controlling ferrite in stainless steel welds

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my company has a job welding 347 stainless steel piping.the diameter range is 8" thru 20" and all thicknesses are schedule 100.the code isASME B31.3.we are going to use the gtaw process with ER347filler metal with an argon purge and an argon shielding and a maximum interpass temperature of 350 degrees max.the job specifications call for ordering filler metal and ferrite testing 100% of the final welds with a FN range between 2 and 10.it was suggested to use ER 16-8-2 or ER308 filler metal for better control of ferrite,but it was rejected by the engineering company of record.my question is in 2 parts as follows1) are they any specific guidelines to follow during welding to keep the ferrite between 2 thru 10?2) if we exceed the FN number 10, is there a way to reduce the fn number or would the weld have to be cut out and re-welded?thank youcarlkukol
Reply:Ferrite content in austenitic SS welds is primarily controlled by filler metal and base metal chemistry and the percentages of each of these that ends up in the weld pool.  Cr and other elements promote ferrite, while Ni and other elments including N, promote austenite.  Diagrams are used to predict the ferrite based on composition and weld dilution.  Control of inert shielding and amount of nitrogen picked up from the atmosphere is also a factor.  Heat input and cooling rate may be a factor also, can't remember about that.  See links below.A minimum ferrite of up to around 12% is commonly specified to help prevent solidification cracking (hot cracking).  A weld that solidifies as 100% austenite is crack prone because sulfur in the steel in not soluble in austenite and it is the last to solidify, thereby forming a weak liquid fracture path along the austenite grain boundaries, that is easily pulled apart by thermal weld stresses.  Sulfur is soluble in ferrite, and thus a small amount of ferrite forming during solidification holds the sulfur in solution and prevents the liquid sulfur segregation.A maximum ferrite level is only important for prolonged service at high temperatures where it can cause formation of a brittle phase called "sigma".If your pipe is not at elevated temperatures, ferrite higher than 12 and higher is not a concern.http://www2.steel.sandvik.com/sandvi...Form&url=nafta  In Sandvik site do a seach (see binoculars) on "ferrite", then go to references on WRC, Delong, Schaffler.http://www.esabna.com/EUWeb/AWTC/Lesson5_1.htmSeach on ferrite, solidification cracking, hot cracking, microfissuring, and Cr/Ni equivalent in stainless steels, and you should find some more info.We recently bought some 308L filler that came in with a WRC ferrite number of 12 based on the chemistry, and had to write a letter of justification to our customer to allow this since they had specified filler ferrite of5 to 10.Last edited by pulser; 04-09-2007 at 11:09 AM.
Reply:operating conditions for this job is 825degress farenheit at 3200psi
Reply:I've seen recomendations that suggest that sigma formation is not considered an issue at temperatures less than 550 C (1022 F).
Reply:I just worked on a job where we had to have the FN number between 2-6 but the filler had an FN of 2 so we have a 4-6 range, which is somewhat tight and it was all 100% LPT and 100% Ferrite tested every six inches on over 2 miles of weld and there is not a way to decrease the ferrite in a weld that has already been laid into the joint, grind it out, re-weld it, if you want to control the FN while welding WATCH THE INTERPASS TEMP, WATCH THE INTERPASS TEMP, and did I mention to watch the interpass temp?  I know you mentioned 350F but 300F would be better and with the job I was on we welded 3 passes over 6 foot sections and by the time the root was in the beginning to the joint was at about 150F so the 2nd went right in then we waiting 30-60 minutes and made sure that the whole joint was at about 150F (side note: we ran at 200 amps ALL day long so the heat built up pretty quick) and laid the cover on, our base metal was 304H and the filler was 308H, I hope that was enough back info but the key while welding is the interpass temp