[Markus]

I agree with Len.

You must consider the effect of the backing gas on the molten steel. In the example of 75/25, CO2 is an active gas, and when heated to plasma state will disassociate, and can deposit carbon in the weld, and form oxides in the steel. Most mixes will have some active component, the reason for the mix, and, as stated, most are costlier than 100% argon. Nitrogen is not a big issue for mild steel (though argon makes a smoother root), but can cause "nitrogen embrittlement" in Austinitic stainless steels like the 300 series.

I will use nitrogen on SS in two cases. Vent piping that sees no pressure and would require a lot of argon to purge, and the outer vessel of a large vacuum-jacket vessel. In the first case, it's strictly economy. In the second it's both economy (due to the huge amount of argon it would take), and simplicity, as I break the vacuum with nitrogen, and therefore don't have to wait for atmosphere to be displaced by the purge gas.

I've made backpurge containment from aluminum foil and tape, with the tape two inches from the weld, when tape residue from the heat isn't an issue (It's a bitch to get "oxygen clean" when that's required).

Steve S

quote but can cause "nitrogen embrittlement"

Hmm how is this possible? Some of 300 series austenitics grades are alloyed with nitrogen. Reason for this is nitrogens ability to form austenitic phase and also it helps to prevent carbides forming.
I been told too that in some extreme cases you have to use argon-nitrogen mix shielding gas to prevent ferrite phase. Usually when 100% non-magnetic weld is needed.

There has also been some discussion that root gas doesn't affect very much on phase formation. Whereas shielding gas can affect quite a lot.

[Otto Nobedder]

Markus,

The way I understand it, the migration of nitrogen into the weld, though minute, has an effect at cryogenic temperatures. Most of the SS process piping I work on is super-insulated, vacuum-jacketed, and cycles from ambient temperature to liquid hydrogen at -423F, so any undesired effect is greatly magnified by the temperature shock. Additionally, the tendency for cryogenic temperatures to make materials "brittle" in the first place is a concern for ANY impurities in the weld.

A failure in my work could have serious, potentially deadly consequences, so I consider all the risks I'm aware of.

Steve S

[Superiorwelding]

Jonathan,

By saying it's setup for 8" pipe, are you implying that the radius of the 'C', if you will, is the same as the outside radius of an 8" pipe?

GreinTime,
Here is a picture of how it sits now. Have you ever made these or something like it before?
-Jonathan

[GreinTime]

Honestly man I can say I've never welded anything where I needed one. Hell I've never even tried to weld stainless or titanium or anything haha.

Sent from my DROID RAZR using Tapatalk

[Markus]

Markus,

The way I understand it, the migration of nitrogen into the weld, though minute, has an effect at cryogenic temperatures. Most of the SS process piping I work on is super-insulated, vacuum-jacketed, and cycles from ambient temperature to liquid hydrogen at -423F, so any undesired effect is greatly magnified by the temperature shock. Additionally, the tendency for cryogenic temperatures to make materials "brittle" in the first place is a concern for ANY impurities in the weld.

A failure in my work could have serious, potentially deadly consequences, so I consider all the risks I'm aware of.

Steve S

Maybe a little off-topic, but this is getting interesting

In temperatures like that material is definetly pushed into it's limits. However I still doubt that nitrogen would affect in bad way. I really don't know, but it just feels odd that austenite phase couldn't deal with all nitrogen. After all, it does have face centered cubid, which does have more room for atoms than ferrite phase for example.

In the other hand nitrogen usually have more inpurities in bottle than argon 5.0 example, when delivered. That can definetly cause problems, but then again there is high purity nitrogen avaible too.

You got interesting job btw. Propably really demanding what comes in standards and how everything is done. What shielding gas you use? Is it 316L btw?

[Superiorwelding]

I figured I would join in on the purge gas discussion. Instead if talking, read this article by Praxair, specifically page 18. It does show Nitrogen as an acceptable purge gas for 300 series SS.
http://www.awssection.org/uploads/longi ... Manual.pdf
-Jonathan

[Otto Nobedder]

Markus,

A little review/research on my part tells me I may have misspoke. What I just read suggests it's "hydrogen embrittlement", and the excess of nitrogen in the weld (somehow) leads to formation of martensite in the grain boundaries of the weld during cooling, leading to the embrittlement. I'll have to study this deeper to give you a solid answer, as some of the information seems to be conflicting. I will maintain the practice of argon backpurge for all critical welds, regardless, as there are no doubts in this practice.

304 is the preferred material for temperatures to liquid helium, at, what, -457F? We use 308 filler rod. I've always disagreed with this practice, though even the engineers won't listen. 308 is considered fine in the industry for temperatures down to -320F (liquid nitrogen), but for colder, a specific alloy (who's long designation I don't recall) is recommended. That said, our primary client has been specifying 308 filler for over 40 years with no reportable issues, so I don't have a strong argument.

Sheilding gas is always argon (process piping is always TIG), and we specify high-purity.

Thanks for the observation... I DO find my job interesting. I also rarely go more than a few days without learning something that surprises me.

Steve S

[Otto Nobedder]

Good catch, Jonathan,

However, AWS does not deal in cryogenic service welds, at least in that document....

I've said before, I use nitrogen for backpurge in vent systems and outer vessel work that does not see cryogenic temperatures. It's economical and effective. I have no problem recommending it to anyone for most piping. The average user will notice little difference (argon will give a slightly "smoother" root, for want of a better word).

The distraction here is apparently mine specifically, due to the conditions my welds encounter. Consider a 10' piece of SS going abruptly from 100F (in summer conditions) to -423F, in a matter of seconds. It will shrink significantly. I'd have to look up the number, but even 1% is 1.2". All piping is designed with expansion loops in it, so all joints in these loops see significant pulls and pushes as the temperature changes around 500*. This is why every niggling little detail is considered, right down to backpurge gas.

Steve S

[Superiorwelding]

Steve
For our study, do you know off hand what AWS or ASME specs you weld to on a daily basis? I would like to study these.
-Jonathan

[Otto Nobedder]

Jonathan,

Beyond ASME IX (which I can share an outdated copy of), our WPSs are considered "confidential" under the new ownership.

They do, in almost all respects, conform to ASME IX.

If you would like a copy (mine is from 2001), I can send it. Just PM me your email. (As a moderator, I have access to that info, but won't take advantage of it. Everything here should be between gentlemen.)

Steve S

[Superiorwelding]

Duh, I knew that, even "borrowed" your copy a while ago. I have found some interesting info on this subject and am reason through it and post my findings and websites later.
-Jonathan

[Otto Nobedder]

Wow, short-term memory loss...

Yes, I forgot I'd already shared that with you.

In almost all respects, what I do conforms to that code. I'd be hard-put to name a specific difference.

Steve S

[Markus]

Markus,

A little review/research on my part tells me I may have misspoke. What I just read suggests it's "hydrogen embrittlement", and the excess of nitrogen in the weld (somehow) leads to formation of martensite in the grain boundaries of the weld during cooling, leading to the embrittlement. I'll have to study this deeper to give you a solid answer, as some of the information seems to be conflicting. I will maintain the practice of argon backpurge for all critical welds, regardless, as there are no doubts in this practice.

304 is the preferred material for temperatures to liquid helium, at, what, -457F? We use 308 filler rod. I've always disagreed with this practice, though even the engineers won't listen. 308 is considered fine in the industry for temperatures down to -320F (liquid nitrogen), but for colder, a specific alloy (who's long designation I don't recall) is recommended. That said, our primary client has been specifying 308 filler for over 40 years with no reportable issues, so I don't have a strong argument.

Sheilding gas is always argon (process piping is always TIG), and we specify high-purity.

Thanks for the observation... I DO find my job interesting. I also rarely go more than a few days without learning something that surprises me.

Steve S

Yup this definetly need further studies and this is perfect material for my thesis works I have already searched knowledge about usage of nitrogen in normal conditions, but never paid attention to such low temperatures. I will try to find some material too about the subject.

Why 304 btw? Wouldn't 316 be a better choice. After all 316L have much less carbon, which help to prevent formation of martensite. Is it possible to get martensite in grain boundaries btw? Wouldn't it be just a new phase along with austenite?

What about formation of ferrite in the metal structure? Wouldn't it be better to have 100% austenite microstructure? Ferrite usually is not very good at low temperatures. For example duplex stainless does not stand as low temperatures as 304. Reason for this is that it microstructure does have around 50 % of ferrite.

It's great to notice that forum has people who also pays attention to microstructure and things like that Of course this forum is mainly not for PHD's, but sometimes it's good to understand the microstructure aswell.

[Otto Nobedder]

...

Why 304 btw? Wouldn't 316 be a better choice. After all 316L have much less carbon, which help to prevent formation of martensite. Is it possible to get martensite in grain boundaries btw? Wouldn't it be just a new phase along with austenite?

What about formation of ferrite in the metal structure? Wouldn't it be better to have 100% austenite microstructure? Ferrite usually is not very good at low temperatures. For example duplex stainless does not stand as low temperatures as 304. Reason for this is that it microstructure does have around 50 % of ferrite.

All good questions, raising my curiousity evern more.

I can answer the 304 vs 316 question, though. Economy. 304 is proven and more than sufficient. Since there's a good 4 or five tons of it, the cost difference would be significant.

As to the other questions, I'll have to find time to dig. It's been three or four years since I looked this stuff up, and may not be remembering the details correctly. I hope you'll share anything you find, as well.

As a side note, my research on stress corossion cracking will likely lead our main client to specify duplex stainless for the stainless sections of the outer vessel for the new units they're ordering. The trend I'm noticing is that the problem shows up at about 25 years of service, though the design lifespan is 40 years.

Steve

[Otto Nobedder]

Here's a decent article on the effect differing level of nitrogen have on steels, including weld effects, and the effect of temperature on the failure modes:

http://www.keytometals.com/page.aspx?ID ... =EN&NM=202

It does not specifically mention nitrogen back-purge as a mode of introducing excess nitrogen, but does support the idea that extreme low temperatures and excess nitrogen are a bad combination.

An abstract I read also strongly suggests "economy" isn't the sole reason for choosing 304. Apparently, 316, 321, and 347 are progressively more sensitive to the effect of nitrogen on "hydrogen embrittlement". The abstract alone had no supporting evidence, and I didn't purchase the full text since I can likely find a government document for free.

Steve S

[Bill Beauregard]

Markus,

A little review/research on my part tells me I may have misspoke. What I just read suggests it's "hydrogen embrittlement", and the excess of nitrogen in the weld (somehow) leads to formation of martensite in the grain boundaries of the weld during cooling, leading to the embrittlement. I'll have to study this deeper to give you a solid answer, as some of the information seems to be conflicting. I will maintain the practice of argon backpurge for all critical welds, regardless, as there are no doubts in this practice.

304 is the preferred material for temperatures to liquid helium, at, what, -457F? We use 308 filler rod. I've always disagreed with this practice, though even the engineers won't listen. 308 is considered fine in the industry for temperatures down to -320F (liquid nitrogen), but for colder, a specific alloy (who's long designation I don't recall) is recommended. That said, our primary client has been specifying 308 filler for over 40 years with no reportable issues, so I don't have a strong argument.

Sheilding gas is always argon (process piping is always TIG), and we specify high-purity.

Thanks for the observation... I DO find my job interesting. I also rarely go more than a few days without learning something that surprises me.

Steve S

Yup this definetly need further studies and this is perfect material for my thesis works I have already searched knowledge about usage of nitrogen in normal conditions, but never paid attention to such low temperatures. I will try to find some material too about the subject.

Why 304 btw? Wouldn't 316 be a better choice. After all 316L have much less carbon, which help to prevent formation of martensite. Is it possible to get martensite in grain boundaries btw? Wouldn't it be just a new phase along with austenite?

What about formation of ferrite in the metal structure? Wouldn't it be better to have 100% austenite microstructure? Ferrite usually is not very good at low temperatures. For example duplex stainless does not stand as low temperatures as 304. Reason for this is that it microstructure does have around 50 % of ferrite.

It's great to notice that forum has people who also pays attention to microstructure and things like that Of course this forum is mainly not for PHD's, but sometimes it's good to understand the microstructure aswell.

While few of us on the forum are PHDs we long to have that information. When Jody doesn't want to bore me with too much metallurgy, I wish he'd bore me a little more. This is precious information I can't get anywhere else!

[Markus]

Steve

Great to notice that you're in with 100% attitude

I also found some material about it, but haven't had time to read those carefully enough. I will check those tomorrow or overmorrow and post those up here. With quick look there wasn't said anything about extra low temperatures, but I will check those anyway.

[Otto Nobedder]

Well, I'm "all in" because the information affects my work, so I'd hate to be wrong.

The summary of what I've found so far (and the reason 304 is preferred), is that below a transition temperature (that varies with) delta ferrite is a detriment. Nitrogen tends to reduce delta ferrite in austinitic steels, so alloys that can absorb more (and therefore produce lower levels of delta ferrite) tend to imbrittle more in the presence of hydrogen. Since my work is primarily in liquid hydrogen service (extreme cold AND 100% hydrogen exposure), allowing more nitrogen in solution in molten stainless through nitrogen back-purge is frowned upon.

I still have work to do to firmly support this.

Steve S

[weldin mike 27]

My cats name is Mittens.

[MinnesotaDave]

My cats name is Mittens.

Didn't you forget an apostrophe for a possessive noun? Terrible...

Here is an article on nitrogen back purging from 1975 that I found interesting
http://www.aws.org/wj/supplement/WJ_1975_01_s12.pdf

[weldin mike 27]

I love to learn, but lots of stuff simply flies through my ears.

[Superiorwelding]

My cats name is Mittens.

What does that have to do with the price of eggs in China?
-Jonathan

[Superiorwelding]

My cats name is Mittens.

Didn't you forget an apostrophe for a possessive noun? Terrible...

Here is an article on nitrogen back purging from 1975 that I found interesting
http://www.aws.org/wj/supplement/WJ_1975_01_s12.pdf

Dave took my article!!
I am not finding anything that has not already been mentioned or links posted to. Maybe I just have to look harder. Maybe we need to compile all of this purging information and put it in the Welding Library?
-Jonathan

[Bill Beauregard]

My cats name is Mittens.

What that mean?

[weldin mike 27]

Hey,

Sorry folks, not too many simpsons fans here. The picture, Ralph, is a paticularly dull boy, who when faced with brain taxing tasks, lets loose comments like "my cats.... . Thats how i felt when Steve and the others were deep into their science conversation

Sorry for the hack/confusion.

Mick