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Topic 904L Backing Gas By magnolrr Date 12-04-2019 13:13
Hi everyone.My Name is Magno Rodrigues, I'm new here. I would like your help to solve a question.
As we know, some people use nitrogen instead of argon as a backing gas on stainless steel 304, 316 welding
In case of 900 series, 904l specifically, the WPS order to use a 100% purity argon.
I have read some articles, but not about the situation above.
My question is: Can be used nitrogen istead of argon to purge 904l Stainless Steel? If not, Why?
Thanks
Topic Purging LP Tanks By DNukem Date 03-15-2019 07:06
The question is: What will work best to purge an LP tank. i.e. Displace Gasified Propane?
Cutting into a LP/Propane tank has HUGE RISK! Extreme CAUTION must be employeed! Use an Explosive Level meter to ensure safety!
There is a thread from RCWelding (07-10-2010) with much discussion on this subject.
I did not find any of the following important information in that discussion. Using Dawn soap is discussed in his thread & is warranted.

The question is: What will work best to displace Gasified Propane?
Something to consider in trying to displace Gasified Propane.
Molecular Weights of the fuels/gasses in the discussion by lightest to heaviest.
CO (Pure Carbon Monoxide)        28.011
CO (Exhaust/Impure CO)             28.011+ (unknown wt.)
N3 (Nitrogen)                                28.013
Air (common blend of gasses)     28.966
AR (Argon)                                   39.948
CO2 (Carbon Dioxide)                 44.010
C3H8 (Gasified Propane)            44.097

As you can see the Gasified Propane is the heaviest of all of them with CO being the lightest.
If you are trying to displace the propane gasses the openings MUST BE ON THE BOTTOM/FACING DOWN to allow the volatile gasses to escape.
If you are purging the tank and the openings are anywhere other than on the bottom you are asking for serious trouble!
None of the lighter gasses can blend with or move the very HEAVY Gasified Propane uphill to an opening above the bottom.

YES - CO2 will mix fairly well with the Gasified Propane but only close to 50% dilution at best. Don't chance it!  
CO & N3 will "push" the Gasified Propane out the bottom better than any of the others as they are the lightest.
Using "Exhaust" is quite hazardous as it has unburned oxygen/fuel components within the "Impure" Carbon Monoxide, nullifying the safety factor.
Accordingly - N3 (Nitrogen) is the best purging gas as it is very light and has NO OXYGEN or Fuel component! BUT, the openings MUST be facing down!

Source: http://www.stadealer.com/uploads/1/8/4/1/18412059/coalarminstallationheight.pdf
Topic Having issues with 316H stainless passing X-Ray By Lawrence Date 06-06-2016 11:46
If the only issue is "suck-back" and the root passes are being inspected and are ok, your assessment that the second pass might be too hot (too much current or two slow travel) makes a lot of sense.

From a welders standpoint for GTAW, 316, 308 and 321 fillers behave pretty much the same, Meaning operators usually don't need to change technique.  If they have good success with one, they usually are ok with the others.

Things like consistent root openings (don't what it is), root faces (don't know what it is) and operator experience can play a role too... You say the welders are experienced, but I would wager from the information given that the issue is technique.

200 amps is pretty high if you look at the old "rule of thumb"  one amp per thou. of thickness.  The root pass probably at most deposited a bead about .125 eh?

Are the second passes (hot pass) being applied freehand or are the welders walking the cup?

If the operators are using higher current to accommodate a very large filler wire diameter (you didn't mention diameter) or if they are using high current to move faster, but not actually moving faster, it might cause the suck-back you mentioned.

Are the pipes being rolled or welded "in position"  Are the defects at a common place (like 6'oclock at the bottom? or 12 o'clock at the top?)  What is the bevel angle?

Are standard purging practices in place? or is the inside of the pipe possibly pressurized by backing gas?   Are you using Argon or Nitrogen to purge?

What is the pipe diameter?

Probably Al wanted some of the information I'm asking about  :)
Topic Nitrogen purge for open-root GTAW 310 SS By js55 Date 02-24-2016 16:43
I've seen nitrogen used in GTAW shielding for duplex SS's. Though it is uncommon and is used in very small volume. Perhaps 1% to 2%. For the reason mentioned in Lawrence's post. It destabilizes the arc.
The idea fro duplex is to assist in restoring N content to the weld metal that is lost in welding.
outside of this I do not ever remember seeing it used for shielding. It is common in purges however.
Topic Nitrogen purge for open-root GTAW 310 SS By another1 Date 02-23-2016 00:03
I realize that nitrogen purge is common enough for closed-gap pipe welding, and that there are potential concerns with nitrogen in the shielding gas where it is exposed to the higher temperatures and arc-side of the puddle.  What about open gap GTAW where the purge and shielding gas are more commingled?  Does anyone have experience or references to confirm this is a non-issue or otherwise.  I found a paper on closed gap welding indicating non-issue, but wondering about open-gap/open-root.
Topic Stainless Steel Sugaring By Lawrence Date 01-22-2015 16:12
Make an unpurged CJP on a plate, or a pipe and cut out the pup.

Go ahead, flip over to the sugared side and try pickling, passivation, acids, rain dances, voodoo, the power of positive prayer and anything else you care to throw on that sugared metal...  It will still be nasty until it is removed mechanically.

I'm pretty unclear though on the whole process...  The backside of the weld is "inaccessible" yet can be accessed in order to place a seal of primer and paint!   but a backing gas (argon or nitrogen) or a heat sink to avoid burn thru (sugar) cannot be placed ?

I've seen miles of sweet sweet austenitic weld melt-thru that was covered with inexpensive nitrogen... Heck, the air separations plants are trying to give the stuff away practically :)   It's not as shiny as argon backed melt-thru... but it sure can be passivized.

Why oh why Tim  ?
Topic OXYGEN CONTENT By 803056 Date 09-06-2014 19:04
Rules of thumb: 

1)   use enough purge gas to displace the system volume by a factor of 6. Good enough for most applications, but not for Titanium or other reactive metals.
2)   use a U-groove with a 1/32 to/16 inch root face, 1/4 inch bevel radius, and NO root opening.

The volume of gas (6 x the system volume) should reduce the Oxygen to about 0.3 to 0.4% and Nitrogen down to about 1.25%.

The system must be dry, i.e., free of moisture. If the system was used, it has to be dried. Drain and purging the system initially with dry Argon (Nitrogen is a poor second choice)  may be necessary to dry the system.

If a U-groove with a zero root opening is not used, the joints must be taped with a suitable tape to seal the open joints.

Clean the inside of the pipe as well as the groove face and outside. Swab the pipe ID for a minimum distance of 4 inches from the groove to be welded. Acetone or Isopropyl alcohol should work. The preference is Acetone followed by alcohol. Swab until the ID is clean.

Seal all openings to stop the free flow of purge gas.

Vent all high points. Air is lighter than Argon, so it (air) will rise to the high points. Typically a 1/16 inch hole is sufficient to allow the air to escape.

Do not use Nitrogen as a purge gas. Nitrogen is not inert and it is an interstitial element and will alloy with certain base metals (certain stainless steels and reactive metals). In the case of other metals, it may cause porosity (carbon and high strength low alloy steel).

Do not over pressurize the system. A Magnehelic (regulator) can be used to prevent over pressurization.

Best regard - Al
Topic welding duplex 2507 pipe By MRWeldSoCal Date 07-21-2014 17:37
rolled alloys has some decent information if you can find it.  I know that GTAW is prob the best manual process for welding it becuase of the lower heat input.  Purging is a must, welds quite nice.  I dont some FCAW-G on it.  We found that for the Purge we used 98% argon 2% Nitrogen, and same for the shielding with GTAW.  With FCAW-G I dont remember what gas we used, but the wire we got from ESAB.
Topic Fastest way to cut pipe? By CWI555 Date 12-25-2013 14:25
Agreed. Laser would make short work of it. However, for stainless, it's still thermally cutting it. A nitrogen assist purge would be necessary.
Topic another aluminum diesel fuel tank welding question?? By Sourdough Date 12-01-2013 15:59
Chris, nitrogen negates oxygen. If you purge a tank with nitrogen, there will be no oxygen whatsoever to ignite. I do this all the time, as well as hot taps on live gas lines. No oxygen = no ignition.

If there is a return line to the tank, you can purge with nitrogen. Purge it, seal it back up and weld the crack.

Don't pass these jobs up. This kind of thing will set you apart from other "welders"!
Topic another aluminum diesel fuel tank welding question?? By Chris2626 Date 11-30-2013 15:45
I know i have seen from time to time people asking about welding on aluminum diesel fuel tanks and i have a guy wanted me to weld on two of these tanks he said it has been flushed really well with water but you still don't want to trust that. I have also read a lot and a friend told me he usually will fill that tank completely with water and leave the water in that tank as he welds on them or purge it with argon or nitrogen.

You guy have never steered me wrong on anything here, if I completely fill them with water will it be ok to weld on these aluminum tanks?? I will be using tig process and was thinking of going up on the price as well for the danger factor.

Thanks
Chris
Topic Purging Issues with 98% Argon/ 2% Nitrogen By ssbn727 Date 06-05-2013 04:23
That Australian link (first one out of the 3 listed) sure does spell it out when warning of the use of O2 with N as part of the backing gas and it is especially prevalent when using orbital welding to join high purity tubing... For example:

"If a consumable is used it must be at least as corrosion resistant as the tube or pipe material. Otherwise, the narrow weld could corrode rapidly if the tube was used in a corrosive environment. Purge gas must be dry and is normally argon, although low oxygen nitrogen can be used (even for duplex tubing). However, if there is excessive leakage into the arc, then phase balances can be disturbed and cause either cracking, poor toughness or lower corrosion resistance."

So the results are just the opposite of what one would desire when welding DS Steel & SDS Steel if one would include low amounts of Oxygen as part of the purging gas mixture if the joint and purged area surrounding the joint is leaking while welding and the same goes for low amounts of N which in itself doesn't make sense either since one would want a Nitrogen rich atmosphere as the purging/backing gas in order to prevent N loss from welding... I have enjoyed this thread and I appreciate your posts fschweighardt.:lol::cool:

Respectfully,
Henry
Topic Purging Issues with 98% Argon/ 2% Nitrogen By ssbn727 Date 06-05-2013 03:49
Here's another article...

Shielding and Backing Gas Pure argon (Ar) shielding and backing gases can create weldments with sufficient corrosion resistance.

However, N loss is not uncommon to a depth of 0.5mm from the surface of the weld. To correct the phase balance and improve corrosion resistance of the weld, it is beneficial to have additions of 1-2% N in the Ar shielding and 90% N and 10% hydrogen(H) in the backing gas. Nitrogen contents above 2% in the shielding gas can cause degradation of the tungsten electrode for GTAW processes.

The addition of H to the shielding gas is not recommended as it may cause H absorption in the weld.
Back purging should be maintained on the joint until at least 6mm of weld metal thickness has been deposited.
The oxygen content of the back purged volume should not exceed 0.25 (2500ppm).

Since DSS have relatively high chromium contents and relatively low thermal expansion, an oxide scale appearing as an oxide tint is produced during welding that is typically thin and difficult to remove. The appearance and amount of heat-tint produced during welding can be minimized with low levels of oxygen (below0.25%) in the  shielding and backing gases, with minimal moisture in the backing gas, and with limiting contaminants on the surface prior to welding. Hydrogen in the Ar backing gas can adversely affect the appearance of heat tint, and the base metal’s surface finish. Shielding gases suitable for the various gas shielded processes are listed in Table 5.

Table 5.    Shielding gas general recommendations.

GTAW      99.996%Ar, Ar+2%N 2, Ar+5%N2

GMAW     Ar+1%O2, Ar+30%He+1%O2, Ar+2%CO2, Ar+15%He+2%CO2

FCAW      Ar+1%O2, Ar+20%CO2, Ar+2%CO2

PAW        99.996% Argon

There's another Table 6. which covers DSS and SDSS consumable specifications.

I'm not going to add the filler metal recommendations because they can be found in the very same article as a .pdf here:

http://stainless-steel-world.net/pdf/SSW_DEC07_fluor_LR.pdf

Well, that's it for now.:lol:

Respectfully,
Henry
Topic Purging Issues with 98% Argon/ 2% Nitrogen By fschweighardt Date 06-04-2013 23:51
You may be sucking in some air from your root gap.  Are you taping it off?

Adding O2 to make the stainless "more stainless" seems waaaaay off to me.

Cr2O3 is the passive film that makes stainless, well . . stainless.  If you have some elemental chromium hanging out on the root side of the weld, as soon as you expose the root side to air, that Cr is going to grab all of the oxygen it needs right now.

As I am sure you know, Al2O3 is the same type of film responsible for aluminum's corrosion resistance.  Ever have to add some nice O2 into your Al shielding/purge gas to make sure the Al did it's passivation thing?  Probably not.

I bet this guy read that you can try to shoot for .5% O2 in the purge, as long as the O2 comes from air.  Reason is that if you have .5% O2, you also have ~2% N2, which is far and away the most commonly specified mixed gas for duplex GTAW..Seems like you already are using the 2%N2 for your gas, maybe both shield and purge.

This link
http://www.docin.com/p-448996075.html
allows you to see a Weir materials manual for duplex, which contains the .5% PPM nugget of wisdom quoted in full below.  Keep in mind that Weir seems to have invented Zeron 100.

2.WEIR MATERIALS, Guidelines for Welding, ZERON 100
Back Purging:- When welding Zeron 100, it is recommended that commercial purity argon is used to displace the air behind the joint. The oxygen content of the resulting argon/air mixture must be monitored and controlled to ensure that sufficient nitrogen is retained in the backing gas “mixture’ in order to inhibit loss of nitrogen from the weld pool. In practice the oxygen level of this mixture should be controlled at approx 0.5% (5000ppm) oxygen monitored at the start of the welding sequence. In this way, a positive partial pressure of nitrogen is maintained behind the joint thus preventing nitrogen loss from the root bead, whilst the oxidation produced remains acceptable. Passes subsequent to the root pass can be made with minimum oxygen contents.
The backing gas composition should be monitored at the joint line using a portable oxygen monitor immediately prior to starting or re-starting welding in order that consistency can be maintained. Adhesive tape low in sulphur and chloride is used around the open joint seam, and the tape should be removed progressively during the welding sequence.
Topic Purging Issues with 98% Argon/ 2% Nitrogen By MRWeldSoCal Date 06-04-2013 15:46
Im running 6G pipe tests for my crew.  I have to run a 98/2  Argon/ Nitrogen backing gas...  The client wants a .5% Oxygen content so that the inside sugars just slightly to improve corrosion resistance.  We are welding on a Super Duplex SS, and its a 6" sch. 40 coupon. 

My trouble is getting a clean seal for my purge.  I can barely get it to .55%  Im running it at 35 cfm till I get close with my meter then backing it off to 5 CFM and it will slowly start to climb back out.

Any Ideas?

Two of the times I used metal caps then taped them, the last times I used cardboard caps and those times it was leaking somewhere...
Topic asme tank By WELDICCAWSCWI Date 05-23-2013 09:47
Shane no offense here but didnt i just post the NB   serial numbrer .the two pipes in question
have pipe  caps welded on them  thats how it  was pressure tested. The owner of the vessels reo
says to weld it as per asme B31.3 . the drawings do not show any flanges they  do not show how the
pipe is connected!!!!! the other outlets all haved weld neck val.ves that were welded at manufacturers facility
amd pipe was welded in field  to valves per asme b31.3the dwg posteed by joey has nothing to do with my post .so i dont knows why it was  posted here
again i am not an AI iinspector . the only reason thjey want to install flanges is so we have  a way to purge the pipe
without filling the entire vessell with argon or nitrogen  there is no other way to  pirge  the vesseld
tyhey are fabricaterd from stainless and are approx 10  foot dia 30 feet long . they are 02 cleaned  so
flux is not an option
Topic FCAW-G Super Duplex Stainless Gas inclusions? By MRWeldSoCal Date 04-29-2013 15:00
We have a bottle of 98-2 argon/nitrogen we are using for a purge, either way the PQR will most likely get back welded at the root.  not running any nitrogen on the FCAW shielding gas though
Topic FCAW-G Super Duplex Stainless Gas inclusions? By ozniek Date 04-27-2013 12:05
Hi Al

Nitrogen is often added to the purge gas and sometimes the shielding gas for GTAW, as it tends to give better corrosion test results. There is some debate if the 2%N is desirable for anything after the root pass, but we have had good experience using it on DSS as purge and shielding gas (For GTAW) all the way through thickness. Having said this, we have also had good results using plain Ar for purge. I have no experience with N in the shielding gas for FCAW, and doubt if it would be beneficial.

Regards
Niekie
Topic FCAW-G Super Duplex Stainless Gas inclusions? By 803056 Date 04-26-2013 19:17
My guess would be that you are too short with the electrode extension (contact tip to work distance).

The CTWD is one of those important parameters that some codes do not consider. However, the use of too short or too long a CTWD can create all sorts of havoc. Ignoring the CTWD can sink a job pretty quickly. You can usually find the proper range for the CTWD on the manufacturer's product page. ESAB recommends 1/2 to 1 inch for that particular electrode running at 25 to 28 volts and 227 to 567 ipm for wire feed speed. I would start some where around the middle of the recommended ranges. The shielding gas listed is 75% Ar and 25% CO2. A flow rate of 40 to 50 CFH would be the norm for a 5/8 inch gas nozzle.

Nitrogen is an alloying constituent for this alloy. You might want to rethink using nitrogen in the shielding gas or the purge gas.

Al
Topic FCAW-G Super Duplex Stainless Gas inclusions? By MRWeldSoCal Date 04-25-2013 19:20
Tomorrow will be the judgement day for this, we have ran successful PQR's for tig 3/8 plate  with a 98-2 argon nitrogen top gas and purge gas... then did a solid wire 2G with pulse spray and that also passed... We were worried the flux core wouldnt pass the G48 corrosion test but I think it will.  Just cut the 3/4 plate for this next round and were going to run it 2G so im sure all will be well.   Its funny cause we have all "old school" guys here that think the XMT 450 is the devil and their deltaARC 451 is the only way to go...  So im going to give them a roll of shield bright tomorrow and have them run it their way and see what they come up with.
Topic welding on diesel tank By Sourdough Date 09-20-2012 03:33
Nitrogen purge is all you need. Nitrogen will purge the oxygen. I do it all the time.
Topic Backing gas By ozniek Date 12-02-2011 11:13
Hi vdao

The codes I use do not generally specify the use of the backing gas. They merely state that if you need to use backing gas to get an acceptable result in your PQR test, then you need to also use the same for production welding. The need for backing gas may be specified in client specifications, but generally it is a matter of basic welding engineering. Given certain materials, with certain processes, you just need to protect the root from oxygen (or nitrogen) to get an acceptable result. Typically GTAW welding of stainless steels.

The easiest way to purge a plate coupon (there are a number of ways) would be to tack weld an angle iron onto the back of the plate, and seal the sides with tape. (e.g. masking tape) Apply the gas from one end of the angle iron, and make a small drain hole in the other end to prevent excessive pressure build up when closing the weld. (Or else it will cause root concavity.) Place a piece of masking tape along the top of the joint, and progressively remove it as the weld progresses. For most stainless steels, ensure you keep the root purged till you have about 6mm of weld deposited.

Regards
Niekie
Topic Welding Duplex 2205 with GTAW multipass with 2%N/98%Ar By 464238 Date 10-10-2011 19:36
We fabricated several Duplex Piping Spools using 98% AR 2% Nitrogen as a shielding gas but we used 95% Ar and 5% Nitrogen per API 938-C for one of our clients a couple of years ago. We had a zero percent RT failure and welded from 3/4" to 10" piping and the thinnest we welded was .093" and thickest .900"

Take a look at API 938-C

it is essential to get a good purge
Topic diesetank welding By welderbrent Date 09-16-2011 03:39
Tyler,

I have to agree with SOME of the other posters.  You can't be too safe with fuel tanks, diesel or gas.

Fire and/or explosion takes three elements: Fuel, Oxygen, Heat.

Since diesel has a fairly high ignition temperature it is not difficult to keep temps relatively low, though any 'arc' will be enough with all the other factors.

Thus, the key ingredients are Fuel and Oxygen.  I choose to rinse/purge a tank several times with Simple Green.  It cuts fuel residue considerably.  But there will still be enough fuel residue in the pores of most tanks to ignite.  Even a cheap explosive atmosphere tester will give a danger reading.  No matter the tester reading, I then fill the tank with either Nitrogen, Argon, or CO2.  Any of these will displace the Oxygen in the tank and also displace much of the fuel fumes removing them from the equation. 

So...
1) Keep the Heat factor as low as possible,
2) Remove/reduce the Fuel factor by detergent cleansing and then gas purging,
3) Finally, Remove/reduce the Oxygen factor by gas purging.

REMOVING just one of the three legs of the Fire/Explosive triangle will prevent explosion, BUT as I am not ASSURED that I have TOTALLY removed that one leg I prefer to reduce to the greatest extent all three legs. 

When the Explosives Fumes meter gives a satisfactory reading it is SAFE to weld.

Personally,  I just pass on most tanks anymore.  Especially gas.  I still do some diesel tanks.  All depends on each individual case.

Have a Great Day,  Brent
Topic diesetank welding By rlitman Date 09-14-2011 16:41
I'd suggest dry ice. 
Water will displace fumes where it is, but you need an air headspace where you can weld, and any remaining fuel will float on that water.  So, water will reduce the volume for the fumes, but not eliminate it.

A nitrogen purge will work if you can keep a constant flow of nitrogen going in, to prevent any air from entering.
A CO2 purge can be better because being heavier than air, it is more likely to stay in the tank, IF the opening you're welding on is at the top (if the opening is at the bottom, then nitrogen would be the better choice).
If you have enough dry ice, it will continually release CO2 gas, so you would have the continuous overflow of purge gas you want that will keep any air from entering.
Oh, and when you're done, there's no cleanup.

If you can't find dry ice, ask the local ice-cream truck.
Topic diesetank welding By cmays Date 09-14-2011 03:26
In my experience simply "smelling for fumes" won't cut it. We usually empty them completely and purge with nitrogen for ample amount of time before welding. Calculate your vessel volume in cubic foot and the determine the purge time, adding more for redundancy.
Topic Storing 7024 By GusTX Date 09-01-2011 00:42
If you purge any tank with Nitrogen you will be able cut it without it becoming a bomb.

Just do not use truck exhaust, I for one would like you to keep building BBQ pits.

It look's good
Topic Mig welding 308 and 309 By DaveBoyer Date 08-15-2011 04:47
Nitrogen is cheap. It is an accepted purge gas for brazing copper refrigerant tube, but I have not heard if using it as a purge for arc welding stainless pipe.

Nitrogen at high temperatures can harden at least some steel alloys, and can cause nitrogen embrittlement, but I don't know the details.
Nitriding, a case hardening process is an example of this.

There is a family of stainless steels [Aqumet 22 and Nitronic 50 as examples] that are strengthened by additions of nitrogen, in a similar fasion to how others [and steel] are strengthened by additions of carbon. The nitrogen does not combine with chrome like carbon does, so it does not affect corosion resistance.
Topic Mig welding 308 and 309 By Chris2626 Date 08-15-2011 03:46
Dave thanks for the website that really was a good read and still reading. I've used the trimix before but it was for ballistic steel which I can understand because you needed that extra punch to penetrate. Second day I was at this job I got into it with the shop foreman and well I've learned my lesson... I'm there to learn to fit and thats it. If they want to keep using the wrong gases and not purging stainless pipe it's on them but while i'm there I will do my best to do quality work. Another thing I notice well when I took a pipe welding test there they were using nitrogen to purge while i have always used argon. I found my pipe test came out like crap although I hadn't welded stainless pipe in about 2 years or so there really was no color at all in my weld root wasn't sugar coated but didn't have much color maybe it was because i was rusty I don't know.
One person told me it was because I was not purging with argon but I really don't know.
Topic Interesting Report of Pressure Vessel Failure By ssbn727 Date 07-07-2011 03:13
Is it diesel fuel Tommy?

If it is, then use Nitrogen instead beforehand to neutralize the vapors and if you have access to a "fuel gas/vapor sniffer" (fancy name for a combustible gas/vapor analyzer) then use it before going ahead with your Argon purge if you even need to do so.:wink::cool:

Remember - SAFETY FIRST!!!

Respectfully,
Henry
Topic What cert for wellheads? By Superflux Date 03-31-2011 03:33
Oxygen is not used due to it's instability with anything flammable (even more so under pressure), as in it could go boom with even small amounts of grease, oil, etc.
Nitrogen is probably used because it is CHEAP, readily available, and not very reactive.
Up until the mid 1980's, we routinely used Nitrogen for purge gas when GTAW on stainless pipe. Filling a large system with Argon back then was considered cost prohibitive for all the most critical of applications. I'm not referring to small cheap plants either. These were large scale facilities by the largest Oil& Gas companies in the world.
Topic Welding inconnel and hastelloy pipe? By 803056 Date 02-07-2011 21:49
Nickel is one alloy family where weave beads are a definite "no-no." Look who’s saying not use weave beads!

The oxide on the surface is very tenacious and must be removed by abrading each bead with a grinding disk, clean file, or carbide rotary file before depositing weld over the previous weld bead.

Do not use preheat and maintain a low interpass temperature. Surfaces should be dry, but allow to cool until warm to the touch if you need to warm the surfaces to dry moisture before welding. 

Clean the inside of the pipe to remove oxides (use an abrasive wheel or carbide rotary file) and wipe down with solvent soaked rags. Cleaning should extend at least 1 inch from the toe of the weld. Tape all joints until you are ready to weld and then peel the tape just before welding. Don’t leave the tape on small diameter pipe (4 inch or less).  Scorched tape will produce CO and CO2 that will contaminate the weld pool. The purge gas should be argon and the flow sufficient to reduce the oxygen and nitrogen to less than ½ percent. The system must be free of moisture as well. Heavy oxide deposits on the root surfaces are a sure indication of insufficient purge. The root surfaces should be silver if the purge is performed properly.

Use clean rags and clean acetone or MEK (you can still get MEK, right?) to wipe the joints before welding. Sulfur, phosphorus, lead, zirconium, and boron will result in cracking problems. Do not use “rent- a-rags” for cleaning. The solvents used by the cleaning company may contain harmful chemicals that will cause the nickel alloys to crack.

The weld pool is very sluggish and does not wet in. It can be described as like welding with toothpaste. You have to put the weld where you want it.

Best regards - Al
Topic Long runs purging advice By 3.2 Inspector Date 09-30-2010 13:43
Good question.

If you purge with Nitrogen first and later replace it with Argon, how would you know that all the Nitrogen has been replaced with pure Argon?
Since you mention Argon, I guess your WPS has that listed as purge gas.

I would experiment with rice paper as already mentioned in this thread.

Not to sound rude, but this should had been considered at an early stage of construction. (maybe you did and this is the only option)

3.2
Topic Long runs purging advice By scrappywelds Date 09-30-2010 01:25
Well there is using purge dams on both sides of the joint and purging through the fit. Put all the root in except for a small hole for a football needle to purge through. Puy in most of the hotpass, then pull ouot the needle fill in the hole then imedately back up and do the rest of the hotpass. There is the solarflux option ofcourse. There is also flux coated filler rod and flux cored filler rod. I have had passed x-ray with all of them. probably best bet is to bring in a tanker truck with nitrogen and one of those super flowmeter like pushing 200 or 300 psi. This is the option we used on a big natural gas line system with good success. Best advice for solarflux is not to mix with water use denatured alcohol. Mix it in two different batches, one thin and one thick. Apply thin layer to both insides of joint, then the thick layer on top of that. Good luck.
Topic Long runs purging advice By PipeIt Date 09-29-2010 17:59
We are putting the final touches on about 2000' of 6" stainless schedule 40 open butt, fuel carrier line (JP8 Jet Fuel) anyway this is 100% X-RAY job we have shot 100% to date, the purging time is killing us.

We used Dams for the fabrication of the straights etc, I hate to use Solarflux but won't rule that out not sure that would work on open butt, We haven't used that product before any advice on this product would be appreciated.

Something we are considering but I want advice from the experts first is doing a pre-purge first blowing it out using Nitrogen once we get the oxygen out then before welding drop the CFH and introduce the Argon.

Thanks,

PI
Topic Not Alloy 20 Now Hastealloy By JeremyW83 Date 09-21-2010 01:59
I am also looking for any suggestions on the following:

I was reading online and it said that the interpass temperature limit was 300*.  Is this accurate for all wall thicknesses?  Also in the past I have gone about an inch to either side of the weld joint to check the temperature will this work?

What amperage ranges are the best for root to cap?

At this company we like to use nitrogen to purge the pipe but this has always been with 304L & 316L.  Is this acceptable for Alloy 20 or do I need to insist using all Argon?

I have never welded Nickles and would be interested in how it is welded and how it could help me with alloy 20? 

Are there any special tapes that I need to use cause they usually buy blue painters tape?
Topic Cutting a propane tank By low_hydrogen Date 07-10-2010 02:31
I hate to even get involved in one of these threads about cutting propane tanks.  First off stop useing the exhaust off a gas engine your loading the tank with unburnt fuel.   A diesal truck is a lot better way to go, nitrogen being the safest option.

Your going to get a lot of comments from so called experts, about how they have been filling them with water and cutting on them for years and nothing has happened.

I know people who do it this way, I have done it that way, and people have died doing it that way so i would never suggest that some one attempt to purge a propane tank with water.

Purging the tank with nitrogen 4 times the cubic feet per hour IMHO is the safest way to attempt a cut out.  Diesal is another common method but you will still from time to time get the "flash".

A gas engine especially over 2000 rpms is expelling unburnt fuel that you will light with a torch!!!!

I heard two guys burnt to death cutting on an old pressure vessal in marlow ok this week, at a salvage yard were they cut on them all the time and have never had an accident.  So it's alway better to take every precation available, it's better to save your life than save time and money by taking short cuts
Topic Welding titanium By ssbn727 Date 03-07-2010 08:23
You're right about Titanium being relatively easy to weld as far as weldability is concerned except for one, or two, or even more than three small, yet very important factors that is a must to be considered when welding Ti outside of an enclosed inert atmosphere in order to achieve consistent, metallurgicly sound, successfully welded joints of any configuration...

If the bag did successfully shielded the Ti from being contaminated by the atmosphere, then I wouldn't say that the person lack experience because they did it that way as long as the person who used this method carefully considered all of the inherently important factors that must be incorporated with the use of such a temporary enclosure such as calculating with an added factor of assurance that the volume of atmosphere inside the temporary enclosure was removed from the enclosure at least 10 times it's volume...

And an ultra high purity grade of argon (UHP grade Helium is  also normally used on thicker Ti material due to increased penetration.) was used along with a minimum -50 f dew point, and finally the person welding took extra care in ensuring that the metal was properly cleaned of any potential contaminants, and wore gloves that would not leave any type of residual contamination on either the filler metal or the base metal itself along with cleaning the filler metal just prior to the start of actual welding also, then I don't see anything wrong with advocating that method at all IMHO...

In fact, in the field where there are not as many controls with respect to the environment, we also set up temporary enclosures along with incorporating the same methods used in our main fabrication facility also only because of the type of stringent NDT methods used that was also a customer requirement beyond what the code we were working to required as well. ;)

Titanium being welded out of a glove-box type configuration whereby the joint is enclosed in a temporary inert atmospheric containment vessel, is engulfed completely by Argon...
If one is to weld without the use of such containment, it is required that the joint have at least two of three types of shielding surrounding the joint during welding...
Titanium, whether it be a CP (Commercially Pure) grade, or any of the Ti weldable alloys, are both reactive to certain elements found in the atmosphere ( Nitrogen, Oxygen, Carbon Dioxide, etc. ) mainly because of the element Titanium itself along with some of the other rare earth elements (Ti is no longer considered nor should it have been considered an "Exotic" metal since it is the ninth most abundant element found on earth and the fourth amongst metals, much like Aluminum once was thought as an "exotic" metal also.) which may also be used as alloying elements in certain grades of Ti...

Hydrogen is another element which will cause contamination in Ti welds which is why the metal must be not only as clean as possible from any thin layer of hydrocarbons or moisture, but also the handling of the Titanium must not be done with bare hands and appropriate gloves which will not leave any type of hydrocarbon residue must be used... 

And when one is one is welding a binary phase type of Ti, or a Ti alloy such as an Alpha-Beta type, prevention of the metal from reacting to the atmosphere which is made up of mostly nitrogen and at the very least 18% oxygen along with smaller percentages of other gases and water in the form known to most of us a relative humidity, is the most important factor to consider when welding because most of, if not all of the elements found in the atmosphere cause the metal, whether it be commercially pure or a Ti alloy - to react badly to them and there's no getting around that... This is why it is strongly recommended that Titanium be welded using a very high purity grade of argon as well as at least a -50 F dew point also in order to prevent any moisture from being introduced into the inert shielded weld zone and possibly contaminating the welded joint.

Now this doesn't mean that one must weld the Ti in a totally enclosed inert atmosphere in order to achieve metallurgicly sound, successful welds where there is limited to no reactivity to the atmosphere surrounding the joint being welded. However, to say that one can achieve metallurgicly sound successful welds without the use of at least a backing shield on the joint engulfing the backside of the joint prior, during and after welding as well as extending the time of gas post-flow being used for the GTAW torch and not having to hold the torch in place once stopping to ensure coverage of the solidifying weld pool for at least as long as the extended shielding gas post-flow rate is set to and finally, not being concerned with welding the Ti at the lowest heat input possible is to almost a guarantee that most of the joint will be contaminated, the weld will indeed be brittle even though it may "look good" and is relatively "easy to weld," the weld will eventually fail in short order if it hasn't already failed once the weld has been completed without the aid of some sort of backing gas shielding for the backside of the joint...

So one of the two important factors is the fact that the backside of the joint must be shielded prior to, during and after the welding is completed at least until the base metal along with the welded area has cooled down to certain safe temperature which can be different according to (The USN uses below 600 F, and Titanium Fabrication Corp used below 450 F even though Timet, which is one of the world's foremost producer and experts in Titanium recommends below 800 F which probably the maximum safe temperature where the metal will not react to the atmosphere surrounding it.) various groups who are welding Titanium... Then there is the inescapable fact that if the solidified weld metal itself that is now behind the weld pool as the welder advances along the joint being welded, is not properly shielded from the atmosphere, and is exposed to the atmosphere before it has cooled down to a non-reactive temperature, then the weld will indeed become contaminated and as such, the Ti will have reacted!!! Hence the term "Reactive metal." and one can easily notice it by the color of the weld and it's adjacent area...

So, the welding of Titanium without a trailing shield is very limited to relatively small sized joints where the risk of contaminating the solidifying weld behind the torch is very low indeed! ;) Still, I prefer to use one of those "Monster" type cups along with the larger than normal sized gas lens diffuser/collet bodies for welds that are short in length and can be completely engulfed by the gas shielding plume once I have completed the weld and extinguished the arc Now some of the trailing shields shown in pictures may seem to look bulky, but there are designs that are much more streamlined and more operator friendly to use with a GTA welding torch

If the shielding gas post-flow rate is set at a normal setting when one is welding Aluminum, carbon, or stainless steels, the Ti will react badly at the weld crater once the welding has stopped, and if one uses a dabbing technique when manually feeding the filler metal into the molten weld pool and inadvertently pulls the filler end that was just shielded out of the gas plume even ever so slightly, one will be introducing contaminated Ti filler metal into the weld pool and therefore contaminating the joint...

Here is another interesting foot note:

Titanium Weld Color:A Potential Indicator of Weld Quality

A common question when welding titanium is, How important is the discoloration of the weld surface?

Weld color can be an indicator of an unacceptable weld, but everyone involved in titanium fabrication—designers, engineers, welders, fabricators, and inspectors—should keep in mind that discoloration should be used as part of the decision-making process and not the sole factor in weld acceptance.

The chart below shows a range of colors that may develop when titanium is exposed to atmospheric conditions at elevated temperatures. The color change is an indication of the thickness of the surface oxide (rutile) that develops and, thus, an indicator of the degree of weld contamination that may have occurred. With minimal or no oxidization, the surface color will be silver or light straw, while in severe cases of oxidation, loose, flaky white or gray deposits form.

If a contract specifically requires evaluation of weld color, the AWS D1.9 Structural Welding Code— Titanium can be a useful reference. The code specifies the inspection requirements, qualifications, and responsibilities for individuals evaluating welds and provides detailed color acceptance criteria in the form of Table 5.3.

When discoloration is found in a weld, the welding team should examine the welding equipment, filler wire, and work environment to understand the cause. For example, a contaminated tungsten welding electrode is evidence that the quality of the primary shield gas has been compromised in the welding torch lines, whereas contamination at the end of the filler wire could be an example of a poor wire manipulation technique.

Ineffective shielding can lead to discoloration of the weld as indicated in the chart below. Possible causes of ineffective shielding are the location of the welding area, such as near a door or vent where the air current flowing across the weld can cause the inert gas shield to fail to some degree. It is imperative to perform welding in an area free from drafts to prevent hot weld metal from being exposed to atmospheric oxygen and nitrogen if bright, shiny silver welds with minimal contamination are required.

Unlike GTAW, GMAW presents new challenges to the issue of weld discoloration. GMAW produces a titanium vapor in the weld arc that deposits onto the cooler weld surface as finely divided titanium particles. Because of the reactive nature of titanium, these particles will combust spontaneously upon exposure to atmospheric oxygen once outside the protective weld gas shield. The nature of the resulting black deposit on the weld makes it difficult for inspectors to use color guidance charts as a reference for visual inspection of weld color. It is therefore very important that every manufacturer establish and approve welding inspection procedures applicable to the specific welding processes adopted.

I could go on and on but, I'll end this post by adding these links which I consider very good ones regarding the proper welding of Titanium without an enclosed atomspheric vessel and hopefully these links will be educational to whomever wanting to learn more about welding Titanium and its alloys! ;)

http://www.millerwelds.com/resources/articles/TIG-gtaw-titanium-welding

http://www.timet.com/pdfs/ti-handbook.pdf

http://www.weldingtipsandtricks.com/welding-titanium.html

http://www.kobelco.co.jp/english/titan/files/details.pdf

http://www.aws.org/wj/june02/feature1.html

Larry will like this one about a hybrid plasma/Gas metal arc experimental welding process for use on Titanium called "Super MIG!":

http://www.spaceformtech.com/pdfs/SuperMigTitaniumWelding.pdf

http://www.wtia.com.au/pdf/TGN-MS-02%20Titanium.pdf

http://www.weldreality.com/Titanium%20Data.htm

http://www.struers.com/resources/elements/12/38860/e-Structure%203_EN.pdf

There are 2 pictures attached which show good examples of what is a good sound weld and a bad one should look like... On of them was posted by my esteemed colleague Al in a previous post and I wanted to include it along with the color chart in the second picture so that one could use Al's picture to distinguish the acceptable weld from an unacceptable one using side by side comparison with the chart, so take a look at both pictures. ;)

TITANIUM WELD COLOR TITANIUM WELD QUALITY

BRIGHT SILVER - ACCEPT BUT REMOVE DISCORATION IF MULTI-PASS WELDS

SILVER - ACCEPT BUT REMOVE DISCORATION IF MULTI-PASS WELDS

LIGHT STRAW - ACCEPT BUT REMOVE DISCORATION IF MULTI-PASS WELDS

DARK STRAW - ACCEPT BUT REMOVE DISCORATION IF MULTI-PASS WELDS

BRONZE - ACCEPT BUT REMOVE DISCORATION IF MULTI-PASS WELDS

BROWN - ACCEPT BUT REMOVE DISCORATION IF MULTI-PASS WELDS

VIOLET - UNNACEPTABLE. REMOVE IF MULTI-PASS WELDS

DARK BLUE - UNNACEPTABLE. REMOVE IF MULTI-PASS WELDS

LIGHT BLUE - UNNACEPTABLE. REMOVE IF MULTI-PASS WELDS

GREEN - UNNACEPTABLE. REMOVE IF MULTI-PASS WELDS

GREY - UNNACEPTABLE. REMOVE IF MULTI-PASS WELDS

WHITE - UNNACEPTABLE. REMOVE IF MULTIPASS WELDS


A positive feature of welding titanium is the color of the weld beads which will give a good indication of the effectiveness of the inert gases on protection of the parts from the atmospheric gases... The aim when welding titanium should always be to produce a bright silver weld. Any discoloration outside the silver weld indicates that some reaction with oxygen has occurred either during the actual welding or during the cool down period.

Any weld discoloration should be cause for stopping the welding operation and correcting the welding problem. Light straw-colored weld discoloration can be removed by wire brushing with a clean stainless steel brush, and the welding can be continued. Dark blue oxide or white powdery oxide on the weld is an indication of a seriously deficient purge. When the discoloration takes place, stop welding immediately and review the causes of the oxide reaction.

Also, if one uses the search function and types in Titanium, you'll find plenty of posts with some interesting links to some really good articles on Titanium welding as well as metallurgy in this forum, so enjoy the reading!!! :) :) :)

Respectfully,
Henry
Attachment: welding-titanium-colors.jpg (30k)
Topic bbq smoker By Fritz T Katt Date 03-04-2010 22:07
I don't see why taking the valve completely off, rinsing with water and then letting sit for 24 hours wouldn't work?

All of the LPG should be washed out by the water, then any fumes SHOULD evaporate in the 24 hours. If not, is a nitrogen/argon purge so hard?

Isn't Liquid Propane Gas an oxymoron?
Topic bbq smoker By low_hydrogen Date 03-01-2010 21:19
9 out of 10  times the filling it with water will work, the exhaust fumes is a horrible way to do it, your just loading the tank with combustable gas.  The only 100 percent safe method is to purge it with nitrogen. IMO

I know I'll hear a lot of comments about, I've been doing it this way with water for years, no problems or I have been hooking my power stroke exhaust to gas tanks before I weld on them and nothings happened to me.
And that great but the reason no one comments on how it didn't work for them is because they are dead......  cubic feet times 2, nitrogen that's my two cents
Topic titanium? By 803056 Date 02-19-2010 23:26
Hello Joe;

You have already received some good advice. All I can do is add my confirmation on a couple of points.

Shielding Gas: Argon purchased to AWS A5.32 is "pure" enough for welding titanium. Use the largest gas nozzle you can and if possible, use a gas lens to permit higher flow rates than would be possible without the gas lens. A trailing cup can be easily fabricated using thin gage stainless steel, a diffuser, and a "cape" made from a piece of "fire blanket.” The idea is to contain the argon gas to displace all oxygen and nitrogen, as well as hydrogen from the weld puddle (gas nozzle's function) and the weld as it cools to a temperature sufficiently low that it no longer reacts with atmospheric gases (the function of the trailing cup). You can also build a "bubble" or a chamber out of plastic film stretched over a wire frame constructed from welding rod. The larger volume of the chamber will require extensive purge and it will require a good volume of argon to lower the oxygen/nitrogen content to less than 0.5%.

Purge: You most definitely need to purge the ID of the tube. Block the ends with stainless steel wool that has been rinsed in 90% isopropyl alcohol and air-dried. You need to allow the air in the tube to be completely displaced by the argon. The (flow x time) should be sufficient to displace the system volume about 7 times to reduce the oxygen content to less than 0.5% (maximum). Air is less dense than argon, so there needs to be venting at the high points in the system to allow the air and excess purge gas (argon) to vent. I would use too high a flow rate with the thin wall tubing. Initial purge flow can be about 10 cfh and then it can be reduced to about 5 cfh if the vents are small. You do not want a positive pressure inside the system that will "blow-out" the molten weld puddle leaving a gaping hole where the weld puddle once was. 

Cleaning: Wire brush all the joints with a stiff bristle brush that has austenitic stainless steel bristles after the piece and the wire brush have been cleaned in isopropyl alcohol to remove all traces of oils (including fingerprints). Do not handle the piece with your bare hands or using gloves that are not perfectly clean. Wipe down any fixturing or tools with the alcohol before using them.

Sweat or the use of chlorinated solvents can contaminate the titanium and it will cause it to crack if the part is reheated (as in the case of stress relief or annealing).

Rods, filler metal: Wipe down all filler metals with the alcohol before using them. It would be a good idea to clean them using stainless steel wool that has also been cleaned in alcohol and air-dried before using it.

Henry said it all when he addressed cleaning and the need to keep everything clean.

If you do everything correctly, the weld, exterior and root side, will be as silver as a new quarter. The discoloration will occur if there is contamination. It will go from silver to a light straw (pale yellow), to gold, to purple, blue, black, and finally gray ash. You need to strive for silver. Any more than a hint of yellow is a kiss of death unless you are doing a sculpture and you are looking for pretty colors for an entirely different reason.

Nearly anything will contaminate titanium. Sweat, fingerprints, oils, chlorinated solvents, dirty rags or welding gloves, dust in the air, etc. Some of the shops I have worked with have people tasked with vacuuming and cleaning all day long just to reduce the possibility of contamination. Adhesive tape will give off fumes when it is heated which will be absorbed by the weld puddle and it will cause contamination.

Grinding disks and other tooling must not be used on anything other than the titanium. Carbide rotary files are usually preferred over grinding disks.

This sample shows the affects of varying levels of atmospheric contamination. Wire brushing the weld will remove the colors derived from oxidation, but the damage is done and it is permanent. Once contaminated, the weld and adjacent base metal has to be removed before a structually sound weld can be made.

Best regards - Al
Attachment: TitaniumContaminationLevels-Color19-Feb-2010.jpg - Varying levels of contamination (186k)
Topic anhydrous ammonia By shorthood2006 Date 12-29-2009 03:02
ya damn sure dont want to sniff or smell anhydrous...it will drop you to your knees.It will consume the air you breath. I would purge with nitrogen from top to bottom.
Topic anhydrous ammonia By low_hydrogen Date 12-29-2009 00:56
Does anybody know which is heavier anhydrous ammonia or nitrogen? nitrogen has a lower specific gravity than air but I am not familiar with anhydrous.

Anhydrous ammonia is generally not considered to be a flammable hazardous product because its temperature of ignition is greater than 1,560 degrees F and the ammonia/air mixture must be 16 percent to 25 percent ammonia vapor for ignition.

I'm curious which method is better to purge nitrogen in the bottom, purging any gas out the top or from the top pushing it out the bottom?   Have a old anhydrous tank around 250 cubic feet that we will be cutting on and welding some xx fittings to. 

Gonna purge it to be on the safe side curious as to which method to go with pugre up or down?

Topic Hilarious! By low_hydrogen Date 11-24-2009 23:19
I've actually purged tanks before with propane.....but I won't do it anymore,  it aint worth the risk.  I dont tell any one how to do it.  I've had people ask me how to and It ain't worth somebody getting killed.  metal is cheaper than hospitals or funerals.  just go buy or build a new one.

Every year some body wants a propane tank cut down for a wood stove, or a smoker. Had people that want fuel tanks cut on to make overhead storage tanks for corn etc.
people around still do it but I'm not going to do it just to prove I can.  And I don't give advice to people who are set on killing their selves just to save a buck!
If It has to be done we are purging it with nitrogen or they can get somebody else to do it period!

And their are no shortage of idiot's around here that just fill it with water or smoke it out with a diesal pick-up and a hose on their tail pipe,  And yes they get buy with it, some for years, but all it takes is being wrong once!!
Topic Hilarious! By low_hydrogen Date 11-24-2009 01:11
Nitrogen??  hell fill it with water than cut on it!!    so and so down the street has done it that way for 60 yrs and he has never killed anybody.

Hell they use to purge gas tanks with propane to weld on them? and u wont cut one in half?

As long as its full there is no oxygen,  and you can't have an explosion without oxygen?

It hasn't had any thing in it for over a year?

I washed it out at the carwash with the soap and hot water its clean!

You welders think you know it all my grandad use to weld on propane tanks with a torch and baleing wire.

If you have ever heard any of these statements you might know a redneck! and if I had a dollar for everytime I have heard one? be like $75 bucks?
Topic Strange purge technique on 304?? By Stephan Date 08-17-2009 06:02
Hi Henry,

thanks again!

Quote: "I guess I should have been more thorough."

No, as usual you were thorough in the absolute widest extent!

Of course, from the pure practical side to fixing the issue, all the responses, like those ones coming from yourself, from Lawrence, Dave, Steve and Shane are stringently pathbreaking.

My response whereas was more a bit more pointing to the general physical feasibility of using CO2 as a purge gas in general - at least as it may have no direct contact to the arc or W-electrode respectively, in a first order.

You know, there are so many investigations on purge gases like Argon, Nitrogen + H2,..., etc. But I personally have never seen an investigation by now, dealing with CO2. Of course the assumption of dissociation and both electrode destruction and surface oxidation lies near, but anyway, how would it appear as far as the gas does really only 'purge' the surface and displacing hereby the pure Oxygen? Hmmm... yet perhaps a bit of a too 'academic' question for the forum - at least as I interpret the 'rating'. But this, I know, we are fortunately having the same opinion about... ;-) 

Certainly however, I also wish that Tommy may go to solve his application most properly and therefore your and the other replies are far more suitable. I guess anyway, that Tom would have not conducted his announced test using the final and expensive part.

All the best,
Stephan
Topic Strange purge technique on 304?? By ssbn727 Date 08-15-2009 23:44
Tommy,

Think basic chemistry for a moment... CO2 = Carbon =more carbon migrating into the weld pool which will bring up the carbon content in the stainless, which will in turn increase the potential of forming together with the chromium to create chromium carbides inside of the tubing at the root... Compounded by the fact that there are also two atoms of Oxygen in the CO2 molecule so, you would also increase the amount of oxygen the root of the weld would be exposed to resulting in more oxidation than you would expose the root of the weld if you did not use any back-purging at all and therefore, increasing the potential for increasing the amount of what is known as "sugaring" as well as porosity at the root of the weld and that's just what would happen at the root of the weld!!! I didn't even talk about what would happen to the base metal adjacent to the weld at the root yet!!!

The HAZ or heat Affected Zone will also be exposed to the extra carbon via possible migration, or through absorption, the extra increased amount of Oxygen in the atmosphere in side the tube will be replacing the amount of primarily Nitrogen in the air atmosphere that it will be replacing which in turn, will increase the amount of oxidation which will occur at the heat affected zone if NO CO2 via dry ice - purging were to be used... In other words, you would be doing more damage than with no type of purge at all!!!

Now I could get more in depth as to what would actually occur at the molecular level as well as what specific changes would occur at the metallurgical also but, I do not want to bore you with so much theory, so much research data that supports what would actually happen, or so much data that would literally have you spinning in your seat saying: "Huh???"

In other words, there's no need for me to go anymore in depth than I already have at this time. if there are questions after-wards, I'd be happy to attempt to answer them to the best of my ability but, the main reason I went the basic chemistry route was to rather simplify in terms that you would readily recognize since you already have quite a bit of experience with welding stainless steels as opposed to potentially losing your attention as to what would occur if CO2 were used compared to no purging at all by inundating you with so much data that you would be saying "Huh???"

So that's what would happen in a nut shell Tommy! ;) The two options I mentioned earlier are good options, and the option which includes the use of a "Y" connection with valves as well as a flowmeter/regulator downstream from the "Y" connection could be even more efficient with respect to the amount of purging gas to be used by incorporating into the tube, the use of water dissolving temporary purging dams which could easily be washed away after the welds are cooled down to room temps after-wards! This would more importantly reduce the amount of wasted purging gas tremendously which in turn would leave you owing your friend less Argon after the work is completed!!! :) :) :)

Respectfully,
Henry
Topic Effect of purging gas in stainless steel By 803056 Date 06-24-2009 12:43
I believe the PMC may have a valid opinion on the cause of the tint assuming there was sufficient purge to begin with.

My experience is that most welders do not allow sufficient time to adequately purge the pipe spool. Next would be the failure to "seal" the system from leakage to prevent oxygen and nitrogen from leaking into the spool faster than the purge gas can displace it. 

I had one case where the pipe spools consisted of several joints each with 1/16 inch root openings at each joint. They didn't tape the open root joints and they used a 1/4 inch diameter hose to feed the argon purge gas into the pipe spool. They did switch to a liquid argon supply and they installed a filter on the liquid argon tank to no avail (no kidding!).

The contractor insisted there was no way to obtain a proper purge and my client would simply have to live with the results they obtained.

I disagreed, changed the purge setup, timed the purge to displace the volume six times and obtained perfectly silver root surfaces.

They then claimed that they wouldn't make any money on a job if they followed my instructions.

The system they installed was completely dismantled.  They followed my instructions on the re-installation and got the silver root surfaces required by the client and contract. As they say, "There isn't enough time or money in the bid to do it right the first time, but there is to do it right the second time."  

Darwin's theories were right and they hold true in the business world as well as the natural world.

Since Chuck is no longer available to provide us with his council, I suggest some literature available from the Nickel Development Institute. They have several papers available on the affects of root oxidation (heat tint) in corrosive environments.

May our good friend Chuck rest in peace.

Best regards - Al
Topic Purging ss pipe tig By 803056 Date 06-16-2009 14:55
Let the purge gas flow long enough to displace the internal volume of the pipe spool seven times. That should dilute the air in the system down to about 0.75% air, which is about 20% oxygen with the remainder mostly nitrogen. The oxygen content should be sufficiently low enough for most welding operations and will result in a shiny silver root when welding stainless steel or even titanium. 

It is best to limit the flow rate so as to minimize mixing of the air with the purge gas. High rates of flow increase turbulence and mixing. Total volume is simply flow rate, (yes, my little buckaroo, a flow meter is a necessary complication) read from the flow meter, times the number of hours (or fractions thereof). So, assuming the pipe spool has an internal volume of 1 cubic foot and if a flow rate of 15 cfh is used, it would take about 1/2 hour to completely purge the system. If you increase the flow rate to 30 cfh, the purge time is reduced to about 15 minutes. This approach is very conservative because it assumes complete mixing of the air and purge gas. The use of the oxygen meter will most likely reduce the purge times considerably.  

I prefer U-groove edge preparations with a root face limited to about 1/32 inch and zero root openings to minimize leakage of the purge gas. Complete joint penetration is not a problem if the root face is limited. Multiple joints are easily accommodated. Bladders or dams can be used to minimize the volume of gas and time required to obtain an acceptable purge. There are soluble paper dams that flush out with water when the system is hydrotested. There are some clients that object to the soluble paper dams, so in that event, inflatable bladders can be inserted to reduce the volume of the system during purge, but then the bladders have to be removed after welding is completed.

It is best to check the completeness of the purge with an oxygen meter. The use of a cigarette or "stink sticks" to monitor air (oxygen) content are not as definitive as the oxygen meter and I've seen problems with oxidized roots using those alternate techniques, i.e., they give false or misleading indication the purge is complete.

Weld the joint closest to the purge source, i.e., purge hose connection, first and progress toward the furthest joint. Weld branch connections toward the top of the spool last based on the assumption that the air is lighter than the purge gas (argon), thus will rise toward the top. Vent holes should be located in the upper most portions of the spool and at each branch to allow the air to "rise" and escape. Reduce the flow rate once welding has begun to reduce the tendency to produce concave root surfaces (suck back), but allow the purge gas to flow while welding to minimize air reentering the system. Don't forget that oxygen isn't the only contaminate to consider. Nitrogen is not truly an inert gas and it can cause complications if it isn't excluded from the spool. Nitrogen can be used as a purge with some stainless steels, but it is also an alloying constituent with certain alloying systems. Being a conservative fellow, I prefer to use argon in all cases to minimize the opportunity of a welder using the wrong purge gas. Gas is cheap compared to replacing the welds.

The use of tape is "iffy" to seal the open joints. There is an aluminum tape available specifically for welding. It is somewhat pricey, so companies, being the cheap SOBs they are, use duct tape, masking tape, and other inexpensive alternatives. The sticky stuff is very sticky when heated and it leaves a residue on the surfaces of the pipe. The adhesive is a contaminate that can be vaporized by the heat of welding. The vaporized adhesive components can easily be drawn into the shielding gas and contaminate the weld. This is not desirable. If you tape the joints to minimize the volume of purge gas escaping the spool, buy the "good stuff", pricey as it is.

The use of a gas lens or diffuser is not necessary in my opinion, but if it works or satisfies the welder, then use it. I do recommend pipe caps on the ends of all open pipe or fittings. Plastic end caps are fine if the heat of welding does not cause them to overheat and produce vapors or melt. I prefer socket end caps that slide over the end of the pipe or a plug that can be inserted snugly in a fitting. Some small amount of gas leakage is fine, after all, the air needs someplace to escape so there is no pressure build-up to produce concave roots or worse yet, a blow out of the final weld as the root pass is just about to be completed!

The magnehelic gage limits the internal pressure to prevent "blow-outs". You still have to time the purge to ensure it is complete.

I almost forgot, water is also a problem. The system must be completely dry and free of moisture. This problem occurs most often with modifications to existing systems or in the field when the pipe spool is left over night. The humid air can cool and condense overnight, in which case the entire pipe spool may have to be heated to a temperature higher than the dew point to ensure all the moisture has evaporated. It is true the purge gas is "dry", but it would take a very long time to rid the system of condensate using purge gas alone.

Best regards - Al
Topic N2 Backing Gas For Stainless Steel (austenitic) By Shane Feder Date 03-05-2009 19:53
js55,
As you are probably well aware argon and nitrogen as purging gases work in different ways. Argon is heavier than air so once your argon level has risen above the weld level you should have 0% oxygen, whereas the nitrogen combines with the air to change the composition of the gas in the chamber. Therefore it is harder to confirm complete removal of all the oxygen (if not using a purge monitor) and if welding commences prior to complete removal of oxygen the weld may be acceptable but not as higher standard as an argon purged weld.
Hope that makes sense.
There may be other metallurgical reasons that Nanjing was alluding to.
Regards,
Shane

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