low ferrite in weld of Duplex steel
low ferrite in weld of Duplex steel
(OP)
My welder has welded a test coupon for PQR in Duplex (UNS S31803) of 30 mm thickness with double welding .The electrode used was E2209-15 of dia 3.2 mm, the ferrite reported on MTC of electrode is 36%. I did not measured ferrite content during & post welding in weld, the coupon was subsequently solution annealed at 1140 deg C for 75 mins.
The test result reported by Lab shows ferrite in weld only 25% whereas HAZ shows 51% & PM shows 59%. Now we need compliance as per NACE MR0175 where in ferrite in weld required is between 35-65%.
What could be the reason for low ferrite in weld, the heat input during welding was 0.8-1.0 kJ/mm.
What I will need to do to meet the 35-65% ferrite level. I do remember that ferrite in weld of austenitic tends to reduce post solution annealing, does same phenomenon occurs in Duplex steel?
The test result reported by Lab shows ferrite in weld only 25% whereas HAZ shows 51% & PM shows 59%. Now we need compliance as per NACE MR0175 where in ferrite in weld required is between 35-65%.
What could be the reason for low ferrite in weld, the heat input during welding was 0.8-1.0 kJ/mm.
What I will need to do to meet the 35-65% ferrite level. I do remember that ferrite in weld of austenitic tends to reduce post solution annealing, does same phenomenon occurs in Duplex steel?





RE: low ferrite in weld of Duplex steel
- Exact Nominal chemistry of the filler metal, as recorded in the MTR
-Calculate the "Cr" and "Ni"quivalent as per DeLong Diagram.
How the solution anneal was carried out following holding @ 1140 Deg C, water quenching, forced air cooling or still air cooling??
Appreciate your input.
Thanks.
Pradip Goswami,P.Eng.IWE
Welding & Metallurgical Specialist
Ontario, Canada.
ca.linkedin.com/pub/pradip-goswami/5/985/299
RE: low ferrite in weld of Duplex steel
The limits apply to as-welded deposit.
Using constitution diagrams to predict ferrite number for as-welded deposit is the way to go. This would give some idea of ferrite number before applying heat treatment.
Birken
RE: low ferrite in weld of Duplex steel
Nick
RE: low ferrite in weld of Duplex steel
-DSS solidify primarily as ferrite at approximately 1425°C (2597°F) and partially transform to austenite at lower temperatures by a solid state reaction.
-If the cooling rate is rapid, very little ferrite will transform to austenite resulting in an excessive ferrite phase at room temperature.
-Consequently, the cooling rate of duplex welds must be slow enough to allow the transformation of approximately 50% of the ferrite to austenite.
SMAW consumables had approx 36% ferrite, as welded ferrite readings are not available.Take the worst case scenario of 35% ferrite in the weld,when this alloy is soaked @ 1140 DegC for 75 minutes,some ferrite will start transformation to austenite.If this alloy is rapidly cooled or quenched, austenite will remain untransformed , causing higher austenite percentage in the weld.
The sugegsted ways to solve this problem:-
- if solution anneal is called for, perform the same @ relatively lower temperature 1050 DegC , shorter time, 30 mins.
- forced air cool(sufficiently fast upto 800 Deg C, to avoid sigma) and see the results
-otherwise weld a fresh coupon with welding consumables having better ferrite:austenite ratio (45:55 or 50:50)and solution anneal as suggested above.
The comments from the originator would be appreciated.
Thanks
Pradip Goswami,P.Eng.IWE
Welding & Metallurgical Specialist
Ontario, Canada.
ca.linkedin.com/pub/pradip-goswami/5/985/299
RE: low ferrite in weld of Duplex steel
Thanks for your input, below is the chemistry reported in MTR.
C:0.04,Si:0.65,Mn:1.0,P:0.02,S:0.002Cr:23.2,Ni:9.1,Mo:3.1,Cu:0.11,N:0.17
FN:36%
Apology for wrong soaking temp in above write up, the actual soaking temp was 1040 deg C & cupon was water quenched.
Appreciate further input from all of you.
RE: low ferrite in weld of Duplex steel
Some things additional to the need for ferrite verification prior to any PWHT;
1. Be aware that values for measured vs calculated ferrite may vary greatly.
2. There's a substantial difference between Ferrite number and % ferrite at the higher values. They're considered to
be the same up to about 10, but over that the gap widens considerably.
3. The welding technique can greatly influence as deposited ferrite. SMAW is especially prone to nitrogen pickup and
that's a ferrite killer.
4. In my experience, SMAW electrodes often run on the low side - around 25% - 30% ferrite. Your calculated value
is on the low side of the spec, but on the high side of what I've found to be the norm. The experience of others
may vary on this.
RE: low ferrite in weld of Duplex steel
Can you put some more light how this FN 25 can be converted to volume fraction?
RE: low ferrite in weld of Duplex steel
The exact ferrite number of the filler with WRC 1992 diagram comes out to be 34.09 FN.
Reduction in FN is unavoidable during any heat treatment. To attend this matter and meet the specifications, an appropriate filler may be considered.
You need to predict FN for the weld joint (considering exact composition of base metal and consumable) and then leave some margin of reduction during the heat treatment. That would help in meeting the specifications.
I had the chemical composition of duplex S31803 (recently purchased) and I used that with chemical composition of filler you provided. Predicted FN of the weld with WRC 1992 came out to be 66.41 FN (see attached file)
This shows a large reduction in FN because of solution annealing putting question on this treatment. As you mentioned, the coupon was water quenched.
Birken
RE: low ferrite in weld of Duplex steel
I've attached some info regarding conversion of % ferrite to FN.
RE: low ferrite in weld of Duplex steel
I still don't know if you need to pwht at all..
Nick
RE: low ferrite in weld of Duplex steel
RE: low ferrite in weld of Duplex steel
" The ferrite content in weld metal root & unreheated weld cap shall be determined in accordance with ASTM E562 and shall be in the range of 30% to 70% volume fraction."
Can any body put insight on interpretation of "unreahated weld cap".Does this mean that ferrite is to be checked before PWHT of weld & the section does not talk any thing about ferrite of PWTH DSS material.
RE: low ferrite in weld of Duplex steel
Reheating, e.g. by means of "TIG dressing" has influence on the ferrite content, and may give the weld a ferrite content outside the desired range.
This is why TIG dressing of DSS welds is often discouraged. Refer to e.g. API TR 938-C, annex C.
RE: low ferrite in weld of Duplex steel
RE: low ferrite in weld of Duplex steel
Solution annealing is required on weld as I am fabricating A928 DSS pipe.
Today I conducted 2nd trial on weld cupon & post solution annealing result is shocking.
ferrite on PM is also gone down to 25-29%, weld ferrite is also 25%.
This time I kept interpass temp as 100 deg C max & heat input between 1.0-1.2 KJ/mm.
base metal & weld consumalbes are same as first cupon. before PWHT (solution annealing) ferrite in weld was avg :32%
cycle used as follows
heating rate: 200 deg C/Hr
soaking temp : 1050 deg C
soaking time: 30 mins
Qenchning: water immerson
RE: low ferrite in weld of Duplex steel
RE: low ferrite in weld of Duplex steel
1. Due to high possibility of ferrite formation the filler metals for duplex ss has high amount of Ni which is austunite stabilizer. If you get low ferrite in weldmed & alos in PM means, i suspect the heat treatment furnace may not reach the proper temp or time specified.
If there is no issue with furnace on reaching temp or soaking then the chemistry of the weldmed need to be verified for sufficient ferrite stabilizer.
Nitrogen is a strong austunitie stabilizer and nitrogen pick will directly affect the ferrite content so shielding gas need to be taken care.
The hand book can be useful to refer the parameters.
http://www.kskct.cz/images/materialy/en/avesta.pdf
RE: low ferrite in weld of Duplex steel
RE: low ferrite in weld of Duplex steel
The 2209 filler is designed for as welded use. You need to actually use 2205 filler.
What shield gas are you running? Ar with some He and N? You may be running more N than you need.
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