rokohn
Posts: 32
Joined: 8/4/2006
From: California
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With submarine steel, ductility (ability to bend and absorb a shock) is more important than the hardness (ability to resist penetration). Hardness and ductility are mutually exclusive properties in steel.
Armor plating for the side belts on BBs and CAs is actually a compound material. The part of the plate facing out has been impregnated with carbon (the cementing process) leaving a surface with a very high hardness and strength but no ductility (think of glass). The back of the plate is left alone, leaving a backing of high ductility but lower strength. The plating will break up the smaller incoming shells on the hard surface. But any shell powerful enough to get past the hard surface of the plate, will be slowed down by the back half as the plate absorbs the impact of the shell. (Actually 80-90% of the plate thickness is ductile). If possible, the shell will be slowed enough so that it will not bust through the plate intact and will explode while in the plate, minimizing damage.
With Battleships, there is enough room for defense in depth and there will be additional armor plating several feet away from the first (primary) armor plate. This is to stop the remains of the bursting shell, or to slow the shell again if it breaks through the primary plate. Remember that the timer in the shell is counting down to the time it will explode, (we are talking milliseconds here) therefore, by slowing the shell down you will increase the chance that the shell will explode before it gets into the "meat" of the ship. A feature of the internal primary plate of the Iowa's, is that the hull plating will strip away the windscreen of the shell and start the timer before the shell ever hits the primary armor plate.
The armor plating used on decks will not have the hardened (cemented) surface. Bombs are not armor piercing the same way shells are, (which allows for a much greater bursting charge in a bomb), so the armor deck is designed to slow the bomb down (or stop completely) as it goes through the ship.
In the case of the submarine, the hull must bend and contract as the pressure increases requiring a material of good ductility. The strength of the plating is increased by alloy additions, as opposed to heat-treating for high hardness.
As a submarine skipper, I would not be worried about a belt of 50 cal. or drum of 20mm rounds, but a good dose of 40mm would give me pause to worry. (I will ignore range considerations for the nonce)
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Submarine Amour - 
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