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Up Topic Welding Industry / General Welding Discussion / gas mixture for Stainless Steel
- - By gokhan ayhan Date 08-28-2007 05:53
Hello Everybody,first of all i am very glad to be a member of such a great forum.

My question is , during PQR qualification of 316L type austenitic steel i used ER316LSi GMAW wire with gas mixture Ar+5%CO2 than i got lot of porocity, than i tried gas mixture Ar+3%O2 mixture i got same problem again.Finally i tried Ar 78% + CO2 20% - O2 2% than there was no problem.So,is that gas mixture applicable to a StainlessSteel ? and  i wonder if 20% of CO2 has any harmfull effect to stainless steel like an inter granular corrosion?

Regards,
Parent - By Kix (****) Date 08-28-2007 13:53
In my research of trying to get away from using a helium trymix gas for stainless i found that using a 98/2 or 97/3 argon/co2 works really well while seeing a more colorful bead with the 98/2.  However, this was with a 308L wire on a 304 base.  I noticed considerable difference in the wetting action of 308L and 309L wires with the 98/2 argon/co2 with the 308L wetting in much nicer and giving a more consistent quality weld.  I allso have read that when using more then 4% co2 in your gas mixture for stainless you will start to introduce unaceptable amounts of carbon into your joint.  As for a gas for 316L maybe an argon/o2 mix would help wet that out fairly nice and be a lot cheaper then a helium trimix.  If that doesn't go for ya then a helium trimix might be your best option for consistent quality.
Parent - - By Sean (**) Date 08-28-2007 17:18
Wow, thats a lot of CO2 for GMAW stainless.  Try a 98 - 2 Ar/O2 as mentioned.  Large amounts of CO2 will affect the corrosion properties of your stainless...   The excess carbon in the arc will form carbides. in and around your weld.
Parent - - By terrasonic Date 08-28-2007 18:15
Wow, I'm wondering now.  I've always just left it at 75/25 just like the CS shielding and all clients have approved it.
Parent - - By js55 (*****) Date 08-28-2007 18:45
A couple of things. Clients don't always know whats best for them. But most often they do, and generally far more about their intended services than the fabricator does.
I've welded miles and miles of SS using 75/25 with no problems. Short circuit to be sure, but that also means root passes exposed to the corrosive medium.
And while it is true that additional carbides will contribute to the formation of Cr carbides generally assumed to be detrimental to corrosion resistance(it will also contribute to greater strength-take note of the high carbon stainless steel material specs (H Grades)-meaning high carbon in SS is not always detrimental), it may still be perfectly adequate (or even better)for the service intended. The thing to do is, since 75/25 is generally less expensive, is to open an honest dialogue with your customer. Make them aware of your awareness of this metallurgical fact. SS isn't always applied to a service that is so marginal as to render the additional carbon problematic. Learn the alloys-the differing C levels in the alloys and what they are for.
Parent - - By reddoggoose (**) Date 08-28-2007 22:50 Edited 08-28-2007 23:02
I have qualified procedures with 95/5 argon C02 mixes,though corrosion resistance was not the important factor. Js55 and myself had a debate a while back regarding which was better to use, trimix or 75/25 argon/co2. Js55 was right when he said that C25 did give very nice arc characteristics, though I still have not finished my direct comparison for lowest possible parameters between the two gases. As mentioned previously in the thread, excessive CO2 can pose sensitization problems. However, depending on the application and media the weld is exposed, then sensitization may or may not be a problem and further testing may be needed. If in doubt I try to stay with the filler metal manufacturers recommendations.

What I don't understand is why porosity is occuring in the first place. I don't believe the porosity issue lies in the type of gases you are using. Many companies use 95/5 argon/CO2 in a stainless spray transfer successfully. I would maybe consider understanding why the porosity is occuring before changing gases.

Jeff, I haven't forgot about you, when or if I ever get a chance to finish the comparison I'll be sure to let you know how it works out. Take care.
Parent - - By js55 (*****) Date 08-29-2007 13:33
rdg,
"Jeff, I haven't forgot about you, when or if I ever get a chance to finish the comparison I'll be sure to let you know how it works out."
No problem. Those with the funds (our companies and/or clients) don't often cooperate with our desire to experiment and learn something new. Even if it could very well benefit them. I understand. We take our shots when we can.
And I would agree, in recognition of your getting us back on track, the gas mix will not be the variable responsible for porosity. Though theoretically I think the additional energy from higher CO2 mixes would tend to alleviate porosity (He is supposed to do this too) by slightly slowing down cooling rate and allowing time for the gas to escape, I doubt actual results would be apparent beyond a normal scatter band. It would be a tightly controlled experiment indeed that would reveal such a result. I know there has been a lot of work along these lines with H2 and Al comparing He mixes of varying percents.
Parent - - By gokhan ayhan Date 08-31-2007 04:04
Thank you everybody for  informative posts,i will also ask to manufacturer and decide.

Regards,
Parent - - By Tommyjoking (****) Date 08-31-2007 08:43 Edited 09-01-2007 06:41
WOW short as it is I love this thread.....I agree/think along the same lines as Kix  if the pure(er) argon is not doing it for you go with some helium mix....all that co2 must cause a contamination issue.....just interesting the perspectives on this and why....time to do some research.

EDIT:  dumb way for me to say that I rekon JS  ....It just seems a high mix of co2 would not work well on SS ...but thats just conjecture on my part.  I have almost no experience with stainless GMAW....And gas mixtures per application is somthing I really need to learn more about because I am lacking in the ability to choose it on my own with confidence in certain applications.  Hence my interest in the thread.
Parent - - By js55 (*****) Date 08-31-2007 13:24
tj,
While CO2 I suppose can be contaminated, as much as any gas, CO2 itself is not a contaminant. Its essentially a combination of elements in gaseous form. Carbon's alloying properties are well known, if not well understood in many circumstances, but what is often overlooked is oxygens beneficial properties as well.
Many think in terms of deoxidation, deoxidation, and therefore oxygen must be eliminated. But much research has been done that confirms that oxygen content can be critical in the developement of acicular ferrite in carbon steels and actually improve toughness properties. Much of the balancing of alloying elements such as Si, Mn, Ti, and others is done with an eye towards not reducing oxygen too much. Especially with SMAW electodes and SAW fluxes.
And consider this from a theoretical standpoint, there is in some instances concern over the strengthening and hardening of weld deposits using ER70S-6 (as opposed to S-2 or S-3) wires due to the higher Mn contained. Higher oxygen could actually relieve the weld deposit of Mn by oxidizing it and therefore improve toughness and ductility. This would come in handy if someone found themselves with a spec that required some hardness control and did not wish to adjust their procedures.
Just theoretical on my part right now, but I wonder if there has been research to confirm such thinking. My guess is there has. I just haven't found it yet.
Parent - By js55 (*****) Date 08-31-2007 13:34
Actually if I may add a bit, the alloy volume percent reduction properties of oxidizing atmospheres is well known (anybody welded with E321 lately?).
I just dont' know of any specific papers that deal with using it or controlling it for specifically Mn to keep hardnesses down.
Up Topic Welding Industry / General Welding Discussion / gas mixture for Stainless Steel

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