T34 high hardness armor troubles...

[critical mass]

I´d like to direct attention to the excellent job of I.G. Dzheltov and A.J. Makarov summing up the development of soviet high hardness, I-8S armor (T34 armor production designation, original experimental designation was MZ-2) after primary source studied here:

https://t34inform.ru/publication/p02-1.html

Some general observations.

The armor for A-34 prototypes was specified to offer maximum resistence vs soviet domestic 45mm AP shells. Previous armor, such as 1-P derivates were homogenious, medium hardness, Silico-Mangenese steels and offered good protection vs bullets (Dk projectiles) but didn´t satisfy in resistence vs larger calibre projectiles (37mm to 45mm). Silico-Manganese steels was good in small section thicknesses but had large adverse scaling effects and poor shock resistence.

The alternative steel used for thicker section thicknesses by then was classical Krupp steel, alloyed with Nickel and Chromium and some molybdenium. All armor steel was specified as low carbon to allow for good weldability.

The research in armor was directed initially along these lines:

1. search for a chemical composition of steel, which, after appropriate heat treatment of the armor, would provide high anti-shell resistence and presence of tough properties (no splits, breaks and splits from the rear of the armor)
2. The composition of steel and the technology of smelting must ensure that it is alloyed with critical nickel and molybdenum from scrap metal only.
3. The requirement of high armor resistance poses the problem of developing armor of high hardness. The chemical composition of the steel should provide a viscous fibrous fracture, high toughness and plastic properties despite high hardness, good through-hardenability, satisfactory weldability with minimal heat affected tempering zones and have technological properties that make production of armored parts as easy as possible.

Experimentation of the new Kh-3 experimental steel begun already 1937, initially for 20mm and thinner plates, resmelted from 1-P and KhNM naval scrap metal with the following composition:

Carbon:0.3%-0.35%, Si: 1.1-1.3%, Mn: 1.1-1.4%, Cr:0.8-1.0%, Ni:1.0-1.2%, Mo: 0.2-0.3%

This Kh-3 steel was improved through 1939 by a reduction of carbon (to increase weldability), and a small increase of nickel, silicon and manganese. Because Nickel is a toughening agent and because carbon works in combination with manganese as an embrittling agent, these two measures greatly improved the toughness and ductility of the steel under high hardness conditions. This allowed the material to meet A-34 armor specifications for shell proof armor, for elements of 25 to 45 mm section thickness inclusive. kh-3 grade steel was made by the duplex process in the open-hearth basic furnaces using 1-P and 11-S scrap metal and obtained the following, final chemical composition:

Carbon:0.22%-0.27%, Si: 1.0-1.4%, Mn: 1.1-1.5%, Cr:0.75-1.1%, Ni:1.0-1.5%, Mo: traces (.25%)

Specifications called to resist 45mm APBC at 680m/s @0°. Entrance holes were not allowed to be larger than 1.5 cal diameter, exit holes should not exceed 3.0 times the calibre of the attacking shot (135mm for the 45mm & 229mm for 76.2mm). In addition to 45mm the material was to be tested for 37mm resistence and 76mm shock resistence, too. A fair hit was counted only if the edge of the impact hole was at least 3.0 cal distant from the next free edge.
Ballistic tests on the prototype hulls were slightly better than expected. The 45mm plate was safely resisted 45mm AP to ca. 690m/s at 0° impact. The designation was changed from Kh-3 to Mz-2 late in 1939. All projectiles attacking the plate broke up (similar projectile failure mode as if engaging face hardened armor).

Fairly extensive ballitic trials were conducted early in 1940 to systematically assess the limits of MZ-2. Dzheltov and Makarov give good summaries of these trials with some charts. Because of the lack of 76mm ordnance, three slabs of Mz-2 each in 40mm, 45mm and 50mm thickness were shipped to the Izhor factory for 76mm ballistic trial.
The resistence of Mz-2 to 37mm and 45mm at 0°, 15°, 30° and 45° was superior to any other material tested until this point. When fired at 45mm plates, the 45mm AP was always completely shattered (through base), when engaging 35mm plates, the 45mm APBC in 11 out of 12 cases received only nose shatter (cavity intact). At this point the authors fail to recognize that the high resistence to uncapped AP was a function of shatter effects (projectile deficit rather than inherent plate strength), a type of projectile failure, which as they correctly point out, was aided by the high hardness of the relatively thick sectioned Mz-2 (460+-40BHN).

The commission didn´t let Mz-2 pass with flying colours until the tests with overmatching shells (76.2mm) were concluded at Izhor plant. Additionally, Mz-2 plates of 30mm (68% ejected plugs when hit) and 35mm (40% ejected plugs when hit) thickness were deemed unsatisfactory due to a high tendency to spall badly when hit by 37mm and 45mm.

76mm trials were conducted 16th to 17th feb. 1940 at Izhor. The results of these trials were quite catastrophic. Out of six plates tested, four broke up into pieces when impacted by 76mm (catastrophic plate failure), and the plugs ejected from the back of the plate were larger than allowed. In detail, the following tests were made at 0° impact:

50mm plate #914427-5-5
1st impact: 76.2mm APBC striking at 596.8 m/s. Hole entrance: 150mm x 130mm, hole back: 215mm x 185mm [satisfying]
2nd impact: 76.2mm APBC striking at 548.2m/s. Plate split up. [failed due to catastrophic plate failure]

50mm plate #914427-5-3
1st impact: 76.2mm APBC striking at 549.8m/s. Plate broke up into two pieces. [failed due to catastrophic plate failure]
-no further shots attempted-

45mm plate #915641-11-2
1st impact: 76.2mm APBC striking at 506 m/s. plate resisted impact [satisfying]
2nd impact: 76.2mm APBC striking at 556m/s. Plate holed (170 x185mm & 250 x 175mm) [failed due to oversized back plug]
additional impacts at 30° obliquity:
1st impact: 76.2mm APBC striking at 549.8 m/s. plate resisted impact [satisfying]
2nd impact: 76.2mm APBC striking at 607.8 m/s. plate resisted impact [satisfying]
3rd impact: 76.2mm APBC striking at 662 m/s. plate holed (120 x80mm & 200 x 120mm) [satisfying]

45mm plate #915626-12-4
0° impacts:
1st impact: 76.2mm APBC striking at 506 m/s. plate holed (155x80mm &280x260mm) [failed due to oversized back plug]
2nd impact: 76.2mm APBC striking at 499m/s. plate resisted impact [satisfying]
additional impacts at 30° obliquity:
1st impact: 76.2mm APBC striking at 628m/s. Plate broke up into four pieces. [failed due to catastrophic plate failure]
-no further shots attempted-

40mm plate #913537-D
0°impacts:
1st impact: 76.2mm APBC striking at 451.3 m/s. plate holed (85x105mm & 140x150mm) [falied due to low ballistic resistence]
additional impacts at 30° obliquity:
1st impact: 76.2mm APBC striking at 599.8 m/s. plate holed (140x100mm & 180mm) [satisfying]
2nd impact: 76.2mm APBC striking at 549.0 m/s. Plate holed (170 x185mm & 250 x 175mm) [failed due to oversized back plug]
3rd impact: 76.2mm APBC striking at 499.8 m/s. plate holed (130 x80mm & 160mm) [satisfying]

40mm plate #914427-3-7
0°impacts:
1st impact: 76.2mm APBC striking at 497 m/s. plate holed (77x170mm & 220x270mm) [falied due to low ballistic resistence & oversized back plug]
additional impacts at 30° obliquity:
1st impact: 76.2mm APBC striking at 498.4 m/s. Plate broke up into three pieces. [failed due to catastrophic plate failure]
2nd impact: 76.2mm APBC striking at 493.4 m/s. Plate plate broke up into four pieces [failed due to catastrophic plate failure]
-no further shots attempted-

As a consequence of these trials, the commission did not agreed upon the use of high hardness Mz-2 grade steel for ballistic protection of T-34 tanks.
"completely unsatisfactory. When hit 76-mm armor-piercing projectiles, both at the normal and at an angle of 30 ° they broke up. Only two plates of the six tested did not broke up, and their penetrations had too large a size of breaches for the holes.."

What we see here is an example of huge scaling effects. Much like face hardened armor the Mz-2 plates suffered a significant degradation when attacked by overmatching projectiles, despite causing break up of these projectiles (to a less severe degree, for sure). Even ignoring the plate cracks, the critical velocity for 0° impact was 720m/s for 37mm AP, 690m/s for 45mm APBC but only around 500m/s for the 76.2mm APBC, a substantial drop of resistence above the level of scaling encountered normally.

Then NII48 was immediately tasked with finding a quick replacement for Mz-2 but time was running out. Initially, reconsideration was given to the medium to low hardness 11-S grade steel with rather high amounts of Chromium and Nickel present.
However, NII48 argued against it´s use for A-34 tanks due to less hardness and higher carbon content, allowing for worse resistence (NII48 does fail here to understand that the resistence of Mz-2 was a function related to the inferiority of the 45mm APBC test projectile and not a criterium of the strength of the armor) and worse weldability (the latter certainly beeing correct). Instead NII48 suggested 7-PSK grade steel, a low carbon "Krupp type" nickel-chromium steel, but heat treated to high hardness. Since Krupp steel was known since 50 years, experience was at hand to demonstrate it would work, and a timely solution was sought for, giving NII48´s suggestion credence and weight.

Meanwhile, the Mariupol factory conducted a crash program on it´s own to refine it´s Mz-2. It concentrated research on a different heat treatment to obtain the required level of toughness when attacked by overmatching projectiles.
In march 1940, the metallurgist achieved a measure of success by adopting thermal softening heat treatments for Mz-2. However, they realized that the requirements for 45mm protection and 76mm shock resistence were in conflict with each other. If the armor was softened too much in order to obtain good crack resistence vs 76mm shells, then the critical velocity of 45mm APBC projectile impacts was lowered, too.
Once again we see a problem here in the initimate level of understand how armor works with projectile damage. The research of Mariupol intending to soften up Mz-2 was heading into absolutely the right direction but they only looked at the chart values without investigating the correlation between 45mm projectile break up and the critical velocity. This failure will have consequences in the future not only against overmatching shells and shock effects (mentions of 88mm HE shell hits ripping of the T34 turret, f.e.) but also against more improved AP projectiles than soviet 45mm domestic APBC, APCBC-HE projectiles which could defeat high hardness armor intactly aided by a cap and high nose hardness levels.
Mariupol asked ABTU RKKA for a decision and the suggested improvements were ultimately rejected in favour of the high hardness but brittle, original Mz-2.
The requirement of good shock resistence vs overmatching projectiles was deleted from the Mz-2 specifications as it was deemed sufficient to have ballistic protection of the A-34 vehicle only for 45mm shells. It was noticed that Mz-2 steel was "almost in full compliance with specifications" and the original high hardness Mz-2 was finally approved for T-34 mass production.

No need to judge. What sounds like kind of a scandal was a rational decision in light of the danger of running into a T-34 production delay. Mz-2 armor didn´t met the official specifications, NII48 eventually changed the specifications to let the armor pass, designated now I-8S, and mass production started at Mariupol in april 1940.

Further problems with spall and too excessively large back plugs (4 times the calibre) were encountered with 40mm plates. The Mariupol plant once again approached ABTU RKKA with the suggestion to exchange the 40mm plates with 45mm plates, a proposal which was declined. Eventually, improvements of the heat treatment improved the 40mm plates to meet specifications by mid 1940. In the 2nd half of 1940, the designation was once more changed from I-8S to 8S, to show it´s advanet among the mass production steels.