Logo from Scott Cunningham.
Logo from Scott Cunningham.
ace hardening (FH) is a method used to increase the armour hardness of the surface of armour plate while the rear side of the armour plate remains at its original hardness. FH was first used for Naval armour, and was variously called “Harveyised” (after the first person to patent it, H. A. Harvey, in 1890) and “Carburised” armour, among other names. FH armour was used for vehicle armour mainly by Germany in World War II.
Face hardening is carried out by taking a slab of RHA and heat treating it again, but on one side only. The heat treating is time consuming and results in a warped plate which must then be flattened in large presses. German manufacturers were able to handle plates up to and including 50mm thickness (with production oversizes up to 55mm), and tried 80mm FH on the early Panther glacis. Later the Germans found a way to use a heavy electrical current flow through the steel to induction-harden one face. Both methods were used until the end of the war.1
The purpose of the hardened face is to shatter an incoming projectile’s head before it can penetrate. The Germans found it resisted Soviet uncapped AP and APBC projectiles quite well, when the armour plate thickness was around the same size or not too badly overmatched by the projectile (such as Pz.Kpfw.IV 50mm front armour vs. Soviet 45mm or even 76mm AP or APBC).1
“…there are some situations where FH is inferior to RHA.”
However there are some situations were FH armour is inferior to RHA. If the incoming projectile does not shatter, then it is possible that the FH armour in the projectile’s path would punch through the plate back where it acts as a second solid shot projectile, increasing target damage. This is made worse by the fact that such a plug of armour can be ejected from a brittle FH plate at striking velocities well below those where the projectile itself can penetrate the plate.4 Shatter could be eliminated by the use of hard steel piercing caps, such as those used in APC and APCBC projectiles. Uncapped AP and APBC projectiles which significantly overmatch the armour plate were also unlikely to shatter.1
Another problem with FH armour is that if it does succeed in shattering a projectile fired at a high angle to the plate, then the shattered projectile’s middle and base is likely to dig into the armour plate, instead of ricocheting as it would if it had remained in one piece.4
Britain and the USA tested projectiles against FH armour as a matter of course until about 1943, but rarely used it on production vehicles due to its expense, difficulty in working it, and its relatively poor resistance to German APCBC in comparison to RHA. Germany used FH armour extensively. The Germans were faced with APC and APCBC from the Western allies only, not the Soviets, so their decision to use FH armour weakened their tanks against Western guns but strengthened them against Soviet guns.1
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