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Up Topic Welding Industry / Metallurgy / purge P91
- - By Easy638 (*) Date 04-11-2008 02:13 Edited 04-11-2008 02:24
What problems can occur by not purging a GTAW 9% Cr (P91) root weld

I am a welder from NYC on a new powerhouse on Long Island
I have always made these welds with a purge
I know I am going to get sugar and a dead puddle
Hard to weld on My end
Am I going to create any Metallurgy problems, down the road?
This is not My call but from QC to do it?
Parent - - By gwg (*) Date 04-15-2008 09:49
The most common problem will be not passing an RT of the weld root, if an RT is required. Also, the risk of the weld puddle absorbing oxygen and hydrogen can result in inferior mechanical properties of the weld root.
Parent - By Jim Hughes (***) Date 04-25-2008 13:06
Just curious, How is it not going to pass RT? I don't see anything in ASME about rejecting sugar. And how does it affect the mech. properties in a negitive way?  The chrome content is there for it's excellent corrosion resistance and strength at elevated temperatures. The corrosion ability can be impeded by the amount of oxide that forms on the weld. (what is refered to as sugar on the root side of the weld) This has little or no effect on the mech. properties that I know of. We have qualified WPS with black to sugar looking roots and passed the mech. properties part of the process. You would have to perform a G4, or G48 or G78 corrsion test to determine if passivity is being impeded and to what degree.  P91 chrome content helps the strength of the  material at elevated temps. Mostly because of it's use in the power industry where corrosion resistance is a factor but not as much as high chrome material that is in a hostel chemical enviroment.

Thanks
Jim
Parent - By 3.1 Inspector Date 04-18-2008 09:15
Purge during welding on steel above 3% Cr is a must.
If you dont purge it will oxidate - like welding SS without purge.
Parent - - By MBSims (****) Date 04-19-2008 01:34
Let me explain it like this - did you ever see the movie Armageddon with Bruce Willis and the drill crew that were sent to blast an asteroid?  Remember the big stalagtites sticking up all over the asteroid when they were trying to land?  Well that's what a GTAW root pass on P91 looks like with no purge.  It looks even worse on RT film.  Don't take my word for it, weld up a test coupon and see what you think about it then.
Parent - - By pipewelder_1999 (****) Date 04-19-2008 03:21
Do you think any of the ASME B&PV codes will ever address root oxidation as a defect ? Is there a reason they dont?
Parent - - By 3.1 Inspector Date 04-19-2008 10:17
They do - in terms of porosity.
Parent - - By pipewelder_1999 (****) Date 04-19-2008 11:22
I don't understand what you mean. I have never welded P91. Does the lack of purge cause porosity ?
Parent - - By 3.1 Inspector Date 04-19-2008 11:47
Well, I call oxidation a kind of porosity as the oxidation consists of oxygen(holes)
On an x-ray film it will look like porosity.
Parent - - By pipewelder_1999 (****) Date 04-19-2008 13:47
If it shows as a rounded or elongated indication on the film similar to porosity or an inclusion then it would be rejectable. I have never seen what it looks like on P91. I have seen some on 5% chrome and 300 series SS and never noticed anything that looked like porosity or that would be rejectable as a rounded or elongated indication.

Thanks for the comment.

Gerald
Parent - - By js55 (*****) Date 04-25-2008 13:54
Sugaring can be rejected on the basis of the possibility of masking other indications. An accomodation in virtually every code I know of. And certainly in ASME.
Especially in crack sensitive material such as Grade 91. Its hard enough to see cracks sometimes on a radiograph much less with a big ole black mess shadowing the image.
Parent - - By pipewelder_1999 (****) Date 04-25-2008 14:25
I agree with the concept of interfering with the interpretation of the radiograph if the condition does do that.

But do you feel that option indicates the condition should ALWAYS be rejected?  I understand that the amount of root oxidation can be quite severe. I

Does sugaring show up as a "Big Black Mess". Anything added to the surface of a weld such as slag or oxides would reduce the amount of radiation getting to the film and thus turn the image lighter.

I do appreciate the response and do see the point regarding the masking of discontinuities.

Have a good one.

Gerald
Parent - - By js55 (*****) Date 04-25-2008 14:49
pipewelder,
There is no justification for an inspector to reject sugaring if it is of the variety of just a blackening of an otherwise smooth root pass, unless it exists at the specification level. Though its poor workmanship. But, if the stuff is so bad that you get that 'curling' phenomena then it will definately register a shadow that in my opinion is justifiably rejectable. The other thing is, as has been made clear is the "Power" application of the alloy most generally. And we're talkin steam. Main steam comin right out of the boiler. Steam that is really flying through the pipe (if you've ever seen a steam blow it'll scare the crap out of you). It is my belief that if sugaring is of the 'curling' extent that it can very well create turbulances that can accelerate corrosion. Similar to the justification for match boring and ID transition envelopes.
Parent - By pipewelder_1999 (****) Date 04-25-2008 15:04
I agree that in conditions related to flow, internal contamination etc.. this could be a fitness for service type of discontinuity that would need to be addressed by project specifications.

The code is not a replacement for engineering it just says what it says and doesn't say what it doesn't say.
Parent - - By Jim Hughes (***) Date 04-25-2008 16:55
It looks black to the eye but not on film. Even with D4 film it's going to be hard to determine cracking unless its pretty severe. Since you would be concerned about cracking in this material the NDE method of choice would be UT or UT phased array. That is what we are doing. You can have a root bead just above flush that it sugared and you don't see this big curling black thing that was talked about earlier. On P91 it looks like fur or furry with very small nodules. These nodules show up on film as a lighter image. I have never seen it bad enough to cause a flow problem. Again you are seeing oxides forming which would impead pasivity. ASME Sec IX is allowing WPS for P91 to be qualified using the STT or Pipe Pro processes with out purge. Mainly because it does not effect the mech. properties.

Jim
Parent - - By pipewelder_1999 (****) Date 04-25-2008 18:44
I understand the image that oxidation produces. I haven't seen P91 material though. I understand that any extra material whether it be slag, oxidation, purge paper etc.. shows up on the radiograph as a lighter area unless that material is taking the place of a denser material or that material has the ability to absorb more radiation(tunsten, lead).

By your statement above "ASME Sec IX is allowing ......." makes me think there is something restricted somewhere else. Or are you just referring to the STT Short Circuiting process. Its still short circuiting is it not ? Anyway thats another topic.

Is ASME considering restricting the qualification without purge ? I have not seen where this is addressed but I did hear that there were going to be some changes related to P-numbers etc.

Your statement regarding the mechanical properties is one of the things I have oft considered. A fully formed root bead in a circumferential pipe weld in a steam system with oxidation would not impair the mechanical properties of the joint in most applications. There are a many SS and Nickel Allow boiler tube welds in superheaters with plenty of sugar. I have never fixed a leak at one. Has anyone seen a failure in a steam system resulting from root oxidation ?

I make the above statement as an opinion and based on things I have read for years. I stand to be corrected by any as I am only a welder that knows (or thinks he knows) stuff about welding. Any chance to communicate with someone with a degree or formal training is highly valued by me.
Parent - - By Jim Hughes (***) Date 04-26-2008 13:35
Pipwelder 1999,
When I said allowing, I did not mean that at one time ASME disallowed it. I was saying that our corp. QA/QC dept. has recently qualified WPS using the STT and PipPro machines on P91 material without purge. We as a company are  starting to use these processes that have been used in fab shops for a few years now in the field on boilers. The reason I said it does not neg. effect the mech. properties is because we qualified the procedures to Sec. IX and passed all the mechanical tests. The oxidation that forms on the back side of the root is(we call suraging) surface and does not go all the way through the root. If P91 material was being used in hostile chemical eviroments it would not work because pasivity has been impeaded by the oxidation (sugaring) that has formed due to no purge.

Jim
Parent - - By pipewelder_1999 (****) Date 04-26-2008 18:56
I got it. It may have been my misunderstanding of the wording.

I do imagine like SS  that it welds much better with a purge.

Thanks
Parent - - By 3.1 Inspector Date 04-27-2008 08:17
And the Cr content will be well below 9%
I once caught a welder doing a repair of the root without purge, the Cr had dropped to 3% (it was P5 material)
Parent - - By Jim Hughes (***) Date 04-27-2008 15:54
3.1
are you saying that because of no purge the chrome content dropped to 3%?  You say P5. But was it P5A or P5B?  I have PMI welds that did not look like it had a very good purge at all and never saw the chrome content drop that much, maybe 8.5 to 8%.  Think about P91 material for a min. here. If purging plays such an important part what do you thinks happens to that root pass on one inch thick material at 1430 F? Which is what our WPS says is the PWHT range. Are you purging though the PWHT part of the process of welding P91 material? As for Cr. depletion. As long as you are adding filler metal to that weld, chromium depleation is not happing for the most part, in fact even if you dry washed over it, it would not deplete the chromium to that degree. Although I have not tested that theory on P91 material I have tested it on 300 serious stainless and found after dry-washing a filler pass one time it did not deplete the Cr. very much at all.

I will say this, puging does help the weldability a lot using the GTAW process on the root pass but I don't belive that it effects the mechanical properties that much and I don't believe it causes Cr. depletion. I am willing to stand corrected if anyone knows of any studies to the contrary.

Jim
Parent - - By 3.1 Inspector Date 04-27-2008 15:59
I did not understand half of what you wrote....
But yes, no purge in welds above 3% Cr will cause a loss in Cr.
Parent - - By Jim Hughes (***) Date 04-27-2008 17:59
3.1
and how do you Know this? Please give some detail.

Jim
Parent - - By 3.1 Inspector Date 04-27-2008 18:20
I did the PMI myself on that particular weld
Parent - - By Jim Hughes (***) Date 04-28-2008 11:46
3.1
I believe there are some other issues that your dealing with. Maybe wrong filler metal. I have never seen that much of a variance in Cr. content after welding. I have not seen any studies on the amount of back purge dilutes Cr. content. I have looked through all of EPRI's studies that I have on P91 and found nothing. Maybe your on to something. :)

Jim
Parent - - By 3.1 Inspector Date 04-28-2008 12:08
Jim,

According to my welding engineer;
During oxidation of P5, P9, P91, etc.... the Cr is "lost"

I really cant explain it aswell as I would like to - english is not my mother tongue.
Parent - By Jim Hughes (***) Date 04-28-2008 14:56
3.1
That's who you need to be listening to. :)

Jim
Up Topic Welding Industry / Metallurgy / purge P91

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