Jeff,
I agree 100% with what you have said about Chuck's explanation. This is excellent.
But I agree also with you when you say that this topic could generate an interesting discussion, and...
Once again you were right when I can read what the other appreciated fellows have already stated and supplemented after Chuck's and your "boost". By the way, thanks a lot "5354" for appreciating my hints on "uploading documents". If you allow I would like to pass on your thanks directly to John Wright. Without him I would never have been able to upload any kind of document in the forum. ;-)
Honestly, Sirs, I must admit to have never had the luck to weld or process respectively, Duplex- or even Superduplex-Steels. Thus you can easeful call me a "droughty theorist".
Nonetheless I am very interested in these materials since - for me - they do represent a very specific and "non ordinary" group of metallic alloys, having truly superior properties by being a greatly sophisticated mixture of mainly two different metallurgical phases.
This again makes them not so very easy to handle for welding - which normally is a clue for making them interesting regarding weld-processing. Now in combination with the shielding gases to be used my interest is doubled, since the different properties of the shielding gases, just as already described by Chuck and you, have - from the physics point of view - surely a deep influence on the later welding result or microstructure.
I have busied myself a time back relative intensively with the effects of active shielding gas constituents in inert gases, mainly Argon, and their physical influences in welding aluminum and its alloys. That was the time when in Germany the Gas-suppliers have advertised their micro-doped shielding gases containing extremely low amounts of active constituents as in particular Nitrogen.
Basically, and only for a better understanding, I am meanwhile known for sometimes being a "savage dog" in technical discussions or arguing respectively, due to I do not agree to pay for somewhat what is only being good advertised without being proved by hard technical facts and thus to have a significant and technical measurable benefit for the user and/or the welder. Here I am - once again - 100% with Henry, when he says: (quote) "...as you should know most of their recommendations when it pertains to welding gases are shall we say, ridiculously exaggerated towards one's overall welding performance, quality and budget..." When the gas suppliers have developed the micro-doped shielding gases, e.g. Argon + 0.0025% Nitrogen for "...significantly improving the welding results in aluminum alloys (e.g. depth of fusion, reduction in porosity,...)..." I have asked myself if this could really be the truth when Argon is being doped with such as homeopathic values of active gases and thus I have tried to find an answer by having a closer look on the physical coherences when using shielding gases in Gas Metal Arc Welding. As you can surely imagine this is like - as "King Henry" (SSBN 727) stated in another post - "...opening Pandora's box..." - since the entire subject is so tricky that my head was truly aching that time. However, I could find out that there is an effect to be observed, since the mentioned coherences are so complex that also minimal contents of substances or elements are - by intricate interactions - truly able to change the physical behaviour, the composition of, and the effects being accomplished, by the arc-plasma. That was surprising for me and I had, nonetheless, doubts at that time. But intermediately there are newer investigations (methodologies have progressed) in the general field of shielding gases and their influences - also in the field of iron-based alloys - that these facts could be approved and solidified. The funny thing is again, that the physicists have also up to now no real idea what the welding arc - in particular in Gas Shielded Metal Arc Welding - actually is - exceptional the agreed and well known physical models! There are so many different questions, begging to be answered, that - I would like to cite Prof. John J. Lowke (my undisputed idol in arc physics) - "...there is more than enough work to be carried out for future generations of interested Researchers!"
Whole and short, I am sure that likewise and in particular in Gas Shielded Metal Arc Welding of Duplex-Steels or Superduplex-Steels shielding gases and their compositions can and will play an important role in the later behaviour of the base metal or weld-joint, respectively.
Well, my problem though is, now, after I have read what problems have occurred in processing Duplex (original post of jrupert), i.e. "birdnesting" was the initiation for the shielding gas question, I am no more sure to asking my questions about the coherences in GMAW Duplex-Steels with various shielding gases and the resulting outcomes to you - the experts being more than "droughty theorists" but being "practitioners + theorists" and thus universally experienced in processing these materials.
Anyway, although I have also my personal opinion on the reasons in charge for "birdnesting" (personal experiences), however please permit to "capture" the opportunity to making actually a sidestep off the original reasons for the topic of "jrupert" and ask some questions concerning the processing of Duplex-Steel(s) by using the GMAW-process and to honestly hope to be - once again - enlightened by you.
· As you have already stated I guess that it should be of great interest to basically know about the grade of Duplex-Steel to be processed. My latest information - and I beg your forgiveness if I am not fully informed about the actual progression in developing those steels but I honestly have had a look to the "www" - is, that there are meanwhile modified Duplex-grades available, compared with the well known "standard-grade" 2205. Now my question would be - in particular with regard to the United States of America and the specifications valid there - how are these different grades to be treated in Welding. As far as I have interpreted the procedures in welding different materials correctly, in the U.S. you have to prepare a WPS for each application and base/filler-material combination. How would e.g. the procedure "jrupert" has spoken of and being valid for a particular Duplex-grade be used for Duplex-Steels different to this particular grade? Has every specific grade/process application its own WPS to be prepared or are there some "intersections" possible? This means, does, for instance, a modified grade WPS (more complex in composition e.g. Superduplex) include applications of a "standard" grade e.g. 2205?
· Resuming. What about the material parameters to be considered beside the material composition, e.g. wall thickness, joint-preparation geometry, etc.? As far as I know in particular Duplex-Steels having a higher sensitiveness in regard to "heat input" and further on the respective cooling rate for achieving an adequate ratio of Ferrite and Austenite growth. Presuming the welding process and its specific parameters, representing important factors as heat input or thermal efficiency, were fixed. How are different parameters as mentioned above (wall thickness, different joint geometries and thus weld-metal volumes,...) taken into account in particular with regard to cooling rates? Would it, for instance, be possible to prepare a WPS for a specific Duplex-grade, having a specific wall thickness and joint geometry, and extend this WPS on base materials having the similar composition but different wall thickness, joint geometry, etc. by varying e.g. the heat input by "varying the welding speed appropriately", "using different droplet transfer modes",..., to achieve e.g. mechanical-technological, or corrosion testing-results being actually represented by and achieved through following the original WPS?
· Henry has asked for clarifying the "filler metal question". As I have heard once from a Spanish welding engineer being employed in shipbuilding and who has investigated the feasibility of using Laser-GMA-Hybrid Welding in joining pure Duplex-Steel (2205) and dissimilar material-combinations (2205/AH36 structural shipbuilding steel), this specific industrial sector has also specific requirements on the Ferrite/Austenite ratio. As far as I remember, they require a minimum austenite value of 30% in both weld-metal and Heat Affected Zone. Is this - from your experience - a value which can be generalized in welding Duplex-grades? Or have different industrial fields (Chemical processing,...) also different requirements on the Ferrite/Austenite ratio to be obtained after welding? How strong - to come back to Henry's question - can the choice of filler-metal (solid wire, slag forming flux cored wire) affect the mentioned ratio under otherwise similar conditions (e.g. chemical composition)?
· Coming now to the very interesting question of different shielding gases. As Chuck and you, Jeff, have already excellently explained the possible differences in using different shielding gases not only by achieving different arc- or droplet detachment/transfer behaviour, but also influencing directly or indirectly the austenite/ferrite balance, I guess that this issue is a very important one and worth for having a closer look upon. When I have busied myself with the shielding gas subject matter mentioned above, in particular the influence of Nitrogen in Argon, I had also a closer look on the influence of this element in welding austenitic nitrogen containing steels. And what I have found out at that time was the fact, that Nitrogen as a shielding gas additive can negatively affect the weld pool purity or solidification (porosity through supersaturation), the arc stability (through degassing) and the outer seam appearance (through slag- or oxide layer generation). These drawbacks again were to be observed in a not so easy to understand relationship basing on the different base metal nitrogen contents and the shielding gas nitrogen contents. Due to between the arc-plasma (gas physics) and weld-pool (solid-state or fluid flow physics) occurring interactions it was - at least at that time - impossible to calculate the later results from the point of equilibrium conditions. But what the most investigations have shown was, that Nitrogen as an additive in shielding gas mixtures (2- or 3 component mixtures) and welding Nitrogen containing austenitic steels can have not only positive, but also negative effects. O.k. to cut the long story short, my question to you, my appreciated colleagues and experts in the forum, is: "Due to Duplex- or in particular also Superduplex Steels containing higher and reasonable Nitrogen amounts and one can also suppose that - depending on the used welding-processes, fillers, welding conditions,..., - these - for the metallurgical microstructure - important and desired amounts of Nitrogen will be reduced by the process, higher Nitrogen contents in the shielding gas whereas can negatively affect the welding behaviour ("bubbling", spattering through degassing,...,), what is the major way to "compress" these reductions?" Is it only a higher Nickel content within the filler metal to balancing the base metals Nitrogen content and thus to ensure that both Heat Affected Zone and welding joint have sufficiently high amounts of austenitic phase for ensuring the corrosion resistance properties?" By the way, I do not know, therefore I request your understanding when I am asking. In regard to what I have mentioned above, "Boiling"- or "Bubbling" of the molten pool due to the physically-chemically reactions of dissociating molecular Nitrogen into monatomic Nitrogen and the further interactions between the molten metal and the gaseous constituents, I would like to cite once again Henry who has stated: (quote) "...In the .pdf attachment image, is that surface porosity I see or is that on the root side of the weld deposit..:". Could it be that the seam surface, to be seen on the pdf-image, is a result of such a "boiling-reaction" induced through Nitrogen, either in the base metal alone or in both base metal + shielding gas (90% Ar+7.5%CO2+2.5%N2)? Would be extremely interesting - "welder5354" - to hear somewhat about the arc- or process-stability when preparing the specimen!
Enough for now.
Please forgive me for asking such kind of bothersome questions but please believe me, where when not here would I find both friends and experts who could answer the queries I am honestly interested in!
Thanks a lot for reading and... responding, in advance!
Best regards to you and all,
Stephan