How carbon dating works chemically
This is only one of several competing hardness scales, but one of the most widely used, so I use it in place of such possibly more accurate hardness scales (97.5 RB = 20 RC = 238 Vickers = 226 Brinell (minimum RC and Vickers), 251 Vickers = 240 Brinell, and 832 Vickers = 739 Brinell).
Though Brinell testing is not usually used at such high hardness values, a hardness of 66 RC is roughly 757 Brinell, 67 RC is roughly 775, and 68 RC (highest RC used) is roughly 794--these Brinell values would need a test ball harder than tungsten carbide to reach (diamond? A slash (/) means "face maximum/back average" for face-hardened armors.
Very hard materials are usually also brittle and suddenly fail over a large area when the applied force exceeds the shear or tensile strength of the material. Most impurities in a vat of molten Iron weigh a different amount than the Iron and, if given a long enough time, will eventually either float to the surface or sink to the bottom of the vat, allowing the vat to be solidified as is, and then the top and bottom layers of the Iron ingot cut off, removing the impurities, after which the remaining Iron is for further processing (sometimes going through this purification process more than once).
- Projectiles of the "COMMON" type (see above) that usually employed some kind of nose fuze (though most U. World War II HC designs could have their nose fuzes replaced aboard ship by a solid steel nose plug and used as rather weak SAP-type projectiles (see COMMON, above) relying on their impact base fuzes against unarmored or very lightly-armored targets) and that had a very large explosive filler charge (4-10%). A major problem is that the more thoroughly mixed the impurities are and the smaller the particles, the longer this takes (it may take essentially forever for some materials that mix well with Iron). A method of purification that was not used during the Age of Ironclads, but is used widely today, is called Zone Refining.
NOTE: The hardness values given here are typical for the given plate type, usually with a range of about 20-30 up and down for a hard face and 5-10 up and down for the rest, centered on the given value.
Many specification have step-values for metal properties at certain pre-defined thicknesses, complicating evaluations over the entire thickness range.
Base-fuzed designs usually were very similar to armor-piercing designs, except for their 50-200% larger explosive charge and generally lighter construction (the larger the filler charge, the lighter the projectile body was).
Nose-fuzed designs were of the large-filler "HIGH EXPLOSIVE/HIGH CAPACITY" (HE/HC) type (see below), with those dedicated to anti-aircraft use with time or, later, VT nose fuzes also called "AA Common." Base fuzes were always impact types ("Base Detonating" (high explosive filler) or "Base Ignition" (black powder filler)) using the inertia of a weighted firing pin thrown forward on impact to set off the sensitive primer, with or without an internal black powder short-delay element.
Note that Iron alloys are somewhat temperature sensitive and older forms, especially up to the end of World War I, tended to get brittle when the temperature dropped below the freezing point of water, though this had less effect on the tougher, Nickel-alloy steels; most post-World War I steels were much better due to the reduction in the amount of impurities in the metal and tighter quality control, so that much lower temperatures were needed to cause any increased brittleness.
If the material can "give" under the load fast enough, it can keep its net force below the tensile strength and not break or tear open until there is literally no more metal left to stop the force (soft taffy or high quality wrought iron can approximate this), which will prevent brittle behavior.