HY-80 is a high-tensile, high yield strength, low alloy steel. It was developed for use in naval applications, specifically the development of pressure hulls for the US nuclear submarine program and is still currently used in many naval applications. It is valued for its strength to weight ratio.
The "HY" steels are designed to possess a high yield strength (strength in resisting permanent plastic deformation). HY-80 is accompanied by HY-100 and HY-130 with each of the 80, 100 and 130 referring to their yield strength in ksi (80,000 psi, 100,000 psi and 130,000 psi). HY-80 and HY-100 are both weldable grades, whereas the HY-130 is generally considered unweldable. Modern steel manufacturing methods that can precisely control time/temperature during processing of HY steels has made the cost to manufacture more economical. HY-80 is considered to have good corrosion resistance and has good formability to supplement being weldable. Using HY-80 steel requires careful consideration of the welding processes, filler metal selection and joint design to account for microstructure changes, distortion and stress concentration.
Submarines
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 their 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 constrained not merely by yield strength, but also fatigue strength. As well as the obvious need for a hull strong enough not to be crushed at depth, the cyclical effect of hundreds of dives over a submarine's lifetime mean that fatigue strength is also important. To provide sufficient resistance to fatigue, the hull must be designed so that the steel always operates below its endurance limit; that is, the stress due to pressure at depth remains less than the fatigue strength for an indefinite number of cycles.
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 , 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 , and a crush depth of . Bureau of Ships conducted a research program for developing higher strength steel for ship and submarine construction. During testing, a variant of special treatment steel (STS), a homogeneous Krupp-type armor steel developed by Carnegie Steel in 1910 and commonly used for deck protection, with modifications in carbon and nickel and the addition of molybdenum, became known as "Low-carbon STS"; this steel showed the best combination of all the desirable properties. Low-carbon STS became the forerunner of HY-80, and was first used in 1953 for the construction of , a small diesel research submarine. Albacore tested its eponymous teardrop hull shape, which would form a pattern for the following US nuclear classes.
Although the operating depths of submarines are highly secret, their crush depth limits can be calculated approximately, solely from knowledge of the steel strength. With the stronger HY-80 steel, this depth increased to and with HY-100 a depth of .
The first production submarines to use HY-80 steel were the Permit class. These reportedly had a normal operating depth of 1,300 feet, roughly two-thirds the crush depth limit imposed by the steel. , the lead boat of this class, was lost in an accident in 1963. At the time, this unexplained accident raised much controversy about its cause and the new HY-80 steel used was looked at suspiciously, especially for theories about weld cracking having been the cause of the loss.