Austenitic Steel and Duplex Steel not Suitable for Thermal Processes - Why?

What did those people say during the phone call that remains not understood?

What is this some sneaky spam? What makes you think these people you label "source" are any authority? poot, you're starting to look like some kind of company shill...prepare to be flushed

https://app.aws.org/wj/1998/11/kotecki/

That is not strictly true, there are different considerations

About stainless steels

The ss are unique in that they combine High-Temp Strength with good oxidation resistance. Applications at 1000 F. to 1500 F. are frequent, per example: The stress value for ss 304 pipes compared with Low-Alloys Cr Mo is one reason to preferred use ss than others alloys for Temp operations over 1050 F (The behavior of the ss 316 is also very acceptable)

The most common criteria currently use for design hot piping is given in ASME B31.3 (Including systems subject to pressure or vacuum working over a range of Temps. Up to 1500 F).

Perhaps the reasons to follow are rather relative to coefficient of thermal expansion and, importantly, the cost.

About Duplex

Duplex is a different matter ( 20 to 22 % ferritic). But remember that this material (Duplex and Superduplex) are frequently use in heat exchangers operating in a Temp range of 100 C to 150 C. The problem of duplex and super duplex operating at temperature is Hydrogen embrittlement. Different matter.