Shielding Gas for 316L GMAW Pulse Spray Transfer

[steris]

We have a single side weld v-groove joint made from 1/4" thick 316L with backing. It's welded in flat position and we require full penetration. We're looking to change the weld process from GTAW to GMAW. For this joint we're sensitive to both chromium carbide as well as oxide discoloration. We are ideally looking to move toward a GMAW pulse spray with .045" wire. My question is regarding the best shielding gas to use. I seem to be getting mixed information regarding the use of 98/2 AR/CO2 versus 98/2 AR/O2.

Some sources say that the use of O2 is better because it gives better arc stability for spray transfer. The absence of CO2 means you're not introducing carbon into your weld pool, thereby reducing risk of carbide precipitation. However, other sources say CO2 give much better penetration. Additionally, with only 2% CO2, the carbon content is low enough that you don't have to worry about carbides and that the use of pulse stabilizes the arc enough so the added benefit of O2 disappears. It seems that each welder and gas supplier has a different opinion. Does anyone here have any thoughts on this?

Further, I was also looking into the addition of N2 into the gas as that's supposed increase penetration. However, I can't seem to find references that recommend a specific percentage. Any input on that would be much appreciated.

Thanks for the help!

 

[EdStainless]

my 2cents.
No CO2 and no O2.
If you want better penetration add He to the Ar.
You have to be careful with N2 adds because the alloy absorbs it. In 316L we never went above 5% N2.

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[steris]

Thanks for your reply, Ed. From all the information I've gathered, it seems that He is not advisable for spray transfer and it's predominantly used for short-circuit only. We've used tri-mix in the past for short circuit transfer but haven't tried that for pulse spray. Do you use an He mix for spray or pulse spray? If so, what's your mix ratio? Thanks again!

 

Early gas mixes for SS welding in pulse spray were tri-mix, with around 68Ar - 30He - 2O2 (I'm going by memory). A bit expensive and unique to a very limited number of WPSs in most shops. The traditional He mix for short arc has more like 70% He, and is not useful for pulse spray.
I believe these days Ar-O2 mixes are successfully used. However I would avoid CO2 altogether on SS.
Call Hobart or Lincoln for tech support. They will have programs they can easily load into your power source's controller that are not in the standard menu that came with the machine.

"Everyone is entitled to their own opinions, but they are not entitled to their own facts."

 

[kingnero]

Can't comment on the metallurgical implications of adding few % O2 or CO2 in Ar, but this vastly improves the weldability as opposed to pure Ar shielding.
Have seen many tests in dip, spray and pulse transfer, all using Ar with ~ 2% CO2, which is very common where I live, with positive results.
Again, I focus on mechanical properties, I can't comment whether or not there has been an increase in the carbon content in the deposited metal, and whether or not that has an adverse effect for eg. corrosion protection.

Don't add N2 for penetration - change your other welding parameters.

 

[EdStainless]

I'll second the 30% He for this type of welding. If you can get the 2% CO2 go ahead, I don't like it but it should be safe at those levels.
Yes in conventional TIG we would run 65% He 32% Ar 3% N2

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[gtaw]

You might consider SG-HeAC 7.5% 2.5% (90% Helium, 7.5% Argon, and 2.5% Carbon dioxide).

Best regards - Al

 

[kingnero]

Why are there multiple suggestions for welding 1/4" thick 316 with a He-mixture?
Price goes up steeply with He, and 98% Ar with either CO2 or O2 will do the job just fine.

 

[EdStainless]

I come from a world where the welds were all heavily tested (UT usually) and expected to be polished to a fine finish.
It didn't work with O2 in the weld gas.

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[steris]

I agree, we've used He-AR-CO2 90/7.5/2.5 Tri-Mix in the past but He is VERY expensive. Also, as far as I know, Tri-Mix is not appropriate for spray transfer. We've only every used Tri-Mix for short circuit transfer. Our welders and the literature I've read seem to agree with this assessment. In terms of my question here, the decision for us between 98/2 AR/CO2 or 98/2 AR/O2.

I think at the end of the day, we'll have to run a bunch of tests with both gases to determine penetration, fusion, traverse speed, total heat input, discoloration, and probably send out for some corrosion testing too.

Thank you all for the input!

 

[kingnero]

OK, Ed, fair enough.
Where I work, most of the NDT is X-ray (and certainly for "thinner" work like 1/4"), and using Ar/CO2 there have been no problems (so far).
Can't comment about the polishing, have no experience there.

OP needs to carefully consider his application then. Basically, if not critical, I'd stay away from He because of the price. If critical, apparently one should stay away from CO2/O2 mixtures for other reasons...

Edit, as we've crossed posts: if penetration is an issue, consider Pulse welding. I'm sure there'll be an opposition to this as well, but stainless welds really nice in a pulse arc(= this is spray transfer), and has noticably better penetration than DC spray transfer.

 

[gtaw]

This is another mixed gas used to qualify a stainless procedure: SG-AHeC-60%/38%/2% 60% argon,38% helium, 2% carbon dioxide. The process was GMAW pulsed transfer with type 316 stainless.

Best regards - Al

 

[pandithan]

Direct Helium or Argon will not have wetting action which leads to undercut and moreover, surface tension is more without addition of oxidation gases in small portion. Both Argon plus 2percent O2 or 2.5% Co2 are good for best wetting action, penetration etc

 

[EdStainless]

The biggest issue with fluidity in SS welds is related to the S content of the metal.
In applications where we know that there will be a lot of autogenous welding we order higher S content.
The broad range is 0.005-0.017%, but I prefer 0.007-0.012%. This is used for welded tubing and hygienic service tubing.

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[steris]

I'm planning on welding up test pieces to using both AR-O2 versus AR-CO2. I'd like to send them out to see if either one is more resistant to corrosion. Does anyone have any recommendations of appropriate tests to determine resistance to pitting and chloride corrosion? Given that the differences may be slight, what test would be most likely to show this difference? I'm a little at a loss determining the most appropriate of the various testing methods...

 

[EdStainless]

Find a lab that can fo G150 electrochemical corrosion testing.
A solution of either 1% or 5% NaCl that has been slightly acidified (pH=5) and test slightly above room temp (for control reasons), say 40C. You will get a measured pitting potential as a result.

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[steris]

Thanks Ed! Would that test also show evidence of either sensitization or chloride stress corrosion cracking?

Edit: Sorry, just read through the test standard and it's clearly only meant for pitting. I'll give that one a go for pitting. I'd welcome suggestions for other corrosion modes too.

 

[pandithan]

Why do u want to change from GTAW to GMAW because for this thickness GTAW is best, If required u can 2% H2 in Argon shielding to have penetration and Finish of weld.

 

[steris]

We want to switch because we’re a production shop and GTAW is too slow.