At the behest of @Lord_James, this shall be the thread for general discussion of conventional passive metallic armor. Whether it's steel, titanium, magnesium, exotic laminates of all three, this is the thread for it.
In answer to your earlier question, Lord_James, relatively small amounts of boron, in steels that have the appropriate levels of carbon, form intergranular barriers that dramatically slow the diffusion of carbon out of the austenite crystals during quenching. Long story short, this means that the depth of material that can be effectively hardened is much greater.