[Superiorwelding]

Man it's been a while since we visited this thread. I have a small job in the garage welding up the four corners of a 14ga SS drip pan and wanted to make a quick back purge box and thought I would revive this thread all in the same motion. I bought a inert female to pipe male adaptor and raided my SS scrap bin and this is what I came up with. Ideally if I make one again I would make it smaller as it took 25-30cfh to get a good purge due to the volume. It might work with less flow but I didn't want to grind anything out. Inside the box is steel wool I bought a while back at Harbor Freight and the fitting is silver soldered on. The purge box worked out great. I wish I had better penetration on the back side of the weld but it is just a drip pan. Sad part is this pan will be used on a machine that is getting scrapped in a few months. Someone slipped on some oil and got hurt and the company changed their policy that all machines must have drip pans. Me and a friends company have been getting all we can as it is extremely easy money and work. Next I get to metal finish all the corners and ship it. Now to wait 90 days to get paid
-Jonathan

[Braehill]

Does anyone know a place to buy a small (1'x1') sheet of perforated copper in light gauge? I have a little cheap tabletop press brake that will bend up to about 18g or less and I want to make some boxes up. I already have a sheet of thin copper that I can use, so I just need the perforated. Thanks.

Len

[dsmabe]

@Braehill I checked my normal go to, metalsdepot.com, if you can't find what you are looking for you can get 1'x1'x.032 brass for about $35. But you would still have to drill the holes in it.

[Braehill]

Thanks. I'm hoping I don't have to drill all the holes. I could always get some perforated stainless and just silver solder it.


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[dsmabe]

https://concordsheetmetal.com/store/perforated-copper/
I did a quick search and found this, it's $151 for 2'x2'

[Superiorwelding]

Len,
This is a supplier we use when we need perforated plate. Been go to us so far.
http://www.mcnichols.com/products/expan ... MgodenwA8w
-Jonathan
Edit; for the right price I will drill out some copper for you

[Rick_H]

Does anyone know a place to buy a small (1'x1') sheet of perforated copper in light gauge? I have a little cheap tabletop press brake that will bend up to about 18g or less and I want to make some boxes up. I already have a sheet of thin copper that I can use, so I just need the perforated. Thanks.

Len

Mechanical metals.com is in PA

McMaster has brass sheets, but I don't see copper

[Rick_H]

Len,
This is a supplier we use when we need perforated plate. Been go to us so far.
http://www.mcnichols.com/products/expan ... MgodenwA8w
-Jonathan
Edit; for the right price I will drill out some copper for you

That's funny! I also use McNichols just not sure about small qnty.

[Superiorwelding]

Making purge boxes is fun. I have all sorts of crazy ideas. I made a smaller box tonight and polished them up a little. I need to get down to my metal supplier so I can build more!
-Jonathan

[Superiorwelding]

And yet another back purge block. I actually did a little brushing on this one. Not sure the direct use of this one just popped into my head and thought it would look cool.
-Jonathan

[Wes917]

And yet another back purge block. I actually did a little brushing on this one. Not sure the direct use of this one just popped into my head and thought it would look cool.
-Jonathan

These look a lot like ones I used to use when I did aerospace work. I have a couple ideas in my head also for some, for special applications. Did you put stainless steel wool inside to help disperse the gas evenly?

[Superiorwelding]

I am open to other design ideas if you are willing to share. I saw a few that Jody has made and want to copy them when I get time and materials. I put steel wool in them, not SS but it will work. Originally I wants to order some SS wool but never got around to it. I woul also like to try different meshes like TIG diffusers use. Will have to go eBay shopping soon. Do you have any pictures of your boxes?
-Jonathan

[Braehill]

One of our traveling welders came in one time to do a repair and had a duffle bag of different ones that he had made (probably 20) over the years. He said while most of the guys drank and would go to the saloon, he would stay at the site and make purge boxes.

He had one that was outstanding from the others, it had three sides that had piano hinges on them and you could open or close them as needed. The long side was even split down the middle so you open just half if you wanted. He also didn't use the perforated metal, he just used some SS mesh that he balled up and put inside.

I think the guy who's paying for their own Argon seems to be more consrvative on the size boxes that they make.

Len

[Wes917]

I am open to other design ideas if you are willing to share. I saw a few that Jody has made and want to copy them when I get time and materials. I put steel wool in them, not SS but it will work. Originally I wants to order some SS wool but never got around to it. I woul also like to try different meshes like TIG diffusers use. Will have to go eBay shopping soon. Do you have any pictures of your boxes?
-Jonathan

We did have some that had fine ss screens in them similar to gas lens' like your talking about. They work great, but the screens would get burned up over time and would need replaced, while the ones we made similar to yours would last forever. All depended on the job. We would also put a "dispersion" bar if you would running through the center. You'd have your gas in, then come to a "T" that had a bunch of small holes drilled "sorry forget what size we used" then tacked the bar to the ends of the back up, then ss wool, the the screen over top. These worked really well to keep an even flow over the whole back up. Sorry no pics weren't allowed phones in the shop. We'd also build gas into our copper heat sinks, but again no pics.

[kiwi2wheels]

I am open to other design ideas if you are willing to share. I saw a few that Jody has made and want to copy them when I get time and materials. I put steel wool in them, not SS but it will work. Originally I wants to order some SS wool but never got around to it. I woul also like to try different meshes like TIG diffusers use. Will have to go eBay shopping soon. Do you have any pictures of your boxes?
-Jonathan

What about the stainless pot scouring pads, like machining swarf ? Or are they too coarse for even gas distribution ?

[Superiorwelding]

So to get back to our discussion about nitrogen used as a purging gas, here is a great article in the January 2015 edition of The Welding Journal.
http://www.nxtbook.com/nxtbooks/aws/wj_ ... ex.php#/20
-Jonathan

[Otto Nobedder]

Thanks, Jonathan,

That supports my position against N2 backpurge on cryogenic welds. Austenitic SS, and 304 in particular, is one of few metals that become more ductile at low temperatures, and anything that disrupts the grain structure creates a stress point/grain boundry, and in a weld, a sharp line of such.

Steve S

[Markus]

Thanks Jonathan!

Nitrogen pick up on open root didn't surprise at all. Same thing as you weld with shielding gas that contains nitrogen. But when done with closed root, that is another story. Pick up will decrease quite much, I guess.

Also increase of nitrogen in the metal up to 1100 ppm's. Is it really that big deal? What about 304N, which contains around 1500 ppm's of nitrogen?

Well not an easy task to deal with. Specially for me as I'm not that familiar with formation of delta-ferrite yet.
Btw is solidification cracking that big risk nowadays, as we have AOD-converters? In other words we have cleaner base metals than in 60's.

[Braehill]

I think since the cost of Nitrogen is so much lower than Argon that most people are more likely to raise the flow rate and this will then shorten the time for there to be a total exchange of purge gas. Like I said before, if the Nitrogen doesn't stay in the area long enough to turn into N1, I think there's a lot less chance of having a substantial Nitrogen pick up. If it's present though, there's going to be pickup and I still think that the job will dictate how much is acceptable.

I welded a water knock out for a O2 analyzer the other day and used Nitrogen because the end use is at 3 psi and the corrosion resistance properties are not that critical. If I were to weld up the same system for our condensate system I definitely would have used Argon because it's at elevated temps and a very low PH liquid coming through at 170 psi.
I feel Steve's work involving cryo piping would also warrant using Argon. Vents and other open ended pipes like an exhaust in my mind would be fine with N2.

I haven't done the testing and don't dispute the findings of the guy who did, but sometimes good enough is just that, good enough and can't justify the added work or the added cost to use Argon when Nitrogen (in my case) is readily available and accessible. Most things I deal with already have a Nitrogen purge built into the system and only require a valve to be opened. If it has to be and can be removed then I will use Argon whenever I can and I think it's always the 'best' thing to do in those situations. We don't have a lot of weld failures for the amount of extreme temps that we have in service throughout the plant. Most of the failures in our plant are caused by pipe itself in the HAZ being attacked by the low PH liquid that get trapped behind the welds reinforcement.

Len

[Otto Nobedder]

...I feel Steve's work involving cryo piping would also warrant using Argon. Vents and other open ended pipes like an exhaust in my mind would be fine with N2...
Len

Agreed, and that's my approach exactly. Anything outside of process pressure, particularly drains and vents, are purged with nitrogen because it's quick, easy, and cheap. Anything in the cryo process pipe gets argon. Frankly, it doesn't come from my pocket, though failures could get in my pocket, so I'll not take risks on the unknown.

Steve S

[Markus]

@ Steve
Do you mean that purging with nitrogen is easier and cheaper?

@ Len
Purging with nitrogen is said and also studied that you actually get better corrosion resistance than with argon. For example in duplex grades nitrogen as a root gas forms a thin layer of austenite on the surface of the metal, which will improve corrosion recistance. So it's actually more ideal to use nitrogen than plain argon.


What comes to ferrite in 304 and 316 grades. It's known that ferrite prevents solidification cracking, but it's also known that inpurities like sulfur and phosporous in steel actually does create that hot cracking. Nowadays we have cleaner grades avaible, so do we actually need that ferrite phase in 304L and 316L for example?
What if we get fully austenitic microstructure in 304L without hot cracking, why it wouldn't work on cryo service?
In other words why low temperatures need that 4-26% ferrite, if cracking haven't been happened during solidification


About that article Jonathan posted. It was actually funny that they only handled open root welding and then claimed that nitrogen as a purge gas will reduce ferrite content about 50 %. Well of course it will as you force nitrogen in to the arc! When welding nitrogen alloyed grades like 316LN you have to use shielding gas with nitrogen mixed in it. That will quarantee that you end up with fully austenitic phase.
So article gives you easily wrong idea that nitrogen purging will reduce ferrite phase significantly. That's not actually true. When nitrogen atoms can't get on the arc, for example when welding with non open root, there won't be significant ferrite lost. Or lost at all.

Also it was funny that they completely ignored nitrogen-hydrogen mix. That would be better purge gas than nitrogen and perhaps even argon.

[Otto Nobedder]

I read something very interesting, by accident, while reading the Welding Engineer's Handbook.

I think we already established that Nitrogen is quite soluable in a molten puddle of austenitic SS, from another article one of us quoted.

I'm going to paraphrase for now, as it's 1600+ pages.

"While the addition of N2 to austenitic stainless steels can have benefits in toughness, concentrations above .25% can lead to Chromium Nitride precipitation in the weld zone, leading to cracking."

Remember, this is a paraphrase. I'll try to find the exact quote, and post back.

Steve S

[Otto Nobedder]

The exact quote:

In reference specifically to X5CrNiMo1713 (a European designation for an austenitic SS):

"More than 0,25% Nitrogen should not be used, because then the steel precipitates Chromium Nitrides CrN2 at the grain boundaries, which ... cause an IC susceptibility of the steel."

(IC=Intra-granular cracking).

Steve S

[Markus]

Good catch Steve!

IGC (intergranular cracking) is a form of corrosion type, right?

Well there is maximum rates for nitrogen as alloying element in each grade and that 2500 ppm is very high! Sure it can do harm in the alloy. After all it nitrogen is very strong austenite and carbide former.

Little reminded now. Even normal 316L stainless grades can have nitrogen as an alloying element. It all depends what Mill have to had use in their process to get the right structure. As orbital welder I check material certifications trough very closely and every now and then they seem to add nitrogen in the mixture. Perhaps to control ferrite content or so.



Nitrogen in welding is very different story then. There are cases when you need to add nitrogen in your metal. For example you have to weld 316LN (nitrogen alloyed) stainless steel. With pure argon you will lose precious nitrogen from the metal due to evaporation etc. Therefore maximum corrosion resistance is lowered. That is why you have bring nitrogen into the steel and this is done with Argon-nitrogen mixtures.

AGA.png (24.69 KiB) Viewed 2430 times

[Source: Page 5 http://www.aga.is/internet.lg.lg.isl/en ... 101350.pdf]





So that was nitrogen in the shielding gas. What about in the root, specially in closed type root. Very different story!
Nitrogen in root gas will only affect little or not at all to the composition of the metal. Why is that? As I have already mentioned here, there need to be much energy to get nitrogen into metal. Welding arc will provide enough energy for that, but because root gas and shielding gas are usually in different sides of the weld therefore there won't be problems with nitrogen as a root gas.
Open root weld then again is a very different story, because there is risk of shielding and root gases mixing up.

[Otto Nobedder]

So, the question at hand is,

Does Nitrogen (as a pack-purge) give enough opportunity in an open-root weld to increase the alloying of N/N2 to a level that can produce Cr2N precipitation in a weld's HAZ, and, therefore the risk of intragranular corrosion/cracking?

This "unknown" is why we don't use nitrogen as a back-purge when the service temperature is -423F. Could this nitride precipitation provide a path for hydrogen embrittlement? (The service IS liquid hydrogen.)

I can't help but be interested in this.

Steve S