Duplex S31803 vs. S32205 - Purge Gas for GTAW Pipe Welding 1

[Guest102023]

Trying to decide between 100% Argon and 100% Nitrogen for the backing gas. We are working in the 1/4~1/2 inch range of BM thickness, 1~6 inch diameter.

I would like opinions drawn from actual experience, and if the requirements differ for the two alloys.

We will do damming and purge gas oxygen testing of course.

.

"If you don't have time to do the job right the first time, when are you going to find time to repair it?"

 

[gtaw]

Some grades of stainless steel utilize nitrogen as an alloying constituent. They should be purged with argon. Few welders appreciate the difference between the different stainless steel alloys, those that can utilize nitrogen purge and those that should be purged with argon, so it is prudent (safe) to use argon for all purge applications.

Nitrogen is not an inert gas. Highest quality and minimum heat tint will be attained using argon gas for the root purge on all grades of austenitic stainless steel.

My feeling on the subject is simple. If you can afford the price differential between carbon steel and austenitic stainless steel and optimum properties and corrosion resistance is needed, you can afford to use argon to protect the root surface from oxidation.

Best regards - Al

 

[gtaw]

My bad, I just reread the post and see that the question is in regards to duplex stainless steel. I would still go with the argon.

Best regards - Al

 

[Guest102023]

Thanks gtaw,

My research is also telling me argon, but I am up against a spec that mandates nitrogen.

"If you don't have time to do the job right the first time, when are you going to find time to repair it?"

 

[EdStainless]

I would purge with Ar + 5% N2.
You will be using N2 in the weld gas correct?

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

 

[Guest102023]

EdS,
No, argon for the shielding gas. I guess my real question is whether nitrogen can be absorbed into the molten metal.

"If you don't have time to do the job right the first time, when are you going to find time to repair it?"

 

[Guest102023]

Nitrogen from the backing gas, that is.

"If you don't have time to do the job right the first time, when are you going to find time to repair it?"

 

[DekDee]

brimstoner,
Tried to copy paste from a paper "Guidelines for the fabrication of Duplex Stainless Steels" by the International Molybdenum Association but unable.
They recommend Argon backing gas with up to 3% Nitrogen for the more highly alloyed Duplex Stainless Steels.
While the addition of Nitrogen increases electrode wear, the addition of Helium partly offsets that.

Hope that helps,
Cheers,
DD

 

[metengr]

For backing 100% welding grade Argon gas. For shielding gas, Ar + 3% N₂ for the root followed by 100% Argon for fill.

 

[Guest102023]

Does the use of S32205, with its higher N content, obviate the need for additions to the shielding gas?

"If you don't have time to do the job right the first time, when are you going to find time to repair it?"

 

[racookpe1978]

Why does the "soec" reqire nitrogen for back gas purge? Are you sure it does not specify the high-Ar/low-N2 mix?

 

[metengr]

Does the use of S32205, with its higher N content, obviate the need for additions to the shielding gas?

Not necessarily

Look here on page 7 and 8.

http://www.outokumpu.com/SiteCollec...-2205-hpsa-imperial-outokumpu-en-americas.pdf

 

[Guest102023]

Well the first trial results are in, on 2" sch. 80 pipe coupons. Two passes GTAW (3/32") + one pass SMAW (1/8"). Interpass temp. was 100°F max.
Ferrite was measured on metallographic sections, and agree reasonably well with magnetic measurements made in the shop. Spec is 35~60% α, the same for all welding processes.

α content:

1- Root weld - 60% α
2- Root HAZ - 30% α

3- Cap weld - 35% α
4- Cap HAZ - 65% α
No deleterious phases observed.

Conclusions:
a) 1&2 tell me that I do not need nitrogen in the backing gas, and that heat input could be higher.
b) 3&4 present a dilemma. If I increase heat input and/or interpass temp I risk having too little ferrite in the weld, but I need slower cooling to keep α from going too high in the HAZ.

Comments, advice?

"If you don't have time to do the job right the first time, when are you going to find time to repair it?"

 

[metengr]

Did you run or have the chemical analysis of the base material and weld metal?

 

[Guest102023]

Base metal is S32205 (no MTR quickly available). Filler is Avesta ER2209 Rod (3/32") and Avesta 2205PW Stick (1/8"). MTRs attached for all materials. (You'll notice the size is wrong for the stick, but it is the same product that was used.)

I am stuck as to how to tackle the cap pass problem. No deleterious phases observed, so there is scope to increase heat if necessary.

.


"If you don't have time to do the job right the first time, when are you going to find time to repair it?"

 

[metengr]

The data you reported does not seem correct to me, 3 and 4 should be reversed. The cap pass HAZ measurement seems odd. Either an error occurred on sample prep or recording or you have some type of chemical segregation going on in the base material. I would double check.

 

[Guest102023]

Thanks Metengr, I will do that.

"If you don't have time to do the job right the first time, when are you going to find time to repair it?"