WHAT IT IS

Krupp hardened armor is face-hardened nickel-chromium steel plate from Fried. Krupp of Essen, introduced 1893–94 and licensed worldwide inside a decade. The working name in English is Krupp cemented armor, KC, or KCA. “Cemented” here is the old metallurgy word for carburizing the face — packing carbon into the surface — not Portland cement.

One plate does two jobs that compound iron/steel sandwiches had split: a glass-hard high-carbon face to break or strip an armor-piercing cap and nose, and a tough, lower-carbon back that holds the plate together and catches fragments. Harvey armor (1890–91) already had the thin cemented skin. Krupp added chromium to the alloy, gas cementation instead of charcoal packs, and a deep “decremental” quench borrowed from Gruson chilled cast iron so the hardness fell through 30–40% of the plate thickness instead of stopping after about an inch.

Ballistic shorthand from contemporary trials: about 10.2 in of early Krupp ≈ 12 in Harvey; about 11.9 in Krupp ≈ 13 in Harvey on thicker belts. A 15% quality jump over Harvey on plates above ~5 in is the 1937 U.S. Navy history figure. That is why every fleet that could pay the royalty switched.

NAMES — KEEP THEM STRAIGHT

Harvey / Harveyized nickel-steel. Charcoal cementation, nickel steel, thin hard skin (~1 in), oil then water quench. 13 in Harvey ≈ 15.5 in homogeneous nickel-steel.

Krupp / early Krupp armor (1893–). Ni-Cr alloy plus gas carburizing plus deep face quench. Wikipedia sometimes treats this as a short-lived step before “KCA.” In shop talk it is already KC.

Krupp cemented / KCA / KC a/A (“old type”). The 1894–1918 production standard. After interwar improvements Germany renamed the wartime recipe KC a/A so the new plate could be KC n/A.

KC n/A (“new type”). Krupp late-1920s/30s. Cleaner steel, molybdenum, tougher back, face ~41% of thickness, hardness drop almost linear. First famous use: 15 cm turret faces on Deutschland. Meant to deal with hard AP caps after Jutland.

Krupp soft / Qualität 420. Same Ni-Cr melt used as homogeneous (not face-hardened) armor and structural plate. Schneider had a similar analysis as early as 1889; Krupp’s fame is the face treatment plus the deep quench.

Non-cemented cousins (Japanese VH on Yamato, some later experiments) keep the deep hard face by quench only and skip the carbon-gas week. That is not classic KC.

ALLOY

Typical late KCA analysis (U.S. Navy 1937 textbook):

C 0.35   Ni 3.90   Cr 2.00   Mn 0.35   Si 0.07   P 0.025   S 0.020

Early American KC ran lower carbon (~0.27), Ni ~3.75, Cr ~1.75. Soft Krupp / Qualitat 420 is quoted 0.35–0.4 C, 1.75–2.0 Cr, 3.0–3.5 Ni. Sulfur and phosphorus are enemies: they make the plate tear in the forge and crack on impact. Manganese and silicon help hardenability and tie up leftover sulfur. Chromium is why the face can be quenched deep without the back going glass-brittle.

Cemented skin itself is not 0.35% C. After two to three weeks the face layer is about 1–1.5% carbon, quenchable to ~650–680 Brinell typical, up to ~700 on the best Krupp and Terni faces. Back of an old-type KC plate sits around 225 Brinell; n/A back is harder (~240) and tougher. Nathan Okun’s tables give Krupp 1894–1918 tensile 92–105 ksi, yield 61–71, elongation 18–22%, face/back hardness 680/225.

KC n/A adds Mo (about 0.4% on thin plates, under 0.3% on thick), tensile 112–117 ksi, yield 85–90, elongation 22%.

HOW A PLATE WAS MADE

1. Melt a clean low-carbon Ni-Cr heat. Forge and roll to thickness. Anneal and machine to shape — you cannot bend a finished KC belt.

2. Cement the face. Harvey packed bone charcoal on the hot face for two to three weeks at ~1200 °C. Krupp put the plate in a sealed oven and blew illuminating gas (methane / coal gas; acetylene is also named) across the face so carbon soaked in faster and more evenly. Depth of the super-hard skin is still only about an inch; the “deep face” is the quench, not the carbon.

3. Heat the cemented face hard enough that austenite runs 30–40% (later ~41% on n/A) into the plate.

4. Quench. Jets of water or oil on the hot face first, then both sides. Jets matter: a still bath grows a steam blanket and the face does not harden. Krupp used oil and water; Harvey was water-only after an oil dip. Plates were not allowed to go cold between certain steps.

5. Draw / anneal the back so it stays fibrous. That back elasticity is why KCA threw fewer chips and cracks than a plate that was hard all the way through.

Carbon high enough to harden well also tears in the press and spalls on the range. The whole art is living on that line.

WHERE THE HARDNESS COMES FROM

Two stacked ideas:

- Harvey skin: 1–1.5% C martensite, shatters uncapped or soft-capped noses. A hard AP cap (post-Jutland, and German/AH practice earlier) eats that skin before the nose arrives. That is why n/A and later non-cemented faces exist.

- Gruson-style deep chill: hardness declines through a third of the plate so the body of the shell is smashed after the nose is already ruined. Okun’s point: the cemented film does its work in the first instant; the deep face does the rest.

HISTORY AND THE CARTEL

1890 — Harvey cements a French nickel-steel plate at Washington Navy Yard. 1892–94 — Admiralty trial; British firms first refuse, then license around Harvey. Harvey United Steel / Harvey Syndicate ties Vickers, Armstrong, Brown, Cammell, Krupp, Schneider, Carnegie, Bethlehem and others into a price and patent pool. Harvey dies 1894; the pool keeps his name and later sells Krupp’s process through the same vehicle (~$50/ton royalty in one U.S. congressional write-up; tests no harder than European ones).

1893–94 — Krupp issues Ni-Cr plate. Buys Grusonwerk and the chill-cast know-how. 15 Sep 1895 — public 11.8 in plate at Meppen takes three 12 in AP shot at ~1,993 ft/s in the press accounts that sold the process. 1898 — Carnegie sends men to Europe; 19 Jul 1898 first U.S.-made Krupp plate at Indian Head. 1900 — U.S. ships begin receiving KC. By the 1930s a U.S. officer could write that most armor of the previous 25 years was Krupp cemented, maybe 10% better than the first lots.

Use: vertical protection from about 8 cm (Krupp/Witkowitz) or 4 in (everyone else) upward — belts, turrets, conning towers. Decks stayed homogeneous; you do not cement a deck the same way.

AFTER KC

WWI AP caps reduced the value of a thin cementite skin. Interwar answers: thicker/tougher faces (KC n/A), or skip cementing and quench a deep face (VH). Homogeneous high-tensile steels (STS and cousins) took decks, bulkheads, and later tank armor where a brittle face was a liability. Battleship belts of WWII were still KC-family plates in Germany, Britain (CA), the U.S. (Class A), Italy (Terni cemented), Japan (VH on Yamato’s sides). The 1894 recipe is the parent, not the last word.

STATUS

Obsolete as a current naval specification. Historically it is the plate that closed the Harvey decade and defined capital-ship side armor until aircraft and plunging fire made decks the expensive problem. Surviving KC is museum belt and turret armor. The metallurgy — low-S/P Ni-Cr steel, case carbon, differential quench — is still the textbook for any face-hardened plate.

IN ONE SENTENCE

Krupp hardened armor is nickel-chromium ship plate with a gas-carburized, jet-quenched face a third of the way through so the front breaks the shell and the fibrous back keeps the ship from wearing its own armor as shrapnel — Harvey’s skin on Gruson’s deep chill, licensed to every fleet that could afford the royalty.