Hogan!

Thank you so much for your kind words!

As I have written to Henry a minute ago, please forgive me for not having already responsed on your fine post, but I have (fortunately) lots of work to do presently.

I personally have an idea of what you mean - hopefully.

Give me a little more time to think about. What I mean is, I do not think that the SMAW is detracting Aluminum from the high-alloyed-FCAW-deposit. From my point of view Aluminum might being used for the slag-system of the FCW...

As far as no one else has responded until I'll find time (I am sure someone of the appreciated colleagues would have done so) I beg your patience.

Thank you and regards,
Stephan

hogan,

good feeling being "back".

Most of work I have mentioned has been done and due to I have seen no one else has found time to response, now I would like to take the chance to humble reply your interesting post. I am honest, I have considered and, yes, I have hesitated a time due to it might be reasonable to write a relatively short answer. You know, the subjects matter is not so easy to explain and the balance between the share what might being necessary to describe and the will of keeping it short and simple by the way of description is a true "adventure". Thus of course - once again - I do honestly hope, to finding the right words.

Before I begin to write down what I am considering, I would like to inform you about three phone conversations I had in regard to your topic. You have to know, although I had to work and thus having had no time to attend the forum, I haven't completely been inactive intermediately. I know personally the head of the R&D-Department of »BOEHLER Welding« in Austria. As you certainly know, »BOEHLER« is one of the worldwide leading companies in manufacturing high-alloyed welding-consumables, and in particular also high-alloyed flux-cored-wires. I have asked him, moreover another colleague from »BOEHLER Germany«, and my old fellow, working with »ESAB Germany«, if they would have ever heard about the issue that there is a deterioration of mechanical properties when using specific welding-processes, just as SMAW, for re-work already FCA-Welded seams. They, so far, did not, and they told me that my - or let's better say  y o u r  - question was a kind of premiere for them. Now it is up to you by perhaps saying: "O.K. this answer is sufficient for me!" or... to read further on.

Recently I have read a very interesting article in the 01/07 edition of the AWS Welding Journal, where a fellow (under the rubric "Q&A") had asked "Does the temperature of the stainless steel weld metal have an affect on ferrite measurements?" (see also: http://files.aws.org/wj/2007/01/wj200701/wj0107-20.pdf ) The fellow who has answered this query was - as probably always - no one else than the famous Dr. Damian J. Kotecki, among many other responsibilities technical director of LINCOLN Electric's stainless and high-alloy product development. Once again he has replied the inquiry in an incomparable way, so, that no further questions would have remained. But what I have liked most was the honest sentence stated: "I didn't know the answer to your question, so I undertook a test." What a sentence, isn't it? Pure honesty coming from - my personal point of view - one of the greatest experts in welding at all. And this is what I meant when once I stated "...there is no one who knows really  e v e r y t h i n g  in welding..."

Also my highest goal ever is to be honest, and thus I would like to respectful cite Dr. Kotecki when I must admit: "I do not know exactly the answer to your question", and unfortunately, I haven't likewise the chance to execute an examination. But, I had a period in my life where I have busied myself very intensively with the welding of high-alloyed base materials and the sequences being responsible for hot-cracking and particularly for Intergranular Corrosion of welded high-alloyed steels, about what I have written an article for a German professional journal at that time. So what I would humble like to try subsequently is, to use my own knowledge and describe some sequences of which I mean they could be important and helpful in regard to your question and finally to hope you might agree to these.
I suppose it will be unnecessary to talk about the fundamentals of mechanisms for creating high-alloyed-steel materials. Too many - good - books have been written by good authors to explain these sequences.

For specific information in regard to creating welding consumables for high-alloyed steels I would like to recommend a sound link, coming from the LINCOLN Electric. I hope you may not already know it(?):
http://www.lincolnelectric.co.uk/knowledge/help/article.asp?PID=978

Firstly and since you have spoken from "...rob AL...", I do honestly hope you do not mean high-alloyed heat-resistant base materials, containing Aluminum for forming a dense surface oxide-layer. In this case the coherences might be more complicated since these materials are being welded with dissimilar fillers. Assuming you may not have meant heat-resistant and Aluminum containing base-materials, my very first assumption was, that Aluminum does not play the major role in achieving the mechanical properties of the weld deposit. I have researched - thanks the Internet is a wonderful tool to carry out - for the materials data sheet of high-alloyed flux-cored-wires in the quality I have mentioned below. And what I found was great, see also the attached »Material_Safety_Data_Sheet.pdf«.

As you can recognize there, Aluminum as chemical compound »Aluminum Oxide« takes only a relatively small share among all other compounds being used for creating the flux-cored-wires for welding high-alloyed steels. Therefore I must repeat myself when guessing, it is used only for specific sequences in generating the correct slag-composition for protecting the molten droplets and the weld-pool.
Considering that there are no further details known in terms of base- and also filler-metal composition I would like to deal with only one well known base- and filler- material. The AISI 347 base-material - which is a Niobium stabilized 18 Cr/10Ni austenitic steel, and the adequate filler for this material, the ER 347 - which is 19 Cr and 9 Ni also Niobium stabilized. I hope you may agree to this combination due to the German or European standardization in terms of high-alloyed materials is different as you know and I have derivated this combination from the German Standards. As you see I would like to talk only of  o n e  base- and filler-material since I guess it might be enough and every type of material more would complicate the explanations unnecessary.
When I have read your kind question the first idea running through my brain was: "If even there might be a deterioration of the FCA-welded seams quality being re-worked by SMAW, it might be founded on intermetallic compounds!" And now it comes. I am sure: "The almighty god joins everything to its predetermined place." Hence I was so positively impressed and surprised when I have opened the forum to see what has happened in the mean time and could read the topic:

»My 316L stainless steel weld is magnetic!!«

posted by "Farshid". What should I say? There are so many great replies on this topic that words have failed me! And I thought by myself: "Wow, certainly a few of the greatest experts on the forum (Chuck, js55, Henry, Sourdough...) have opened up a gap wide their treasure chests of personal expertise, to reply on Farshid's topic!" And as a great benefit for myself many of the replies in regard to the mentioned topic, have gone to the same direction I had theoretically considered to use for replying your post! I am talking about the austenitic weld deposit and its non austenitic constituents, in particular and basically - the Ferrite (Deltaferrite). Once again and in particular the links embedded by Henry (»ssbn727«) were excellent to explain the "Farshid" query and I have read immediately the: http://www.egmrs.org/EJS/PDF/vo291/151.pdf , dealing with the effects of low temperature aging of 316L austenitic steel weld metals on transformation of Ferrite phase. That was my basically consideration also on your topic, Hogan: "What about the transformation of Deltaferrite in high-alloyed steel weld metal deposits when re-working already welded FCAW-seams by using different welding-processes?"

So far so good. When I have read the paper mentioned above I was impressed, no doubt. It deals in a quite detailed way with the sequences being observable when putting a specific high-alloyed-steel weld-metal-deposit to higher temperatures for let them affect over longer periods of time. But forgive me when I am wrong, the fully understanding of the paper might require - from my point of view - a bit of a kind of "background" in terms of used abbreviations and specific technical terms, since the subject of matter the paper dealt with, is certainly not the easiest one in welding. Here we are. Due to I would like to try to explain subsequently (very partially and fundamentally!) what is going on when welding high-alloyed steels, I would like to describe firstly what prudent people - like M. Shafy in his paper "Effect of Low Temperature Aging of Type 316L Austenitic Stainless Weld Metal on Transformation of Ferrite Phase" - mean, when they are talking about Phase Transformation etc.

So my access to an attempt for finding out if there might be a deterioration in regard to high-alloyed FCA-Welded seams when using other welding-processes (e.g. SMAW)  for re-working these welds, is to ask and reply some very fundamental questions as follows:

1.  What is (strongly simplified) Deltaferrite?
2.  Anton L. Schaeffler - What was (strongly simplified) his contribution?
3.  How to use the »Schaeffler-Diagram«?
4.  What does the »Ferrite Number« mean?
5.  What kinds of "phases" can emerge from the Ferrite?
6.  Can the Deltaferrite - contained within a high-alloyed-steel weld-metal-deposit - be transformed into severe and dangerous brittle phases only by using arc-welding processes?

Ad 1:   What is Deltaferrite?

It is most commonly known that high-alloyed steels are basically defined by their microstructure. I am sure everyone might be able to name some "austenitic", "ferritic", or "martensitic" steels. When we talk about a base material like AISI 347, we are talking about an austenitic material being stabilized with an element having a great affinity to Carbon for creating relatively stable Carbides to prevent "Intergranular Corrosion", namely Niobium. In Germany we are using another kind of system for designating high-alloyed steels. Thus the mentioned AISI 347 is being named in Germany: X6CrNiNb18-10. It is good recognizable that the material has a Carbon content of ~ 0.06% mean, a Chromium content of  ~ 18% mean and a Nickel content of  ~ 8% mean. Furthermore the steel is - as mentioned - stabilized with Niobium in a defined Carbon/Niobium ratio of 10, i.e. the material contains ~ 10 times the content of Carbon and thus ~ 0.6 % Niobium. Whether of what the chemical composition of those materials in detail is, the most important elements affecting the high-alloy-material microstructure are Chromium - which is a strong ferrite forming element - and Nickel, which is a strong austenite builder. Chromium in a pure 2-component-Iron-Chromium System constricts the field of where the Austenite exists beginning with contents of ~ 13% Cr and is mainly "responsible" for the growth of a surface Chromium-Oxide layer making the steel "stainless". Nickel whereas extends the field of where the Austenite exists. In a pure 3-component-Iron-Chromium-Nickel System one would be able to achieve an austenitic microstructure which is stable down to room-temperature.
Important is, that - in relation to the ratio of Chromium and Nickel (and other specific elements where I would like to come later on) - the alloy can have different ways of solidification from the molten condition. Simplified one could say: "The more Chromium the more Ferrite and the more Nickel the more Austenite." And thus, the higher the Chromium-  - or Ferrite generating elements - content, the stronger the forces for a primary ferritic solidification of the molten material. And otherwise, the higher the Nickel - or Austenite generating elements - content, the stronger the forces for a primary austenitic solidification of the molten metal. Hoping very much so far it is understandable what I wrote, since it is a strong simplification and I request already now the forgiveness of all the "materials scientists" on the forum. What's important is the fact, that pure Austenite is a "critical" microstructure, due to its susceptibility for hot-cracking, I request your understanding for not further treating this fact. But, sometimes pure Austenite is required due to its excellent properties in regard to corrosion resistance or e.g. very low values of magnetic permeability, please see also the detailed points, been given by Henry ("ssbn727") and others in their replies to Farshid's post. However, due to the Ferrite - being formed while the molten metal is solidifying - has a positive effect on different properties of the material (e.g. hot-cracking-resistance, increase of the yield-strength...) it is mostly wanted that there is generated a specific, but little amount of Ferrite when the material is being cooled down from its melting-temperature to room-temperature. The term »Deltaferrite« again has its origin in the utilization of the commonly well known »Iron-Carbon-Diagram«, where the different breakpoints - from room-temperature to melting-temperature - have been named by using Greek characters, i.e. »Alpha-Iron (magnetic) = Alphaferrite«, »Alpha- or Beta-Iron [old designation] (nonmagnetic by passing the Curie-temperature) = "Betaferrite" «, »Gamma-Iron (nonmagnetic) = Austenite« and »Delta-Iron (nonmagnetic) = Deltaferrite«. In the Iron-Carbon-Diagram it can be recognized that Deltaferrite is being formed above the temperature of t = 1390°C. Finally one can say, Deltaferrite is being generated from the melt, whereas Alphaferrite is being generated by the transformation from Austenite to Ferrite (Gamma-Iron to Alpha-Iron). What's also very important, is, please see also here the replies been made by the appreciated colleagues mentioned previously several times (topic of Farshid), the time for quenching or cooling down, respectively, the material from its melting-temperature. Here one can say, so far there are Ferrite-generating elements (come to later on) in a sufficient amount, resulting to a microstructure contains both Ferrite and Austenite finally, the amount of Deltaferrite is as higher the higher the cooling rate is. Nevertheless - Duplex-Steels excepted - the amount of Deltaferrite should not exceed 20% at all.

Ad 2:   Anton L. Schaeffler - What was his contribution?

Well what we have "spoken" about until here was a strong restricted view on the sequences being necessary for creating a share of a specific microstructures constituent, called »Deltaferrite«. We have heard that this kind of microstructural constituent has a positive effect on specific material properties (yield-strength, resistance against hot-cracking...) and thus it is mostly desired when high-alloyed-steel weld-metal-deposits cool down from melting- to room-temperature. What we have to distinguish therefore - since there are different amounts of Ferrite and Austenite forming elements - are high-alloyed-steels, being real "stable" Austenites (i.e. pure Austenite apparently "free" of any Deltaferrite) or "metastable" Austenites (i.e. containing Deltaferrite whose amount is among others affected by the cooling rate and the amount of Ferrite forming elements).

Well, a time ago there was a very intelligent man, named A. L. Schaeffler, and I am certain you have already heard of this man or perhaps even worked with its famous contribution to verify which amount of Deltaferrite in Welding dissimilar steels is sound and which is not.
Schaeffler was willing to find a way of convenient and accurate selection of welding-consumables - at that time stick-electrodes - for using it in joining dissimilar steel-materials (un- low- and high-alloyed). You know the problem in welding dissimilar steels is to find a filler material which meets the requirements of both base materials. This is a huge problem since due to the dilution of both dissimilar base materials by the filler metal its microstructure can be altered significantly compared to its original condition. Schaeffler therefore has carried out bead on plate welds using 3/16" austenitic stick-electrodes and different base-materials for finding out their final microstructure by chemical and metallographical investigations. Due to the previously mentioned it has to be denoted that the use of the subsequently created Diagram has to be restrained by these restrained attempt-conditions. The very first "draft" of his created "Schaeffler-Diagram" has been presented in 1947 in the Welding Journal's Research Supplements (601-s - 620-s) and was named »Selection of Austenitic Electrodes for Welding Dissimilar Metals«. I assume you would be surprised when having a look on it, since it doesn't look like the very well known diagram been presented in 1949 ("Constitution Diagram for Stainless Steel Weld Metal" ("Metal Progr." Nov. 1949). Mentioned by the way, the first draft was founded on the fundamental work of "B. Strauß" and "Ed. Maurer" the real famous German Researchers and Developers of the KRUPP "VA"-Steels.

What Schaeffler did was - simply expressed - amazing, since he did not only judge the main-alloying elements Chromium (Ferrite-builder) and Nickel (Austenite-builder) but he has calculated two equivalents for further important alloying-elements - Molybdenum, Silicon and Niobium as Ferrite-builders and Carbon and Manganese as Austenite-builders. Within these equivalents the elements above were being used for defining their "strength" in regard to work as Ferrite- or Austenite-builders. Thus the "Chromium"- and the "Nickel-Equivalent" have been invented, please see also the Schaeffler__jpeg. One can see that Molybdenum in its worth for building Ferrite is equivalent to Chromium, Silicon works 1.5 times more and thus stronger than Chromium and Niobium counts only 0.5 times the worth of Chromium and thus 50% less than it.

Considering the above mentioned, the Schaeffler-Diagram »Chromium-Equivalent« can be stated as:

Cr Eq. = % Cr + % Mo + 1.5 % Si + 0.5 % Nb.

Moreover one can see that Carbon works 30 times stronger in forming Austenite compared with Nickel and is thus a very strong Austenite-builder. Manganese again has only 0.5 of the worth of Nickel in forming Austenite. Thus the Schaeffler-Diagram »Nickel-Equivalent« can be written as:

Ni Eq. = % Ni + 30 % C + 0.5 % Mn.

Ad 3:   How to use the Schaeffler-Diagram?

What can be recognized in the jpeg »Schaeffler_« are different fields of microstructures being expectable when different material compositions, cooling conditions etc. can be stated. And now I would like to come to your question hogan. When you are writing that you have heard that high-alloyed FCA-Welded seams are being deteriorated by using SMAW we should use basically the Schaeffler-Diagram (I will come to the DeLong-Diagram and its Ferrite-Numbers later on) to see if this might show us the right direction in estimating the final microstructure of the material. Therefore we simply have to calculate the base materials Chromium- and Nickel-Equivalent first. As I have mentioned above "we are going to use" a AISI 347 base-material which has a mean composition of:

Carbon (C):     max. 0.08%
Silicon (Si) :    max. 1.0 %
Manganese (Mn):  max. 2.0%
Phosphorus (P):  max. 0.045 %
Sulfur (S):    max. 0.015 %
Chromium (Cr):   17...19%
Nickel (Ni):    9... 12%
Niobium (Nb):  min. 10x %C (max. 1.0%)

Thus one calculate the Chromium-Equivalent by:

18 Cr + 1.5 Si + 0.3 Nb = ~ 19.8

and the Nickel-Equivalent by:

10 Ni + 1.8 C + 1 Mn = ~ 12.8

Now we are going to set these both equivalents by fixing an "x" into the Schaeffler-Diagram (see also jpeg Schaeffler_1).

Now we're having a look on the filler material for the mentioned and calculated base-metal. Since we have to weld similarly we use an ER 347 welding-consumable, e.g. a flux cored wire having a composition of mean:

Carbon (C):     ~ 0.035%
Silicon (Si) :    ~ 0.5%
Manganese (Mn):  ~ 1.4%
Chromium (Cr):    19.4%
Nickel (Ni):    9.5%
Niobium (Nb):  ~ 0.5%

And thus we can calculate the Chromium-Equivalent for the filler wire by:

19.4 Cr + 0.75 Si + 0.15 Nb = ~ 20.3

and the Nickel-Equivalent for the wire by:

9.5 Ni + 1.05 C + 1.4 Mn = ~ 11.95
  
Now we can fix these filler equivalents by setting an "o" into the Schaeffler-Diagram, see also jpeg Schaeffler_1.

One can see that both materials are to find in the field of a Deltaferrite-content between 0% and 5%. Assuming there will be an intermixture of both materials of approx. 50%, i.e. both materials will be molten off in same ratios, one can assume that the final solidified metal has a Deltaferrite content of ~ 3... 5%, which is - and this has to be emphasized - only an approximation. However, one can suppose that there is no extensive hot cracking to expect on the one hand, and no kind of excessive danger for corrosion on the other hand - once again emphasized - in most cases!

However, joining similar materials is not the main field for using the Schaeffler-Diagram, just as already mentioned. This is in fact to find, in joining dissimilar steels just as un- and high-alloyed ones. And herein the Schaeffler-Diagram shows, what it is able for (please wait, I will come later on to DeLong and his Ferrite-Numbers). You know, although there are a few very sophisticated diagrams available intermediately, just as the Welding Research Council (please see also jpeg WRC_Diagram), in many cases the good old "Schaeffler" is working very well and its Diagram is the basis for every Diagram that has followed afterwards. Due to, please let us make a short skip to another kind of welding-application using different base- and filler materials. Since I do not know what kind of materials you have meant when you have posted your topic, I would like you to virtually "join" an unalloyed steel (comparable to your "A 36" structural steel - I hope I am right) and the high alloyed base material AISI 347. But this time we should use another filler wire, e.g. a massive wire electrode comparable the ER 312 containing approx. 30% Cr and 9% Ni and which is used often in joining difficult to weld steels etc.

O.K. first we are going to calculate again the Chromium- and Nickel-Equivalents for the unalloyed structural steel "A36":

Cr Eq. = 1.5 Si = ~ 1.5

Ni Eq. = 6 C + 0.5 Mn = 6.5

Now we can mark these equivalents by setting an » x « to the Schaeffler-Diagram (see also jpeg Schaeffler_2). As you can see the materials position is being located within the field of Ferrite + Martensite.

Now we set the point for AISI 347, just as already done before, by setting a » o «. Subsequently we have to connect both marked positions by setting a wider line (called also Dilution Direction Line), see also jpeg Schaeffler_2.

Assuming now that both base materials being intermixed by welding in an equal ratio (= 50% of each), we can divide the lines distance in its centre, see also jpeg Schaeffler_2.

Now we calculate the Chromium- and Nickel-Equivalents for the filler wire:

Cr Eq. = 30 Cr = ~ 30.0   and

Ni Eq. = 9.5 Ni + 3.6 C = ~ 13.1

Now we mark these equivalents by setting a black square. Afterwards we connect the centre of the both base material equivalents with the equivalents position of the filler wire (black square), see the yellow Dilution Direction Line in jpeg Schaeffler_2. And now we can finish the work by assuming a ratio of dilution for the used welding process (GMAW using massive wire electrode). This is being set by approx. 30%. By dividing the yellow Dilution Direction Line distance by 10, we achieve 10 equal sections where every section stands for 10% of dilution. In case of a dilution ratio of approx. 30% we can locate the final weld-metal-deposits microstructure beginning always at the point of filler materials equivalent, i.e. the black square in this case (blue cross in jpeg Schaeffler_2). As you can see the finally solidified weld metal has an austenitic + ~ 18% Deltaferrite microstructure and thus a high resistance against martensitic induced cracking on the one hand and pure stable austenite induced hot-cracking on the other hand (I request your forgiveness for all these simplifications until here).
In conjunction with the ratios of dilution it should also be mentioned that different welding-processes has of course different behaviours of diluting the base material. Although there are many different data in regard to this fact I would like to list the well known approximately ratios of dilution as:

SAW - Strip Surfacing: 8...15%
SAW - Wire: ~ 50%
GTAW: 15... 30%
SMAW: 15... 30%
GMAW: 25... 40%

What I would have liked to show by describing the previous, is a bit of the background needed for - hopefully - understanding the subsequent facts in regard to whether the microstructures constituent called "Deltaferrite" can be altered into brittle phases and thus being the reason for a significant deterioration of the already welded high-alloyed seam by using other arc-welding processes, in particular SMAW. But before I would like to come to the mentioned brittle compounds I want to make a very short side trip to DeLong and the Ferrite-Numbers, since of these ones is being spoken also in many coherences when talking about Deltaferrite containing high-alloyed weld-metal-deposits.

Ad 4:   What does the "Ferrite-Number" mean?

Well to keeping it short and simple. After Schaeffler has created his diagram - described previously - there have been executed many other investigations and attempts to see and prove if it really would work in practice. And it did and it does also today. But of course there were other researchers who have modified the diagram by integrating other important alloying elements into the original Schaeffler-Diagram. One of those was W.T. DeLong who has integrated the strong force of Nitrogen (N) in regard to form austenitic microstructure. Already in 1960 (Metal Progr. February 1960 - "A modified phase diagram for stainless steel weld metals") DeLong has evaluated 600(!) pieces of specimen by mean of Magnetic Gauge. Here he has found out that up to a Nitrogen content of 0.055% (using 19Cr 9Ni or 19Cr 10Ni + Nb stick- electrodes, respectively) the Deltaferrite contents calculated by using the Schaeffler-Diagram and his magnetic measurements of Deltaferrite agree very well. But for the evaluation of distinct Nitrogen contents (different to 0.055%) DeLong has determined a new "course" for "Ferrite-Lines" from 0... 14% Deltaferrite, for the area of Chromium-Equivalents from 18... 26. Therefore he has suggested a new formula for calculating the Nickel-Equivalent which was:

Ni Eq. = %Ni + 30 x % C + 30 x % N + 0.5 % Mn

Here one can see that also Nitrogen has the ability to form an austenitic microstructure and that, approx. 30 times stronger than Nickel itself induces austenite (!).

DeLong has thus created the so called "Ferrite-Numbers" which is another way of designation for the amount of Deltaferrite content integrating the effect of Nitrogen as a strong Austenite-former basing on magnetic gauge values. Until 6% Deltaferrite both Schaeffler and DeLong agree in the amounts of Deltaferrite-contents. Above this point the Ferrite-Number Lines in the DeLong Diagram deviate in comparison to the Schaeffler-Diagram, see also jpeg DeLong_Diagram. As promised, I have kept it short.

Ad 5:   What kind of phases can emerge from the Deltaferrite?

As heard the Deltaferrite has a number of advantages in terms of mechanical properties of the material. But moreover it has also some disadvantages to be aware of. The most negative fact is, that Deltaferrite can be transformed into so called intermetallic and thus brittle compounds or phases, respectively. These, among others, are - mentioned by the way - those phases the appreciated colleagues have spoken about in regard to replying Farshid's topic. The problem is, that Deltaferrite is a relatively unstable constituent which can be affected by temperature by time. Therefore different compounds can be formed from the Deltaferrite, which can deteriorate the weld metal properties due to their physical character - they are brittle. These phases are - as mentioned above - by underlying the law of thermodynamical equilibrium, influenced and formed by temperature and time the temperature can affect on the material. Subsequently I would like to list the most important kinds of phases and want to describe them, and their conjunction with the Deltaferrite they are being formed of, very shortly.

·  Sigma

There are two different fundamental mechanisms known this phase is being precipitated, whereas I do not want to treat further the mechanism of decomposition of carbides in stable austenitic steels, since it would - from my point of view - complicate the matter in an unnecessary way. Basically, in a pure alloy-system of Iron and Chromium this brittle phase has a composition of 52% Iron and 48% Chromium. In metastable austenitic steels the non-magnetic Sigma-Phase reduces the materials ductility and increases its hardness (700... 800 Hardness Vickers). »Sigma« can also increase the materials corrosion susceptibility. The phase is being precipitated only above Chromium-contents of ~ 16%. It has been proven that the phase is being formed by a decomposition of Deltaferrite into Austenite and Sigma-Phase. within a temperature range of 500°C... 900°C and has a maximum forming-rate at ~ 800°C. Weld-metal containing higher amounts of Deltaferrite is more susceptible for forming Sigma-Phase. Mainly the Sigma-Phase is being formed by exposing the metal temperatures within the described range.

·  Chi

Alloy systems based on Iron-Chromium-Nickel-Molybdenum can precipitate another equilibrium-phase, called Chi-Phase. This constituent is formed within temperatures between 760... 980°C and can be dissolved again at temperatures above 1000°C. The brittle Chi-Phase has been among others also been investigated in steel AISI 317 (X3CrNiMo18-12-3). Although both Sigma- and Chi-Phase have different crystallographic modifications (Sigma = tetragonal - Chi = Body Centred Cubic) both phases have been found to have a quite similar chemical composition with AISI 317. Increasing contents of Niobium constrain a formation of Chi-Phase.

·  Laves (or Eta-phase)

Molybdenum alloyed high-alloyed and Niobium-stabilized austenitic stainless steels can form in temperature ranges between 600... 1000°C the so called Laves- or Eta-Phase, a compound of Iron and Molybdenum (Fe2Mo). However, for forming this phase longer time ranges on elevated temperature levels being necessary (e.g. ~ 6 hours at 800°C).

I request your understanding for not further treating the details in thermodynamical mechanisms for forming intermetallic phases. I am sure there are appreciated colleagues on the forum (Henry?) having good hints for specific information and can attach some links for more in depth information. From my point of view it is - here and now - firstly more important to try to clarify the question whether arc-welding processes can cause a deterioration of already welded high-alloy-weld-metal-deposits and thus I would like to come to the final question to be answered:

Ad 7:   Can the Deltaferrite - contained within a high-alloyed-steel weld-metal-deposit - be transformed into severe and dangerous brittle phases only by using arc-welding processes? 

Due to what has been said until this point and by the current knowledge in this field, the question must be answered basically with "Yes, this might happen."
Due to the forming rate for intermetallic phases - and in particular we are talking only about Sigma-Phase subsequently - is relatively increased in Deltaferrite compared with Austenite, in weld-metal-deposits having a relatively high content of Deltaferrite, Sigma-Phase can be formed already by the affecting through the weld-heat. In those cases the weld-seam-intersections can be subjected to an embrittlement(!). Therefore it is being recommended to avoid any unnecessary additional warm-up of the already welded seam. Particularly higher alloyed Molybdenum and Niobium containing weld-metal is susceptible to form Sigma-Phase. Already > 3% of Molybdenum in weld-metal can increase the forming rate for Sigma-Phase in an unacceptable amount and can provide Sigma-Phase contents in multi-layer welded seams. The forming rate for Sigma-Phase is additionally increased by Niobium containing high-alloyed weld-metal-deposits. Thus - if we assume that welding over an already deposited weld-metal and having a "critical" weld-metal composition - it might happen that Sigma-Phase can be formed by the weld-heat affecting and the weld-deposit is subjected to an embrittlement. This again would deteriorate the mechanical properties of the seam been welded over.

Finally I am unfortunately not able to say if there is any difference in arc-welding processes being used for re-working the seam and thus to say if the SMAW is being better or worse than e.g. GTAW, but as I have written at the beginning of my response, the mentioned fellows working in depth in this field, do not mean that it might be so. Therefore I would recommend to reduce the "re-work heat input" down to an absolute minimum whether which welding-process is being used for re-working.
The only thing I would like to mention - real finally - is, SMAW in welding high-alloyed steels can be accompanied by an increased absorption of Nitrogen by too long arc-lengths. Nitrogen again - as described above - is a very strong Austenite-builder and thus can influence the non-balanced forming of microstructures constituents in a significant way. But how far this displacement can deteriorate the properties of the already deposited and subsequent re-worked weld-metal, I am - unfortunately - not able to say.

By the way, Sigma-Phase can be dissolved and thus the embrittlement of the material can be deleted, by solution-annealing the material at temperatures between 950°C... 1050°C.

Oh, I guess the very crucial point comes at last, of course the filler-material composition for being used for re-working the already welded seam has to be properly chosen. But I guess also, this to mention is certainly unnecessary...

My very best regards to you and apology for the late reply,
Stephan