The Soviet Union is the only country that has ever built submarines from titanium and thatís some years back now. The rare metal is light and durable, and is most suitable for pressure hulls for a submarine. But it is too expensive, and is very hard to work on. In the West, titanium is used on aircraft and on their engines, but on a much smaller scale.
NATO calls Soviet titanium submarines "Sierra". The Soviets planned three types -- Sierra I, II and III. "Karp" and "Kostroma" were the type "I". "Nizhniy Novgorod" and "Pskov" were the type "II". But, with the demise of the Soviet Union, no type "III" was built.
Their underwater displacements are 8,000 -- 9,000 tons. They can submerge to an amazing depth -- 750 meters. They are extremely fast, making some 37 knots underwater. They were planned specifically to search and destroy U.S. strategic missile submarines.
On Feb. 11, the U.S. Los Angeles-class submarine "Baton Rouge" SSN-689 collided with the Sierra I "Kostroma" off Kildin Island in northern Russia. The U.S. submarine was so badly damaged that she was retired in June of the same year. The Russian submarine continued to serve after repairs.
I have recently heard about the Sierra-class submarines. Last December "Pskov" (type II) returned to the Russian fleet after an extended modernization work. And, on the last New Year's Eve, "Nizhniy Novgorod (also type II) returned to the submarine base Vidyaevo in northern Russia after an extended patrol. The crew enjoyed a brief Christmas leave (in Russia, it is January 7th) with their families. They will go off on another patrol as soon as their submarine is replenished.