War History Online posted this photo on their Facebook page.

26-inch thick armor from Japanese Yamato class battleship, pierced by a US Navy 16-inch gun. The armor is on display at the US Navy Museum.

I found this Ballistic Test Report which I believe to be about this specific piece of armor.

Ballistic Tests on the IJN Shinano’s Turret Face ArmorBy Nathan Okun
Updated 31 August 1999

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SOURCE:

U.S. NAVAL PROVING GROUND, DAHLGREN, VIRGINIA, REPORT #5-47, “BALLISTIC TESTS AND METALLUGICAL EXAMINATION OF JAPANESE HEAVY ARMOR PLATE” (November 1947)

SUBJECT:

Test of 26″ (66cm) Class “A” Main Armament Turret Face (Port) Plate, originally for IJN SHINANO, the third Japanese YAMATO-Class super-battleship (converted into an aircraft carrier, instead, and sunk on its way to final fitting out yard by a U.S. submarine), which made up far left side of turret face looking from inside turret out of gun port, with “D”-shaped cutout making up about half of left curved gun port forming center of long right side of plate.

PLATE MATERIAL:

Japanese Vickers Hardened (VH) face-hardened, non-cemented armor (used only on YAMATO-Class battleships)

COMPOSITION OF STEEL:

ELEMENT.....PERCENT (By Weight)

CARBON-------0.48
MANGANESE----0.39
PHOSPHORUS---0.016
SULPHUR------0.026
SILICON------0.18
NICKEL-------3.67
CHROMIUM-----2.13
MOLYBDENUM---0.06
COPPER-------0.12

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TENSILE TEST (SLOWLY INCREASING PULL) DATA:

COUPON_ Y.S._ T.S.__ %EL._ %R.A

LONG.__ 68.5_ 99.4__ 22.3_ 45.6
TRAN.__ 70.1_ 98.2__ 25.5_ 62.2
NORM.__ N/A__ 93.16__ 7.0_ 20.3
(Average of 6 coupons)

CHARPY V-NOTCH IMPACT DATA VS TEMPERATURE (°F):

COUPON# -105 -78 -50_ 0__ 50_ 100 <-TEMP.

LONG.#1_ 10G 23G 35G 73FG 75F 95F
LONG.#2_ 14G 20G 31G 77FG 94F 80F
TRAN.#1__ 8G 20G 30G 56FG 73F 70F
TRAN.#2__ 9G 20G 26G 48FG 71F 69F

DEFINITIONS:

LONG.=Longitudinal (parallel to plate face; long plate axis direction)
TRAN.=Transverse (parallel to plate face; short plate axis direction)
NORM.=Normal (in thickness direction)
Y.S.=Yield Strength (x1000 lb./sq.in.)(0.2% elongation point) (sq.in.=original cross-section)
T.S.=Tensile/Ultimate Strength (x1000 lb./sq.in.)
%EL.=PERCENT ELONGATION (% coupon had stretched when it broke)
%R.A=PERCENT REDUCTION IN AREA (% coupon cross-sectional area had shrank by at narrowest when it broke)

(Above data averaged from two coupons in each direction)

CHARPY V-NOTCH TEST=Hammer snaps off one end of coupon while other end is held in a vise with a notch-shaped groove put into coupon just above vise grip where coupon will fold and split apart (the higher the number, the greater the toughness)
“G”=All GRAIN (brittle fracture) at break point
“F”=All FIBER (ductile tearing) at break point
“FG”=Both GRAIN & FIBER MIXED at break point, more FIBER than GRAIN

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HARDNESS (ROCKWELL “C”):
(Ave. of two tests; measured from plate face surface directly into plate)

SURFACE:_46.5 (Tempered surface lost some carbon)
0.75":___50.0 (Maximum hardness of plate)
5.00":___36.0 ("Undrillable" face ends and transition layer begins)
9.25":___15.0 (Inner edge of back layer reached)

Hardness varied in smooth “ski-slope” curves between these points–at 0.75″ hardness increase reversed direction abruptly. Back hardness varied from ROCKWELL “C” 11.0 to 15.5 in long undulations to back surface.

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STEEL QUALITY:

Steel had many tiny pieces of dirt and so forth, being about the same as pre-WWI British Vickers Cemented (VC) KC-type armor steel in quality (VC was used for the first time in the Japanese battleship IJN KONGO, built in Britain, and manufactured in Japan under license thereafter), from which the unique Japanese armors New Vickers Non-Cemented (NVNC), the homogeneous, ductile form of VH used in a number of Japanese post-WWI warships, and VH itself was derived (this steel was not up to U.S., British, or German post-1930 steel quality). Carbon content was raised above VC steel level to increase ease of hardening, some copper added to allow some nickel (in short supply in Japan) to be removed (but not much), slight amount of molybdenum added to increase hardenability still more, and the cemented (carburized) thin surface layer used in VC (and in most other, foreign face-hardened armors) was eliminated with no loss of resistance from VC quality (a good design point). Surface of plate face was very smooth, unlike rough, pebbly surface of cemented plates, such as U.S. Navy Class “A” armor.

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TWO BALLISTIC TESTS:

PROJECTILE USED:

U.S. Navy 2700-lb 16″ Mark 8 Mod 6 AP with inert filler (“BL&P”) (last version of this projectile manufactured during WWII)

OBLIQUITY: Near-normal (0°)

TEST #1 on 16 October 1946 (IMPACT #33443):

POINT OF IMPACT: Upper center near joint with turret roof at 0.33° obliquity from normal where plate was 25.99″ thick.

STRIKING VELOCITY: 1992 feet/second (607.2 m/sec)

RESULT: Complete penetration and plate snapped in two through impact between side edge and upper end of curved gun port hollow. Hole more-or-less cylindrical, with little difference between front and back of plate. Numerous small cracks also put in plate around impact. No damage to projectile indicated, though projectile had considerable remaining velocity and ended up in the Potomac River, never being recovered. Considerable amount of lamination noted in hole (layering effect parallel to face, much like pages in a book glued together). The upper portion of this broken plate is now on display at the U.S. Navy Memorial Museum at the Washington Navy Yard, Washington, DC, just in front of the old Gun Factory building which houses much of the museum today.

TEST #2 on 23 October 1946 (IMPACT #33459):

POINT OF IMPACT: About halfway between first impact and base of plate, centered between cutout for gun port and left edge of plate, at 0.5° obliquity from normal where plate was 25.98″ thick.

STRIKING VELOCITY: 1707 feet/second (502.3 m/sec)

RESULT: Projectile nose tip only penetrated 21″ (53.34cm) into the plate, though punching a hole entirely through. Projectile was completely undamaged (merely lost its windscreen and AP cap, as usual). Plate had exactly the same thing happen to it as with the first test, with numerous small cracks, many laminations, and a complete break through hole between left edge and curved gun port cutout.

CONCLUSIONS:

The U.S. Navy Ballistic Limit (complete penetration minimum velocity with this projectile at normal) estimated at 1839 feet/second (560.5 m/sec), plus or minus 3%, which gives it about a relative plate quality of 0.839 compared to U.S. Class “A” armor (estimated, as no such super-thick plate was ever made in the U.S.). This was about the same as the best WWI-era British KC-type armor, which was what the Japanese were trying for–they had not attempted to make improved face-hardened armor, as the U.S. Navy did during the 1930’s, for actual ship installation.

The plate was excessively brittle internally, with too much “upper bainite” crystal structure due to too-slow cooling.  This was due to using the same pre-WWI British Vickers KC-type armor-hardening techniques on plates over 17″ (55.8cm) thick, for which they were never intended. This problem was solved during WWII, but no more VH was ever made except for some thin experimental plates. Brittleness did not seem to reduce resistance to penetration, though cracking might cause problems due to hits that ricocheted off.

Note that one of these experimental plates–7.21″ (18.3cm) VH plate NPG #3133–was patterned on Krupp KC n/A (probably from data traded with Germany during WWII) and was tested by the U.S. Navy at the NPG using 335-pound 8″ Mark 21 Mod 3 and Mod 5 (the latter with the super-hard AP cap, which turned out to be required to penetrate that plate intact) during this same test series. It was found to be THE BEST PLATE OF ITS THICKNESS RANGE (6-8″ (15.2-20.3cm)) EVER TESTED BY THE U.S. NAVY, even though its steel was of the same rather poor quality as the other VH plates tested!!! This caused the U.S. test conductors to state that obviously they did not understand what it took to make a high-quality Class “A” plate, since the 7.21″ VH plate should not have been so good from everything they thought they knew about face-hardened armor!!! Obviously the Japanese could make armor as good as anyone if the specifications had required it!

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FINAL COMMENTS:

At about 40,000 yards, the U.S. Navy 16″/50 firing a 16″ Mark 8 Mod 6 AP projectile (the later Mod 7 and Mod 8 designs were post-WWII, so I usually do not count them and they were no better ballistically, to my knowledge) will hit at about 45° downward angle and 1607 feet/second (489.8 m/sec). Just as with a point blank hit at 2500 feet/second (762 m/sec) and 45° obliquity, this hit too will barely hole the plate as the projectile is hitting at 0° (normal) obliquity, though not completely penetrate it. Any slight barrel wear will lower the muzzle and striking velocities and no holing will occur at THESE OR ANY OTHER ranges, as mentioned. However, this is so far above any real fighting range (even with radar it is hard to see the target due to the earth’s curvature interfering, especially in any kind of imperfect seeing conditions) that I do not even consider it in my computations, while putting the gun barrel up to almost touching the enemy turret is also a pipe dream in real life! Thus, no holing or complete penetrations, ever, though possibly some cracking of the plate and possible jamming of the turret if the crack-off plate piece is dislodged badly enough.

Therefore, these plates are the only warship armor plates that could not be completely penetrated by ANY gun ever put on a warship when installed leaning back at 45°, as they were in the actual turrets!!! Even to completely hole the plate all the way through at that inclination requires a brand new 16″/50 Mark 7 or German 38cm SK C/34 gun at  point-blank range firing the latest versions of their respective AP projectiles; it might be cracked at a lower striking velocity, but no hole put entirely through it! AND THEY SAID GUNS HAD COMPLETELY OVERMATCHED ALL ARMOR–*NOT SO*!!!

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Some more photos of this plate.