I don't see why PWHT would be required.
I presume that you are using matching filler (low C and N) and Ar shield (both sides).
This alloy does not harden by HT, it is a ferritic SS.
Double check and make sure that it is less than 0.025% C.

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P.E. Metallurgy, consulting work welcomed

It is required for Sec. VIII Div. 1.

Also required for B31.3.

And Sec. I and B31.1

I can see why now that I think about it.
This alloy cannot be HT to a higher strength, it has almost no C in it.
But on solidification there can be martensite formed (from austenite) which would have lower ductility.
It really helps to keep weld power input lower with these alloys.
The PWHT is actually an anneal, usually 1450-1500F. And for as short of a time as is practical to minimize grain growth.
You can slow cool down to about 1150F, but you must then rapid cool down below 700F to prevent the formation of intermetallics.
In light gage material I have seen projects done with no PWHT, but I didn't write the weld specs so I don't know how they got there.

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P.E. Metallurgy, consulting work welcomed

Thank u all!

1.4521 or UNS S44400 is a Ferritic stainless steels and is assigned to P-No. 7.1 acc. to ASME IX as well to Material Group 7. 1 acc. to ISO/TR 20172.
In compliance with:
• ASME VIII/1 Edition 2021, Table UHA-32-2
• ASME B31.3 Edition 2020, Table 331.1.1
general PWHT is required.
But in compliance with General Note (a) of ASME VIII/1, Table UHA-32-2 and ASMB31.3, table 331.1.3 there is a exemption to PWHT for the above Steel (carbon content ≤0.08%) if:
• Nominal material thickness ≤10 mm)
• Weld filler metal is A-No. 8, A-No. 9, or F-No. 43 composition
However, it is recommended to perform PWHT in a range between 700 to 750°C. As a result of this the carbides precipitated coagulate and stress reduction is achieved at the same time. Both factors lead within certain limits to an improvement in ductility. However, it is no longer possible to get rid of the coarse grain in the heat-affected zone. When using austenitic filler metals consideration must be given to its tendency to precipitate intermetallic phases (embrittlement) in the 600 to 900°C temperature range.

Regards - Juergen

UNS S44400 is P#7 Gr. #2. The exemption in Sec. VIII Div. 1 is only for UNS S41003. B31.3 has no exemptions for P7 Gr.2 materials. I can't speak for the ISO codes.