[MW:12512] PWHT temperature below tempering temperature

Experts,

Pls advise, Client spec as below for PWHT

"For quenched/normalized and tempered steels, the PWHT temperature shall be

such as to avoid an unacceptable decrease of mechanical properties of the

parent material; PWHT temperature shall be maintained at least 20 degC below

the tempering temperature."

And Client recommend to use PWHT temperature is 593–629 deg C, but as per B31.3 it is 593–649 degC. Client reduced 20 degC in the max. temperature range(649-20=629)

Material: CS(A106-B > 20mm)

Is it correct as per spec?

Regards,

Stri