[Superiorwelding]

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

Ok now it makes sense. Stick with us we all are learning a thing or two.
-Jonathan

[Otto Nobedder]

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

At this point, I'm slightly embarrassed to admit that I got the joke...

[weldin mike 27]

Im 32 this year, and the Simpsons has been going for around 24 years. It kind of shaped a large part of my life, sad? Maybe, Embarrasing? Not really. Ask an older person how The Brady bunch or Dallas shaped their life. Hacked again. I really should stop that.

Mick

[Braehill]

I'll be 50 in a month and Marsha Brady was the star of more than a few no nonsense dreams for me. So I'll not have you talkin smack on the B Bunch.

Len


Sent from my iPhone using Tapatalk

[Otto Nobedder]

That's a first...

I think this thread hijacked ITSELF!


Steve S

[weldin mike 27]

Hey,

Not talking smack at all, merely a generational reference. Buffy the vampire slayer did the job for me.

Now no more hacks, i promise .

Mick

[MinnesotaDave]

Tool time

image.jpg (18.64 KiB) Viewed 1517 times

[Markus]

Nice picture Dave

But back to business. Here is little video about nitrogen. As said Nitrogen atoms have a really strong bond between each others and breaking that bond needs a lot of energy and high temperature. Shielding gas definetly produces enough pressure and energy to do that, but what about root gas?

https://www.youtube.com/watch?v=zmvJ54kRpjg

[Markus]

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

Hmm good job.

I don't fully understand this one "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"

Why it would be more imbrittle in the presence of hydrogen? Due to lack of ferrite it can then absorb hydrogen?

[Superiorwelding]

Now we will see who will still follow this thread.
I pulled up random information on a lot of the subject matter mentioned so far. Enjoy.
Transformation of Delta-Ferrite
http://www.aws.org/wj/supplement/WJ_1986_05_s122.pdf
Welding Parameter effects on SS weld metal Ferrite
http://aws.org/wj/supplement/WJ_1978_04_s109.pdf
Massive-type delta-ferrite to austenite phase in steel
https://www.youtube.com/watch?v=5RQlVZbYPrk
Lecture that may bore you
https://www.youtube.com/watch?v=CUvWB402fL8
-Jonathan

[Otto Nobedder]

Jonathan,

I will probably wait for Saturday to study those. Got some personal issues side-tracking me at the moment, but I'm not going to be distracted from this. It's getting interesting.

Steve S

[Superiorwelding]

Jonathan,

I will probably wait for Saturday to study those. Got some personal issues side-tracking me at the moment, but I'm not going to be distracted from this. It's getting interesting.

Steve S

Steve,
This is very interesting. Some of those links don't necessarily have anything to do with purging, but are quite interesting. I hope we are not loosing our audience, we are getting into very advanced topics now.
-Jonathan

[Braehill]

I don't know what I can add to the metallurgy portion of this post but I throw out some bits about the gas. Nitrogen is diatomic in it's natural state and when it's heated to a high temp it becomes monatomic. When this happens it's expansion rate increases violently and becomes a reactive gas instead of it's normal inert state. This reaction in the presense of Hydrogen can cause enough temp increase to also make the Hydrogen atoms seperate (also diatomic) and this might be what causes the H atoms to impregnate the grain structure because of their reduction in size. This last part is just a theory of mine and has no backing.

Nitrogen as a back purge is not subject to enough heat usually to go through this atomic change. Because of it's low cost it is usually being delivered at a much higher flow rate, therefore spending very little time at it's critical temps. I'm confident that with proper sheild gas coverage that the effects of using it as a back purge will keep it's detriment and benefits to a minimum either way.

Len

[Bill Beauregard]

This is very interesting. Some of those links don't necessarily have anything to do with purging, but are quite interesting. I hope we are not loosing our audience, we are getting into very advanced topics now.
-Jonathan
"What could go wrong?"

Superiorwelding
Weldmonger

Posts: 672
Joined: Thu Jan 24, 2013 10:13 pm
Location: Wooster, OH

Not to worry, you're way over my head, but I'm paying attention. Like any good puzzle, if I ever figure it out I'll feel smarter than before.

[Markus]

I don't know what I can add to the metallurgy portion of this post but I throw out some bits about the gas. Nitrogen is diatomic in it's natural state and when it's heated to a high temp it becomes monatomic. When this happens it's expansion rate increases violently and becomes a reactive gas instead of it's normal inert state. This reaction in the presense of Hydrogen can cause enough temp increase to also make the Hydrogen atoms seperate (also diatomic) and this might be what causes the H atoms to impregnate the grain structure because of their reduction in size. This last part is just a theory of mine and has no backing.

Nitrogen as a back purge is not subject to enough heat usually to go through this atomic change. Because of it's low cost it is usually being delivered at a much higher flow rate, therefore spending very little time at it's critical temps. I'm confident that with proper sheild gas coverage that the effects of using it as a back purge will keep it's detriment and benefits to a minimum either way.

Len

Exactly.


So far I have found only one decent article of using nitrogen as back purge. Sadly that article is already linked here.

(http://www.google.fi/url?sa=t&rct=j&q=& ... 8261,d.bGQ)

In that article studies are made in India and pointed to AWS. There isn't said anything about low temperatures. However test outcome shows that nitrogen can be safely used as back purge.

How reliable this article is then? Honestly I don't know. It does look reliable, but everything what is made in China or India makes me little sucpicious due to experience of some of their products quality.

[Superiorwelding]

I found this article on FB and thought I would post it here to continue our discussion on purging and nitrogen. I thought it was very informative and I have contacted Regis Geisler of Lincoln Electric to see if he has any other reports for the Nitrogen discussion we had going on here.

http://www.fabricatingandmetalworking.c ... day-again/

-Jonathan

[Markus]

I found this article on FB and thought I would post it here to continue our discussion on purging and nitrogen. I thought it was very informative and I have contacted Regis Geisler of Lincoln Electric to see if he has any other reports for the Nitrogen discussion we had going on here.

http://www.fabricatingandmetalworking.c ... day-again/

-Jonathan

Good article! Thanks for sharing.

Although it was about shielding gas, not root gas, it will definetly help us to better understand effects of Nitrogen in stainless grades.

Some things I just don't understand yet. Why do people want ferrite in microstructure? Nowadays L-grades have so little carbon in it, that hot cracking etc should not be a problem. Referring to some articles at least.
Also isn't austenitic phase more spacious than ferrite? Would austenite absorb better then and form more uniform structure? Which would help to prevent cracking etc.


Btw here is guide for choosing root gas from Polysoude.
Source: http://www.google.fi/url?sa=t&rct=j&q=& ... 0277,d.bGE

[Braehill]

Markus,
I just used your article to convince a young engineer to let the welder use Nitrogen to purge a 6" 2205 duplex pipe that would have taken 60 or more cylinders of Argon to purge.


They were about to start the x-ray when I left. When I go to work tonight I'll find out the results.

Len


Sent from my iPhone using Tapatalk

[Wes917]

Seems I missed a bunch and should have been paying more attention as this topic has gotten very interesting.

I searched for this topic as I've been doing a little research on back ups as we are looking for a little better coverage. I've dealt with this company before and their top notch. http://www.pwt-online.com/products/prodGSD.htm. Some other similiar in style http://www.arc-zone.com/index.php?main_ ... 5_228_1106

Now to go back and read through all the other posts

[Superiorwelding]

Wes917,
I think you will like the read although it might take quite a while to digest everything.

Markus,
I am working on a response to your questions.

Len,
Very interested in your results!
-Jonathan

[Otto Nobedder]

Nitrogen as backpurge should have no effect on the x-ray, unless the person(s) doing the root pass are completely spoiled for argon, and simply perform poorly.

I've been trying to recall the origin of the information I offered about nitrogen on 300-series SS for cryogenic service. I think it was a site specific to welding for sub- liquid nitrogen temperatures, specifically H and He, and the site was geared more toward the recommended filler rods for that service temperature. The Nitrogen caveat "may" have been specific to a recommended filler alloy.

I'll dig a bit along that path, and see if I can find it.

Steve S

[Braehill]

It was found after I left to go get some sleep (night shift) that the original weld did not have complete penetration and the whole pipe had to be removed and all the welds cut out and rewelded. The weld repair with the N2 back purge turned out fine, but this was prior to them finding the other issue of incomplete penetration. Meanwhile when they removed the section of piping they were able to purge it out with Argon.

I was actually looking forward to see if the weld repair held up in service. Bad news is there's always something around here that springs a leak and I'm sure we will get another opportunity to do some real life testing. In the meantime we can still theorize and fill each other's brains with more mush.

Len

[Markus]

Sorry to hear Len

As already said x-ray won't show the difference between purging gases.

Nitrogen purging in Duplex is more than recommended, as the metal itself contais Nitrogen and when welding some of the nitrogen will be lost due to evaporation etc. Some articles even recommend to use argon with 2% Nitrogen as shielding gas.
This will provide that there is at least same amount of Nitrogen in the weld itself, that untouched metal does contain. So corrosion properties will stay same.


About that ferrite in cold temperatures in 300-series. Here is a little quote:

The toughness of the austenitics relies on their fcc atomic structure. The presence of either ferrite or martensite can limit the cryogenic usefulness of the austenitic stainless steels.
The small levels of ferrite usually present in wrought austenitics is not usually detrimental.

Source: http://www.bssa.org.uk/topics.php?article=41

More links:
Materials for cryogenic service
http://www.google.fi/url?sa=t&rct=j&q=& ... 0277,d.bGQ

Let's keep up the good work!

[Otto Nobedder]

Thanks, Markus,

I've read the first link, the one your quote is from, and I think you're on the right path. That clarifies the effect of ferrite below a transition temperature. I've already cited some information on increased ferrite in teh weld zone from nitrogen in solution in molten steels including austinitic SS. I also noted the description of special "low ferrite" filler alloys for cryogenic service, which fits with what I commented in my previous post.

I'm completely convinced that nitrogen packpurge is perfectly fine and economical for 304 at ambient to high temperature, and low temperatures possibly to -320F (liquid nitrogen). I do in fact use nitrogen as backpurge for all non-pressure vent piping regularly, if I haven't mentioned it before. It's the liquid process piping under pressure that I'm most concerned about.

I'm about to read the other link you posted. Hopefully we can tie all this together into a solid answer.

Edit: As interesting as that link was, the weld joints discussed were MMA, SMAW, and SAW. No discussion of GTAW and associated backpurge. Does strongly support controlling the quantity of delta ferrite.
Interestingly, it gave me information on a non-cryogenic issue I'm dealing with... Passivating against Corrosion Stress Cracking. About the middle of page 5 it's discussed, and mentions a post-weld heat treatment. I'll have to dig more on that, as well.

The more I learn, the more I discover I don't know...

Steve S

[Superiorwelding]

I came across this pdf on purging in general and wanted to add it to this thread. This thread will make a great place to come back to for references on purging and all the other topics we have discussed.
http://www.gowelding.com/met/purging.pdf
-Jonathan