[bocklund]

Steels can undergo a transition to becoming brittle when they get cold (called a “ductile-to-brittle” transition). It’s important to know what the properties would be like in this regime and -70C is enough to get there (even 0C can be enough, depending on the alloy).

The reason this person may have thought the -70C test eqs stupid is because a sub will never be working in conditions much colder than the freezing temperature of water (which is not strongly pressure dependent, btw), since the water would want to freeze - not good for the boat.