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If you would permit me to add to your excellent response Bozaktwo1, I wholeheartedly agree with everything you said with respect to HY steels used in submarine hulls as I personally have a wee bit of experience with them (Helped construct in some way or another 14 688-Los Angeles class, and the first four Trident-Ohio class "Super-Boomers" SSBN's, otherwise known as FBM's also). I also helped maintain a few of the older boats which for quite a few of them have met their fate after some thirty years of service in the scrap heap out near San Francisco's Mare Island complex where eventually some of the metals are recycled to be used in constructing new "boats."
The only thing that I question, and it really may be nothing to worry about except for the fact that for that thickness of .750", I would slightly increase the preheat temp to 200 degrees F, and maintain an interpass temp of 300 F max. as you mentioned earlier. I mean 125 degrees F is a good starting point but, as soon as the thickness of HY steels extend beyond .500" the safe bet is a least 200 degrees F in order to insure that ther is no "HIC, or HAC" (Hydrogen Induced Cracking, or Hydrogen assisted Cracking). Just another "Bubblehead" giving his two cents worth!!! ;) ;) ;)
Respectfully,
Henry