Saturday, 8 February 2020

Thick Skin of German Beasts

German Tiger, Panther, and King Tiger tanks are some of the best known vehicles in the history of tank building and still occupy the minds of armoured vehicle enthusiasts. Their enormous size, thick armour, and long powerful weapons created a reputation of all-destroying and nearly invulnerable tanks. However, if everything is more or less clear with their size and armament, then the issue of armour is much more difficult. This is the first in a cycle dedicated to the creation of armour for the Tiger Ausf.H1, Panther of all variants, and the Tiger Ausf.B, their assembly, and trials of the armour in the USSR, USA, and Great Britain. To start, let us talk about rolled armour that the hulls and turrets of Tigers and Panthers were composed of almost fully.

German industry produced two types of rolled armour: homogeneous of medium hardness and surface hardened. Before production began, the company would present samples to confirm that it had the technological ability to produce armour of a certain quality.

The requirements for armour makers were set by the Supreme Command of Land Forces (Oberkommando des Heeres — OKH), the 6th department of the Weapons Agency (Waffen Prüfen 6, or Wa Prüf 6 for short). For homogeneous armour the requirements included chemical composition and hardness, ballistics trial conditions, and in some cases thermal treatment requirements. The armour was split into seven ranges, each of which with their own requirements: 5-14.5 mm, 16-30 mm, 35-50 mm, 50-80 mm, 85-120 mm, 125-160 mm, 165-200 mm. Of the listed categories the 5-14.5 range had special requirements, but since it was not used in the aforementioned tanks then it can be omitted.

One of the two Tiger Ausf.H1 tanks first captured by the Red Army in the winter of 1943. This tank was tested at the artillery proving grounds and gave some alarming revelations about the ability of Soviet anti-tank artillery to combat the new type of tank.

The use of surface hardened armour was limited to 16-30 mm, 35-50 mm, and 50-80 mm ranged, but the majority of this kind of armour was 30-50 mm thick. Requirements published in July of 1944 no longer included heterogeneous armour, and its production began to wrap up.

It is interesting that during the inspection of captured Panther Ausf.D and Panzerjager Tiger(P) or Ferdinand vehicles much attention was paid to surface hardened armour. A NII-48 report dedicated to the study of foreign armoured hull production published in 1945 states:
"The fact that the Germans used heterogeneous rolled armour in the most vulnerable parts of the Panther and Ferdinand vehicles (front and sides) is newly revealed in the current work. This could be done as armoured steels with a high carbon content were used by the Germans.
It is known that heterogeneous armour is more resilient against sharp-tipped shells and bullets with a caliber that is less than or close to the thickness of the armour.
Considering that modern armies widely use sharp-tipped shells and anti-tank armour piercing bullets, the use of heterogeneous armour by the Germans deserves attention.
Given the above, it is reasonable to discuss the possibility of organizing production of heterogeneous heavy tank armour in this country, hardened from one side using high frequency electric current."
Heterogeneous surface hardened armour was produced by flame hardening homogeneous armour. In addition to all the requirements for homogeneous armour, requirements for heterogeneous armour included the surface hardness and depth of hardening. Ballistic trials were chiefly performed before the hardening and surface hardened armour was not shot at.

The tolerance for the hardening thickness was 5%. Pitting on the surface of the armour was permitted with the requirement that the depth was not more than 3% of the thickness of the plate and no more than 10% of the surface area is affected.

Each factory was supposed to perform trials and obtain a certification for each batch of armour. The certification lasted for 2 years. If a plate from the batch did not pass trials, the company was supposed to replace the batch at no charge. The manufacturer provided their own equipment for measuring the rolled armour. The thickness was measured and the surface was inspected for cracks, pores, and other defects. 

The appearance of the Panther Ausf.D in the summer of 1943 was another unpleasant surprise. The front armour was much more powerful than that of the Tiger and could not be penetrated by an 85 mm AA gun. However, the Panther Ausf.D had much more variable armour: the thinner sides could be penetrated by any gun down to the 45 mm.

The inspector would then check the certification of the armour against requirements of the Wa Prüf 6. Hardness trials using the Brinell method. Usually hardness was checked in five places on the plate.

The final stage of the acceptance trials was ballistic testing. To pass, the back of the plate could display either a clean bulge from a hit or a bulge with short cracks. Plugging or through cracks were unacceptable. For certain ranges of armour penetrations without cracking were accepteble.

Chemical composition

German rolled armour had a high level of carbon (up to 0.26-0.53%) and carbide-forming chrome (up to 3.2%). Its amount decreased with the increase of thickness for 85-120 mm, 125-160 mm, and 165-200 mm ranges. British scientists considered that this was done to attempt to reduce cracking during hardening.

A high carbon content with a high amount of carbide forming elements allowed medium hardness armour to be obtained at high tempering. This ensured a lack of shock during hardening and a reduced tendency to crack during welding.

The requirements for chemical composition of homogeneous armour, depengin on thickness, weer as follows:

16-30 mm

C
Mn
Si
Cr
Ni
Mo
V
P
S
March 1941 to November 1944
0.43-0.53%
0.55-0.85%
up to 0.40%
1.25-1.55%
-
0.40-0.60%
up to 0.30%
Up to 0.05%
Up to 0.05%
After November 1944
0.41-0.49%
0.40-0.80%
0.50-0.80%
0.45-0.75%
-
-
-
Up to 0.05%
Up to 0.05%

35-50 mm

C
Mn
Si
Cr
Ni
Mo
V
P
S
March 1941 to November 1944
0,43–0,53
0,55–0,85
Up to 0,40%
1,25–1,55
-
0,40–0,60
0,15–0,30
Up to 0.05%
Up to 0.05%
After November 1944
0.41-0.49%
0,60–1,00%
0.50-0.80%
0,75–1,05
%
-
-
-
Up to 0.05%
Up to 0.05%

 55-80 mm

C
Mn
Si
Cr
Ni
Mo
V
P
S
June 1942 to November 1944
0,37–0,47
0,55–0,85
up to 0,40
1,25–1,55
0,40–0,60
0,15–0,30
up to 0,05
up to 0,05
After November 1944
0,41–0,49
0,80–1,20
0,20–0,50
0,90–1,20
up to 0,05
up to 0,05

85-120 mm

C
Mn
Si
Cr
Ni
Mo
V
P
S
August 1942 to June 1944
0,32–0,42
0,30–0,65
0,15–0,50
2,00–2,40
0,20–0,30
up to 0,05
up to 0,05
After June 1944
0,37–0,47
0,60–0,90
0,20–0,50
1,60–1,90
up to 0,15
up to 0,05
up to 0,05

125-160 mm

C
Mn
Si
Cr
Ni
Mo
V
P
S
As of June 1944
0,30–0,40
0,60–0,90
0,20–0,50
2,30–2,70
1,00–1,50
up to 0,15
up to 0,05
up to 0,05

165-200 mm

C
Mn
Si
Cr
Ni
Mo
V
P
S
As of June 1944
0,28–0,33
0,60–0,90
0,20–0,50
2,80–3,20
0,90–1,10
up to 0,05
up to 0,05

The Germans ran into a shortage of molybdenum first. By late 1942 it was excluded from small and medium thickness armour, by June of 1944 it vanished from armour of all thickness. Nickel began to turn up in armour in significant amounts as of 1943. Even though it was absent from the requirements, it appeared in the armour of Tigers and Panthers. NII-48 reports:
The decrease of chrome and molybdenum with the addition of nickel was perhaps not just a sign of economy of alloying elements, but an attempt to increase ductility, as chrome-molybdenum steel like the kind used on the Tiger has increased brittleness when fired at, as established by NII-48 trials.
By mid-1944 nickel was removed from all thicknesses of armour except 125-160 mm and 165-200 mm, where it remained until the end of the war. By the fall of 1944 the use of vanadium was also nearly stopped. The main alloying element that was used throughout the entire war was chrome. 

A Tiger Ausf.B before firing trials at the proving grounds. The new tank was not very surprising after the study of the Panzerjager Tiger(P), which had similar armour. Nevertheless, this was the most heavily armoured tank of WWII.

It is interesting to compare the requirements for German armour and the values actually obtained from captured German tanks. In October of 1945 the Mariupol factory and the Moscow branch of the TsNII-48 performed studies of German tank armour to determine its resistance to penetration as well as to evaluate its advantages and disadvantages compared to domestic armour.

Two Panthers Ausf.A and two Tigers Ausf.E were used. Plates were cut out of the tanks and were put through the regular quality control that tank armour went through. Samples were taken to determent fracture properties, hardness, and chemical composition. After samples were taken the plates were put through firing trials following a special program. Excerpts of those trials will follow in the relevant sections, but let's focus on the chemical composition.

For Panther tanks, samples of the upper front plate (50-80 mm category), lower front plate (50-80 mm), and rear plate (35-50 mm) were taken. The following chemical composition was obtained:



C
Mn
Si
Cr
Ni
Mo
V
P
S
Upper front plate 1, 80 mm
0,42
0,85
0,29
1,72
0,07
~0,018
~0,018
Upper front plate 2, 84 mm
0,51
0,74
0,27
2,17
0,22
Lower front plate 1, 60 mm
0,34
0,40
0,25
2,30
1,15
0,30
0,019–0,025
0,019–0,025
Lower front plate 2, 64 mm
0,43
0,76
0,28
1,87
0,42
0,20
Rear plate 1, 40 mm
0,41
0,80
0,25
1,76
0,08
0,019–0,025
0,019–0,025
Rear plate 2, 40 mm
0,41
0,96
0,23
1,89
0,22

One of the Panthers that was used to obtain armour samples at the Mariupol factory.

For the Tiger Ausf.E, let us examine the samples taken from the lower front plate, the previously unseen 85-120 mm range:


C
Mn
Si
Cr
Ni
Mo
V
P
S
Lower front plate 1, 100 mm
0,32
0,45
0,27
2,00
0,12
0,22
0,016–0,020
0,016–0,020
Lower front plate 2, 102 mm
0,33
0,78
0,28
2,61
1,73

Values for the 125-160 and 165-200 ranges can be taken from the Tiger Ausf.B produced in July 1944. The 100 mm and 150 mm hull plates, as well as the 180 mm thick front turret plate, had the following composition:
C
Mn
Si
Cr
Ni
Mo
V
P
S
0,34–0,38
0,58–0,70
0,17–0,36
2,05–2,24
1,17–1,30
0,10–0,16
0,014–0,025
0,014–0,025

It is evident that, with the exception of the last two ranges, the chemical composition requirements were not met, especially the chrome content, as there is too much in most cases, and in the nickel content.

The following conclusions were made by NII-48 specialists, given in their report dated 1945:
The armour from four captured German tanks shows a wide range of chemical compositions and types of steel.
Armour of the same thickness taken from two tanks of the same type can have different chemical composition. Carbon content in German steel can range from 0.32 to 0.57%. 100 mm thick armour usually has less carbon. Increased carbon content (0.40-0.57%) can be seen in medium thickness (40-82 mm) medium hardness armour.
It is known that domestic rolled medium hardness armour contains no more than 0.34% carbon, which is significantly different from German armour.
Chrome content is within the range of 1.67-2.30%. Increased chrome content (over 2.0%) is observed in 60-100 mm thick armour. Nickel and molybdenum do not appear in all inspected samples. Plates 40 mm thick have no nickel or molybdenum, all others from 60 to 100 mm thick have either nickel in the range of 0.77-1.73% or molybdenum in the range of 0.20-0.30%, both, or neither. In all 80 mm plates molybdenum was absent.
It is known that domestic rolled medium hardness 49s type armour used in thicknesses up to 80 mm has such necessary hardening materials in it as nickel and molybdenum. As we see below, the crystalline fractures observed in German medium and low hardness 100 mm thick armour are caused by insufficient amounts or complete absence of nickel and molybdenum in the steel. Vanadium and tungsten was not found in the samples.
Such a variety of types of armour steel can be explained by the volatility of the German metallurgy industry in its ability to access alloying materials (ferro-alloys). The biggest objective of the companies, it seems, was to achieve proper tempering at minimum alloying for each thickness.
Requirements for hardness

Similar to chemical composition, each thickness range had its own required hardness. The following hardnesses were acceptable for each range:

Thickness. mm
5–14.5
16–30
35–50
50–80
85–120
125–160
165–200
Harness. HBW
415–473
311–352
279–324
266–311
220–265
220–265
220–265

The required hardness decreased as thickness increased. In 1941 Wa Prüf 6 gave the order to reduce the harness of the 5-14.5 mm thickness category that was used to horizontal armour plating. In July of 1944 a similar order was given for the 16-30 mm range.

A Tiger Ausf.E tank used in the described trials. Markings for cutting out the sample are shown.

The requirement for surface hardened armour was 555 HBW. The depth of hardening varied depending on the overall thickness: 2.5-4 mm for 16-30 mm, 4-6 mm for 35-50 and 55-80 mm. The depth of hardening was measured by making cuts and measuring the hardness on each side of the plate. The hardness measurements were made for the aforementioned tanks:

Panther Ausf.A:
  • UFP #1, 80 mm thick: 302-211
  • UFP #2, 84 mm thick: 241-255
  • LFP #1, 60 mm thick: 311-320
  • LFP#2, 64 mm thick: 320
  • Rear #1, 40 mm thick: 311-320
  • Rear #2, 40 mm thick: 302-320
Tiger Ausf.E:
  • LFP #1, 100 mm thick: 285-302
  • LFP#2, 102 mm thick: 255-269
Tiger Ausf.B, July 1944 production:
  • UFP 150 mm thick and LFP 100 mm thick: 269-241
  • Turret front, 180 mm thick: 255-241
These inspections led to the same conclusions as with the chemical analysis: the same type of plate can have a wide range of hardnesses and the technical requirements were not met. NII-48 reports:
The 40 and 60 mm thick plates is always medium hardness. 100 mm thick armour can have medium or low hardness. Out of six samples taken from 80 mm thick armour, in three cases the armour was medium hardness and in three cases it was low hardness. Out of two samples taken from 100 mm thick armour, the armour was medium hardness in one case and low hardness in another case. This kind of difference happens not only on two tanks of the same type, but on components that have identical names and function."
Surface hardened armour had outside hardness of 477-555 HBW and 269-341 hardness on the inner side. The hardened layer had a pronounced boundary. The depth was 5 mm.


Krupp factory being bombed, October 1944.

Mechanical properties of German armour were analogous to those of domestic armour.

Thermal treatment

Thermal treatment requirements only existed for 5-14.5 mm thick armour, the rest went through an ordinary hardening and treatment process. The hardening and treatment process temperatures at the Krupp factory are given in the following table:

Thickness, mm
5–14.5
16–30
35–50
50–80
85–120
125–160
165–200
Hardening, °C
900–920
860–880
860–880
860–880
860–880
850–860
880–900
Treatment, °C
250–280
550–570
560–580
570–600
590–610
620–640
630–650

Until June of 1944, hardening was done in oil with the exception of 5-14.5 and 55-80 mm ranges. For the 55-80 mm range hardening in water was first done in June of 1942. The transition to water in June (at some factories in October) was forced by bombing of oil processing plants. This led to difficulties linked to cracking armour is the process was not followed. To minimize risk of cracking, some manufacturers removed the plates from the hardening baths before they cooled fully, which could reduce the effectiveness of the heat treatment.

Fracture of the lower front plate of a Tiger Ausf.E tank. The crystalline fracture with large lamination can be seen in the middle.

Here we should inspect the range of fracture types of German armour that often came up when studying captured tanks. This was due to the quality of the heat treatment. An investigation was carried out on the first captured Tiger Ausf.H1, which was reflected in the NII-48 report:
Samples for microscopic examination were cut from the 100 mm thick front plate (crystalline fracture) and 100 mm thick turret platform front (fibrous fracture) in order to discover the varied character of 62-100 mm thick armour.
Microscopic examination revealed that the microstructure of the sample that gave a fibrous fracture consists of thin sorbite aligned along the martensite.
The microstructure of the samples that gave a crystalline fracture consists of a much less aligned much rougher sorbite and even pearlite with a significant amount of ferrite. 
One of the causes for the difference in fracture is the quality of heat treatment. The quality of metal of the German Tiger tank, based on the slating and layering, is not high.
Microstructure fracture of the lower front plate of another Tiger Ausf.E tank. It is clear that the quality of the fracture differs.

The same conclusions were made as a result of an inspection of Panther tanks produced in 1943-1944, published in the "Tank Industry Herald" journal issue #1 for 1945:
The armour of Panther tanks has varying fracture behaviour, from fibrous to purely crystalline. There is no connection between chemical composition and fracture type.
A lack of consistency in fractures (among components with the same thickness and type of steel) is shown in a number of studies of German armour steel. This allows one to state with confidence that the Germans do not have a fracture test for quality control of heat treatment.
A lack of this control would be possible if there was a well mastered and strictly followed heat treatment process. However, a wide range of fractures indicates that this process, if it is established at all, is not followed. This is also confirmed by the large variations in the range of hardnesses, which in itself would make obtaining identical fracture results impossible.

Ballistics trials

Germans typically took two control plats from a set that produced 25 hulls. However, the conditions of testing and acceptance differed for each range of thicknesses.

The method piqued the interest of both Soviet specialists as well as the Western Allies. The process was designed to avoid modifying ammunition and determining its velocity, which made the process simpler. The Wa Pruf 6 provided the shells. They were held at a certain temperature before firing. Explosive filler was not used. Each range of thicknesses had different angles specified, depending on the penetration of the selected shell.

For the first trial, the plate was installed at a certain angle and 3-5 rounds were fired. If the penetration was not achieved, the angle would be decreased by 10 degrees and the next volley was fired. The angle would continue decreasing until a penetration was achieved. If the plate was placed vertically and still could not be penetrated, a larger caliber was used.

The following ammunition were used:
  • 2 cm PzGr fired from the Flak 30 for 16-30 mm thick armour.
  • 3.7 cm PzGr fired from the anti-tank gun for 35-50 mm thick armour fired from 100 meters.
  • 5 cm PzGr 39 from the 5 cm Pak L/70 (likely a typo in the translation) for 55-80 mm thick armour fired from 100 meters.
  • 7.5 cm PzGr 39 from the Pak 40 for 85-120 mm thick armour fired from 100 meters.
  • 7.5 cm PzGr 39 from the Pak 40 for 165-200 mm armour fired from 100 meters.
No data exists for the 125-160 mm range. For the 180 mm plate the OKH had a special requirement: at normal only one penetration with the PzGr 39 without cracks or spalling is permissible. It is not likely the the Pak 40 was actually used, the 7.5 cm Pak 42 L/70 is a more likely candidate.

According to existing data, in the second quarter of 1944 more than 30% of batches failed the first trials, 15% failed the second trials, and about 8% failed the third trials. The biggest cause of failure was spalling. This result forced the Germans to develop preliminary methods of checking metal quality. Attempts were made at several factories.


Results of firing 85 mm sharp-tipped shells at the side armour of a Tiger Ausf.E tank (83 mm). Spalling can be seen.

Ballistic tests of German armour were performed many times in the USSR. Let us study the results of the trials performed jointly by the Mariupol factory and Moscow branch of the TsNII-48. The plates were installed at normal, 30 degrees, 45 degrees, and 60 degrees. The 45 mm gun was used to test 40 mm thick armour, 60 and 80 mm thick armour was tested with the 76 mm divisional gun, 80 and 100 mm thick armour was tested with the 85 mm gun.

21 plates were tested in total. The limit of rear damage (PTP) and limit of complete penetration (PSP) were established. The results of the trials, presented in the NII-48 report, confirm the above conclusions made in other tests.
40 mm thick plates:
The biggest difference in the PTP and PSP took place during firing at normal. The difference in  PTP was 82 m/s, the difference in PSP was 55 m/s. All three plates have almost identical chemical composition and hardness. The different toughness can be explained by the quality of heat treatment. The plate that showed the best results (rear of Panther tank #2)  had a fibrous fracture. The plate that showed medium results (front sloped roof from Tiger tank #2) had a dry fracture, and the worst performing plate (rear of Panther tank #1) had a fibrous fracture with small crystalline traces. Two plates out of three tested at normal had four cases of spalling from the rear side, one of which was excessively large (4 calibers).  
60-64 mm thick plates: 
The varying toughness of plates is explained by the chemical composition, thickness, and quality of fracture. Plate #2 (lower front plate of Panther tank #1) that had lower PSP and PTP compared to plate #6 (lower front plate of Panther tank #2) was 4 mm thinner, had worse fracture characteristics, and contains less carbon with minimal difference in other elements.
Plate #14 (upper sloped front plate of Tiger tank #2) that fractured on the second shot is different from the two aforementioned plates in its much higher carbon content (0.57%), lower quantity of chrome and nickel, absence of molybdenum, and fracture properties (small crystals).
The cause of cracking can be traced to poor chemical composition (high carbon content, low content of alloying elements) and no fibers in the fracture.
80-82 mm thick plates: 
All seven plates fractured when tested at normal, with the exception of plate #20 that only gave cracks. The chemical composition and quality of heat treatment had an effect on these plates. Plates ##15, 16, and 17 (upper right side plates of Tiger tank #2), alloyed with chrome and without any nickel, fractured on the first and second hits.
Plates #18 and 19 (upper left side plates of Tiger tank #2) which contained about the same amount of chrome but also nickel fractured after the third or fourth hits.
Plate #18 (tested at 30 degrees) and plate #20 (rear of Tiger tank #2), which had less carbon and less chrome than plates #18 and 19, but more nickel, had cracks running through them after hits 3 and 4.
Plate #3 (upper front plate of Panther tank #1) had the same chemical composition as plates ##15-17, but fractured on the first hit from a 76 mm gun.
Plates #18 and 19 contained nickel and had a hardness of 3.6-3.7 (Soviet reports used impression diameter to represent hardness given a 10 mm ball and 3000 kg pressure - author's note), had fractures with small crystals, all other plates with or without nickel and hardness of 3.5-3.6 had fibrous fractures with crystalline traces along the entire cross section. Plates #15 and #16 had acceptable back spall.
Trials of the 100 mm thick plates gave no results, as the first plate fractured after the second hit and the second plate gave many cracks after the first shot. The overall conclusions were as follows:
  1. Toughness of the 40 mm plate is higher than required of domestic armour of the same thickness.
  2. Toughness of the 60 mm plate is about the same as required of domestic armour of the same thickness, but lower than that of armour produced at the Mariupol factory. 83-84 mm thick low hardness armour has comparatively higher toughness.
  3. German 40, 83, and 100 mm armour has a tendency to brittle failure, especially 83 mm.
  4. German 40 and 63 mm medium hardness armour as well as 83-84 mm low hardness armour has good durability. Medium hardness 82 mm armour and 100 mm medium or low hardness armour has low durability.
  5. The pattern observed in domestic production is confirmed: brittle failure correlates with the fracture properties.
The same properties were observed when studying the armour of the Tiger Ausf.B: unsatisfactory fracture characteristics and increased percentage of brittle failures as armour thickness increased. 80 mm plates from the sides and the 150 mm thick front plate were used for trials. Due to a lack of domestic armour of this thickness, it was compared with 90 mm thick sides of IS-2 tanks and the 160 mm lower front of the experimental Object 701 tank. The results gave similar toughness of German and Soviet armour, with the significant advantage of the Soviet armour in ductility.

Overall, it is clear that the German system of quality control could not ensure production of rolled armour of necessary quality. A large range of hardnesses, chemical composition, and fracture properties within even one vehicle did not allow Germany to maintain satisfactory toughness during production.




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