The PQR is based on QW-403.9 and select the thickness range as per QW-451.1 (5mm to 2T). Now if impact test is required, then can we consider QW-403.6 instead of QW-403.9 for thickness range i.e starting from 16mm upto 2T.

Please advice

In my experience, your impact range will be as specified in the constructing code. Which code are you constructing to?

What is constructing code meant for. Our design code is ASME Section VIII Div (1). The meterial is going to be impact tested, then can we change the thickness reange (5mm to 2T) as per QW-451.1 to 16mm to 2T as per QW-403.6. Waiting for your reply.

Your question is not precise enough, therefore I assume as follows:
a) the welding process is SMAW, SAW, GMAW or GTAW or a combination of these processes
b) your thickness range without impact test requirements is acc. QW-403.8 (not acc. to QW-403.9) 5 mm to 2T
c) in compliance with a), b) and table QW-451.1 T of the test piece is 10 mm < T < 38 mm

With the assumption above and acc. to QW-403.6 the qualified base metal thickness range is as follows:
test piece 10 mm < T <= 16 mm qualifies for T to 2T
test piece 16 mm < T < 38 mm qualifies for 16 mm to 2 T.

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Kind regards,
Juergen Fleischfresser