Highly modified form of
Vickers cemented (VC) armor. Used 1910-spec VC deep-hardening process, but increased Carbon content to 0.5-0.55% and eliminated cementing to reduce cost so that the maximum face hardness was about 510-520 Brinell with standard heavy production plates (some experimental plates differed, as will be discussed below). Same metallurgical composition as
new Vickers non-cemented homogeneous armor. This armor was unusual in that it had a 7% plate thickness layer at and just behind the plate face surface that was similar to the cemented layer of non-German KC plates, in that the hardness went from circa 450-480 Brinell at the surface (where the Carbon content also decreased due to the effects of the post-hardening temper used), increased steadily to the maximum hardness at the back (inside) edge of this region (rather deeper into the plate than with cemented armor and, of course, much softer). At this point, the hardness rapidly dropped in roughly a straight line or a "ski-slope" to the minimum face layer hardness of 350-370 Brinell (about 20 Brinell hardness points above VC) at about 18-23% of the plate's thickness from the face surface, and finally dropped more rapidly in the transition layer in another, steeper ski-slope to the back layer's usual hardness of 200-210 Brinell at the 35% plate thickness point, which is the same as in VC armor. The plate-to-plate variation in hardness pattern was remarkably small, giving a rather narrow variation in quality, though retaining the quality level of circa 1919 British CA through improved metallurgical skill - production plates still could not shatter soft-capped APC projectiles at normal, as this kind of improvement was not considered (and the armor manufacturing recipe was given to Japan prior to the major improvements in this plate quality in Britain).
Minimum production armor thickness raised to over 11" (28cm) since that was the thinnest vertical armor used in the
YAMATO Class ships and maximum thickness was 26" (66cm) for turret face plates. By retaining the old Vickers water/oil quenching process on plates well above the thickness it was designed for, all VH plates in the thickness range 17-26" (43.2-66cm) did not cool fast enough deep inside and formed brittle
upper bainite at their centers, which did not make them less effective as armor, but did cause them to snap in two through the impact point on any solid hit, which could result in secondary effects such as jamming turrets (the Japanese investigated the problem and solved it, but by that time no more battleships were being built).
Experimental VH plates were made by the Japanese during World War II and some of these and many production
MNC,
NVNC,
CNC, and VH plates were brought to the U.S. Naval Proving Ground, Dahlgren, Virginia, and to the British Naval Proving Ground after the war for testing. Most of these plates, including the thick, production VH plates, showed that they were not much better than their World War I counterparts (which is not really bad for VH considering this was the best showing of any non-cemented armor actually used aboard ship and that the Japanese were not trying to make a better armor, just make British 1910-quality face-hardened armor cheaper). An experimental 7.21" (18.15cm) VH plate (#3133 at NPG, Dahlgren) seems to have been made from a
German KC n/Aspecification added to an otherwise standard VH plate. It had a 535 Brinell maximum face hardness 3% into the plate, which is harder than all heavy VH production plate, though only by a small amount, with the hard point the closest to the face surface (about the same distance from the face surface (0.22" (0.55cm)) as with most cemented non-German KC-type plates), and a back hardness of 210-215 Brinell, which is slightly higher than production VH, but still well below average foreign World War II-era KC-type plates. The steel used was identical to the production VH plates that had been tested previously (rather dirty steel again of circa-1910 quality) and the hardness curve showed a typical VH-style pattern for the high-hardness portion. However, the major difference between this plate and all other VH plates was that the transition layer was much wider, extending to 43% into the plate (57% unhardened back), almost identical to German KC n/A, though with a higher average hardness than KC n/A in the decrementally hardened region. Since Germany and Japan were allies during World War II, it is not surprising that the Japanese may have obtained such information on German Krupp armor and made test plates to compare it with their own armor.
The U.S. test personnel at the U.S. N.P.G. did not expect it to be much different from the production VH plates, especially due to its relatively poor steel quality.
However, this plate was found to be the best face-hardened plate of its thickness ever tested at the U.S. N.P.G.! (The next best plate of similar thickness was also an experimental non-cemented face-hardened plate of 7.6" (19.3cm) thickness made by Carnegie-Illinois Steel Corporation during World War II, which was only slightly inferior to this VH plate.) It required the late-World War II, improved, super-hard-capped (650-680 Brinell all the way through) U.S. 8" (20.3cm) Mark 21 Mod 5 AP projectiles to completely penetrate this plate in "effective" condition at 30° obliquity (the standard U.S. armor test in World War II) - the older Mod 3 projectiles, similar but with a maximum cap hardness of 555-580 Brinell, were torn up badly even when they completely penetrated (a rare feat against these U.S. projectiles!) and needed a much higher striking velocity to do so. Against the Mod 5 projectile, my calculations indicate that this plate was 1%
better on an equivalent thickness basis than an average KC n/A plate (probably because the VH plate does not lose any resistance due to the existance of the thin, but very brittle, cemented layer used in KC n/A armor), meaning that it would take an average 7.28" KC n/A plate to replace this 7.21" VH plate - a pretty big improvement from regular VH armor in a single step! The U.S. test personnel were at a loss to explain this, but to me it seems that the Japanese personnel used the face tempering process of KC n/A, possibly with some of their own expertise added, in addition to just increasing the chill thickness. Japanese metallurgists (and most other technical personnel) obviously were (and are still) just as good as anybody else when allowed to set their own standards of excellence, as the post-World War II world has discovered big-time! Also, a rather unusual 15" VH plate was obtained by Britain at the same time and when tested showed a very high quality, also. However, this plate also had a non-cemented surface hardness of 575 Brinell, which I did not know was possible, and the British could not duplicate this plate when they tried to make two 12" (30.5cm) scaled copies in two different manufacturing plants - the plates ended up rather like improved standard production VH armor in composition and ballistic results, with only about a 500 Brinell face layer in both cases, so the British could not figure out how such a face layer was made, either.