100mm in the German rear, or how Krupp envied Soviet armor
Krupp AG investigation
One of the unpleasant discoveries of the Germans in Soviet Russia was the heavy Tanks KV. First and foremost, their impressive protection. It got to the point that Nazi metallurgists were forced to examine captured equipment for any successful solutions they could borrow. Modern archives contain numerous testimonies of such events. For example, in the fall of 1943, after the disastrous Battle of Kursk for the Reich, a 100mm piece of armor was brought from the front to a proving ground in Hillersleben, Germany. It's worth noting that the proving ground belonged to the Wehrmacht, so it's highly likely that the armor came from a tank.
Apparently, the Germans captured a relatively rare KV-85, as no other domestic tank could boast such protection. The KV-1 had no more than 90 mm of armor in the gun mantlet, and the IS-1 only began production in the fall of 1943, when Krupp's engineers were already studying the captured artifact.
It's surprising how quickly this tank armor sample fell into German hands. The first production KV-85s were deployed to troops on the southern front in September 1943 and proved quite effective against German 88mm shells. This is precisely what piqued the interest of enemy metallurgists. Judging by the testing program, at least one KV-85 fell into German hands as early as October, four to five weeks after the vehicle's appearance at the front. Engineers from Kpupp AG recovered a sample of captured armor from Hillersleben for study in late October 1943.
The KV-85 was a rarity at the front and was a transitional model to the IS series.
The first step was to study the chemical composition of the steel. And here, the enemy felt burning envy: the nickel and chromium concentrations were impressive—2,28% and 2,41%, respectively. By that time, the Germans were already experiencing a shortage of alloying elements and could not afford such wastefulness. These are crucial components of armor. Nickel is responsible for impact toughness, while chromium is responsible for hardness and hardenability. Soviet metallurgists managed to maintain a low phosphorus concentration (0,024%), which prevented the armor from becoming excessively brittle. The results of the German research clearly point to the 71L armor grade used for the KV-85 tank's turret.
A captured sample of 100mm armor undergoing testing in Germany.
The next stage of the study involved mechanical testing. Standard samples were cut from the plate, some along the rolling direction of the metal, some across (the properties of rolled steel typically differ slightly in these two directions due to the microstructure elongated during the rolling process), and subjected to tension testing on a tensile testing machine. This test allows us to determine two key strength parameters: yield strength—the stress at which the metal begins to plastically, or inelastically, irreversibly deform—and tensile strength—the maximum stress the sample can withstand before ultimately failing.
The ultimate strength was 149 kg/mm²—an enormous value for armor steel of the time. At the same time, the steel retained a relatively high ductility—elongation of 11–12%, contraction of 36–44%—meaning the metal wasn't extremely brittle, despite its extremely high strength. This is an ideal, difficult-to-achieve combination of characteristics for armor: the stronger the steel, the more brittle it tends to be (prone to cracking and spalling when hit by a shell), and vice versa. Finding a balance between strength and toughness is a perennial challenge in armor metallurgy.
Soviet martensite
A month after the initial armor studies, the Germans became concerned about production technology. Metallurgists suspected that the USSR had developed advanced alloy steel processing technologies. One of Krupp AG's internal correspondence documents contains the following:
Es liegt uns daran, die Vergütungsart der Panzerplatte festzustellen (Normal- oder Warmbadhärtung?) Wir bitten Sie, entsprechende Gefügeuntersuchungen vorzunehmen. (It is important for us to establish the method of heat treatment of the armor plate (normal hardening or hardening in a warm bath?). We ask you to carry out an appropriate study of the microstructure.)
We are talking about fundamentally different approaches to heat treatment of alloy steel:
Normal hardening — the classic method: the steel is heated to austenitizing temperature, then rapidly cooled (usually in water or oil), producing a hard but brittle martensite structure. It is then tempered at a moderate temperature to relieve internal stresses and increase toughness at the expense of a slight loss of hardness. This is a reliable, proven technology, but not the most perfect—martensite, even tempered, always remains relatively brittle by nature.
Quenching in a warm bath, or isothermal (bainitic) quenching — a much more complex and progressive technology, known in English-speaking metallurgy as austempering. Instead of rapidly cooling to room temperature, the steel is held for some time in a molten salt or metal bath at an intermediate temperature (usually 250–400°C), which allows the austenite to transform not into martensite, but into bainite—a structure that, while having comparable hardness, has significantly higher impact toughness and is less susceptible to cracking. In German documents, this is literally called "tempering through an intermediate stage"—this is bainite, an intermediate structure between martensite and pearlite.
How did the Germans distinguish between martensite and bainite in armor? For this study, a small piece was cut from a plate sample, the resulting surface was carefully ground and polished to a mirror shine. The next step was etching—treatment with a weak solution of nitric acid in alcohol—which unevenly dissolves the various structural components of the metal, thereby revealing grain boundaries and the internal crystalline architecture invisible on the unetched, perfectly smooth surface. After etching, the sample was placed under a metallographic microscope and examined at significant magnification—up to 500x. This allowed for the detection of the finest structural details hidden from the naked eye.
It is difficult for a layman to distinguish martensite from bainite (below)
German metallurgists, observing the unusually high performance of Soviet armor—high strength combined with acceptable ductility—suspected that Soviet engineers had employed precisely this advanced, elusive technology. The logic behind this suspicion is clear: achieving both very high strength (149 kg/mm²!) and acceptable toughness through conventional martensitic hardening is more difficult than through a bainitic structure, which specializes in precisely this combination of properties. If this suspicion were confirmed, it would mean that the Soviet tank industry possessed a technological advantage in heat-treating armor—meaning the Germans should urgently adopt this experience.
A German archive contains a document containing the results of a crystallographic study of tank armor from the USSR.
It's worth noting that the Germans spent a long time examining the Soviet armor—two whole months. Only on December 20, 1943, did the metallurgists' final report appear. There was no hot bath involved. Under magnification, the structure of the Soviet armor plate was found to contain the most ordinary, classic tempered martensite—meaning the metal had undergone the completely standard, world-famous process of simple quenching in water or oil followed by tempering.
Although the Germans were clearly somewhat disappointed (if only they knew the conditions under which Soviet armor had to be cast), the examination of the trophy gave them a new idea. What if they retained the same chemical composition, corresponding to cast 71L armor, and carried out a progressive heat treatment, in the form of a hot bath? Whether the Germans succeeded or not... story He's silent about it. Given the growing shortage of alloying elements in the Third Reich, such experiments could have been of purely scientific interest. There was no talk of any mass production.
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