Titanium is the most ideal material in the marine environment. its use can greatly improve the battle effectiveness, reduce maintenance costs, prolong service life and improve concealment of naval vessels and equipment. 
it has been widely used in pressure-resistant shells, seawater piping systems, superstructure and other components of deep submarines, underwater submarines, surface vessels, ships and other equipment by the Russian, American and Chinese navies.
Pressure resistant hull
There are many examples of using titanium as pressure-resistant hull at home and abroad, which are mainly used for deep submarine manned spherical and submarine hull.Ti-6A1-4V, Ti-6A1-4VELI (corresponding to the domestic brands TC4 and TC4ELD) are the major choice of titanium materials for deep submersible manned spherical hull at home and abroad. Many deep submersible manned spherical hull use steel in the early stage, but they are replaced by titanium materials in the later stage, which can reduce weight, increase the depth of submersion and increase the service life.
For example: (1) The US Alvin deep submersible was constructed in 1964 with HY100 high strength steel (33.8m m plate thickness) and a submergence depth of 2000 m. When it was reconstructed in 1973, the pressure-resistant hull was replaced with titanium alloy (49m m plate thickness), the submergence depth increased to 3600 M. Ti-6A1-4VELI was also used for its auxiliary box and high-pressure air vehicle; (2) Japan's "Deep Sea 2000" manned spherical hull were prepared by rust-flipping, followed by the construction of "Deep Sea 6500" using titanium materials to prepare manned spherical shells to increase the depth of submergence.
In the application of submarine pressure hull, Russia chose titanium materia as the earliest material. It is the most advanced and mature country in the world using titanium to make pressure hull. The only titanium material selected for all-titanium nuclear submarine manufacturing is IIT-3B titanium alloy (corresponding to domestic number TA17). In recent years, it is also more preferred in the construction of submarines in China. The brands mainly focus on TA17, TC4, TC4ELI, TC11, pure titanium, etc.
In addition, many countries are currently building large-depth manned submersibles and large-depth all-titanium weapons and equipment. The domestic "Fighter" deep-sea manned submersible successfully challenged the deepest Mariana trench in the world on November 10, 2020, reaching a depth of 1090m, fulfilling the Chinese deep-sea dream. The manned spherical shell of the deep-sea submersible is made of high strength, high toughness and damage tolerance titanium alloy.
