Welding DSS to CS 1

[nickt1960]

A contractor has approved a procedure using 309 SS to weld a A36 support shoe to the re-pad on S31803 DSS piping (repad is of the same piping material).

We normally specify a DSS filler for welding DSS to CS.

We are thinking that 309, being a high alloy austenitic stainless in dilution with DSS, may form deleterious phases, eg. Sigma that could cause cracking? Many company specs require a photomicrograph, when qualifying a PQR, to confirm there are no brittle phases, but I'm sure they haven't done it.
Any thoughts?

 

[metengr]

From the information that I have reviewed (source; Practical Guidelines for the Fabrication of Duplex Stainless Steels by the International Molybdenum Institute) for dissimilar metal welding to duplex stainless steel, the recommended filler metal of choice is E/ER 309L or for increased corrosion resistance E/ER 309 MoL.

The concern with welding the duplex stainless steel is to assure proper balance of Ni and Cr to obtain a 40-50% ferrite/austenite duplex structure in the weld; the use of E/ER 309L or MoL will assure this balance. The addition of Mo is for increased corrosion resistance to match that of the duplex stainless steel base metal.

Second, and equally as important is heat input. The preheat and interpass needs to be as low as possible to avoid the harmful formation of intermetallic compounds along the duplex stainless steel side of the weld joint.


I don't understand why one would need a photomicrograph as part of the procedure qualification. I suppose it could be used to evaluate the % ferrite/austenite contents in the weld deposit, but if you use 309, this would not be necessary. As long as you follow good welding practices and qualify the weld procedure using tensile and bend tests in accordance with ASME Section IX for guidance, this should be all you need.

 

[EdStainless]

I would rather use the 309 than a duplex filler for welding to CS. The extra Ni in the fully austenitic filler will assure that the weld is not overly ferritic.
If you look at the duplex fillers you will see that they have a lower ferrite value than the base metals. When welding to a fully ferritic material your filler needs to be even more austenitic.

Unless low temp ductility is an issue I would not be worried about the microstructure.

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[GRoberts]

309 would have less chance to form sigma when being used in this application than a duplex filler metal. The sigma forms from the ferrite, and the 309 weld would have less ferrite than a duplex weld. The properties of the duplex base metal are probably the most critical, so follow good duplex welding practices as metengr suggests.