Hello All,
Hopefully some of you can chime in on this. One of the guys I work with was in a recent discussion with an engineer about moisture in metals and preheating. The engineer said that moisture cannot be absorbed into metal and is only on the surface. I tend to believe otherwise. What do you all think?
By surface he must include the oxide layer. My take is that the thin oxide layer can absorb moisture but the core metal can not.
That is that you see when you preheat. The thicker the oxide layer the more mositure you will see when heating. Try taking a grinder and smoothing the surface. I bet you won't see any moisture if it is bright and shiny - at least that has been my experience.
OB
This is an old discussion that derives from the manual preheating of steel with a torch.
The common phrase "Heat it until you knock the water out.", comes from the moisture that can be seen appearing on the surface of cool metal as it heats up.
Most think this moisture is being "sweated" out of the steel because the steel has absorbed it.
This moisture does not come from the steel, as steel does not absorb moisture (except in the case on a thick oxide layer as OBEWAN noted).
The moisture is a direct result of outside air condensating on the steel surface as it heats up.
Tim
I must admit to being one who thought the steel absorbed it. That is why I come to this site. Learn something new every time I do. Thanks guys.
That would be the first case of moisture condensing on an object as it gets hotter that I have ever seen. Does moisture condense on your coffee cup or your iced tea?
I am far from an authority, but I have actually tried to read up on this a bit a while back, and the consensus seems to be that there is a potentially a small amount of moisture in the oxide layer, and the rest is a product of combustion from the torch. You can easily test this by heating on one side of the plate and observing what happens on the other side.
But it sure is.
Look, the metal starts out cold. Forget about the fact that it's getting hotter, it starts out cold.
Combustion creates CO2 and H20 (ok, complete combustion, but you get the point). Combustion gases are very hot. Hot air can dissolve much more water vapor than cold air.
When that hot moist gas comes into contact with the cold surface, you get condensation, as the water vapor becomes supersaturated when the gas cools. It is really that simple.
Oh, and yes, I DO get condensation on my iced tea, and the back side of a piece of sheet metal getting heated by a torch will not have condensation on it.
Also, you do NOT get condensation on sheet metal when heated by a TIG torch, or by a carbon arc torch.
There is no water inside metal to "sweat" out.
The oxide layer on steel does not contain appreciable amounts of water. It is not that porous. Aluminum oxide, when built up into thicker layers such as from anodizing is kind of like a sponge, but the volume of water that appears on the surface of a piece of metal that has been exposed to a flame is significantly greater than the total volume of any oxide layer, even on anodized aluminum.
I am not an expert as many of you are, but I also feel that the steel itself does not absorb moisture. What we have is just as basic as a warm and cold weather pattern clashing,and what do we get,condensation in the form of rain. That is just a simple fact of life.
The moisture observed when preheating a cold piece of metal with a torch is the by-product of combustion, i.e., H2O, condensing on the cold surface.
Any hydrocarbon fuel, whether it is wood, coal, oil, or a gas such as acetylene, liquefied natural gas, etc., produces waters as one of the by-products of combustion. Even gasoline produces water as a by-product of combustion when it is used in the automobile engine. That is why you see the condensate dripping from the cold tail pipe of the car on a cold morning. Once the base metal or the muffler system of the automobile heats to a temperature above the dew point, condensation ceases because the moisture will stay in the gaseous state.
We had to do an experiment in physics once upon a time, long, long ago. We had to measure the BTU or the heat content of natural gas. The experiment consisted of a boiler, two flow meters, two thermometers, and a Bunsen burner. We measured the volume of gas consumed by the Bunsen burner set under the boiler. We measured the temperature of the water going into the boiler and the temperature exiting the boiled as well as the volume of water fed through the boiler.
The old professor had everyone place a saucer under the Bunsen burner. Once the experiment was started one team asked for another boiler. Theirs was leaking. Then another team asked for a different boiler because theirs had also sprung a leak. Then a third group asked for a different boiler as well. The old professor turned to me and said, "Al, my little welder friend, explain to these genius' where the water is coming from."
I answered back that it was simply one of the by-products of the natural gas burning and condensing on the underside of the boilers. They continued to condense the moisture because the boilers where kept cool by the cold water running through them. He said, "See, even a welder understands the principles of chemistry and combustion."
As long as the temperature of the boilers was below the dew point, the moisture from the flame, i.e., the by-product of combustion, condensed on the cold metal. The same principle applies to preheating base metal with a torch that burns a hydrocarbon fuel with air or oxygen.
If you were to heat the cold base metal using an electric heating element or infrared lamp there would be no evidence of "sweating" from the base metal, surface oxide, or other sources unless of course there was ice or free standing water on the surface to begin with.
On a very cold morning you can see water condensing on the surface of a cold plate if you make a small tack weld with E6010 or E6011. The cellulose in the flux covering produces water as a by-product of combustion. The water, in gaseous form, condenses on the cold metal immediately around the arc. As soon as the plate heats up slightly, the moisture no longer condenses. So look quick because it doesn’t last more than a couple of seconds. It is this very same moisture that disassociates in the welding arc that provides a wonder source of hydrogen for absorption by the molten weld pool.
If any of you have a diesel driven welding machine, you may have encountered a situation called, “Wet Stacking”. That’s where the engine is idling at such a low speed that the exhaust system does not heat up sufficiently to keep the exhaust gases in the gaseous state. The moisture condenses inside the stack. Not a good thing.
Best regards – Al
Just another though on the subject, if there was actually water in the base metal, it would infer that there were voids in the metal that would act as reservoirs to hold the moisture. That could present a problem for the beverage industry because their metal cans, measuring a few thousandths in thickness, would tend to leak.
If the base metal, e.g., the steel, was porous, as inferred by the presence of voids necessary to hold the water, submarines would tend to only sink, never to regain sufficient buoyancy to be refloated. Ouch!
Al
thought I would add my 2 cents, metals actually tend to have some non-zero rate of diffusion for water vapor, and generally do carry micro voids and porosity which traps some non-zero level of moisture. If you think about it delayed hydride cracking is generally due to dissassociated hydrogen from water chugging it's way down grain boundaries and stress risers.
But in the macro sense, no metals generally are not porous and don't "hold" moisture in any great quantity relative to their own weight or volume. I've been doing quite a bit of research in this field and I won't say more, but you won't really offgas any moisture from a piece of metal in anything above maybe a few parts per million. Preheat just helps give you above the temperature where condensation would play a factor and also helps drive away surface contaminants by burning them off.
All of that being said, oxide layers, flash rust, and especially surface contaminants like hydrocarbons (shop oils) dirt, mill scale etc all do their job at holding moisture adherent to the surface, for things like dirt or flash rust it will hold on to the very end and get dissassociated by the arc. Again unless you welding with parts you roll around in the grass outside usually these levels aren't big enough that you can notice them with anything besides science instruments and metallographic examination like electron microscopes or other destructive methods. So unless you have a half million of equipment around that can analyzer water vapor in the parts per million range you'll never really know it's there.
Wish I could say more, but it's really not prevalent above ~100 ppm range in controlled atmospheres. If you aren't welding in a vacuum chamber or bubble chamber you probably have a lot more pertinent sources of moisture than the metal, namely how humid it is, surface contaminants and the likes.
Simple answer: No metals don't offgas or retain significant moisture.
Thanks Al.
That is probably the best explanation I have every heard/read about the moisture on steel when pre-heating.
If my simplistic mind has put this together correctly, the easiest way for many of us to think about it is relative to 'Dew Points'. Just as the dew that accumulates on the front lawn when atmospheric temperature conditions come together just right. And, your example of the exhaust pipe as well.
Good explanation with excellent and understandable examples.
Have a Great Day, Brent
By 803056 
Date 07-04-2012 19:36
Edited 07-04-2012 19:46
Another example of dew point is the sweat on the cold water pipes on a humid summer day.
I remember the time my wife told me we needed to replace all the plumbing because the pipes were all leaking. I asked if all the pipes were leaking or just the cold water pipes. She looked at me kind of puzzled. "How would I know the difference?" she asked.
I said touch the pipe that is dry and the pipe that is wet. If the wet pipe is hot, we have a problem. If the wet pipe is cold, we have a while yet before we need to replace them.
She accuses me of being “Mr. Wizard”. That kind of tells you how old she is. That television show has been off the air for nearly a half a century!
This is the same lovely lady that told me she needed a new car one cold winter day. I asked her, "Now why should you say that you need a new car? That car only has 100,000 miles on it?"
She said there is water running out of the tail pipe. Mind you, she has been driving nearly half a century, but she never noticed water coming out of the tail pipe before.
I asked her, "Is the water clear or is it colored?"
She said, "What other color could it be?"
I said, "If it is dripping green, we have a problem. If it is clear, it is fine."
"I want you to go look at it." she said.
"It’s your car and you are already dressed to go out." I told her.
A couple of minutes later she trudged back into the house and said, "It’s clear."
"No problem. It’s good for another 100,000 miles." then I rolled over and went back to sleep.
Best regards - Al
That was a great explaination.
I thought of the products of combustion too but did not thnk to post it because I was not 100% sure. But I do know about water in my car exhaust. And that is one reason exhaust systems used to rust out sooner for people who had a very short drive to work - that is before stainless steel exhausts.
I've never noticed condensation from a cellulosic rod, but it makes sense. They would have enough water to cause that.
You can get the equivalent of wet stacking in a gasoline engine, if you only make really short trips, like going to the corner deli, stopping, and going home a little while later, but one 20 minute drive, and it's all good. Those cars "only driven to church by that little old lady on Sundays" can be in much worse condition inside the engine than they would appear by looking at the paint.
Very well put.
It is not condensation, it is the by-product of combustion. Granted, there is some moisture in the covering acting to bind all the materials to the electrode, but think of it this way. What happens to paper when you introduce it to a very hot heat source? It burns. The by-products of combustions? You got it right again, CO2 and H2O.
Best regards - Al
The byproduct of combustion is water vapor (this is essentially steam). The water that appears on the surface of the metal is liquid, hence it has condensed.
It IS the byproduct of combustion, but it is condensation when you see it on the cold metal surface.
Another way to look at it, is that the metal is above the dew point of the ambient air, so water isn't condensing on it outside of the flame, but the metal is below the dew point of the gas within the envelope of the flame, so you see condensation.
This is an excellent thread...I have learned a thing or two.
Now down to a "real world" point......light preheating on steel structures before final weld out is a sound process/technique. Don't complicate it the issue by bringing up different alloy combinations. On mild steel, a36 common structural stuff it is simply beneficial to the weld deposit quality...yea?
Ok, I've re-read your comment, and paper (and the cellulosic coating on 6011) does indeed have moisture content locked inside, aside from the water liberated from combustion.
A google search tells me that paper is from 2% to 12% water. Firewood has lots of water in it. Like paper, you can heat it, without burning it. When it turns black, you've made charcoal, and driven off all the water. The difference in weight between charcoal and the wood that it came from, was all water.
So, burning 6011 will drive off some water. I suspect though that the combustion of the cellulose will create more water, than what was simply absorbed into it though (I just haven't bothered to do the numbers). It's just that cellulose and sugars are loaded with hydrogen atoms.
If the steel absorbs water you could use it for window screen !! LOL
