You will loose strength if you PWHT above the tempering temperature of the T-1. Get a certification from the manufacturer that specifies the tempering temperature they used, then make sure you do not go within 25-50F of that temperature. However, watch for cracking. T-1 is susceptable to PWHT cracking, and is not really designed for PWHT applications. We have had fair luck with PWHT on thin T-1, but haven't tried anything very thick.