You need to check your fabrication code - I think Tech Pub 1688 will have the info but your contract will specify a code.  I think it's 150F min preheat and 350F max interpass, but don't hold me to that.

As per NAVSHIPS/NAVSEA???? spec. (Mike is probably correct regarding the Tech publication number), preheat should be 200 degrees F and 300 degrees F interpass temperature for thicknesses 3/4" and up...

I have welded miles of HY-80 up in good old "Rotten Groton" CT @ General Dynamics Electric Boat Division main shipyard on both 688 Los Angeles class attack boats, and  Ohio/Trident class "Super Boomers" Fleet Ballistic Missile firing Nuclear Submarines and at least 70% of the steel used in both of those submarine designs was made up of HY-80 Q&T steel.

Anything less than 3/4" T one could use 150 F for preheat, but you still want to stay at 300 F for your inter-pass temperature, and a tempering bead would be required to be deposited on the cover pass/layer when we welded HY-80 if SMAW was used as the process and the cover pass was a multi-pass deposit.

Memories!!! :) :) :)

Respectfully,
Henry