First, do you understand the term impact testing? If you do, the purpose of UCS-66 is to provide rules for what materials need to be impact tested based on design temperature and materials which can be exempt from impact testing depending on classification, thickness of the material and design temperature.
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.