Armor quality of the Tiger I

[seppw]

Attached are Zahlentafel 9a and-b from BAMA RH8-1319 ("H. Knüppel, Entwicklung von Hochleistungsgranaten auf Grundlage des Homogengeschosses", originally classified SECRET, dated 1942, reporting on the improvements of APC mass production techniques for Pzgr.39 series APCBC-HE, unpublished primary source).

Notice that the lower table shows the effect of optimum hardness. The upper tabulation therein was for tests employing a much higher hardness (Rc =64.0 nose and Rc=60.0 core), which lead to early break up and outright shatter in some cases. The slight temper of the german AP towards a nose hardness =61.0 and core 59.0 ideally, was exactly on the sweet spot between optimum hardness but acceptable toughness to prevent early break up.
Post ww2, the germans were extremely critical about the british adoption of higher hardness for their AP-shot. They claimed this gives raise to formation of brittle carbide networks in the head of the shell, which are relatively easy to break up. The british thought that the use of a cap would prevent shatter, which it did but they failed to understand that the germans consistently differentiated between shatter failure and break up failure, and that they tested these shells with cap, too.

The upper tabulation, Zahlentafel 9b shows experimentation in the optimum alloying with heat#8079 beeing markedly superior, the results of which I reported in a previous memo. Notice that these are actual test records, not smoothed penetration chart interpretation of the actual tests.
On the base of these tests, WaPrüf issued updated specification standarts for manufacturing of the Pzgr39 to all manufacturers, replacing their respective heat treatments during mid 1942. The first projectiles arriving in not before summer / autumn 1942 on the eastern front.

Critical Mass,
fantastic contribution!
On page 2 a Leuchtgas-Sauerstoffbrenner is mentioned. Usually this device is used to cut steel in a cost-effective but not very accurate manner.
Casting #8079 has a CEV value greater than 0.92.
Was this device used in order to get the approximate shape, which could be corrected in a subsequent step, or in order to reheat the metal?

[critical mass]

Seppw,

glad You found my previous memo useful. The oxygen cutting torch was used for secondary heat treatment, more precisely for the tempering of the base of the projectile ("anlassen"-in german refers to tempering).
The process was first to heat the projectile rough body slab, press it into a positive form matrix by pressing a cavity shaped pole into the hot steel,
then to re heat it again in order to stress relieve the steel matrix.
Then came rough machining of exterior surfaces
then came heating to critical temperature, holding it there at a predefined time for complete transformation to martensite crystal structure and then cooling it quickly under forced water in order to "fix" the desired crystal structure. This gave the required hardness.
However, the projectile would also be very brittle after this treatment. And the base needs to be more ductile / tough than the nose, particularely under sideways impact forces, so ideally You´d want to have a gradual change of hardness -no rapid changes anywhere- from very hard at the nose to relatively soft at where the base plug is seated to protect the explosive cavity.

In order to keep the nose hardness high, the projectile was emersed nose first in cooling water and then tempered by radial oxygene torch heating of the base, while the projectile was step by step lifted out of the cooling water. The timing was important, so that the martensite crystal got spheroized (tempered martensite) and softer towards the base for an optimum, ski-slope shaped hardness contour.

The improved performance of the heat treatment was so successful, that WaPrüf issued a new designation for the re-issued 7.62cm AP to differentiate the good from the poor projectiles. These "7,62cm Pzgr39 rot" fully replaced the "7,62cm Pzgr.39".
These projectiles were monobloc, not two piece welded on tip like many 7,5cm Pzgr39. To the best of my knowledge, the heattreatment practice obtained by Sächsische Gußstahlwerke Döhlen /Freital during their trials with 7,62cm APCBC-HE in spring 1942 demonstrated for the first time that one could manufacture monobloc shells with the same high quality as two-piece AP-projectiles if care has been invested in optimum heat treatment timing.

compare attachments.

[Avalancheon]

7.62cm Pzgr39 rot was tested vs 60mm (120 kg/mm^2), 80mm (110 kg/mm^2) and 100mm (100kg/mm^2 tensile strength) german RHA @ 30°.

I quote from the earliest, succesful trials with the improved heat treatment conducted 1942 with this projectile:
Heat # 8079 (0.49%C, 0.36% Mn, 1.22% Si, 0.58% Ni, 1.56% Cr, 0.12%V, P&S <0.02%), approx. 61.0 Rc at nose and 59.0Rc core
#1 80mm = 656.9m/s -projectile intact through the plate
#2 100mm = 738.6m/s -projectile intact through the plate
#3 100mm = 737.0m/s -projectile intact through the plate
#4 100mm = 763.2m/s -projectile intact through the plate
#5 80mm = velocity not recorded -projectile intact through the plate
#6 100mm = 740.0m/s -projectile intact through the plate
#7 100mm = 749.4m/s -projectile intact through the plate
#8 100mm = 747.4m/s -projectile intact through the plate

source: BAMA RH8-1319, Zahlentafel 9

These projectiles were generally good enough to defeat 80mm @30° and 660m/s while staying intact and penetrate out to 630/640m/s with breakage.
For a V0=710m/s gun, this means that 80mm side hull will only starting to protect against this projectile at about 700/800m -if the target angle is 30°- and beyond ca. 1.5km if the target angle is 0° (as in soviet wartime tests with ZiS-3 against TIGER I side hull).
The front hull also can be penetrated by 7.62cm Pzgr rot, but only at normal, not at the standart 30°condition. If You take V30=740m/s for 100mm than the same velocity would be valid for V0=123mm, which gives a K of about K=2190, which brings the V0 for a 100mm plate to 640m/s for this projectile, very approximately the same range as an 80mm plate will be defeated at 30°.

German service manuals usually expect no worse quality from the enemy than their own. That not only applies to the Tigerfibel but also to the navy fire effect tables (where this principle is explicitely written).

Awesome response, Critical Mass. You answered all my questions and then some!

Its crazy to think that the 76mm F22 gun could get such a performance boost from using properly capped, heat treated shells. If the Soviets had used high quality ammunition like the Germans did, then all they would have needed was the 76mm F34 and 85mm S53 guns. They wouldn't have bothered with anything of a higher caliber, like the 100mm D10 and 122mm A19. (It would be overkill for the task at hand)

To be fair, the Pzgr ammo from 1939-1941 wasn't all that great. But once the Pzgr 39 ammo was introduced in 1942, the Germans had the best quality shells in the world, bar none. The Heereswaffenamt really knew what they were doing.

I'm glad you posted screenshots from the report, this saves me the trouble of hunting it down myself! Theres lots of good info here.

[critical mass]

Yes. It´s necessary to understand why the germans didn´t considered the TIGER-1 immune armorwise against 76mm guns and issued a pamphlet to be aware of angling to augment protection by presenting a more difficult, angled target.
It´s not that the TIGER was exceptionally well armored, it´s rather a function of weakness in foreign AP-shell design to deal with said armor.
The 76mm AAA gun with 815m/s V0 and a good projectile would arguably be enough to frontally deal effectively at long range and variable target angle with TIGER1 and PANTHER (except glacis on the latter). However, the projectiles were of insufficient strength and broke up even against the 82mm sides and 500m when fired with 815m/s initial velocity, leaving dents, only.

[seppw]

In order to keep the nose hardness high, the projectile was emersed nose first in cooling water and then tempered by radial oxygene torch heating of the base, while the projectile was step by step lifted out of the cooling water. The timing was important, so that the martensite crystal got spheroized (tempered martensite) and softer towards the base for an optimum, ski-slope shaped hardness contour.

Right. I assume the shell needs to be harder around the tip, because it's the surface of the tip that's pushing against the target material.
Would it be possible to improve the strength of the shell further by decreasing the hardness gradually from outside to inside?

Yes. It´s necessary to understand why the germans didn´t considered the TIGER-1 immune armorwise against 76mm guns and issued a pamphlet to be aware of angling to augment protection by presenting a more difficult, angled target.
It´s not that the TIGER was exceptionally well armored, it´s rather a function of weakness in foreign AP-shell design to deal with said armor.
The 76mm AAA gun with 815m/s V0 and a good projectile would arguably be enough to frontally deal effectively at long range and variable target angle with TIGER1 and PANTHER (except glacis on the latter). However, the projectiles were of insufficient strength and broke up even against the 82mm sides and 500m when fired with 815m/s initial velocity, leaving dents, only.

Picture related.
https://upload.wikimedia.org/wikipedia/ ... r_I%29.jpg
Does anybody know how tall that hinge is?

[critical mass]

Would it be possible to improve the strength of the shell further by decreasing the hardness gradually from outside to inside?

This process is called "sheath hardening" and was successfully employed first by Grüson chilled cast iron shell in the 1870´s, then by Krupp´s naval ww1 and ww2 APC and finally by the US Navy and US army AP-projectiles after ww1.

The data I have seen suggests very strongly that sheath hardening was tried to be employed in Pzgr39 but all attempts in calibre below 105mm were unsuccessful. In 105mm and 128mm at least no disadvantage was observed. The technique is really useful only in very large, naval calibre projectiles due to the larger section thicknesses possible there.

I attach a comparison of heat#897 and heat#847 for 75mm monobloc Pzgr39. Both heats use the same steel alloy mix but differ only in hardening contouring, with the former using standart and the latter sheath hardening. The standart, decrementally hardened (basically constant hardness in shoulder to shoulder line) is superior in ballistic tests. The sheath hardened example shows a high surface hardened gradually softened down to a mildly (Rc=54.0) softer core hardness but failed in actual ballistic tests.

With intermediate or small calibre shot, the core hardness supports the nose during penetration. If a certain hardness (not too much, mind the Rc=64.0 tests!) is not provided, the softer core would allow bending of the nose (offset damage). This change of shape creates a higher critical velocity. If more stress is exposed, the offset damage will be augmented by signs of compression (upset damage). Past a critical plastic deformation, the projectiles then start to break up with the origin of break up beeing the most stressed area of the fwd bourrolet.
For 75mm and 76.2mm Pzgr39, the minimum core hardness for optimum performance was Rc=59.0. After heat treatment but before fine machining, two examples of shots of each heat were selected and cut in two. Then hardness measurements were performed. If the core hardness was below Rc=56.0, the whole heat was discarded (not even ballistic test was allowed). The heat was marked as potentially "good quality lot" with core hardnesses of Rc59.0 to Rc60.5, though all acceptably hard (> 56.0Rc core hardness for 75mm and 76.2mm) lots were forwarded to ballistic tests and only lots which had both, good hardness contour as well as no failures in ballistic tests were classed as "good quality".

US 75mm and 76mm AP-ammunition was sheath hardened and, I suspect, that was partially the reason for the inferiority experienced when compared with 75mm Pzgr39 in post war USAPG trials.

I have attached hardening patterns sketches made by Dr. Wagenknecht of Bochumer Verein on his 7.62cm Pzgr.39 rot (´1943 pattern with decremental hardening pattern) and Navy sheath hardened 40.6cm L/4.4 Psgr also made by B.V. Notice the isolines of equal hardness are dotted (Zugfestigkeit" or "Linien gleicher Härte")

[Avalancheon]

Yes. It´s necessary to understand why the germans didn´t considered the TIGER-1 immune armorwise against 76mm guns and issued a pamphlet to be aware of angling to augment protection by presenting a more difficult, angled target.
It´s not that the TIGER was exceptionally well armored, it´s rather a function of weakness in foreign AP-shell design to deal with said armor.
The 76mm AAA gun with 815m/s V0 and a good projectile would arguably be enough to frontally deal effectively at long range and variable target angle with TIGER1 and PANTHER (except glacis on the latter). However, the projectiles were of insufficient strength and broke up even against the 82mm sides and 500m when fired with 815m/s initial velocity, leaving dents, only.

I can certainly see why the Germans were worried about the 76mm AA gun. It had a longer barrel and larger propellant charge than the 76mm ZiS-3 gun, which gave it a significantly higher muzzle velocity. With properly capped, heat treated shells, it could have been a major threat.

But that leads me to the question, just how important is the issue of muzzle velocity? The German tests prove that the modified 76mm F-22 gun with Pzgr.39 rot could easily shoot holes through the Tigers front armor. But those guns used a much larger propellant charge than the Soviet guns.

If they had stuck with the smaller propellant charge that was endemic to the guns design, how much would that have reduced their effective range? I'm not sure what kindof velocity they'd get with the Pzgr.39 rot and a standard propellant charge.

One of the shots in that German test recorded a muzzle velocity 656.9m/s. It was against an 80mm thick plate angled at 30 degrees. The shot made a penetration without breakage. What range would that correspond to?

[critical mass]

download/file.php?id=426549&mode=view

round about 500m with an initial velocity of 710m/s (official range table velocity standartized to a wear rate of 10/30 - new gun MV would be 730-740m/s / called"größte V0"). The projectile service acceptance specification as of mid 1942 for the 76.2mm Pzgr 39 rot specified a velocity of 660m/s, and an obliquity of 30°. Two out of two projectiles selected from each heat had to perforate a 110kg/mm^2 plate (~330BHN) of 80mm thickness in intact condition fit to burst. If one failed, another one from the heat was substituted and HAD to penetrate without breakage or the lot would be rejected. This is the minimum projectile quality acceptance floor for service AP.
Older service acceptance specifications for 76.2mm Pzgr.39 specified against 60mm plate of 120kg/mm² tensile strength (~360BHN) at 660m/s.

[Avalancheon]

So, returning to the topic of the Tiger Is armor quality. As you all know, the British were able to capture numerous examples of this tank during the war, and subject them to ballistic and metallurgical tests. They wrote a number of different reports, and the findings difer slightly.

One of these tests was mentioned in Tiger!: The Tiger Tank: A British View. ''Though machinable, it was frequently much harder than corresponding British armour, and in many cases it caused shot to shatter, while the plates were liable to crack and flake. The side plates from turret, hull and superstructure etc, have all been found to flake in - various cases.''

Then there is the A.T. 252 report. ''Unlike previous experiences with Panther tanks, the armour plates, with one exception (hull roof) did not show any marked tendency of brittleness, and their behaviour was not unlike British machinable quality plates.''


The fact that the Tiger in question did not use face hardened armor, and yet was still able to shatter many the projectiles fired at it is very surprising. The British probably fired a number of APC shells at the tank, and yet the caps did not stop them from shattering. They also note that the armor plates had a tendency to spall even when they stopped the incoming shell.

[critical mass]

The german RHA was more closely treated to the 1st ductile-brittle transition point in regard to hardness than british MQ armor. This point offers enhanced ballistic resistence against multiple threats. The armor is not, however, ideal in regard to spalling of, f.e. broken AP, shattering against the target. British wartime practice was not to complain about projectile breakage as long as the break up occured rather late in the penetration (even A.P.C. rarely stayed in effective bursting condition unless farly significantly overmatched in critical velocity or T/D ratio). Nose hardness of late war A.P. was too high, giving raise to the formation of brittle carbide networks in the head of the shell. This is not a problem against soft RHA and CHA but against moderately harder RHA / HHA it can contribute to cases of undesired early break up.

[Avalancheon]

The german RHA was more closely treated to the 1st ductile-brittle transition point in regard to hardness than british MQ armor. This point offers enhanced ballistic resistence against multiple threats. The armor is not, however, ideal in regard to spalling of, f.e. broken AP, shattering against the target. British wartime practice was not to complain about projectile breakage as long as the break up occured rather late in the penetration (even A.P.C. rarely stayed in effective bursting condition unless farly significantly overmatched in critical velocity or T/D ratio). Nose hardness of late war A.P. was too high, giving raise to the formation of brittle carbide networks in the head of the shell. This is not a problem against soft RHA and CHA but against moderately harder RHA / HHA it can contribute to cases of undesired early break up.

So the armor is performing a function quite similar to face hardened armor (by shattering projectiles), without actually being face hardened. Thats a neat little trick. Of course, it wouldn't work against high quality German ammo. Thomas Jentz mentions the results of some Wa Pruf tests in his book, Germanys Tiger Tanks D.W. to Tiger I.

On March 31, 1942, the turret of the VK 45.01 tank was fired at by a 5 cm Pak 38 (L/70) gun, and a 7.5 cm F.K. 16 na gun. The testers concluded that the turret can protect against the 5 cm Pzgr, but not the 7.5 cm Pzgr.

On April 30, 1942, the hull of the VK 45.01 tank was fired at by the 5 cm F.K. 16 na gun. The testers concluded that the hull could not protect against the 7.5 cm Pzgr.

It just goes to show that the Tigers armor was far from impervious. Its resistance depended on the poor quality of the Soviet, British, and American projectiles.

[Mobius]

The german RHA was more closely treated to the 1st ductile-brittle transition point in regard to hardness than british MQ armor. This point offers enhanced ballistic resistence against multiple threats. The armor is not, however, ideal in regard to spalling of, f.e. broken AP, shattering against the target. British wartime practice was not to complain about projectile breakage as long as the break up occured rather late in the penetration (even A.P.C. rarely stayed in effective bursting condition unless farly significantly overmatched in critical velocity or T/D ratio). Nose hardness of late war A.P. was too high, giving raise to the formation of brittle carbide networks in the head of the shell. This is not a problem against soft RHA and CHA but against moderately harder RHA / HHA it can contribute to cases of undesired early break up.

There's not a lot of bursting going on with British projectiles as they are for the most solid shot.

[Avalancheon]

I found this cool picture of a Tiger tank that had clearly been subjected to a ballistic tests. Even though its armor had been repeatedly penetrated, there were no signs of brittle failure. The upper front plate (UFP) and lower front plate (LFP) have seemingly been fired at by two different caliber weapons.



Does anyone know which nation was responsible for this test? It probably wasn't a Soviet test, because they were very erratic and inconsistent with their shot patterns. Their April 1943 trials against the Tiger I is instructive: Testing the front hull, they fired three shots from the 57mm Zis-2, five shots from the 76mm F-34, and then only three shots from the 85mm S-53 gun...

Thats why I think this must have been a German, British, or American test.

[Richard Anderson]

Indeed, probably British. That is either an American or British '7', but in the 1940s it is a tossup whether or not that is an American or British '4'. Probably British.

[Avalancheon]

Indeed, probably British. That is either an American or British '7', but in the 1940s it is a tossup whether or not that is an American or British '4'. Probably British.

What are you referring to? The kindof gun used in the test? Please be more specific with the models.