There are no easy answers to your question and I assume that you are asking it with regard to selling X70 Line pipe with your welding process. For cross country line pipe, Owner/Operators normally define the impact toughness requirements depending on operating conditions (temp., pressure, gas or liquids carrying, etc.)diameter, and thickness. Impact testing with calculated absorbed energy values may be required to assure fracture arrest in a large diameter gas pipe line.
To arrest crack like flaws at high applied stress levels, absorbed energy of 30 to 35 ft-lbs (Charpy V-notch)were often specified at the minimum design temperature if the design stress was 70% of the SMYS for X70 pipe.