HY-100

HY-80 is a high-tensile alloy steel. It is most notable for its use in the construction of pressure hulls for US nuclear submarines in the 1950s to 1980s.

The 'HY' steels are a series of steels with high yield strength, hence the name. Their numbers, HY-80, HY-100, HY-130 refer to their yield strength in ksi, i.e. HY-80 has a yield strength of 80,000 psi (550 MPa), and HY-100 of 100,000 psi (690 MPa).

The need to develop improved steels was driven by a desire for deeper-diving submarines. To avoid detection by sonar, submarines ideally operate at least 100 metres below the Sonic Layer Depth. World War II submarines operated at a total depth of rarely more than 100 metres. With the development of nuclear submarines, their new independence from the surface for an air supply for diesel engines meant that they could focus on hidden operation at depth, rather than operating largely as surface-cruising submersibles. The increased power of a nuclear reactor allowed their hulls to become larger and faster. Developments in sonar made them able to hunt effectively at depth, rather than relying on visual observations from periscope depth. All these factors drove a need for improved steels for stronger pressure hulls.

The strength of a submarine hull is not merely that of absolute strength, but rather yield strength. As well as the obvious need for a hull strong enough not to be crushed at depth, the hundreds of dives over a submarine's lifetime mean that the fatigue life is also an important issue. To provide resistance to fatigue, the hull must be designed so that the steel always operates below its elastic limit; that is the stress applied by pressure is less than its yield stress. The HY- series steels were developed to provide a high yield stress, so allowing this.

US submarines post-WWII, both conventional and nuclear, had improved designs compared to the earlier Fleet submarines. Their steel was also improved and was the equivalent of "HY-42". Boats of this construction included USS Nautilus, and the Skate-class, which were the first nuclear submarines, with the then-conventional hull shape. The later Skipjack class, although of the new Albacore 'teardrop' hull form, also used these earlier steels. Such boats had normal operating depths of some 700 feet (210 m), and a crush depth of 1,100 feet (340 m).

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