WELDING FROM THE OUTSIDE IN 316L PIPES

By Lawrence (*****)

Date 03-04-2006 13:53

Zam

I think all the possible code choices will dictate an Argon or Nitrogen purge for the indside dia. of your pipes.

If your not working to code, I still think all competent advisors would tell you that it is most necessary to internally purge those pipes.

By ZAMORANO Date 03-05-2006 19:51

Thaks for your answer. Where in the ASME code or similar code can I find this recomendation ?
I make this question because my welding subcontractor thinks that the backing gas is not required and as you, from my personal point of view and experience I know that it is obviously required, but I can't find any documentation to show him that he is wrong.
I have also another doubt in the same way. From the begining of the project with the same subcontractor I rejected the usage of Metal Cored Gas Shielded wire for the repairs on pipes where the usage of backing gas was so difficult, because I thought this was not good enough to protect the root and the HAZ. Do you think that it was a good decision ?

By - Date 03-04-2006 15:14

Zam,
Actually, acetic acid is more of a concern for corrosion to the carbon steel and 13% chrome steels. The stainless steels, starting with the 304, stand up quite well to acetic acid. As discussed in previous threads, the corrosivity of acids depend on two things, the concentration percentage and the temperature. Specifically speaking of a 316L steel, it is virtually anti-corossive to the acetic acid except at 99.5% concentration with the temperature at 200C. Even then, it is only calculated to have a corrosion rate of only 0.1-1.0 mm/yr. This means that it is not totally corrosion proof, but very useful with this acid.

But, your concern was about internal corrosion when welding on the outside of some 3mm 316L pipes. 3mm thickness is pretty thin, so to eliminate the possibility of internal sugaring, it would probably be wise to purge the inside of the pipes, just to be safe. Depending on the heat input you are using with the TIG process, you may not encounter massive sugaring, but without a purge you can still have detrimental effects on the inside. Strictly in my opinion, I would recommend that you use an internal purge during this welding. But, on the other hand, I have seen very few instances where a welder would be required to purge the inside of a pipe when he is only welding a support to the pipe.

Chuck

By ZAMORANO Date 03-05-2006 19:52

By - Date 03-06-2006 00:54

Cesar,
I'm not sure the Code addresses your particular circumstance as a "requirement". Unless I'm mistaken, which I'm very often corrected, I don't think the Code Books are books of recommendations, but list essential and non-essential variables necessary to perform Code related tasks. You are asking about Code requirements directly related to purging pipes where supports are being welded. I doubt very seriously that you will find pro or con circumstances directly related to this circumstance. To be honest, your subcontractor is probably right in the fact that the purge is not "required", but that does not eliminate the fact that it might be "necessary" to protect the inside form contamination. You make reference to "my welding subcontractor" on a couple of occasions. This leads me to believe that this subcontractor is working under your direct authority. If you, either as the General Contractor or authoritative contractor, feels the pipes should be purged due to the thin wall pipe, then don't ask his opinions anyway, require it. We know that the reason for purging pipe is to keep the atmosphere from entering the weld pool and causing contamination. If you feel that the welding on the outside is affecting the mechanical properties and corrosion resistance of the inside of the pipe, make the choice for the subcontractor and require it. I do not feel that in every circumstance possible that it is necessary to purge the inside of pipes when welding is done on the inside. I think that wall thickness and heat input and welding process should be the deciding factors. We know that a thin wall pipe (3mm in this case) is more prone to heat related problems on the inside that a pipe of greater thickness (1/2" wall thickness and above).

Now, your decision to reject Metal Cored Gas Shielded welding process for repairs should have been decided by your repair procedure. To arbitrarily say it is not good enough strictly because of the root and HAZ protection leaves a lot for discussion. For example, joint configuration, depth and severity of the repairs, and accessibility should be considered before accepting or rejecting any weld repair process. Cesar, if you have the authority to reject/accept the process to make repairs, then you should have the authority to require/not require the purging of the pipes. Due to the fact that a lot of repairs have limited access, your decision was probably a logical one, but this forum does not have the authority to make decisions without know all the facts. Even then, we are only going to make recommendations (or suggestions) after reviewing all the facts presented to us. There are a LOT of educated and experienced people in this forum that are unselfishly and willing to share experience and latest technology with the rest of the industry, but from a liability standpoint, we are only able to offer our own personal recommendation and suggestion. I'm sure you understand that. Sometimes you might have to make a mock weld using the scenario you are presentiing to us to prove your point to your subcontractor.

Chuck

By matt uk Date 03-04-2006 19:47

it,s always good practice to purge any stainless pipes

By - Date 03-04-2006 23:35

Matt,
I agree with you that it is good practice to purge any stainless pipes, but the necessity sometimes might outweigh that good practice. For example, if the pipes are of greater thickness and the welding is going to be only the welding of supports might not be detrimental to the inside of the pipes. If the pipes are of thicknesses that the TIG process ( a relatively low heat and localized heat input) it may not be necessary to purge the inside. While it is always good practice to purge the inside of pipes under most situations, factors such as wall thickness, weld process, and ability to adequately purge the pipes should be considered. "Good practice" and "necessity" should both be considered before deciding on having to absolutely purge the pipes. I agree that it is good practice, but it is not always possible.

By 803056

Date 03-06-2006 02:39

AWS publishes two recommended practices; AWS D10.4 and D10.11 you should read and have on hand.

AWS D10.4 Recommended Practices for Welding Austenitic Chromium-Nickel Stainless Steel Piping and Tubing provides good guidance on the preparation and welding of stainless pipe.

AWS D10.11 Recommended Practices for Root pass Welding of Pipe without Backing provides guidance on the proper method of providing purge for the root of a weld. It recommends maintaining the root purge for the first three layers of weld. In your situation, welding thin wall pipe/tubing, it is advisable to maintain a purge to protect the ID from corrosion.

The Navy requires the purge to be maintained until the weld has attained a thickness of 3/16 inch or more. Again, it sounds as though this is applicable to your situation as well.

You want to use argon gas for the 316 alloy. While it is hoped the welder will not melt through the pipe wall, there is always the chance it will happen. Nitrogen is quite soluble in the liquid puddle, as the puddle cools, solubility decreases and porosity may result. Nitrogen is also a strong austenite stabilizer and will tend to decrease the ferrite number, thus increasing the opportunity to develop cracks.

You might consider obtaining a copy of a recent book written by Kotecki and Lippold (I hope I have the correct spelling, forgive me gentlemen if I don't) published by Wiley. It is well written and contains a wealth of information on the metallurgy and welding of stainless steels.

You can also visit Lincoln Electric's website and look for a couple of articles written by Mr. Kotecki. Good stuff!

Good luck - Al

By - Date 03-06-2006 03:43

Al,
Your information is certainly informative. I think the application, welding supports to the outside of the pipe, is a little different than what D10.4 and D10.11 addresses. Welding of "open-butt" and joint configurated welding is a little different story. Again, the determining factor should be the thickness of the piece being welded. Using nitrogen as an internal purge will have little or not affect of the phase balance of the structure unless the walls are penetrated. Even with a wall thickness of 3mm, it is unlikely that the welder will actually melt the walls of the pipe when welding supports. More of a consideration would be the protection of the walls to the propensity of depletion of the chromium oxide that is necessary for corrosion protection. 3mm thickness is very much considered thin wall pipe, but unless the welding process is such that the weld actually melts the inner wall, the main concern should not be the phase balance (austenite.ferrite balance), but the damage the heat did to the chromium protective layer. Using nitrogen as an internal purge will have negligable effects on the phase balance of welds being done on the outside of the pipes unless total melting of the walls is generated. Even then, the filler metal (316L) will not be exposed to the atmosphere of the internal part. A general rule of thumb is that if the walls of SS are such that the pipe incurs discoloration, it is likely that the inpervious, invisible layer of chromium has been compromised. This would be of more a concern to the internals than altering the phase balance. As has been discussed in much detail in previous threads, this is the layer that gives SS it's corrosion protection. While nitrogen has an effect of the phase balance (nitrogen being an austenite former) that incurs when the molten weld metal is in direct contact with the nitrogen. Personally, I am not condoning the disregard of internal purge of pipes by any stretch of the imagination, but there comes a time where it is of no benefit depending on the thickness being welded. While cracking is always a possibility, that is more prevelant in the pure austenitic steels where no ferrite is present and that is normally in open butt welds. Welding of supports to the external side of pipe is more in line with fillet welds or seal welding where the nitrogen would not come in contact with the molten weld pool.

By Shane Feder (****)

Date 03-06-2006 10:40

Chuck,
Agree with everything you have said ( I always learn plenty about stainless when you respond to queries) just wanted to add my 5 cents worth.
In my past experience of welding light wall stainless piping the pipe supports have generally been 6 mm thick.
Due to the differing thicknesses of parent metal the arc has to be directed at the thicker metal to obtain equal fusion to both sides of the joint.
If the welder is inexperienced or "not switched on" and has the arc directed at the thinner side by the time he has got sufficient heat into the 6 mm side he will have already "sugared" (we call it cauliflowered) the inside of the pipe.
Therefore, unless you can 100% guarantee that this will not happen it is safer to purge. How can you guarantee that this has not happened if it is in an area of the piping where you cannot visually inspect the inside of the pipe?
Regards,
Shane

By - Date 03-06-2006 13:45

Shane,
You are exactly right. Naturally, it is definitely better to be safe than sorry when it comes to protecting the pipe from possible contamination. I would think that 3mm pipe is right at the border of being very susceptable to corrosion contamination if not protected on the inside. But, as you might agree, I've seen many more times where the pipe is not purged when welding supports than when it is purged. Just because people don't do it doesn't make it right.

Your comments of directing the arc to the thicker part (usually the support) is an excellent observation. That would definitely cut down on some of the possibility of burning through the 3mm pipe. Either way, I believe some damage will be done to the inside of the pipe, most likely the damage to the passive layer on the inside. One other thing comes to mind, and I might be wrong, but I will almost bet that the supports are other than SS. If that is the case, now there are issues not only to the inside, but to the weld area on the outside. But, I'll leave that one alone because it brings up a total different set of circumstances regarding corrosion.

Good points, Shane...

Chuck

By rodofgod (**)

Date 03-19-2006 23:44

Hi All!

By no means an expert on this suject attached to the main pipeline line, however if welding onto a pressure envelope, codes apply! That is, if your welding a support, directly onto the main pipe, then you should treat the weld as you would any other weld applicable to the spec required for that pipeline!
If, however you are welding onto a 'doubling' plate i.e a plate previously welded onto the main pipeline, then you are at the mercy of client specs regards NDT and weld specs!
Of course, welding of the doubling plate would fall under any code that was applicable!

Regards

By - Date 03-20-2006 02:58

Glenn,
What Code(s) are you referring to that specifically sets the standard for welding supports to pipes? In other words, where does it say that internal purge of 316L pipes is mandatory when welding supports? I think it is more an industry "good practice" standard or customer spec for internal purge than a Code requirement. Just for my own personal information, please direct me to the Code(s) that sets the requirements for this. Thank you.

Chuck

By rodofgod (**)

Date 03-20-2006 22:32

Hi Chuck!

After going thru several codes today, I think your right!! It does appear to be a client spec for most of the work we carry out and, like you say an industry 'good practice'! Nice too see somebody 'on the ball'!

Like they say, you learn something new every day!

Regards

By tito

Date 03-20-2006 23:30

Just my 2 cents regarding purge.

The contract document should specify what specifications to follow. If the contract document says that WPS's and PQR's should follow ASME sec IX, then there should be a welding procedure for GTAW for the material to be welded, for the size to be welded. If the WPS was qualified (PQR) WITH backing gas, that is an essential variable. Therefore you cannot weld it without backing gas (QW408.8). If, however they have a WPS and PQR for GTAW, for applicable material type, and to size limitations, that was performed WITHOUT backing gas, then it's their option....only if approved by the customer. Regardless, the contract should spell out what to do. If not, that would be the company requesting the works fault for not specifying.

I could be wrong.....

By - Date 03-21-2006 01:29

Tito,
With respect, I think we are missing the main point here. Following the essential variables relating to Sect. IX used for protection of internal purges for SS pipe is a little different than requiring a purge for an "enclosed" pipe, such as welding on the outside of such pipe (or plate). I realize that this whole thread is all on the same page, but specifically specifying the variables for purging the inside of pipe when welding supports, in my opinion, is not addressed in any Code book other than customer specs, if any. Personally, I know of no Code requirement that addresses every possible scenario such as pipe thickness, grade of support material, welding process required, purge gas required, etc. The particulars can go on and on. I mean, the purge gas for a 316L as opposed to a duplex material purge can be quite different. Therefore, I know of no Code or customer spec that will cover any and all individual scenarios. I think that each process should be evaluated on an individual basis. I can almost guarantee that welding on the outside of a 316L pipe that is 5/8" thick using a SS support will be quite different that if the pipe was 3mm thick and using a CS support. There is no ONE single spec or Code requirement that is applicable to every scenario. Welding on the outside of a pipe (regardless of grade) that is 3mm thick will have drastically different results than welding supports on a 316L pipe that is 1/2" thick. There are just too many variables to be considered for only one set of requirements to cover all. I do not mean to sound disagreeable, but welding SS is a whole different ballgame than it welding CS.

By tito

Date 03-21-2006 14:55

Well that's what I get for not reading and comprehending what the initial post said. I completely missed the thing about welding supports to the outside of the pipe. I thought he was asking about purging for butt welds. Then, as I skimmed through the other posts, once again I missed the point of mentioning supports. Now that I realize the main point of the original post, I realize my post is completely meaningless. Please everyone, disregard my last post!!

I agree with you Chuck, situations like this should be evaluated on individual basis.

You're justified in sounding disagreeable. I was wrong because I failed to pay attention. I'm allowed to screw up at least once in my life. Right?

By Shane Feder (****)

Date 07-22-2007 20:03

Hello guys,
Just trawling through some old posts and noticed this one, wasn't 100% sure of Titos statement about purging being an essential variable so thought I would do some research.
QW 408.8 is not even listed as a variable for GTAW as per QW-256 so that clause can be disregarded.
QW 408.5 has the addition/deletion of backing gas as a non essential variable.
QW 408.9 has deletion of backing an essential variable only for welds in P41 through P49, 10I,10J,10K, P51 through 53, and P61/62 materials.
Even though it probably seems strange from a metallurgical point of view, 300 series stainless steels can be welded with or without purge regardless of what was done on the PQR.
It is the general consensus (amongst my work colleagues) that stainless steel should never be welded without backing gas (purge) so why does the code allow it ?
Why is P10H not included in QW 408.9 ?
Started out trying to answer a question and ended up asking some myself,
Regards,
Shane

By 803056

Date 07-23-2007 12:42

The mechanical properites of austenitic stainless steel largely remain intact even if the root is not protected by shielding gas. That isn't to say the corrosion resistance isn't affect, but ASME Section IX isn't concerned with corrosion properties, only mechanical properties. Considering the fact that the root reinforcement and face reinforcement is removed prior to bend testing or tensile testing, any potential degradation to the mechanical properties of the root pass are mitigated.

It is good arguement in favor of NAVSEA's requirement that the root bends be performed with the root in the "as welded" condition if backing is not used.

Best regards - Al

By ramakant_v (*)

Date 07-23-2007 17:10

Hello All,

I wish I could have joined you guys much earlier. I agree with most of the guys who have shared their experiences that Stainless Steels must be welded with Shielding gas purging from inside. This becomes further necessary if the thicknesses to be joined are so thin. Personally I do not know any code which says that purging the pipes from backside is necessary or a mandatory requirement. But still it is a universally-accepted-good-engineering-practice to provide shielding gas at backside of the weld while welding SS.

I was just going through some of the literatures written by Dr. Kotecki who is a pioneer in Stainless Steel. I found a figure given in AWS D18.1 which mentions the effect of oxygen content (in ppm) on the appearance of the weld at backside. It is really interesting to see the effect and one can easily establish what kind of discoloration one achieves at backside of the weld..!! Moreover It is my personal view that in case of such circumstances making a mock up test pieces would be of great benefit as it can be practically shown how important purging of shielding gas is for such a thin pipes.

Regards,
Ramakant

By 803056

Date 07-23-2007 19:31

I believe most of us are in agreement that if it is worth the cost and effort to use stanless steel, it should be worth the effort to use a proper purge on the root side of the joint.

You mention the appearance of the root side of the weld when proper purge isn't used, but you don't say anything about the mechanical properties, Hummm, very interesting.

Best regards - Al

By Tommyjoking (****)

Date 07-24-2007 06:05

On thicker piping not requiring a full pen weld ///fit with zero gap and weld away. On any ss project where the backside of the joint will reach a liquid or close to liquid temperature some sort of shielding from oxygen should be supplied wheither its specified code wise or not (just because its omitted does not mean you should not do it!). The only way I would weld this job without backing gas was if the customer specifically requested it to be done that way in writing. To weld it without shielding behind the joint in my opinion is a serious disservice to the customer.

Tommy

By ramakant_v (*)

Date 07-25-2007 16:27

Hello,

I am unable to comment on the effect of mechanical properties as I have have not tested any samples which compares results of mechanical properties with and without purge at back side. However, I think there should not be any change in mechanical properties unless there is difference in chemical composition of the weld or there is a severe oxidation at back side.

Regards,
Ramakant

By aevald

Date 07-25-2007 16:48

Hello ramakant_v, I believe that if the backside of the welded parts has any sugaring(extreme oxidation), you will definitely experience a reduction in the mechanical properties that will be exhibited, if it is subjected to various chemical substances you will also likely experience failures of the weld joints as some of the necessary alloying substances would be lost in the sugared portions of the root. There are certainly times when a particular welding process or procedure will provide enough residual shielding due to joint design and welding procedure that you might get away without providing a purge gas or protective flux for the backside of the joint, but, that is generally a gamble that I personally wouldn't want to be responsible for. Just my $.02. Regards, aevald