find the one I was interested in, but I did find two others, one older and one younger. Sometime back in the 1970s, some people at Rensselaer Polytechnic took a piece of steel which they polished, and they had it so they could stress it, and they had a crack in it, and they immersed it in water and put it in a microscope, and they looked at just the crack tip, and they could see bubbles coming out. As they increased the stress, they would see the hydrogen bubbles come out with greater frequency right at the crack tip. Why is that? Well, hydrogen is an interstitial atom. It likes to strain the lattice, and if you strain the lattice, the hydrogen will go to the lowest energy state where the lattice is already expanded. That's at the crack tip. I'm pulling on the lattice at the crack tip.
If the hydrogen was uniformly distributed within the steel — such as my can of Sprite with the CO2 before I opened it up, now I have it in a solid, not a liquid — but if I take that steel with hydrogen everywhere, and I apply a stress, and I have any kind of discontinuity, the hydrogen will migrate to the crack tip. Now, it's a diffusion process and it takes some time. How much time it takes depends on how much hydrogen is present, the thickness of the steel, and a number of other things. But unfortunately for us, in the world we live in, the rate that hydrogen diffuses and can cause cracking happens to be greatest at room temperature.
If I take a notched specimen and I apply a stress and look at different temperatures, and I have a hydrogen-free specimen — this is strength, megapascals yield strength — and I look at hydrogen-containing specimens, right around room temperature is where I get the fastest cracking, or the cracking at the lowest stress. The reason is that at temperatures below room temperature the hydrogen can't diffuse. At temperatures above room temperature, the hydrogen can diffuse fairly rapidly and will come out of the steel — it'll just diffuse out. So if we heat up the steel, we can get rid of the hydrogen. We can reduce the size of this circle, the H2 circle. That's one way to prevent the cracking.
To put it in more concrete terms, this actually comes from sulfide stress cracking in a high-strength low-alloy steel. This is hydrogen sulfide, sulfide stress corrosion cracking. Sulfide stress corrosion cracking is merely another form of hydrogen cracking, but it occurs when you have H2S gas around, which you just as soon not be around because it's actually toxic. You can suffocate, you can die from breathing too much H2S. Anyway, this is time to failure versus temperature. At 25 degrees C, which is room temperature, we have cracking in less than 2 hours. If we cool down, it takes up to 20 hours, because the hydrogen can't diffuse to the crack tip. At higher temperatures I can diffuse the hydrogen out before it can all congregate to do its damage at the crack tip.
§4. Delayed cracking and the 48-hour rule [11:45]
So this quantifies it. I've been telling you that with hydrogen cracking, you have to do something about getting rid of the hydrogen within about an hour to four hours. This quantifies, at least for hydrogen stress corrosion cracking, but we know it for other things, that you've got to get rid of the hydrogen fairly quickly. But because hydrogen takes time to diffuse to the crack, you have time to heat the steel up and get the hydrogen out. That's why if you electroplate — we talked about electroplating of bicycle tubes — if they had plated the bicycle tube and gotten it into a 375-degree Fahrenheit furnace, which is the standard temperature, within an hour, they wouldn't have formed a crack, and the bicyclist wouldn't have become a paraplegic. A crack formed, and a fatigue crack grew from that.
If I go to one of the welding codes — and I'm talking high-strength steels for bridges — this code requires that you not test for cracks in your welds until 48 hours after you've made the weld. Why? Because if you test within the first hour, you might find no crack, and a couple hours later there might be a crack, and you just missed it because you tested too quickly. You have to sit for 48 hours before you can do your non-destructive testing if you're building a bridge or a building out of 100 ksi steel. [Tom holds up a welded HY-80 sample.] The US Navy building nuclear submarines — that's HY-80 — that's a weld about 1 inch thick. It's got a backing bar so the metal doesn't slip through. It's got a heat-affected zone. This was probably made back in the 1980s, but I coated it so it wouldn't rust. The cracks would form in the heat-affected zone if you didn't do something about it.
This is a typical diagram of weld cracking: toe cracks, root cracks, under-bead cracks. These are all hydrogen cracks that form during welding, but not right away. We often call this delayed cracking. It's hydrogen cracking, it's also called delayed cracking, it's sometimes called under-bead cracking. It's got five or six different names because lots of people have seen it lots of times. It still occurs frequently. I get several hydrogen cracking problems thrown at me each year, because — we know about it, we know how to control it, but that doesn't mean that people do.
§5. Sources of hydrogen; stick electrodes [14:39]
Now where does the hydrogen come from? What's the largest source of hydrogen in the world? Water. The ocean. It doesn't come from the ocean, but it does come from moisture, because moisture is everywhere too. So one of the key areas is moisture. What else contains hydrogen? Hydrocarbons contain hydrogen. So any grease or oil that's around, lubricants — I can tell you a story about welding of the Seawolf submarine, and I tracked it down in my opinion to lubricants. The moisture can even be humidity in the air. So one of the common things — and we'll talk about it some more — are stick electrode coatings.
Stick electrodes are still used. Back 40 years ago, 60% of all welding was done with a stick electrode. You take an electrical copper clamp, you clamp the electrode on this end where it has no flux coating, which will melt off and form a slag to pull out the impurities and protect the weld. It has a tip that's been brushed off so you can strike an arc — if you have a helmet on, hopefully, so you don't sunburn your eyes. The light intensity coming from the electric arc will give you a sunburned retina within 10, 15 seconds. If your sunburned retina is not very sunburned, all it does is, for the next couple of days until the sunburn goes away, it feels like having sand in your eyeballs. You can't get rid of it. That's what I've been told. I've never actually had it — I tend to use safety glasses, goggles, or a welding helmet. But if you do it long enough, you'll burn your retina and you'll go blind. How about that? Even better, right?
[Tom holds up sample electrodes.] So we've got the coating here, and the coating comes in various types. This is a basic coating. It has a little number on the side — says 7018. Can you see that? Here's a 6011. I think I have a 7014 here. I'll pass some of these around so you can see different electrodes. Actually, I'll keep two of them plus two others. So here's three different types of welding electrodes. [Tom passes electrode samples around the class.] There are dozens of types of steel electrodes for different steels.
The 6011 — the 60 means 60 ksi strength, the 11 is the type of coating and the type of polarity, AC, DC positive, or DC negative, from your power source. This is a cellulosic coating, and I should be able to bend it about 90 — well, not quite 90 degrees — before it starts to flake off. The coating flakes off depending on how freshly made it is. If it's freshly made, it's kind of rubbery, and I can often get nearly 90 degrees before it starts breaking off. If I take a 7018 now — these 7018s were baked at about 400 degrees Fahrenheit to get rid of most of the moisture, these were baked at a higher temperature, but these have been sitting around in the air, and this one — well, I got about 15 degrees before it started flaking off. That's because it's basically a brittle glassy coating. It worked better this time, didn't it, Jordan? I showed this to her a couple weeks ago.
In any case, the 7018 is baked around 400, this one's baked about 750. There's less moisture in the 7018. The 7018 is known as a low-hydrogen electrode; the 6011 is a normal electrode, or high hydrogen. We don't usually call it high hydrogen — we call the ones that are low hydrogen low hydrogen. So we have different types of electrodes because that controls part of the moisture.
Student: [question about shelf life of electrodes, off-mic]
Yes. Well first of all, they have a one-year lifetime. If you let them sit around for 10 years, they will pick up moisture from the air. In Arizona, not so much, but in New England they will pick up moisture from the air. Typically the 7018s have to be kept in a rod oven at about 200 or 250 degrees Fahrenheit, and used — if you're dealing with a high-strength steel, sometimes used within one hour of taking them out of the oven, because they'll start to pick up moisture right away. It's a question of how much moisture — I'll get into how much moisture, but I think your question is, how long can I leave them out. Cellulosic electrodes, you can leave out for a year, because we didn't try to drive the hydrogen off. The arc for a cellulosic electrode is 60% hydrogen. You're burning off the cellulose and creating hydrogen. We're actually using hydrogen.
The cellulosic electrode came from wrapping paper around an electrode in 1910, and they found, oh, they got rid of the nitrogen porosity in the steel. They didn't know it was nitrogen porosity, but that's what happened. By burning paper and creating an H2-monatomic-hydrogen-CO2 plasma, they excluded all the nitrogen in the air, and they got a good weld. A guy in Gothenburg, Sweden, found that sticking his bar electrode in the mud at about 1910 and welding with a muddy electrode excluded all the porosity that came from welding in the air, in terms of the nitrogen in the air causing porosity. I go through this in my fusion welding class in a little more detail. But that in Gothenburg, Sweden, became the beginning of what's now known as the ESAB company, which is one of the world's largest welding companies. They learned to use muddy electrodes. They were basically using minerals from the mud, but they had a lot of moisture.
Then we learned in the 1940s, when we were doing welding of ships for World War II, you need to dry out these electrodes by keeping them warm. You go down to New London, or Groton, Connecticut, where they're building nuclear submarines, they will issue you a dozen electrodes or 20 electrodes at a time, and you have to bring back the stubs to prove that you used up all 20 of them. They don't want you carrying around these electrodes for more than two hours at a time, and you may have an insulated little canister that you hold them in to keep them warm and keep the moisture off.
§6. Mild steel, high-strength steel, and the welder population estimate [22:14]
Student: [question about mild steel and cracking, off-mic]
Yeah, if you're using mild steel — as you brought up the term the other day, mild steel — it's almost impossible to crack mild steel. The residual stresses from a low-strength steel are so low, that circle is so small, you don't really — you've got to really work. I tried to do it in the laboratory once and I couldn't get it done in a week, with a number of different things. It's the high-strength steels, the things we use for nuclear submarines, for bridges when we want to save weight and we're using 100 ksi steel — you never would weld on aircraft landing gear, that's forbidden, 250 ksi steel, you've got a huge stress circle there. The only thing you can do is get rid of the other two things.
So you've got to get rid of at least one of these, and the higher the strength of the steel, the more you have to be careful about working on several of your approaches to avoiding hydrogen cracking. Mild steel, not a big problem. You can use cellulosic electrodes. You're welding a regular old pipeline — root pass goes in as 6011, cap passes go in at 7018. Those were the same diameter of steel rod, but the 7018 was much thicker on the flux. That's because it contains iron powder in the flux, and it gives you a higher deposition rate. You can weld faster with 7018 than 6011, type of weld you're not welding on something fancy.
Student: [question about excavators or heavy equipment, off-mic]
Right. Yeah, these excavators, a lot of them are using 100 ksi strength material, and it's stupid if you don't control your hydrogen on a high-strength steel. I've seen people lose their lives because of it. There's a lot of, I call it, farmer welding. I once estimated — 30 years ago I was trying to figure out how many welders are in the country. So I did an estimate. This is another story, I'm never going to get to welding. I knew at that time 60% of all the welding was done with stick electrodes. I knew how many pounds of these electrodes were sold each year. I knew how many pounds per hour a welder could put down. Put all this together and I calculated, in order to burn up all those millions of pounds of electrodes, you needed 700,000 welders in the United States.
I thought, well, not everybody's a full-time welder. Even a full-time welder may only be welding half the time. He's out doing something else for part of the time, and a lot of them are auto mechanics, farmers, whatever. So I estimated, if I calculated 700,000 full-time welders, then maybe there are half a million people who would consider themselves a full-time welder, whether they welded 40 hours a week or 20 hours a week, and another one and a half million who were part-time welders. You're a part-time welder, you've welded before, that makes you a part-time welder. I go to some welding conference and I say this, just to give some people some perspective, because I couldn't find it in the literature.
The eighth edition of the welding handbook comes out about early 1990s, and I'm flipping through it when I get my copy, and I see right there near the introduction, it says there are about 2 million welders in the United States. I thought, that's the same number I got. But it had a reference — the Bureau of Labor Statistics. So I called American Welding, I said, what's the reference, the Bureau of Labor Statistics, that has this number? They said, oh, they called us up and wanted a number, how many welders, so we gave them your number. So at least I knew how it was calculated. Bureau of Labor Statistics didn't know how it was calculated and they were using it, but I knew how it was calculated, because I did. That's one of a couple of times where I made some estimate and all of a sudden it became the gospel truth in the world.
The other time is, how much does it cost to build a steel plant. My oldest daughter was working for Clayton Christensen one summer when he was writing his book, The Innovator's Dilemma. She came home — she was in college, it was a summer job — and said, Dad, Clayton wants to know how much it cost to build an integrated steel plant and I can't find that anywhere in the literature. I said, you won't, Rebecca, because no one's built one except a country since — you know, I told you that story yesterday. I said, however, I have a slide that I use, and I'll bring you a copy of it. This slide was put together in a little airport in northern Michigan. I was sitting there with a graduate student, and we'd just visited General Motors, and we were talking about steel plants, and I just sketched out — this is what a mini mill would cost approximately, this is what — and I had $15 billion for an integrated steel plant. So I gave Rebecca my slide, and I'm referenced in Clayton Christensen's book. We just pulled it out of the air at an airport in — oh, Saginaw, Michigan. I was in Saginaw, Michigan, sitting in an airport, nothing else to do, I came up with a number. And there it is — the rest of the world now knows it costs $15 billion. They haven't got a clue where the number comes from, but they will all reference it. There actually is a basis for my pulling out these numbers, but anyway, just pull the number out. There's probably a few others I don't even know about. Just tells you about scientific rigor.
§7. Nautilus and the move to austenitic electrodes [28:48]
Anyway, did you have another question? Well, these little stories keep life going here. So — moisture, humidity. You're not supposed to weld, according to the US Navy, you're not supposed to weld a nuclear submarine if it's more than 85% relative humidity. Do you know how many days in a shipyard right next to the ocean on the east coast of the United States, which is where we build submarines — either Newport News or in Groton, Connecticut — it's above 85% relative humidity? About 200 days of the year it's above 85% relative humidity down there next to the water. Not too surprising. So do they quit welding for 200 days a year? No, they keep welding. They're supposed to be controlling the hydrogen by keeping the moisture out, but in fact they do a few other things on these other circles, and so they don't have big cracking problems.
But they did, when they went to build some of the mockup for the Nautilus submarine in the mid-1950s, which was the first nuclear sub. Before that they just used low-strength steel. The other night, Run Silent, Run Deep was on, and the Olympics hadn't come on, so I just put on this old Clark Gable, Burt Lancaster movie, and that submarine, when they dive deep, they go to 200 feet, because they just used regular old low-strength mild steel, whatever you want to call it, for submarines in World War II. For nuclear submarines, when they came with the Nautilus, they were using HY-80 — that's the piece I passed around — 80 ksi as opposed to 35 or 40 ksi. And so they could dive deeper. I won't tell you how much deeper, because it's classified, but it's a lot more than 200. It is more than double 200, because they did other things on the design and fracture mechanics and other stuff. They can go a lot deeper than 200 feet. But I can't tell you how deep.
When they first started welding some of these egg-crate type of construction — all these stiffeners and stuff in the submarine — they'd welded up during the day, all kinds of residual stresses. So the big circle there — they didn't have very good hydrogen control on their electrodes. They didn't check them out back in 1955 where you could only have them out of the oven for 2 hours, and you had to bring back a stub for every electrode you got. There's some little clerk there counting your stubs before you can get more electrodes. They didn't have all these hydrogen controls. And so some guy welding at night would hear this piece that had been welded during the day, and it'd be a big boom, like a rifle shot. A crack — a hydrogen crack — just ran through the part they had made that day, 8 hours earlier. Delayed cracking. The hydrogen diffused to the crack tip during those eight hours.
What did they do? They had to keep building some of these things. So they used austenitic stainless steel. Has a face-centered cubic structure. There's lots of room for the hydrogen in a face-centered cubic structure as opposed to a body-centered cubic structure. The cracking susceptibility of austenitic stainless steels is much less than body-centered cubic steels. So just to get some of their work done, for their structures and stuff, they ended up welding with austenitic electrodes before they finally learned how to do it successfully in the steels with ferritic weld metal of matching strength. Problem with the austenitics: you wouldn't get the same strength.
§8. Iron-hydrogen phase diagram and quantifying solubility [33:01]
So here is the iron-hydrogen phase diagram, and we'll come up with some numbers of how much hydrogen will be in the steel. I've done the one-atmosphere curve — this is a plot from Carl Zapffe, who was a hydrogen cracking expert 50, 60, 70 years ago. Tenth of an atmosphere, one atmosphere of hydrogen, 10 atmospheres, 100 atmospheres of hydrogen, as a function of temperature. Here's the melting point of iron, and above the melting point, you can dissolve — it says cubic centimeters of hydrogen per 100 gram of iron. That's within 10% of parts per million atomic, or parts per million by weight. Relative volumes of hydrogen, atomic percentage — but if we're interested in weight percentage, that's 30 ppm.
So at the melting point at one-atmosphere hydrogen atmosphere, the equilibrium is 30 parts per million hydrogen, or cubic centimeters per 100 gram of iron. That's just because that's the way you measure it. You basically put your sample into a little chamber and you measure how many cubic centimeters of hydrogen come out of the specimen after you heat it up inside this closed chamber. At the melting point, it rejects hydrogen down to less than about five or six. Then you get down to where the delta iron transforms to gamma iron, and you see the higher solubility of hydrogen in austenitic steels. Then you transform at 910 to BCC again, and you see that the solubility goes down to something on the order of one part per million. So that's why the hydrogen's there.
My secretary will send you, if she hasn't already, a little video that was done by what used to be Norske Veritas — DNV now — and they used to classify ships. Now they do all kinds of metallurgical consulting around the world for big oil companies and people. They have a nice little video on hydrogen cracking, an 8-minute video. I watched it for the first time this morning. You ought to watch it. It's going to show you some of the same things I'm telling you, but it's got videos where they weld on a piece of steel, stick it in glycerin, and then they take a video of the hydrogen bubbles coming out. You can see them in the glycerin. If you do it in water, it would be coming out like my Sprite, all in one big foam. You do it in glycerin because the viscosity of the glycerin lets you actually see the hydrogen coming out, and it's a lot, believe me. It's worth watching. Well, I don't know, you may not think it's worth watching — it's a homework assignment, so you can decide whether to do your homework.
So I'll send you that. It's going to talk about hydrogen cracking in general. There's also another one I'll probably send you that some people in the Netherlands did in the 1950s with similar types of tests. And then I was looking for one that had been done at RPI, a video, I didn't find that one on YouTube. But in any case, there are books, whole books written on hydrogen cracking in steels. [Tom holds up a reference book.] This is the second edition from the British Welding Institute, Welding Steels Without Hydrogen Cracking, and there's all kinds of authors. Frank Coe wrote the first edition by himself, but he had lots of help with the other editions. In that book, there is this plot, which was in Coe's first edition. So Frank Coe put this together, very intelligent man, as opposed to a lot of his co-authors for the second edition. Weld hydrogen level, milliliters per 100 gram of deposit — that's the same as cubic centimeters, so you can think of this as parts per million.