Instead of design temperature, I will term MDMT - Minimum Design Metal Temperature.
UCS-66 deals with Carbon steel materials only. IF rated MDMT is higher than design MDMT, then you have to perform impact test of that material in conjuction with UG-84.
First of all, you need to find out what is governing thickness with the help of figure UCS-66.3. Then find out under which curve equipment material falls as per figure UCS-66. And accordingly UCS-66 figure will help, determine rated MDMT.
Use figures UCS-66.1/66.2 to calculate coincident ratio and deduction in rated MDMT. Use UCS-68(c) if appicable.
After that decide for impact test requirement.