The minimum hydrostatic test temperature is related to the "ductile - fragile transition range" to which carbon steel is subject.
To know this range, a series of impact (Charpy) tests must be performed at different temperatures, starting, say, at 10º Celsius (you translate into Farenheit), and going down and down (for example 5, 0, -5, -10 etc. degrees C) until a sudden drop of ductility happens.
These tests may be performed by the steel mill (not for free, of course) if so required by the client, or they can be carried out by a metallurgical lab. It's assumed that the carbon steel pertaining to the same heat (see note) has the same ductile - fragile transition.
Therefore, the serie of tests made for one heat don't apply to another one.
The ductile - fragile transition of a given carbon steel material (pipe, structural shapes, plates etc.) is not known beforehand unless the tests have been performed, which is not always the case.
Of course, the hydro test must be carried out at a temperature always higher than the ductile - fragile transition one, which is seldom known, as explained before. What to do, then? The applicable codes don't state any particular temperature, for the reason I've already explained.
Back in my days of erector engineer I'd been in charge of the erection of two Combustion Engineering boilers (does Combustion Engineering still exist? I've known that it's been purchased by Alstom). The minimum water temperature for the hydro test was specified as 4ºC by Combustion. Later, I'd been in charge of the erection of a crude and vacuum oil refinery, whose basic engineering had been done by The Lummus Company. In this case, the minimum water temperature had been specified as 15 ºC by Lummus. As you see, opinions vary.
Have you appointed a consulting engineer for the job you're talking about? It's his responsibility to clear up your doubts.
In some cases, the carbon steel material has been specified to show a minimum impact value at a given (usually low) temperature. If this is your case, add a safety margin and that will be your minimum hydro temperature. If not, take as a basis the figures I've mentioned. 35 ºF is almost freezing point and is too low a temperature. Fortunately, the A 53 pipe has withstood it, but if I were you I'd carry out the forthcoming hydro tests at a higher one.
Giovanni S. Crisi
Sao Paulo - Brazil
Note: English being not my mother language, I may be wrong, but I believe that the word "heat" in metallurgy means the molten metal within the furnace that, when ready, is discharged into the molds to make the billets.
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