Krupp armour, later supplanted by the improved Krupp cemented armour, was a type of steel armour used in the construction of capital ships starting shortly before the end of the nineteenth century. It was developed by Germany's Krupp Arms Works in 1893 and quickly replaced Harvey armour as the primary method of protecting naval ships.
The initial manufacturing of Krupp armour was very similar to Harveyized armour; however, while the Harvey process generally used nickel-steel, the Krupp process added as much as 1% chromium to the alloy for additional hardness. Also, while Harveyized armour was carburized by heating the steel and placing charcoal on its surface for long periods (often several weeks), Krupp armour went a step further. Instead of inefficiently introducing carbon at the surface with coal, Krupp armour achieved greater depth of carbon cementation by applying carbon-bearing gases to the heated steel. Once the carburization process was complete, the metal was then transformed into face hardened steel by rapidly heating the cemented face, allowing the high heat to penetrate 30% to 40% of the steel's depth, then quickly quenching first the superheated then both sides of the steel with powerful jets of either water or oil.
Krupp armour was swiftly adopted by the world's major navies; ballistic tests showed that 10.2 inches (25.9 cm) of Krupp armour offered the same protection as 12 inches (30.4 cm) of Harvey armour.
By the early twentieth century, Krupp armour was rendered obsolete by the development of Krupp cemented armour (also "Krupp cemented steel", "K.C. armor" or "KCA"), an evolved variant of Krupp armour. The manufacturing process remained largely the same, with slight changes in the alloy composition: in % of total – carbon 0.35, nickel 3.90, chromium 2.00, manganese 0.35, silicon 0.07, phosphorus 0.025, sulphur 0.020.
KCA retained the hardened face of Krupp armour via the application of carbonized gases but also retained a much greater fibrous elasticity on the rear of the plate. This increased elasticity greatly and reduced the incidence of spalling and cracking under incoming fire, a valuable quality during long engagements. Ballistic testing shows that KCA and Krupp armour were roughly equal in other respects.
Developments in face-hardened armour in the late nineteenth and early to mid twentieth centuries revealed that such armour was less effective against glancing oblique impacts. The hardened face layer's brittleness meant that it was counterproductive when protecting against such impacts. Consequently, alongside face hardened armour such as KCA, homogeneous armour types that combined ductility and tensile strength was developed to protect against glancing impacts. Homogeneous armor was typically used for deck armor, which is subject to more high-obliquity impacts and, on some warships such as Yamato class and Iowa class battleships, for lower belt armor below the waterline to protect against shells that land short and dive underwater.