Weldability is a vary broad term, and contains multiple aspects. Something that can be benificial for constructional weldability, can be more or less detrimental for metallurgical weldablity.
You need to provide more details to get some meaningful answers.
Customer needs to get up to date on how to manage ferrite in welds. Worse, he is trying to micromanage the material and fabrication process, which is generally a futile strategy. Material suppliers (including weld electrode) are annoyingly consistent and similar in the compositions they supply.
In my own experience of testing welds, ferrite number readings have been extremely consistent for 308 and 316 deposits, to the point where for a given welding process I can usually predict FN to ±2 points.
"Everyone is entitled to their own opinions, but they are not entitled to their own facts."
We see people setting a range for FN and specifying how it is arrived at.
That takes care of it.
In hygienic service where we see a lot of autougenous welding there is also a S control, 0.007-0.017% as I recall to improve weld fluidity.
Not pool fluidity exactly; it is about the effect of S on fluid flow and hence heat transfer within the weld puddle. But I think this topic is not relevant to the original question.
"Everyone is entitled to their own opinions, but they are not entitled to their own facts."
For autougenous welded 316 tubes, the skelp has or had controlled chemistry to provide 2-4% ferrite. I assume that the customer's specs were based on a Cr:Ni ratio to assure providing ferrite into the deposited weld metal when autougenous welding of the circ. seams as well to mitigate hot cracking.
