Stud Welding limitations

[gr2vessels]

Gents,
I did the necessary reseach for the limitations on the use of stud weld - Duplex S/S studs 5 mm dia on 2.5 mm Duplex plate, without success.
The welding tests show that there is high ferrite content and high hardness developed at the edge of the weld, including the HAZ, which is pointing towards potential crack(s) of the base plate. The service is a heat exchanger where cross contamination could be a very expensive fluid replacement exercise and plant shut-down.
Is the stud welding procedure suitable for Duplex materials? Are there any particular limitations for this procedure? The picture of welded studs we took, shows large variations in local discoloration of the base plate, as size of welding aureola and colour concerns. Is this an indication of a welding process not quite under control or it is a normal weld discoloration. I don't know if the process is an arc weld process or capacitive discharge one.
I would appreciate some good advice from welding specialists.
Thank you
gr2vessels

 

[metengr]

gr2vessels;
I have worked with arc stud welding in boiler applications, and I have not heard of duplex stud welding, per say. The observations and stud diameter you describe would imply arc stud welding process versus capacitance discharge. The capacitance discharge method provides very localized heating and under pressure does not result in melting of the base metal. This is why it is not classified as an arc welding process (more like percussion welding). However, I would expect that using austenitic studs may be a better approach on a duplex substrate. What exactly is the applications for the studs?

 

[gr2vessels]

Thanks metengr,
Is an Alfa Laval spiral heat exchanger with the PQR quite scary, given the potential of some pressure cycling / flexing of the plate. 75-78 % ferrite and excessive hardness, particularly at the edge of the weld ring around the stud. I could see some cracks in time, when the fluids cross contamination and the heater out of service could cause some serious plant issues.
I wonder also why are they stuck with the duplex studs, when only compliance to NACE MR0175 is required, not certification (mild wet H2S).
I also can recall some comments within our forums, on Alfa Laval spiral exchanger with Duplex for MOC, with bad reflections on the control of their welding procedures. Unfortunatelly, the search is still not working and those comments are a bit blurred now..
The welding process does not address the fast cooling of the plate, which appears to increase the ferrite content and the hardness in weldment and HAZ, probably decreasing the corrosion resistance also, defeating the actual purpose of using duplex S/S.
Perhaps I'm being over cautious and I should seek some additional assurances from AL for their exchangers. The point of austenitic studs could be the correct answer to those worries.
Thanks again,
gr2vessels

 

[GRoberts]

I would expect ferrite to be high for duplex stud welds. Duplex filler metals have increased nickel contents as compared to the base metal to achieve the ferrite balance. You would not be able to get that in a stud weld.