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.
versus 12.7 mm APM2, Em of 1.23-1.62
I want to calculate the weight required to make an armor that can resist 7.62 RUAG SWISS AP, also known as VPAM level 12. I needed a baseline so I took NIJ Level IV and then tried to find the difference of weight so I could get a percentage. The only plate that's still made for this threat is the TenCate CX-950 IC. This plate is 8.93 lbs for a sapi medium and is alumina in-conjonction with soft armor. I then needed to find a Level IV alumina IC, which I found on UARM's website. It's 7.6 lbs, so if we do 8.93/7.6 we get around 1.175, but I put 1.25 considering UARM's plates are often quite heavy. Now that we have 1.25, we can start applying it to silicon carbide and boron carbide. Denmark's group has a level IV silicon carbide plate at 5.95 lbs, so times 1.25 it gives 7.4375. Hesco's boron carbide IV plate is 5.1 lbs, so times 1.25 we get 6.375.
Now, I wanted to know what was the weight for hardened steel. I took MARS 600, which is one of the best armor steel you can get. Using this page, I can easily calculate that you would need about 19mm to stop it. Using a calculator, we know that a full inch sapi medium plate would weight 33.9 lbs. 19mm/25.4mm = 0.748 inch so if we do 0.748*33.9 we get 25.3572 lbs.
The problem is obvious : How is boron carbide 4 times as light as steel? Silicon carbide is 3.4 times as light too? It doesn't make any sense, giving that they are both around 2.2 ME and hardened steel is 1.3 ME, so it should be around 1.7 times heavier for steel. What did I get wrong?