But the discovery off Cuba was from the same team that discovered the wreck of the U.S.S. Maine, scuttled after being raised from Havana harbor. That led to El Wik, which led to Harvey Armor:
Harvey armor used a single plate of steel, but re-introduced the benefits of compound armor. The front surface was converted to high carbon steel by "cementing". In this process, the steel plate would be covered with charcoal and heated to approximately 1200 degrees Celsius for two to three weeks. The process increased the carbon content at the face to around 1 percent; the carbon content decreasing gradually from this level with distance into the plate, reaching the original proportion (approximately 0.1–0.2 percent) at a depth of around an inch. After cementing, the plate was chilled first in an oil bath, then in a water bath, before being annealed to toughen the back of the plate. The water bath was later replaced with jets of water to prevent the formation of a layer of steam which would insulate the steel from the cooling effect of the water. The process was further improved by low temperature forging of the plate before the final heat treatment.
I hadn't known that case hardening wasn't just used on nice revolvers but on battleship plate armor as well.