"Homogeneous tank armour from new silicon-manganese-molybdenum Izhor factory steel (IZ type)"
"To the People's Commissar of Heavy Manufacturing, comrade S. Ordzhonikidze.
Explanation note on homogeneous tank armour made from new Izhor factory steel (manganese-silicon-molybdenum "IZ" type steel).
Until the middle of 1932, tank armour produced at the Izhor factory was made from chrome-nickel-molybdenum steel, containing 0.8-1.2% chrome, 4-5% nickel, and 0.4-0.6% molybdenum. This steel was very expensive, as it required a large amount of expensive imported components. The need for high quality, and, at the same time, cheaper and independently produced armour forced the factory to perform research into types of steel based on domestically produced compounds.
A step in the right direction was achieved with silicon-chrome "PI" type steel, containing no imported components other than chrome. "PI" steel was proposed to two factories, ours and Krasniy Putilovets. It was successfully adopted by our factory. However, the "PI" steel could not fully satisfy our needs, as the high chrome byproducts could not be used directly in production of armour, and, when used in other manufacturing processes, resulted in low quality products.
This forced the factory to explore new types of steel, which combined high toughness and maximum independence from imports with reasonable byproducts. Research work at the factory discovered such a steel at the end of 1932. It was a manganese-silicon-molybdenum steel with the following composition:
- Carbon: 0.29-0.4%
- Manganese: 1.1-1.8%
- Silicon: 1.3-1.8%
- Molybdenum: 0.3-0.5%
- Phosphorous and sulphur: 0.035%
IZ steel (IZ: Izhorskiy Zavod) was extensively researched in our factory. Research was performed in 1932-1933, first on steel cast in the electric furnace, later in a Martin furnace. This research revealed the high armour and mechanical qualities of the IZ steel, and it became necessary to test its combat and manufacturing properties on a wide scale.
In the end of 1933, IZ steel was used for T-35 tanks, in 11 hulls. The amount of defective plates was deemed acceptable for a new type of steel. The number of defective plates was less than with "PI" type steel of equal thickness. However, armour made into hulls showed cracking defects.
These defects are caused by additional stress that the plate experiences during welding. This is especially noticeable in complicated welding processes, or when high hardness plates are being welded. The requirement for high hardness plates (2.7-2.9 mm Brinell diameter) was from an old RKKA request, since, at the time, it was the only way to achieve required toughness.
The factory processed and used high hardness steel according to those requirements, which was one of the main reasons for cracks on IZ steel T-35 hulls. Further research showed that it was possible to use softer steel (up to 3.1 mm Brinell diameter) without loss of toughness. The reduced hardness increases the ductility of the metal, which reduces cracks on the hulls after welding.
All of this was confirmed when 5 hulls for the T-37 amphibious tank were assembled at the "KES" factory in Podolsk. Unlike the hulls produced at the factory with KO type steel, the IZ type steel hulls had no cracks at all.
In April-May of 1934, an experimental batch of 150 welded and riveted T-26 hulls and 42 welded T-37 hulls was produced. The factory has already mastered the new steel and production of armour from it. Despite the difficulty of using new steel, defective product at all stages of the process, as well as product found defective during shooting and crack inspections, was no higher than for PI steel in 1933, and much lower than for PI steel during its trial period. The production data from the most difficult period in the steel's use gives no doubt that tank armour should be produced from IZ steel. This was once more confirmed when the factory held a general trial of armour and armoured hulls produced from IZ steel in order to finally determine the service quality of armour made from it. The trials were held as follows:
A: Armour trials
- Bullet-proofness of 6, 7, 8, 9, 10, 13, and 15 mm armour from 29 batches, mostly forged in Martin furnaces.
- Shell-proofness of 15 mm thick armour, which is the armour mostly used in T-26 hulls.
- Bullet-proofness of armour in very low temperatures (-20, -30, -50 degrees,) and very high temperatures (+100 degrees).
B: Armoured hull trials
- Bullet-proofness of the T-37 hull.
- Shell-proofness of the riveted and welded T-26 hulls.
- Effects of low temperatures (-50 and -70 degrees) and sharp temperature changes (from -50 to -70 degrees, and from +90 to +130 degrees) on cracking and deforming of riveted and welded T-26 hulls.
- Bullet-proofness of a riveted T-26 hull subjected to alternating heating and cooling.
All of these trials were successfully passed by IZ steel armour. This finally confirmed the necessity of producing tank armour using this type of steel. Precise details on testing are recorded in documents with participation from representatives from the RKKA UMM, Armour Council of the NKTP, and the Central Institute of Metals (see the attachment "Materials on the general trials of IZ steel armour"). The main results are given below, along with illustrations.
The data from the research work (composed into a special report), and results from wide use of IZ steel in production, as well as these armour trials allow the following characteristics of main IZ steel qualities to be determined, compared to PI steel.
- Mechanical qualities: the toughness qualities (resistance to tearing and proportionality limit) for IZ and PI steels is approximately the same. Ductility properties (resistance to blows, compression, and bending) shows that IZ steel is more ductile.
- Bullet and shell resistance: IZ armour of lower hardness resists bullets just as well as PI steel, and resists shells better than PI steel, giving a more favourable penetration type in the latter case.
- Armoured hull properties: trials consisting of alternating freezing both types of T-26 hulls to low temperatures (-52 to -70 degrees) and heating them to high temperatures (+90 to +130 degrees), followed by heavy gunfire did not result in cracking defects or a reduction in toughness. This indicates that IZ steel is suitable for use in various climates, and that it is unaffected by sudden temperature changes.
- Technological and economical advantages of IZ steel: due to the impact of high chrome byproducts on the quality of steel, PI byproducts cannot be used for production of high quality products such as tank armour. The factory was either forced to sell these byproducts at a very low price or use them for lower quality products (pipes, frames for locomotives, etc). No doubt, these chromed byproducts also reduced quality of the latter.
With IZ steel instead of PI steel, the factory can solve its byproduct issue, since IZ byproducts may be used to produce high quality components. The factory will reduce its consumption of metals, and increase the quality of products produced at the factory. Additionally, the byproducts contain molybdenum, an element that does not burn out during casting, and the re-use of these byproducts will increase the factory's demand for ferromolybdenum. All this allows reduction of costs and increase in quality.
In conclusion, the manganese-silicon-molybdenum IZ steel is no worse than PI steel in its toughness, climate resistance, and mechanical qualities, and has a series of advantages over it. These advantages include the possibility of reusing all byproducts of the casting process and returning molybdenum into new casts.
Aside from this, the new steel guarantees mobilizational readiness of all factories to a much higher degree due to the production process and the materials used, as well as guarantees equal toughness at lower hardnesses.
Taking all of the above into account, the Izhor factory insists that the issue of IZ steel in the production of tank armour be explored as soon as possible.
Attachments: photo album with 40 photographs.
June 10th, 1934"
The following pages contain technical details. The first is acceptable amounts of carbon and manganese in the steel. The graph is pretty hard to read, but someone was nice enough to write out the composition on the top in large letters:
- C: 0.29-0.4%
- Mo: 0.3-0.5%
- Mn: 11-18%
- P and S: 0.035%
- Si: 1.3-1.8%
- Cr: 0.3%
The next page is the numeric results of all those test above, which wouldn't really be interesting to anyone that's not a metallurgist, or anyone at all with nothing to compare them to. After that, photographs of trials: tearing resistance, compression resistance, impact resistance, bending resistance, microstructures under various conditions. These are probably also not interesting, but, as always, ask if you need them. The following section is more interesting to us: armour properties!
Bullet-proofness testing was done with 7.62x54r armour piercing AU model 1930 rounds and with regular lead rounds at various distances. Here are some photos.
26 AU armour piercing bullets at 26 meters against a 15 mm plate, no penetrations. Plate front is shown on the top, plate rear on the bottom.
20 AU bullets against a 13 mm plate from 200 meters, no penetrations.
36 AU bullets from 450 meters against one side of a 10 mm plate and 17 regular bullets from the other side at 50 meters. No penetrations.
9.3 mm plate, 20 AU bullets from 570 meters and 14 regular from 14 meters. No penetrations.
I think you get the idea, so I'll stop posting pictures and only post more results. An 8 mm plate cannot be penetrated from 600 meters by an AU bullet or from 50 meters by a regular bullet. One bullet causes an indeterminate penetration (penetration due to the same point in armour being struck multiple times). A 7.5 mm plate also resists regular bullets from 50 meters. A 6.4 mm plate can resist the bullet from 150 meters.
Now we get to the temperature controlled trials. The next photograph is of the armour plate in a freezer (it still looks the same).
The frozen armour plate after being shot at and thawing. No penetrations from either bullet type at 50 meters, two inconclusive penetrations. Another plate with a different type of weld goes through the same process, with no penetrations (not even inconclusive ones). Another type gets one inconclusive penetration.
Next, we get to the cooling and trials of entire hulls, with slightly more interesting pictures.
Here is a T-26 hull in a freezer. The label helpfully points out that it is covered in snow. The record at the bottom shows the temperatures that the hull withstood: -10 when inserted into the freezer, frozen to -70 over 4 hours, and then warmed up to -60 degrees over 8 hours. The details for heating are on the next page. The hull was heated up to 90 degrees over 4 hours, and then cooled to 35 over 8 hours.
Same thing for the riveted hull. The pictures shows frost on the side. The hull was at a balmy -8 when placed into the freezer, cooled to -57 over 4 hours, and warmed to -52 over 8 hours. It was then heated to 120 degrees over 3 hours, and cooled to 65 over 9 hours.
Both hulls were then sprayed with armour piercing bullets from a DT machinegun at 50 meters. The welded hull developed one 35 mm long shallow crack. The riveted hull developed no cracks.
Next is the T-37 hull. The hull was shot at with a Maxim machinegun, using 230 regular bullets at 50 meters and 114 armour piercing bullets at 570 meters. No penetrations were found.
Photographs of the T-37 hull before trials and various angles after trials.
Seems that this new miracle steel of theirs was pretty good. IZ steel in thicknesses between 4 and 20 mm was used until at least the start of the Great Patriotic War.