[MW:13364] Oxygen level in purging gas during welding of duplex stainless steel

Dear Mr Martin,

 I am glad to write about DSS on this group,It is always good to understand the metallurgy of DSS before  welding.

           As we all know DSS are two phase alloys i.e 50% ferrite (BCC structure)& 50 % Austenite (FCC structure). The weld joint (weld metal +HAZ) shall also contain the same proportions after welding, but achieving these proportions is a nightmare..!!!   

       

         Usage of  DSS materials is particularly meant where Chloride stress corrosion cracking (CI SCC)& pitting corrosion cracking resistance are required.

         If 50:50 phase balance is not achieved during welding the loss of properties can be acute.

         If ferrite content goes high on the weld joint, it may loss the toughness properties  and if austenite content goes high weld joint will loss CI SCC resistance.

         During welding ,the DSS weld metal solidify  essentially as 100 % ferrite ,at high temperatures around 1300°C the Austenite nucleates and grows with the help of Austenite –stabilizing elements(Carbon,Nickel,Nitrogen,&Copper).The ferrite –stabilizing elements are Cr, Mo, W. (It is advised to have a clear idea about De-Long Diagram&WRC diagram)

       By selecting proper filler wires, welding parameters and shielding gases, the near 50:50 phase equilibrium can be achieved. 

                  i.      Filler wire.  It is always advised to weld with over alloyed filler wires (usually Ni & Cr content shall be high than base material).

                ii.       Welding Parameters.

                    Heat Input should not be too low or too high.

      If heat Input is too low rapid cooling occurs and ends up with more ferrite content.

      If heat input is too high chance for intermetallic compound formation are high.        

               iii.      Shielding gases.

        In order to get good pitting corrosion resistance Ar+2% N max mixer combination can be used for SDSS..

     Since nitrogen is good Austenite stabilizer it helps to form a thin layer of Austenite at root surface, which will the increase corrosion resistance..

     Oxygen content shall be closely monitored during root welding,(max 0.2% is highly recommended  ). 

Convey the following Tips to Welders..

     Advice welder to avoid too many  start &stops during welding,as it leads to rapid cooling..

                  Avoid autogenous welding. (I.e. without addition of filler )..usually welders use to do autogenous welding while tack weld fro fitup purpose& while arc termination on capping  pass welding.

If possible,kindly mail me the  design and  material details  like UNS number ,design temperature etc..if time  permits I will write more on this   topic..

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 regards,

K.Sudhakar

IWE, Welding Engineer,

Larsen and Toubro Electromech,

Muscat, Sultanate of Oman.

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