Less ferrite in Super Duplex

[smart143]

Hi,

I want to qualify a super Duplex PQR in UNS S32760 grade material, Currently I am using mock up piece of 3" sch 80 UNS 32760 & welding with GTAW process with ER2594 filler wire having shielding & backing gas as Ar+2% N2.

I am getting ferrite of 46% in root pass & 36% in hot pass, in subsequent passes ferrite is dropping to below 30%. I have maintained interpass of max 100 Deg C & heat input between 0.8 -1.2 kJ/mm. Project spec ask to use 2% N2 for shielding & ASTM G48 test is required in PQR with 35-60% ferrite in weld.

For a changer when I used pure Argon (99.999%) for shielding I am getting ferrite of 43-46% in weld.
.Is there any relation of gas mixture with respect to ferrite, again I have been advised that to pass ASTM G48 test 2%N2 mixture is must. So I am confused how to get the required ferrite & to pass the ASTM G48 test. A quick response is highly appreciated.

 

[metengr]

You should review the attached and modify your procedure;

http://www.nickelalloys.com.br/Metr...and Superduplex/Duplex Welding Guidelines.pdf

 

[jwhit]

Nitrogen is an austenite former and will result in a reduced ferrite content. We use welding grade argon and have not had issues with the G48 Corrosion tests. On the other hand, we post weld solution anneal all of our superduplex, which should eliminate any nasty phases.

 

[smart143]

I am getting below result of mass losss (pitting corrosion test per ASTM G48 A at 40 deg C ) for SUper Duplex weld cupon (UNS 32760)
249.8 gm/m2/day.This result is awful, I welded the cupon with root & hot pass with 2%N2+ Argon as sheilding & backinng gas, rest of the passes where welded with Argon as shelding gas & 2% N2+Argon as backing gas for 8 mm thickness.

Heat input was maintained bewtween 1-1.2Kj/mm & process used was GTAW with filler as ER2594.Can any body give more insight to pass the above test.

 

[smart143]

Refer attached ASTM G48A test result also, i have doubt about its correct ness too ,though it is from reputed NABL Lab, request an expert to review it for its correctness.

 

[qcrobert]

heat input between 0.8 -1.2 kJ/mm.

May I ask what amps, volts, travel speed you are recording to obtain above values?