WM_S2014_13

Well actually before I do that I'm going to give you a little bit on preheat versus post-heat. So preheat. There's three levels of preheat. Does anybody now remember what level Mike Baumforth said yesterday for martensitic stainless steels, what temperature you said you had to preheat the steel to? It's like 600 degrees Fahrenheit okay. How'd you like to weld on a piece of steel that's at 600 degrees Fahrenheit? Okay it's not very comfortable.

And when they had the problem with the Seawolf submarine, these guys had to go into enclosed containers where the steel had to be at 400 degrees Fahrenheit to prevent the hydrogen cracking. They had to wear blue jelly suits. They were only allowed to be in there for like twenty minutes at a time. So they'd have a little cart like a mechanic's creeper to go underneath a car, and they'd roll them in there, and they'd have to have someone outside because this was a confined space. And they put them in blue, they called them blue jelly suits. They had to wear this suit that had this blue liquid in it that would cool them down because they were going into a 400 degree oven to do their welding. And they had respirators so they could breathe cooler air so they wouldn't burn up their lungs okay. But they go in for you know like twenty minutes and then have to come out and have a rest, and another guy would go in there. It got a little expensive to build that submarine okay. But it's because they had too high strength and didn't have good enough hydrogen control.

So there's three levels of preheat. One is, 50 degree, must be greater than 50 degrees Fahrenheit. One is you heat the steel to, from let's say 100 degrees Fahrenheit to 300 degrees Fahrenheit. And the third level is greater than 300 degrees Fahrenheit. Well what are we doing with each one of these?

The ASME Boiler Pressure Vessel Code says you shall not weld on steel if it's below 50 degrees Fahrenheit. I don't care if it's the mildest steel in the world, you don't weld if it's less than 50 degrees Fahrenheit. The structural welding code, which is probably this, well these two codes — ASME Boiler Pressure Vessel Code, ASME is American Society Mechanical Engineers for all umes — and the boiler and pressure, and the structural welding code which is basically the welding engineers, [American] Welding Society. But civil engineers use it, anybody building bridges and buildings uses this. Anybody building pressure vessels uses ASME code. This says basically, for less than an eighth of an inch you've got to be at 32 degrees Fahrenheit, or zero degrees centigrade. That's if you're [an] eighth to three quarter inch inclusive, otherwise for different steels it might be 50 degrees or 100 degrees. Here's a steel over here and it's going to be 32 or 50 depending on the thickness okay.

Why? They got a minimum temperature. Well obviously you don't want to weld on top of ice right. Why is ASME say 50? Because even though you can't see it, there's a layer of moisture on steel. There's a layer of moisture on that table as we're talking right now at room temperature. But as you get lower and lower in temperature towards the freezing point of water, that layer of moisture gets thicker and thicker. 50 degrees drives off enough moisture that they have found they don't get cracking by welding on steel that's above 50 degrees. So that you've got to be a 50 degree minimum. Surface moisture, you got to drive off surface moisture. No matter how mild the steel is, no matter how low the carbon is, no matter how low the hardenability of the steel is, you still got to get rid of that surface moisture. And that's why the two codes say 32 degrees, 50 degrees whatever. You don't weld on snow, under snow.

Okay, 100 to 300 degrees F, what am I doing? I'm slowing the cooling rate. If I look at a thermal profile for striking an arc, temperature versus time, very rapid rise in temperature up to the melting point and then it cools down on an error function solution for all you mathematicians out there, which is all of you MIT students. But it slows down to some ambient temperature. If I'm heating up rapidly to let's say uh 2500 degrees Fahrenheit, 1200 degrees or 1300 degrees centigrade, or actually it's about 1600 degrees centigrade, and I cool down to room temperature, this stuff gets to low temperatures within a couple of minutes, and I trap all that hydrogen. Remember 25 degrees is the worst temperature I can be at.

If I heat it up to 100 degrees or 200 degrees it will cool down, but it will cool down to a higher temperature, and it will stay at that temperature for this great big piece of steel that weighs several tons for an hour or two. And that allows the hydrogen to diffuse out. I'm taking care of that, I'm taking advantage of that little V-shaped curve I showed you. At higher temperatures, even in this temperature range, I'm gonna not have minutes before I get down to room temperature, I'll have hours before I get all the way down. And it's like doing a bake out at either 100 degrees or 200 degrees or 300 degrees, whatever my temperature was that I preheated to, right.

Above 300 degrees, I'm tempering the steel. And by tempering the steel, turns out steels will lose their hardness if you form martensite in the welding process, and you then hold it at temperature for an hour or whatever. Once you get to 300 degrees centigrade — or 600 degrees centigrade, anyway 600 Fahrenheit — you will actually, 300 you start to see a drop off in hardness. And so if this is 300 Fahrenheit, you actually will have enough time, this is probably log time as I've drawn this, you have enough time in here above what you would have if you were just letting it cool to room time. It's just cooling down and being quenched to room temperature by the steel.

So what am I doing if I start looking at my Venn diagram? Okay the first one, I've got whatever stress I've got, some hardness which is fairly large, but I'm trying to make the amount of hydrogen circles smaller. So my Venn diagram, I'm reducing the hydrogen by getting rid of the moisture. On this one I'm doing the same thing, but I'm reducing the hydrogen by giving more time for the hydrogen to diffuse away. Because this steel is even harder, what am I doing here? I've got my Venn diagram, I'm trying to reduce the hardness and the hydrogen. I'm working on both of them okay.

So, you won't find this explanation anywhere. I made it up a couple weeks ago. No I did, as I was looking at preheat, and said well — and I've always sort of known it, but until I had to teach it to you I hadn't really articulated it. But if you think about a Venn diagram, I actually have, they just talk about preheat and they say this is the preheat use, it's a cookbook, you go to Stout and Doty [Stout & Doty], it tells you. But if you actually want to start thinking about it, and how we control hydrogen, we walk through it this way okay. So that's enough for today. Tomorrow we'll be here and we'll, hopefully we'll finish up preheat and post-heat and all that stuff.