Good question. Perhaps ASM may have some information on that subject. However, they sell information, they don't give it away.
Short of that, you would have to establish it be testing a statistically significant number of samples.
Best regards - Al
Interesting, what code you are reffering to? ASME VIII Div I states the following:
It is recommended that the metal temperature during hydrostatic test be maintained at least 30°F (17°C) above the minimum
design metal temperature, but need not exceed 120°F (48°C), to minimize the risk of brittle fracture.
The test pressure shall not be applied until the vessel and its contents are at about the same temperature. If the test temperature
exceeds 120°F (48°C), it is recommended that inspection of the vessel required by (g) be delayed until the temperature is reduced to 120°F (48°C) or less.
(g) Following the application of the hydrostatic test pressure, an inspection shall be made of all joints and connections.
This inspection shall be made at a pressure not less than the test pressure divided by 1.3....