Generally speaking, the difficulty for a shot to penetrate an armour plate increases with the armour plate's hardness. Very hard armour plates will not only defeat shots by simply stopping them before penetrating, but can also cause the shot to scatter when hitting the armour plate. (WAL 710/908)
The steel's hardness is increased by a combination of alloys and treatments of the armour plate.
There are several different scales with which hardness can be measured. Discussions usually only refer to the Brinell hardness scale, but few know of the different Brinell scales or the other scales used to measure hardness. Finally, the tensile strength can be expressed as tensile strength.
Because the Brinell and Rockwell scales use different methods of measuring hardness, it is not possible to make a perfect conversion. Rather, the scale comparison must be derived from emperical data. The following table has been created based on a combination of tables from Hillfoot Steel Group Ltd. (Brinell 10/3000, Rockwell C and tensile strength), Newage Testing Instruments, Inc. (Brinell 10/3000 and Rockwell C) and Monachos Mechanical Engineering (Brinell 10/3000, Rockwell C and tensile strength).
| Brinell 10/3000 | Rockwell C | Kgf/mm2 | N/mm2 |
|---|---|---|---|
| 600 | 59 | 210 | 2056 |
| 575 | 57 | 201 | 1970 |
| 550 | 55 | 192 | 1885 |
| 525 | 54 | 183 | 1799 |
| 500 | 51,5 | 175 | 1713 |
| 475 | 50 | 166 | 1628 |
| 450 | 47,5 | 157 | 1542 |
| 425 | 45,5 | 148 | 1456 |
| 400 | 43 | 140 | 1371 |
| 375 | 40 | 131 | 1285 |
| 350 | 37,5 | 122 | 1199 |
| 325 | 34,5 | 114 | 1114 |
| 300 | 31,5 | 105 | 1028 |
| 275 | 28 | 96 | 942 |
| 250 | 24 | 87 | 857 |
| 225 | 20 | 79 | 771 |
| 200 | - | 70 | 685 |
| 175 | - | 61 | 600 |
| 150 | - | 52 | 514 |
| 125 | - | 44 | 428 |
| 100 | - | 35 | 343 |
Toughness or elongation is often neglected when discussing armour plate quality, in favour of hardness. Nevertheless, it is a very important aspect of armour plates. If an armour plate has poor elongation, it will be more likely to crack when fired upon.
While the elongation will generally be poor for very hard armour plates, it is possible to improve it by the use of certain alloys, without reducing the armour plate's ability to defeat shots.
When the thickness of the armour plate is less than the diameter of the shot fired at it, the shot is said to overmatch the armour plate. When a shot overmatch the plate, the stress on the steel increases. For flat-nosed low-energy shots, this can result in a plug being punched from the armour plate by the shot, rather than the shot pushing through the plate. Especially hard armour plates are likely to plug formation when overmatched. (WAL 710/492)
For this reason, overmatching shots will be significantly more likely to penetrate an armour plate than a matching or undermatching shot than the increase in shot size alone would indicate.
One of the most mis-understood aspects of armour design is sloped armour. The common argument is that sloped armour offers better protection than vertical armour, simply because the sloped armour offers a thicker horizontal protection for the same plate thickness. If this argument was true, sloped armour would not only fail to offer a better protection for an armour plate of a given weight, but would in fact in some cases decrease the protection of the armour plate.
Consider two armour plates: one is vertical, and one has an inclination of 30 degrees from vertical. The armour plates must both offer a protection equivilant to 100 mm thickness. Obviously, the vertical plate must be 100 mm thick. If using pure trigonometry to calculate the required thickness of the other plate, we would get 100 mm × cos (30) ≈ 87 mm. While this result would seem to indicate a weight saving of 13 percent, it does not take into account that the armour plate would have to be longer to cover the same area.
If the armour plate would have to be 1000 mm tall, the height of the vertical armour plate would naturally be 1000 mm. The length of the sloped armour plate, however, would have to be longer, since sloping the armour would reduce the height covered by a plate 1000 mm long. The exact length is easily calculated as 1000 mm ÷ cos(30) ≈ 1155 mm. Since 100 mm × 1000 mm ≈ 87 mm × 1155 mm, it is demonstrated that the weight will be identical of two plates that has the same horizontal thickness and covers the same area, regardless of slope. Formally, this can be presented as cos(x) ÷ cos(x) = 1.
Since it would appear that the calculation above does not favour neither vertical nor sloped armour (aside from a slightly more cumbersome interior layout for the sloped armour), if the purely trigonomic advantage in horizontal thickness was the only advantage, the thinner, sloped armour plate would in fact be at a disadvantage. Although thinner armour plates will usually have a higher Brinell hardness, the thinner plate is more likely to be overmatched.
If a plug is formed when a shot hits a sloped armour plate, the plug will offer less resistance than the surrounding armour plate. As a result, the shot will turn towards the armour plate, decreasing the effective angle and negating much of the increased horizontal thickness. (WAL 710/492)