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My limited knowledge in engineering tells me:

  • Titanium is expensive, stronger and heavier than aluminium, lighter and weaker than high yield steel.

  • The most significant advantage of titanium is the good strength to weight ratio.

  • In case of submarines, weight is not really a big concern, as long as the weight is manageable (less than the displacement and bigger than displacement - maximum ballast mass).

The mentioned points exclude the use of titanium. Since the designers decided otherwise, and surely had some good reasons, I wonder where my misunderstanding is?

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    $\begingroup$ Does titanium rust / corrode in seawater? $\endgroup$
    – Solar Mike
    Sep 15, 2018 at 21:45
  • $\begingroup$ Wikipedia told me that titanium is virtually corrosion-free. Otoh traditional steel hulls seem to cope with seawater, so I am inclined to believe that corrosion resistance isn't the deciding factor. $\endgroup$
    – Martin
    Sep 15, 2018 at 21:53
  • $\begingroup$ Your point two is a reason for titanium, not against. Point 1 is fairly irrelevant, since it compares density but ignores strength - on that measure, wood would be a much better (and cheaper) material than any metal! $\endgroup$
    – alephzero
    Sep 15, 2018 at 22:18
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    $\begingroup$ Engineers would choose titanium if they can afford it. So basically the flowchart goes like this. Can you afford titanium? Yes, do. No then you will have to look into alternatives. $\endgroup$
    – joojaa
    Sep 15, 2018 at 23:52
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    $\begingroup$ Weight IS a problem. High strength hulls are required to go the desired depths; High strength > high weight ( too much weight and they don't come back up). That is why steel hulls are high strength steel . . They were HY 80 ( high yield 80,000 psi yield) ; The Navy was working on HY 100 last I knew. These higher strength steels bring problems with welding and toughness that I know of and likely more problems = $$$$. But that is what is required to go deep. High strength titanium has similar problems , I doubt corrosion resistance was a significant factor. $\endgroup$ Sep 16, 2018 at 0:40

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the former soviet union had plentiful stores of titanium on hand, so its expense was not an issue.

Being nonmagnetic, a titanium hull would be more difficult to detect with magnetometers.

Note also that in a properly-designed submarine pressure hull, the limiter is not tensile strength but compressive yield strength and elastic modulus, since the hull will fail by compressive collapse like a short column or buckling as for a long column.

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  • $\begingroup$ This statement 'Being nonmagnetic, a titanium hull would be more difficult to detect with magnetometers.' is also holds true for SR71 blackbird. $\endgroup$
    – user14407
    Sep 16, 2018 at 19:23
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    $\begingroup$ yes, and the SR71 was also built from russian titanium, purchased through a shell company so the russians couldn't find out the USA was buying it! $\endgroup$ Sep 16, 2018 at 19:31
  • $\begingroup$ And some in the USA like to pretend they are not getting Ti and Cr from Russia. $\endgroup$ May 3, 2020 at 0:54
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It was more a political decision than engineering. The problems with fabricating thick titanium plate are enormous = expensive, and the Russian engineers did a remarkable job. There is much information on the web, but I could not find information on the particular alloy , thickness, strength, toughness, etc. An interesting summary is "Unravelling a Cold War Mystery".So the titanium hulls technical success and a functional failure, as all these subs have been decommissioned. The cost of the titanium does not justify possible advantages. Remember the political decision Russia made not to have containment for the Chernobyl class nuclear reactors ? Politicians should probably not make engineering decisions.

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All Soviet industries were state owned, including titanium production. So Soviet made titanium submarines to showcase their technological might. THough titanium is strong, corrosion resistant and lighter than steel as well as non-magnetic. This would seem advantageous to any navy....still.

It has numerous drawbacks.....

  • Titanium is not easy to shape.
  • Welding requires inert gasses to displace oxygen, meaning welders would need oxygen masks through the whole work shift.
  • Titanium was horribly expensive, so much so, the designs for ALFA class consumed nearly 1% of Soviet GDP and they only built 6, one of which was scuttled because reactor issues.
  • Titanium is tough, but it's not flexible. It has a terrible Young Modulus ie. It's "Give" or ability to withstand deforming stresses with ability to return to basic level of normalcy is limited, Steel is superior in this regard. enter image description here
  • Titanium is corrosion resistant, but the titanium hulls still required extensive inspection due compression stress after each mission dive. With Many of the subs experiencing hull cracks.
  • The US developed "Austensic Steels" Which were non-magnetic too. Also developed deeper diving torpedoes that relegated any advantage the Alfa had in deep water
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