Hello all, another situation to sol 4

[apiguy]

Hello all, another situation to solve.

We have a new Thermal Oxidizer being constructed. The vent header piping material is SA358 Type 254 SMO. The original WPS specified Argon as a backing gas. When I arrived to inspect the project the welders were using nitrogen as a backing gas. The heat-tint is dark blue on the root surface and HAZ approximately 1 1/4" wide into the base metal. Does anyone have any suggestions regarding the effects of the use of nitrogen on the corrosion resistance of 254 SMO material?

 

[rustbuster1]

Nitrogen is an alloying element for stainless steels including 254 SMO - it usually improves pitting resistance and strength. However, uncontrolled absorbtion of N from the shielding gas is NOT good. The directions I have for TIG welding of 254 SMO make a strong point of calling for "99.9% Argon with a dew point of -77F" for shielding both the back side and the arc. Apparently Nitrogen gas also leads to excessive consumption of the tungsten electrode (not good for alloy content again), although "a small addition of dry nitrogen may be considered."

The recommended shielding gas for MIG is 100% WELDING GRADE ARGON, but up to 25% helium can be added for pulsed arc or short-arc welding. A gas with 69% Ar, 30% helium and 1% CO2 can give satisfactory results.

My position as an inspector would be that welds made with N shielding gas are not acceptable, unless someone proved otherwise, taking the design purpose into consideration.

 

[moseley]

Gentlemen,
Would someone please let me know what this "Type 254 SMO" is? I can't find that material anywhere. Section IX doesn't even metion that type of SA358. Thanks for any input on this.

 

[apiguy]

Moseley,
If you look on page 87 of Section IX the fourth material from the top is 254 SMO. It is listed as SA-358 Type S31254. The "S31254" is the UNS number this material is known by. The 254 SMO is the Manufacturers' trademark name.

 

[apiguy]

Rustbuster1,
It looks like we have the reference material as that is exactly what I have highlighted on my desk. I am having a had time convincing the contractor the welds are not acceptable for the "design purpose"(waste gas collection header for a Thermal Oxidizer). Their position is that a change in backing gas composition is considered a non-essential variable and therefore the welds are acceptable. I have made contact with the manufacturer of the material and he concurs with my recommendations 100%. Just to let you know, 21 of 22 of the welds that were radiographed; failed. The person reviewing the film accepted one, but mentioned there was a hot tear. In all of my experience, a hot tear is a casting defect. The first weld I inspected had a one-inch long, root crack that was open quite a bit. I have requested to see the film in question as I feel the "hot-tear" was incorrectly identified. I'll keep you all informed....

 

[moseley]

APIGUY,
Thanks. And you are 100% right, the change in composition is definately a ESSENTIAL variable. I had to deal with the same problem with P91 material. Again thanks for the reference help!

 

[apiguy]

Glad I could help...Moseley, You have to verify the material P-number. Changing the backing gas composition is only an essential variable for GTAW for the following P-numbers:

QW408.9 For groove welds in P41-47 and all welds of P10I, P10J, P10K, P51-53 AND P61-62.






I have discovered another problem this morning. The welders have been using a filler metal I'm not familiar with. The bare wire imprinting reads:

A2 4831v
The imprint is hard to read, but I believe those are the correct numbers. Does anyone know if this is a high nickel rod. I hope the A2 does not refer to the A2 composition for carbon - half moly.

Any comments.......

 

[Seldom]

Gentleman,
I don’t think I’m too far off base when I say the material in question being a P8 material and the change of gas is a nonessential variable as noted in QW-408.5 but I would suggest referencing QW-401. It appears to me that this paragraph pretty much seals the deal when it states-

“A change in a welding condition which will not affect the mechanical properties of a weldment”

The use of nitrogen as a backing gas has been proven to affect the mechanical properties by causing hot cracking in P8, 300 series stainless steel welds especially when no precautions are taken to minimize the nitrogen from the arc.

 

[Seldom]

aipguy,
You may want to take a look at the filler metal possibly being a US Welding Corp. product. Their 5837-V possibly could resemble your 4831-V which would make sense. NiCrMo-3
Just a thought.

 

[apiguy]

Seldom,
you were right on track with the filler metal suggestion. After an entire afternoon fox hunt, we found the mystery rod. It is AvestaPolariat ERNiCrMo-3. One end of the wire is imprinted "ERNiCrMo-3" the other side only has A2 4831v. The welders had cut the wire in half at the beginning of the project and forgot where it had originated from. I finally found a box that contained a couple pieces of wire that had not been cut. A learning experience for all...... Richard Schram
Mechanical Integrity Specialist
Pharmacia Global Supply Arecibo-P. Rico
rschram@pharmacia.com

 

[MURZ]

Interesting.
First I would like to comment on the weld wire that was used. It has been shown that welds, which solidify as primary austenite, may be very susceptible to solidification cracking. When welding Austenitic Stainless Steel, the weld metal should solidify with a certain percent of Delta Ferrite. Cracking susceptibility is drastically reduced in welds that solidify as primary ferrite. The percentage of Delta Ferrite should be in the range of 5%. ERNiCrMo-3 is a high Nickel welding wire (around 60% Ni) that solidifies with an entirely austenitic microstructure. There will be no delta ferrite in the weld metal. This could be the reason for the cracking that you are seeing. Most of the commonly used austenitic stainless steel filler metals are formulated so that the as deposited weld metal contains ferrite in the range stated above. At this point I would be more concerned with the weld wire that was used in lieu of the backing gas.

I am not saying that I would approve the use of Nitrogen as a backing gas. However, I would be more concerned if they used Nitrogen as a shielding gas. Nitrogen (N2) will be dissociated by the welding arc and this elementary form of nitrogen (single atom) is easily absorbed by the stainless steel. This absorption can form nitrides in the weld metal that are extremely brittle. These nitrides can lead to weld metal cracking either after welding or in service. When Nitrogen is used to shield the backside of the weld, it is isolated from the welding arc. This being the case the likelihood of Nitrogen being adsorbed into the weld metal is small compared to when it is used as a shielding gas.

Hope this is of some help.

 

[apiguy]

MURZ,
When the root is being welded, is the arc not exposed to the Nitrogen? Richard Schram
Mechanical Integrity Specialist
Pharmacia Global Supply Arecibo-P. Rico
rschram@pharmacia.com

 

[welding1]

Using Nitrogen as a backing gas for 304/316 type stainless steel is not uncommon, however you will get a grey tinge as you describe, which is presumably an oxide layer. This will impair corrosion resistance and as you are using a super–austenitic material presumably for its enhanced corrosion resistant properties, it does not make sense to cut corners. Nitrogen + 10% Helium would be better, but to get the best out of this material a good gas purge is essential, 0.015% Oxygen content or less.

Codes and standards don’t really address these problems; the procedures and welding methods should be approved and supervised by a properly qualified Welding Engineer.

John

http://www.gowelding.com

 

[apiguy]

John,
The original WPS was written by a "Welding Engineer" which specified the use of high purity argn. When the welder had problems welding to this procedure, he was told by the "Welding Engineer" to use nitrogen. There is no documentation to support this though. I have reviewed many WPSs' written by this "Welding Engineer" and all have contained errors. I honestly belive that this person is all about collecting money than performing his services. Every welder and procedure has passed all of the required tests according to this guy, but every weld I have inspected has failed. There are no coupons to see, no film available and no other documentation to support that any of the required tests have been made. Sounds pretty fishy to me...

sorry about the soapbox sermon but I had to let my feelings be known.... Richard Schram
Mechanical Integrity Specialist
Pharmacia Global Supply Arecibo-P. Rico
rschram@pharmacia.com

 

[Seldom]

Hi apiguy,
Without trying to be presumptuous of MURZ’s reply, possibly I can help you understand the practical application of his second paragraph regarding nitrogen exposure to the arc in relation to exposure to the root’s backside from my experiences.

The normal joint design (1/8+” min root opening) used by “cup-walkers” shouldn’t be used. This allows the nitrogen backing gas to come into intimate contact with the arc through the root opening unimpeded. The joint design has to be kept very tight (1/64”-1/32”) with the filler rod more or less laid in the joint just ahead of the puddle helping to block the gas for the arc.

It’s one thing to have the nitrogen in contact with the semi-molten backside of the puddle but it’s another issue, as MURZ points out, with the gas in the arc!

 

[welding1]

Welding is one skill, effective purging is another. I have seen many a good welder fail a test because he couldn’t get his purge right. The pipe should be suitably blanked off with dams that allow continuous gas flow through the pipe. At least 6 to 10 volume changes should have occurred in the dam before welding commences. The gas pressure should be reduce prior to completion of the joint.

For the type of material being welded the oxygen level should really be monitored before welding.

If things look suspicious demand a retest in your presence.

Regards

John

http://www.gowelding.com

 

[apiguy]

Thanks John. Richard Schram
Mechanical Integrity Specialist
Pharmacia Global Supply Arecibo-P. Rico
rschram@pharmacia.com

 

[apiguy]

just wanted to update everyone...Today I finally received the PQR for the N2 purge. It came with a date that was about two months after the project had started. X-ray film showed as well....among nearly every defect imaginable, there were numerous cracks the interpreter identified as IP. When I reviewed the film with him, I had to ask how he came up with transverse IP.
Richard Schram
Mechanical Integrity Specialist
Pharmacia Global Supply Arecibo-P. Rico
rschram@pharmacia.com