COMPARISON OF TWO EXPERIMENTAL JAPANESE VICKERS HARDENED (VH) FACE-HARDENED PLATES
WITH STANDARD PRODUCTION VH ARMOR FOR YAMATO CLASS BATTLESHIPS. BRITISH WWII CA CEMENTED ARMOR, KRUPP KC n/A (THICK TYPE) ARMOR, AND U.S. NAVY EARLY-WWII CLASS "A" ARMOR
BY: NATHAN OKUN (8/16/2018)

References:	1)  U.S. Naval Proving Ground Experimental Department Memorandum #1183-45,
		     "Metallurgical Investigation of 13.5-Inch Class 'A' Ex-OKLAHOMA Armor Plate Manufactured 
     		       Prior to 1910" Dated 9/26/1945*
		2)  U.S. Naval Proving Ground Report #5-47, "Ballistic Test and Metallurgical Examination of
		       Japanese Heavy Armor Plate" Dated 11/1947
		3)  U.S. Naval Proving Ground Report #4-47, "Results of Ballistic Tests and Metallurgical
		      Examination of British 14-Inch Armor-Piercing Projectiles" Dated 9/1947
		4)  U.S. Naval Proving Ground Report #4-45, "Ballistic Summary -- Part II:  The Scale Effect and the
		      Ogive Effect" Dated 4/1/1945
		5)  Minutes of the British Admiralty Armour Technical Committee Meeting on 11/20/1947, 
		      Pages 1-3, "Item 4:  Japanese Armour, Plate C #3109/NYC/6687 V.H. 15 Inches Thick"**  
		6)  US Navy Ordnance Pamphlet OP 1188, "Abridged Range Tables for U.S. Naval Guns"
		      Dated 13 June 1944

*Ship was ordered at the end of the year 1911, so all plates were actually made 1912-13, though the armor types used were in some cases (Midvale Plate #2753, for example) those developed prior to 1910, as stated, but not necessarily all of it.
**Plate actual thickness was less than 15" (381mm = nominal "600-lb plate" assuming 40 lb = 1") by an amount that required them to adjust the estimated Penetration ("Through Crack") and Perforation ("Complete Penetration" = U.S. Navy Ballistic Limit (NBL)) Limits calculated from the firing tests upward by 18 ft/sec (see Section III) to estimate the results for a true 15" plate.  This means that the actual plate thickness was approximately 14.82" = 376mm, compared to actual production British 600-lb CA, which averaged 14.7" = 373mm.

I.  INTRODUCTION

During WWII, after production of several plates for the third YAMATO Class battleship IJN SHINANO, VH armor production was halted and SHINANO converted to an ill-fated aircraft carrier with no VH armor (minimum production thickness of this armor was 250mm (9.84"), well above the maximum thickness needed for armor in an aircraft carrier).  During this hiatus through the end of WWII, the manufacturers of VH armor did some experimentation to correct a brittleness defect in the thickest -- over 430mm (16.93") -- VH armor that occurred due to incorrect cooling rates in the center of these extremely thick plates when using the standard British circa-1912 quenching process for the previous Japanese "Vickers Cemented (VC)" armor given to them from the Vickers Company when the IJN KONGO was built in Britain.  A brittle type of the steel crystal bainite forms (there are several kinds of bainite, some with very good properties, but not the one that formed here), instead of the desired hard crystal martensite, deep inside the plate face.  While martensite is also very brittle when first formed, proper post-quench tempering (re-heating and cooling) to remove stress-points from the crystals ("kinks" where cracks can easily start) can make martensite flexible enough to become a "tough" form of steel that will fail slowly enough under projectile impact to maximize the stress on the projectile nose and give adequate resistance to such projectiles.  The Japanese developed a quenching and tempering process to solve the problem, though of course it was never used in actual production VH armor, as no more battleships were built.

During this experimental plate series, at least two never-fired-at VH plates were manufactured using modified heat treatments and forging/rolling methods that were recovered by the US Naval Technical Mission to Japan (NavTecMisJap) and one, a nominal 185mm (7.28") plate -- well below the thickness of VH ever actually used in any Japanese warship -- given the ID #JE-50-3133 (actually 183-184mm (7.2-7.24"), slightly on the thin side, much like most British armor was), was sent to the US Naval Proving Ground, Dahlgren, Virginia, along with a large number of other Japanese standard production plates of both face-hardened and homogeneous, ductile types.  All of these are covered in Reference 2.  Reference 1 has details of late-WWII US Class "A" armor to compare the VH plates to.  Reference 5 has some particulars on the US Navy 335-lb 8" Mk 21 MOD 3 and MOD 5 AP projectiles used to test this plate, the latter not given in detail but being a MOD 3 with a new AP cap of identical shape and weight, but of a new, unique, super-hardened type called a "Triple-Alloy" (Nickel-Chromium-Molybdenum) AP cap.  This new cap was hardened entirely through from top to bottom to 650 Brinell Hardness Number (BHN), with the edge of the cap face where the windscreen threads were cut being hardened to an even higher 680 BHN -- hardnesses about as high as, or even higher in some cases, than the surface of the face-hardened armor it was hitting, especially compared to the non-cemented VH plate type.  The MOD 3 cap was typical of all other US Navy WWII-era AP caps with a thick circa-550-580 BHN face that softened to below 300 BHN where it contacted the projectile nose under the cap (due to a widespread, but incorrect, idea of an AP cap "cushioning" the impact, which the MOD 5 cap definitely showed was completely wrong).  In the MOD 5 cap, only a very narrow unhardened ring at the lower edge of the cap skirt was allowed for the standard US and British AP cap attachment technique of crimping the cap edge into shallow pits ringing the lower nose of the projectile to reinforce the bonding of the cap to the nose by the low-temperature, medium-strength solder used as the primary method to lock the cap to the nose prior to and during the initial stages of armor impact.  The MOD 5's extreme form of hardened cap design, also used in the last versions of the US Navy 130-lb 6" Mk 35 AP projectile, the MOD 9 and MOD 10, gave the shells using them superior penetration ability against face-hardened armor due to the cap's ability to smash a cap-width pit through most of the hard face of the plate more thoroughly and deeper into the plate than previous softer cap designs could, reducing the stress on the projectile nose from the plate's no-longer-solid face layer during the later stages of the penetration process.

The other plate, the nominal 15" ("600-lb") -- actually 14.82" as noted above -- mentioned in Reference 5, was given the ID #JE-50-3109 and was the only VH plate sent to Britain by the NavTecMisJap, along with several Japanese standard production homogeneous, ductile plates, for similar tests.  Reference 3 gives particulars on standard WWII British APC projectiles, here the 1595-lb 14" Mk 1B APC projectile.   The slightly larger 1944-lb 15" Mk 17B APC were used in the tests of this VH plate, but the standard form of this 15" APC projectile is essentially the same in design and ballistic capabilities to the 14" APC projectile, as scaled up.

II.  FACE-HARDENED PLATE PROPERTIES

A face-hardened plate has two major regions, the hard face layer and a soft, ductile back layer, usually, but not always, with a relatively-gradually-softening intermediate layer between them.  The hard face has one purpose:  To damage the attacking projectile so much that it (1) at low-to-moderate obliquity (under roughly 45 degrees from right-angles), has its ability to penetrate the plate reduced enough, by being slowed to a stop in the armor, to hopefully prevent the projectile or its pieces from making it entirely through the plate, even at the cost of pieces of the plate itself being broken off and flying around behind the armor (solid shot cannon balls cause less damage than high-explosive-filled bombs!); and/or (2) has its explosive filler damaged in some way so that it either fails to explode (becomes just another inert cannon ball along with the chunks of armor thrown out the plate's back on penetration); explodes at a lower power (or just burns like a  roman candle); or, if a delay-action fuze is used in the projectile, it explodes prematurely, not deep inside the target behind its armor, as it is designed to do.  Otherwise, homogeneous, ductile armor is superior due to its toughness and ability to more gradually fail under impact, giving it superior stopping power when the projectile damage is not high enough to stop the projectile by itself.  This is especially true at highly-oblique impacts where you do not want the projectile to break up, since if it remains in one piece and you deflect the projectile nose, the rest of the projectile follows the nose and all of it goes away, not just the nose part; in this case, projectile damage of some kinds, especially where pieces break off of the projectile, as on a hard plate face, are counter-productive and make the target MORE, not LESS, vulnerable against that weapon.

You want a face strong and hard and tough enough to do its job of damaging the impacting projectile, but a face layer has some major deficiencies:

	(1)  It is brittle and is thus weaker than an optimally hardened homogeneous, ductile plate with no face, that plate being essentially a face-hardened plate that is all back layer.  Thus, you want to minimize the face portion of the plate as much as possible without compromising its ability to do its destructive job against the attacking projectile.  Against projectiles where the damage caused to the projectile was the same, the thinner-faced Bethlehem Thin Chill (BTC) Class "A" armor, introduced in 1922, was noticeably superior at low obliquity to the then-standard, thicker-faced Carnegie-Krupp Cemented (CKC) Class "A" armor, even though both were essentially the same steel and hardened and tempered in the same general way.

	(2)  The hard face is punched through the plate and out the plate back by the projectile during a holing or complete penetration, since it cannot deform and move sideways out of the projectile's path, as a homogeneous, ductile armor plate material can.  Thus, even an impact where the projectile is stopped can still result in major damage behind the plate, especially in a confined area like a turret, barbette, or conning tower, so the NBL is no longer the place where damage becomes important, but the lower-velocity Holing Limit (HL), where a large hole is punched out of the plate, throwing all of the armor volume into the region behind the plate (sometimes tearing through quite a large region before being stopped).

	(3)  As mentioned, in cases where the projectile is prevented from completely penetrating (the middle and lower body stopped cold or the projectile nose is made to glance off at a highly oblique angle), you do not want the projectile to break up, since as long as it stays in one piece, if you stop/deflect PART of it, you stop/deflect ALL of it.  This also includes the fact that most of these AP projectiles had explosive fillers, so if the projectile remains more-or-less intact and the filler is not excessively sensitive to impact shock (most late-WWI and after fillers), the filler will not go off unless its fuze sets it off, whereas if the projectile breaks apart in a way that includes the filler cavity, the scraping and/or squeezing of the filler by the projectile pieces can cause it to explode or at least burn, causing more damage than the projectile would otherwise cause if the fuze did not work.

	(4)  The hard face fails by brittle fracture in radial cracks and/or concentric ring cracks about the impact point, like what happens when a rock goes through a glass window.  The soft back layer fails by stretching and ductile tearing after it has done its primary job of soaking up the impact shockwave so the hard, brittle face layer does not shatter like a crystal goblet dropped on a tile floor on the initial projectile impact.  Brittle cracking facture along surfaces does not scale well, since the surface area of the cracks is only increasing with projectile/plate size increases as the SQUARE of the scale, while a homogeneous, ductile material that stretches and tears open throughout its entire mass under an impact force has its resistance increase as the CUBE of the scale.  Since the projectile weight and, thus, its momentum and kinetic energy is also going up with CUBE of the scale, only the soft back layer is keeping up with the increasing size of a projectile, even if matched by thickening the plate by the same amount (there is a scaling effect even with homogeneous, ductile armor, but it is very small).  Thus, 
the thicker the face layer, the greater the effect of scale has on weakening the thicker armor hit by scaled-up projectiles, with the effect being rather small for thin-faced armors, but getting rapidly worse and worse as the face thickness as a percentage of the total plate thickness goes up.  Compared to a thin-faced armor like British WWII Cemented Armor (CA), US WWII Thick Chill Class "A" armor has a noticeably worse resistance, all things being equal, against battleship-size shells compared to cruiser-size shells of identical design fired at proportionately thinner plates of the same kind.

	(5)  Cost in money, resources, and time.  The steel used in a homogeneous, ductile plate is essentially the same as used in a face-hardened plate, but a face-hardened plate has to have the additional processes added to create the hard face of the parameters desired.  This includes as a minimum special furnaces to heat the face more than the back and elaborate water-pressured quenching systems to quickly cool the hot plate at the rate and depth desired.  If, as most did, the manufacturer needed/wanted to add a super-hard, thin cemented (carburized, Harveyized) layer to the plate face surface, special carburizing ovens had to be built and maintained, fueled and manned by highly-trained personnel, with the process taking about two weeks, depending on the plate type, thickness, and carburizing process employed (carburized gas jets or solid-pack charcoal on the plate face, either method baking the plate in an oxygen-free environment).  This caused face-hardened armor to cost anywhere from 25% to 50% more than a homogeneous, ductile plate of equal metallurgy, weight, and thickness.

The British were sent by the US NavTecMisJap a single nominally-15" -- I estimate 376mm (14.82"), as mentioned above, (the US-tested '15"' (38cm) Japanese VH Plate #3113 mentioned below was actually almost exactly 38cm everywhere, within 0.25mm (0.01")) -- experimental VH Plate #3109 with a face thickness of 39% -- this face thickness was not defined explicitly in the plate's hardness diagram, since the British arbitrarily used a BHN of 300 as the end of the face (the US previously used the Rockwell "C" value of 35 for this purpose), while I use the actual point where the face's martensitic crystal visually disappears in microscopic photographs.  All mid-WWII-and-after US NPG hardness diagrams also showed this final point, since they changed the definition of "face" for Class "A" armor during WWII to match what I have always used, but the British diagram did not show it, so I had to "eyeball" estimate it as the point where the descending hardness curve of the face leveled off (and actually then went back upward a small amount in this VH plate and in one of the two British new nominal-12" CA plates, one made by Beardmore and one by E.S.C., they compared it to).   I do not think that I am far off.  Also, the maximum face hardness, though a non-cemented plate, was 577 BHN at the surface -- uniquely, the hardness went down, not up slightly as in all other VH plates (usually due to a small loss of carbon on the surface itself from the high-pressure quenching water jets when white hot), behind the surface itself -- only slightly below the nominal 600 BHN minimum for a cemented face, which was FAR higher than the circa-500 BHN maximum for the usual VH plates (including the 185mm VH Plate #3133) and they had their maximum hardness usually about a half-inch or so inside the plate behind the surface itself, not at the surface.  Also, the hardness dropped in almost a straight line from the face surface to the joint with the soft back and the soft back hardness in these plates averaged about 230 BHN, well above the 210 BHN average of British plates and other VH and VC plates.  Plate #3109 was definitely NOT a production VH plate, even though it was within the regular thickness range for such plates.

The rest of the five captured VH armor plates were sent to the US NPG, along with a number of homogeneous CNC, NVNC, and MNC plates.  Except for the single other experimental VH plate of 185mm -- actually 7.21-7.22", slightly under the nominal thickness -- plate, NavTecMisEu Plate #3133, all other NPG VH plates tested by the US NPG had the production standard 35% face (within 1%, which is much more precise than any other plates that I know of!).  These other VH plates were 33.5cm (13.19"), 38cm, and 66cm (25.98") VH plates that had been developed and tested and/or actually made for one of the YAMATO Class BBs, including the 66cm plate for part of one of the main turret faces of the aborted SHINANO.  Note that the British-tested circa-38cm VH plate's face was noticeably thicker than standard VH, though not quite as thick as the 43% value of the other experimental, thin VH Plate #3133 (by the way, no VH armor, other than this one plate, was ever made, to my knowledge, below 25cm (9.84") for actual shipboard use).  The production VH plates all gave better results than the previous British-Vickers-developed 1912-introduced "Vickers Cemented (VC)" armor, obtained by Japan on buying the IJN KONGO that year, that all subsequent Japanese armors were based on, but were not as good as US and any European post-1930 KC-type plates (not counting the Russians here, who did not make any production KC-type armor after WWI).  The elimination for the cemented face greatly reduced the cost and time to make each plate.  The VH production armor was not improved otherwise as much as the US and European face-hardened due to Japanese conservative design philosophy that did not think that any real improvement, other than incidental from better overall technology, was needed in its naval armors compared to their WWI equivalents; only in saving critical alloying elements, such as nickel, or cost, such as eliminating the cemented face, were any improvements aimed at.  Their design philosophy was:  "Better, without very good justification, is the enemy of good enough."  The results speak for themselves:  VH armor can be said to be the first completely successful non-cemented Krupp-steel face-hardened armor ever used in a warship (pre-WWI US Bethlehem and Midvale plates of this type had a number of too-brittle plates that had to be replaced after installation due to cracking, while later WWII-era tests showed poorer performance than the regular KC-type armors (Carnegie-Krupp cemented, etc.) of that period against the modern WWII AP shells).

III.  TEST RESULTS

The British adjusted their 30-degree-obliquity calculated Penetration/Through Crack (close to my FACEHARD Holing Limit (HL)) and Perforation/Complete Penetration/US NBL Velocities up by 18 ft/sec to get these for an actual 15" ("600-lb") plate thickness -- the British used exactly 40 pounds/inch thickness in their "XXX-lb" thickness measure nomenclature, not the true 40.8 lb, so that this meant that British plates were actually 2% thinner than the one inch = 40-lb notation implies) from the actual plate thicknesses and Test Striking Velocities at the four impact points (the actual thicknesses at each point were not given, unlike in US NPG results, and I do not know the rule that they used for the adjustment, which would exactly nail down the actual plate thickness that I estimated at 14.82", as mentioned.  They calculated the adjusted true 15" plate limit velocities as 1740 ft/sec (530.4 m/sec) for Penetration and 1800 ft/sec (548.6 m/sec) for Perforation.  All three of the British 15" Mk 17B APC shells that failed to penetrate broke up, but the single projectile that did completely penetrate was "almost whole" so it suffered some damage that rendered it Not-Fit-to-Burst (the result if not "whole", just like Krupp projectile testing criteria), but this was obviously relatively minor (perhaps a single deep crack) so the projectile's NBL probably was not affected much, if at all.  The complete penetration hole in the plate was 15"x29.9" at the face surface and 34"x48" at the back surface, which was a usual complete penetration result for high-quality face-hardened armor; the other, rejected impacts only caused minor bulging at the plate rear surface and some cracking. These British 15" APC shells, even if not up to the highest Cardonald standard, usually did not break up at low-to-moderate obliquity against up to caliber-thick British CA plate at the standard 30-degree acceptance test velocities even when they rebounded, though some did break.  Since ALL projectiles that failed to penetrate broke up in this case, this Japanese plate was acting "above its weight class" as to its ability to damage British APC shells, just like the 185mm US-tested VH plate could against some US 8" AP projectiles (see below).

Assuming, for a first comparative analysis, that the US-tested experimental 7.22" VH plate was a Krupp late-1930s-production KC n/A plate (its 43% face is close to the KC n/A plate's average 41% face, meaning that using a regular 35%-face VH is not really comparable), hit by the US 8" Mk 21 MOD 3 AP projectile at 30 degrees, we get from FACEHARD v8.0 ARMOR SELECTION #10 & US PROJECTILE SELECTION #19, HL = 1424 ft/sec (434 m/sec) -- roughly equivalent to the British Through Crack Limit when applied to face-hardened armor -- and NBL = 1742 ft/sec (531 m/sec) -- the same as the British Perforation Limit.  This compares to the 1843 +/-18 ft/sec (561.8 m/sec +/-5.5 m/sec) NBL range calculated by NPG test personnel for the unique 7.22" VH plate from their tests.  For the 8" Mk 21 MOD 5 super-hard-capped, but otherwise identical, version of this same shell, US PROJECTILE SELECTION #20, the KC n/A version of the VH plate gives HL = 1506 ft/sec (434 m/sec) and NBL = 1656 ft/sec (531 m/sec), compared to the actual NBL = 1718-1734 ft/sec (523.7-528.5 m/sec), splitting the difference gives 1725 ft/sec (525.8 m/sec).  This VH plate was thus better than average KC n/A armor, though a few superior individual KC n/A plates were better.  This thin VH plate has a FACEHARD v8.0 Q value of ((1725/1656)^1.21 = 1.051) x KC n/A's Q-value = 1.051 x 0.96 = 1.009.  This result, against the MOD 5 projectile that the armor could not damage, shows that the intrinsic quality of this form of VH armor was slightly better than the US armor of that time, adjusting for the different face depth, and about as good as the later late-WWII improved US Class "A" armor used in new cruisers (no more battleships were being built at this time).  This is about a 13% improvement, pound-for-pound, over regular VH, an enormous quality step.

Some details about the US testing of the 7.22" VH plate:

	(1)  The first two impacts were with the MOD 3 shell and the second two were wth the MOD 5 shell. One of each failed to penetrate and one of each completely penetrated, hence the estimate of the NBL values given.

	(2)  The MOD 5 impacts, the last two performed, gave expected results, with the projectiles not damaged by much by the plate, though confirming the high quality of the armor.  The super-hard cap smashed a very deep pit in the face, negating much of the face layer's ability to damage the projectile.

	(3)  The MOD 3 impacts, the first set fired at the armor, were far different.  The one that failed to penetrate suffered severe damage, which was somewhat unusual, but not unknown against US Class "A" plates.  The second impact, at a significantly higher striking velocity, that completely penetrated had the most unusual effect on the projectile that anyone at NPG observing the test ever saw.  Usually, at and above the Holing Limit, the hard AP cap punches a large pit in the face layer as the cap is destroyed, with the projectile body then widening somewhat to be large enough to allow the rest iof the projectile through the face, tearing out the soft back layer in a cone so that the hole at the plate back is wider than the hole at the face surface.  With high-quality face-hardened armor, the hole in the face layer is usually very close to the diameter of the projectile at right-angles to the plane of the projectile path before and after the impact (the projectile will usually be deflected so that its Exit Angle is less -- closer to right angles to the plate back -- than the Impact Obliquity, here 30 degrees) and somewhat longer in the plane as the projectile passes through the plate at an angle (see British 15" VH test hole dimensions).  In this case, the second impact indeed made a large hole in the face somewhat narrower than the projectile, but this plate was so extremely strong that the hole in the face was not widened.  Instead, the projectile nose and body was squeezed inward radially to a smaller diameter and the body lengthened, much like pushing toothpaste through the end of the soft container -- the evaluator used the term "extruded", which I have never seen in any other document describing armor penetration results.  This drastically raised the NBL over what it was assumed it would be when compared to the other, thicker production VH plates then being tested at the NPG.

Let us calibrate my FACEHARD v8.0 results:

Average quality early-WWII Class "A" armor would give, using FACEHARD v8.0, for a 7.22" plate, an NBL = 1693 ft/sec (516 m/sec) for the 8" Mk 21 MOD 5; the US NPG Report on Japanese heavy armor gives an NBL = 1713 ft/sec (522.1 m/sec) for this same average, using their own NPG formulae and test data, or just 1.2% difference from my FACEHARD v8.0 value (and note again that these two averages were created TOTALLY SEPARATELY by the NPG and by me using overlapping, but independently evaluated, data!!).

Similarly, FACEHARD v8.0 gives an NBL = 1782 ft/sec (543.2 m/sec) for the 8" Mk 21 MOD 3 projectile, compared to the NPG NBL = 1762 ft/sec (537.1 m/sec) given in the Japanese armor NPG Report, again totally independently from me, or a 1.1% difference from my FACEHARD v8.0 value (!!! AGAIN).

FACEHARD v8.0 is therefore nailing it as close as it is possible -- just over 1% difference against both projectiles used against the Japanese thin VH plate -- given the somewhat different databases that I and the US NPG used -- lots of overlap, but not precisely the same -- as well as them having to use ORD. SK. 78841 and slides rules to calculate the "F" values and so forth (they are not going to do all of that long-hand arithmetic!) while I have access to scientific electronic hand calculators and digital computers with essentially infinite accuracy for getting correctly rounded numbers.

Assuming the 38cm British-tested VH plate is exactly 15" (as its Penetration and Perforation Limits were adjusted) and also a KC n/A plate (its 39% face is, as with the 185mm plate, close-enough to the 41% average KC n/A face to use KC n/A as the baseline in FACEHARD v8.0), as adjusted, then at 30 degrees against the regular 15" Mk 17B APC shell FACEHARD v8.0 ARMOR SELECTION #10 & BRITISH PROJECTILE #16 gives an HL = 1557 ft/sec I474.6 m/sec) and an NBL = 1657 ft/sec (505.1 m/sec), compared to the actual British-test-personnel-calculated 1800 ft/sec (548.6 m/sec) NBL for that unusual VH plate.  This gives a FACEHARD v8.0 Q of ((1800/1657)^1.21 = 1.105) x KC n/A's Q = 1.105 x 0.96 = 1.061.  Against British WWII CA, ARMOR SELECTION #14, this becomes HL = 1654 ft/sec I504.1 m/sec) and an NBL = 1706 ft/sec (516 m/sec).  This gives a Q that is ((1800/1706)^1.21 = 1.055) x WWII CA's Q or Q = 1.055 x 0.93 = 0.981, though this does not compare exactly due to the very thin 25% face on the typical WWII CA plate.  As noted by the British, this VH plate is WAY better than British WWII CA, even with the advantage of the thinner-faced WWII CA having improved scaling effects against large projectiles, as well as being better than KC n/A, too.

The unique 15" VH plate was even better than the unique 7.22" VH plate, Q = 1.061 versus Q = 1.009, and the latter could DESTROY the average-quality WWII US and foreign  AP shells (US 8" Mk 21 MOD 3 AP with its Navy WWII standard AP cap design and metallurgy) -- well above average compared to most other WWII AP projectiles as to damage resistance since all 6" and up WWII US Navy AP shells was tested at 35 degrees (in some cases 40 degrees), not the typical 30 degrees requirement of most foreign WWII naval AP-type shells.   The 7.22" VH plate required these super-heavy 8" AP projectiles to use the new super-hard-capped souped-up version to get any kind of reasonable result when hitting that plate.

The higher back hardness, face thickness, and roughly straight-line face hardness drop in the two experimental plates, all of which are at odds with all other VH plates, indicates that these two plates may have been heavily influenced by Krupp KC n/A armor then in use in the remaining German battleships, which had similar details and a 41% face thickness (the average between the two Japanese plates is 41%, too, after all).  If so, they wildly exceeded the Krupp results even though using an inferior, WWI-era steel type with an additional 0.55% carbon (as opposed to 0.3-0.4% carbon used in most other Krupp-steel armors), about as high a carbon content as can be used without causing excessive brittleness during the face-hardening processes and requiring extra-careful quality control to prevent cracked plates.

IV.  CONCLUSIONS

To get some idea of how much of an improvement these two plates represent, let us assume that the average Q for all VH armor had been raised to the average of 1.03 from its actual 0.839 (the weakest of the new post-1930 face-hardened armors), keeping everything else the same (35% face, etc.).  The 16" 20-degree-inclined VH belt of the YAMATO (backed by 2" cement and 1" of D-steel) at a Target Angle of 90 degrees (broadside-on) to an IOWA with its 2700-lb 16" Mk 8 MOD 5 (mid-WWII) AP shells fired at 2500 ft/sec would have its complete penetration range, using OP 1188 (Reference #6) for the IOWA's Range Table, go from roughly 19,000 yards to roughly 12,500 yards, which is a considerable increase in the belt armor's resistance.  If the improvement also increased other metallurgical parameters, the resistance might even be somewhat higher and the required range to penetrate lower, especially as these large US AP shells had AP caps similar to the 8" Mk 21 MOD 3, not the super-hard MOD 5 cap.  Would the US 16" shell be "extruded" here, too?  If so, it most definitely would not be in any state to explode properly inside the YAMATO, drastically reducing per-hit damage probability.  To get a high probability of an intact penetration by more of a shock effect on the plate face, the range might have to be dropped by another couple of thousand yards.  The even thicker main armament barbette and turret face VH armor would be virtually impenetrable, even when just trying to make a hole with a rejected projectile, by any shell whatsoever at any range (it nearly was even with its actual lower quality!).  This is an astounding increase in armor strength in a single step, by a small group of obviously superb metallurgists and mechanical engineers working more-or-less alone with very limited contact with anyone else due to the war situation.

Japanese armor metallurgy had gone obviously  WAY, WAY up in the years after they had stopped making the actual shipboard armor and their engineers and scientists could finally do what they wanted to improve things without any interference by "Good-Enough-for-Grandpa" specification  writers.  This is an amazing story about Japanese technical expertise.  (Obviously, this was a foretelling of what the Japanese were going to do to everybody else, most especially the US auto and electronics industries, after WWII as to quality control and low-cost, superior manufacturing systems.)

As a final "kick in the head" due to testing this superior 7.22" VH plate, the US NPG Report #5-47 concerning Japanese armor admitted at its end that the US experts could not understand how that plate could do such terrible things to their standard moderately-hard-capped AP shells, since the basic steel it was made of was IDENTICAL to the rather dirty (by WII standards) WWI-era steel quality used for all of their previously-tested plates.  (As the British test results show, this was not just an abnormal fluke, but a definite, repeatable manufacturing process.)  Kind of late in the game (after WWII when face-hardened armor was obsolete) to figure out that you didn't know what you were talking about for over 50 years, since Harvey Armor was invented by the US in 1891!!!  But at least they DID admit it, which nobody else had the guts, integrity, and/or the smarts to do...

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