HY-80 nuclear submarine steel

Used as the canonical example of an alloyed-for-hardenability steel: 1 inch thick, low carbon, ~3% alloy (Ni, Cr, Mo) for through-thickness hardenability in submarine hull plate.

If I put all this together: hardness is a function of carbon; hardenability is a function of the alloy content, and it's the depth of hardening. If I'm making automobiles out of sheet metal, I can use carbon steels because the sheet metal's no more than an inch thick. If I'm making some big pressure vessel, I'm going to need to use some alloy content. If I'm going to have a hard naval steel — if I'm building a nuclear submarine, here's a nuclear submarine steel, HY-80, inch thick, it's got to have a low carbon content, it's got about 3% alloy content in it. It's got nickel, chrome, molybdenum in this steel for hardenability. Because it turns out if I take that iron-carbon phase diagram and I quench it — I take it up here to the austenite and I quench it down here to the ferrite very quickly — I won't form a body-centered cubic crystal structure, I'll form a body-centered tetragonal crystal structure, which is known as martensite. Martensite is extremely hard. It's an athermal transformation, which means it occurs at near the speed of sound.