soviet armor steel database

[critical mass]

Gentlemen,

I am researching armor grade materials and am in the process of compiling a database for historical specifications of soviet armor steel. Unfortunately, I still have large gaps for some materials properties or chemical analysis (compare attachment). Any idea for good sources (particularely primary) are greatly appreciated.

thanks in advance,
cm

[Stiltzkin]

For T34/85 castings (Korea, 1951 vs 1943 tank comparison from the CIA report) this might be useful, https://www.cia.gov/library/readingroom ... 0001-4.pdf

[critical mass]

Thanks for this. The cast armor had to be changed for the T34/85 (and likely IS cast elements) due to the imperfect nature of 8S grade steel in cast applications. These (by US standarts) inferior castings were noticed in T34/76 (f.e. bow casting) from the previous example studied in the 1943 Watertown report. It has also been mentioned in this regard as very detrimental by Postnikov 2004, 22, 24 and 26 (from where I know that the new material conceived particularely for cast elements was designated 71-L and first employed during 1944).
The CIA tests susbtantiate a significantly deviating chemical analysis (higher Cr and Ni, lower Mn), compared to MZ-2-steel grades. In combination, this change will for a slight reduction of work hardening be able to increase the initial toughness and reduce adverse viscous flow behaviour of the heat during the cast, which is, as I might infere, exactly what they wanted to do. It would be very nice to also know the specification ranges.
Like MZ-2, such steel would be very good in regard to charpy toughness and cold temperature resistence as long as it is treated to high BHN (bypassing temper britellness). It would be a rather poor choice for more common 250-320 BHN applications, due to the increased sensitivity of Cr-MN-Si complexes to temper britellness, which occurred during the 2nd heating as the material has to pass through the higher tempering temperature ranges.
I supoose (my speculation) that´s why they had to introduce yet another new steel mix (42SM for RHA, 43 PSM for roof/bottom ductile plating and 47-L for cast) in the T54 obr. 1953 during the early 1950´s when they also adopted AP-capped projectiles (e.g.BR-412D, against projectiles which cannot be broken up, HHA is worse than RHA due to premature plugging caused by adiabatic shear failure vs intact projectiles).

My mistake was to presume that they also reduced the excessively high BHN levels of the steel, which apparently only occurred much later than I previously assumed.

[Alejandro_]

These (by US standarts) inferior castings were noticed in T34/76 (f.e. bow casting) from the previous example studied in the 1943 Watertown report.

The bow casting was more of a structural element, with very little exposure to hits. it is basically a "plug" that unites the upper and lower front plates.

from where I know that the new material conceived particularely for cast elements was designated 71-L and first employed during 1944).

The new turret was also optimised against higher calibre weapons. It is also likely that the material situation had improved.

Soviet designers were aware of fragility of the early T-34 turrets. In early 1944, only 15-20% of cast towers T-34 had normal strength. Changes to the composition were made and by the end of the year 90 to 95% of the towers met the requirements.

My mistake was to presume that they also reduced the excessively high BHN levels of the steel, which apparently only occurred much later than I previously assumed.

I don't see the point of comparing the steel used in T-34 and T-54, as the requirements were rather different. T-34 armour was optimised against 37-47mm AT guns, while T-54 was against 88mm. The thicknesses were rather different, and this will lead to different characteristics (hardness).

[Stiltzkin]

as the requirements were rather different.

Indeed, as this is without a doubt contextual it is still important to point out that it has rather something to do with their industrial capabilities (or limitations for that matter) and heat treatment procedures, "high speed steel" was still desired. The Soviets salvaged a substantial amount of know how out of their new acquired European territories, post war. This is something certain reports actually mentioned, the improvement was noticed.

[critical mass]

from where I know that the new material conceived particularely for cast elements was designated 71-L and first employed during 1944).

The new turret was also optimised against higher calibre weapons. It is also likely that the material situation had improved.

Soviet designers were aware of fragility of the early T-34 turrets. In early 1944, only 15-20% of cast towers T-34 had normal strength. Changes to the composition were made and by the end of the year 90 to 95% of the towers met the requirements.

Late war, 71-L had one big advantage over MZ-2, the latter was designed for RHA armor, not for cast armor and had more gaseous bubbles in the heat right after pouring the metal in. Such gas bubbles were to be cross rolled out in RHA but would remain unchanged areas of weakness in the metal matrix of cast armor. 71-L was designed for cast processes and led to a less gaseous heat, which avoided these flawed areas of lowered resistence.

Still, the problem is HHA. 71-L cast high hardness armor was distinctly inferior in the ability to resist penetration of capped AP at high obliquity as evidenced by IS3 prototype cast turret trials vs 88mm Pzg39/43. 90-100mm cast HHA (71-L) could be defeated at 59.0° to 62.3° obliquity with velocities of 900m/s (=1050m) and 965m/s (=400m), respectively. To put it into prospect: 80mm german RHA would be entirely safe (reliably no holing) at 60° and 900m/s (1000m). This is a huge comparative reduction of ballistic protection. The high hardness 71-L caused the plate to fail entirely by shear processes (very little plate dishing in), driving out a plug at once, and causing projectile break up -which even inhibited projectile ricochet attempts. Of course, the armor wasn´t specified to german Pzgr 39 types but to domestic AP resistence and it made a good job in regard to the latter.
71-L indeed was very good vs less perfect penetrators (old german Pzgr Gg, US APC / AP or soviet domestic AP-HE ammunition) by facilitating a more complete break up.

My mistake was to presume that they also reduced the excessively high BHN levels of the steel, which apparently only occurred much later than I previously assumed.

I don't see the point of comparing the steel used in T-34 and T-54, as the requirements were rather different. T-34 armour was optimised against 37-47mm AT guns, while T-54 was against 88mm. The thicknesses were rather different, and this will lead to different characteristics (hardness).

The comparison is misleading bacause it has nothing to do with the calibre but with the type of attacking projectile:
The T34´s MZ-2 was designed to resist 5cm which it did well (Pzgr Gg) but failed miserably against 5cm Pzgr 39 APC (lower ballistic limit than 76mm soviet AP-HE). Similarely, IS2 armor is quite instructive here as it was designed to resist 88mm projectiles and roughly similar thicknesses than later T54 armor, yet it featured high hardness armor, which against the thread it was up to against (capped, high quality APCBC-HE), was distinctly inferior against what possibly could have been achived with the later, softer armor.

When theT54 armor was researched, the problems I outlined above for 71-L HHA and other HHA types were eventually realized by the soviets post ww2 when they introduced their own versions of "german", capped APCBC-HE projectiles (f.e. BR-412D, BR-471D,...).

As the germans 15 years earlier, the soviets finally came to the same conclusions: When attacked by a superior projectile, a moderately soft plate would be able to enhance resistence by "giving in" against these indestructible projectiles, and at obliquity keep the turning moment on the intact projectile which would contribute in easing ricochet off past a certain point. Too soft armor would reduce ballistic protection, too and was undesirable, except for in application where increased shock resistence mattered, such as roof and bottom plates (against Mine and HE grenades). What both, the germans in 1937/8 and the soviets in 1951/3 were looking for was the calibre dependent, sweet spot of optimal resistence vs direct ballistic attack with the plate hardness varying up and down slightly with the envisioned threat and section thickness.
It´s no coincidence that early 1950´s 42-SM (T54) soviet RHA armor plate is exhibiting exactly the same physical properties in regard to tensile strength, hardness, ductility and varying hardness with section thickness as can be observed in german ww2 tank armor. Yet, we do not know if the chemical composition matches it, too.

[Alejandro_]

The comparison is misleading bacause it has nothing to do with the calibre but with the type of attacking projectile:

But that is how armour requirements were specified. As you later say "What both, the germans in 1937/8 and the soviets in 1951/3 were looking for was the calibre dependent"

Similarely, IS2 armor is quite instructive here as it was designed to resist 88mm projectiles and roughly similar thicknesses than later T54 armor, yet it featured high hardness armor, which against the thread it was up to against (capped, high quality APCBC-HE), was distinctly inferior against what possibly could have been achived with the later, softer armor.

IS-2 armour was modified via a specific program in 1944. Frontal armour of the chassis (90-110mm@60°) was considered adequate against 88L71 rounds. The problem was that the turret armour could not be improved due to weight and balance.

When theT54 armor was researched, the problems I outlined above for 71-L HHA and other HHA types were eventually realized by the soviets post ww2 when they introduced their own versions of "german", capped APCBC-HE projectiles (f.e. BR-412D, BR-471D,...).

The later changes in steel composition were dictated by economic reasons, and not that they realise that 15 years earlier the Germans were using a specific type of steel. Also, the armour thickness of tanks in 1937/38 had nothing to do with those used in the late 40s early 50s.

[Stiltzkin]

years earlier the Germans were using a specific type of steel

The USSR was relying on espionage and license builds (which it was looking for extensively during the phases from the 20s-30s, primarily in the USA, France and Britain) and after the end of WW2 obtained a vast amount of know how from the newly occupied areas. It was copying German designs as many other patents, this is not a coincidence. Soviet literature or more over the Soviet propaganda apparatus did everything to conceal it, often proclaiming how they were the pioneers of the technology in the field.
Dictatorships cannot show any weaknesses and have to brainwash the population into believing that it is rather their enemies who acquire that particular technology, plain embarassing. The Soviet Union was no innovator and was not introducing nor even selling technology on the global market (or the more confined market of Europe and the US at that time), it was selling resources. Army and resources, that sums it up.

[Alejandro_]

The USSR was relying on espionage and license builds (which it was looking for extensively during the phases from the 20s-30s, primarily in the USA, France and Britain) and after the end of WW2 obtained a vast amount of know how from the newly occupied areas. It was copying German designs as many other patents, this is not a coincidence. Soviet literature or more over the Soviet propaganda apparatus did everything to conceal it, often proclaiming how they were the pioneers of the technology in the field.

Do you have specific examples of "vast amount of know how" being used in T-54, which first appeared in January 1945 and was an evolution of T-44?

[Stiltzkin]

Do you have specific examples of "vast amount of know how" being used in T-54, which first appeared in January 1945 and was an evolution of T-44?

The steel plates, optics and even suspension, synchromesh not even installed before the IS series, they even switched to 3 men turrets and introduced cupolas, in delay with adequate radios, the whole cruiser tank tech evolved from British and US designs, the tank characteristics were taken from french prewar designs (this information is also available in the Journal of Slavic military studies) the whole hispano suiza converted aircraft engines or W-2s to W-55s which were used in the entire T series since the 30s.... you name it it goes on and on. The Soviets even received Pershings in 1945.
Aircraft is the best indicator for the high tech sectors during WW2 (compare Soviet aircraft before the impact and participation of the WAllies and afterwards), you need to analyze subtleties, the interiors (there is a Swedish tank trial evaluation which states that T-72s are practically blind at night due to inferior systems), electrics, comm systems, manufacturing methods or radios. The TsH -16 alone was based on american and german captured equipment, its literally mentioned in their own sources. Yet alone things like the Toshiba-Kongsberg scandal, all tech will have military application, copy it, make a cheap domestically downsampled version, field it https://en.wikipedia.org/wiki/Toshiba-Kongsberg_scandal
The reality is that Western Europe was the most developed part of the world, able to produce high tech on a level the soviets were incapable of doing so, steel plate manufacturing quality, quenching, heat treatment and or machine tools necessary to achieve a better result, skilled labour force to achieve it. They had the advantage of a vast population which could bleed, without impacting the economy because it was not particularly specialized other than to produce basic military equipment. However, failures in the civil economic sector translate into the military sector (one of the most significant reasons to explain their inredibly high losses and one of the reasons why the Union ultimately collapsed). There are simple indicators which enable the assessment of development levels, you will hardly argue that vietnamese troops were comparatively equipped to american troops, such a statement would be plain silly.
Originally I thought that it might be just humor but by now it is just starting to become a nuisance. So spare us the Soviet revisionism of the 21st century in which they are as developed as Germany or the US. The misinforming tankarchive attempts or wikipedia "soviet technological superiority" myth entries are just embarassing products of inferiority complexes and blind nationalism.
E.g. Loss rates and chemical compositions are an indicator for quality. The Soviets did not even possess effective steel stamping procedures before the 50s, when they introduced German steel stamp methods, with East German Scientists and Industrial personnel involved. The AKM was the first cheap mass wide spread Ak version distributed to all belligerents and flooded the centers of conflict in the underdeveloped world, primarily dictatorships.
The whole rocket programme, the R-1 was based on the A-4 design.
Innovation always symbolizes a novelty, setting new standards people are being affected by in everyday life, like the introduction of International Business machines personal computers for everyone and not making cheap smart phone copies in a Chinese factory.
There is academic work on this topic, there is a book by Samuelson who analyzed the Soviet prewar economy and the build up of the war machine. They had connections to the Nazi regime, the Germans could practise maneuvering in the steppes of the USSR in the shadows of the treaties of Versaille, while the Soviets also received German steel in return for resources. For more of this read about the international affairs and diplomatic relations prewar, if you are interested.

[Alejandro_]

The steel plates, optics and even suspension, synchromesh not even installed before the IS series, they even switched to 3 men turrets and introduced cupolas,

What are you on about? USSR had tanks with 3 men turrets even before Barbarossa (KV-1 and T-28/35). Soviet designers were well aware of the advantages but tooling to manufacture the required rings was limited. There was only one factory capable of producing them and it was dedicated to KV-1 heavy tanks.

The same goes with the T-34 suspension. Factories were well versed in the production of the Christie type, and tooling from the rail industry could be used (UVZ).

It is not about the best tank, but the best tank that can be produced in numbers.

the whole hispano suiza converted aircraft engines

You mean like Germans developing BMW radial engines based on the Pratt & Whitney Hornet?

Originally I thought that it might be just humor but by now it is just starting to become a nuisance. So spare us the Soviet revisionism of the 21st century in which they are as developed as Germany or the US.

Perhaps you could read a bit more and spare us all the inaccuracies above? Anyway, who are you to decide what is nuisance or not?

[critical mass]

The comparison is misleading bacause it has nothing to do with the calibre but with the type of attacking projectile:

But that is how armour requirements were specified. As you later say "What both, the germans in 1937/8 and the soviets in 1951/3 were looking for was the calibre dependent"

Ok, I see where the problem comes from. My quote is taken out of context. The soviets in 1953 allowed for a dependency of the ductile-brittle first transition point in RHA hardness to be shifted in relation to armor plate section thickness, not to calibre -my mistake for allowing this subtle difference went unnoticed. That´s why the thinner side armor of T62/T54 is moderately harder than the thicker glacis plates. Exactly like german ww2 tank armor specifications. The reason is that thinner plates could be heat treated slightly harder without increasing their perceptibility to adiabatic shear failure. No such "moving specification" existed during ww2 for soviet domestic armor. If it was specified to HHA, it was hard, irregardless of section thickness.
Soviet armor during ww2 (up to and including 1945) was specified to resist an inferior AP quality (domestic AP and APBC). These inferior shells could be easily and more readily broken up by HHA, resulting in higher apparent levels of protection.
To take the T34 example further, soviet MZ-2 armor was specified to resist domestic 45mm APBC almost at point blanc range even at perfect 90° angle and resist 76mm domestic AP in cracking / penetration at 500m/s under similar conditions (this specification was subsequently deleted from the requirements, when MZ-2 failed to pass. Certainly this can be critisized in a seperate memo). This was a challanging requirement by this time. The soviet 45mm and 37mm firing trials vs Mz2/8S are available in from of charts:
https://t34inform.ru/photo/Ar_01_13.jpg
https://t34inform.ru/photo/Ar_01_11.jpg
https://t34inform.ru/photo/Ar_01_12.jpg
76mm trials in form of a report:
https://t34inform.ru/doc/1940-02-19_MZ-2_76.html
HHA appeared to be good / excellent in these trials because perforation with a completely broken up projectile requires very much elevated excesss velocities in order to drive the fragments all the way through the plate (all domestic AP shattered / broke up on impact).
Mark my words: HHA relied on the inferiority of the attacking projectile for it´s resistence.
The inferiority of the HHA MZ-2 is already hinted by the failure vs 76,2mm domestic AP shells, suffered despite the break up of the attacking projectiles due to the large adverse scaling effects, typical for high hardness and face hardned armor materials.
The ability to break up inferior projectiles, much like face hardened naval armor did to uncapped naval AP at the begin of the 20th century- was also a common experience for early ww2 vs german shells. The pre-ww2 design of the standard 5cm uncapped PzgrGg shattered vs HHA 8S-steel just like soviets APBC did. In exactly this context, one has to understand Guderian´s comments on the perceived superiority of soviet armor steel, which he made before the advent of the Pzgr39 series.

Unfortunately, not everybody continued to use inferior AP-shells and against projectiles which could not be broken up, HHA was offering substantially lower levels of ballistic resistence. Again, this has been demonstrated in numerous armor trials in Germany, Britain and the US. Of course, I have the official soviet´s NII48 armor research institutes 8S penetration and protection charts for captured 5cm Pzgr39 APC (which I tried to post at tankarchives but Your friend Peter Samsonov went into the trouble to filter out my four attempts to post them from his comments). Needless to say, when the projectile could not be broken up, the 8S armor still failed by lower energy adiabatic shear failures (plugging/discing), which made sure that it´s level of resistence vs 5cm Pzgr39 was inferior to plain mild construction steel (do You agree that 5cm isn´t different from the original specification´s 4.5cm?). In numbers: Using the 0° data, The 4.5cm APBC was resisted at approx. 750m/s with 45mm, while 8S failed vs 5cm Pzgr39 at velocities down to approx. 360m/s -which by the standarts of the time was a shockingly poor performance.
However, nothing was mentioned except that the 5cm Pzgr39 was a good design (actually, it wasn´t. According to my data, it was the worst performing of all Pzgr39 series AP bullets).

After that, the soviets still relied on their own, inferior projectiles to base their armor specifications. In this light it might be judged borderline criminal that 1943/44 IS2 armor was still specified to resist domestic AP inspite of the knowledge since 1942 NII48 tests, that german improved AP (Pzgr39 series bullets) did absolutely not behave like domestic AP but it´s possible and even probable that they just didn´t understood the problem. Many people today have problems understanding the effect different failure modes have on penetration mechanisms.

After the end of ww2, the soviets changed their own specifications, not the least because at this time they also included a much higher AP-quality in the test specifications by adopting reverse engeneered german Pzgr39 AP designs and because they specified that armor should resist 88mm KWK with Pzgr39...

IS-2 armour was modified via a specific program in 1944. Frontal armour of the chassis (90-110mm@60°) was considered adequate against 88L71 rounds. The problem was that the turret armour could not be improved due to weight and balance.

Do You know how "adaequate" was quantified? 90-100mm could be defeated at 1000m when angled 60° as evidenced by IS3 turret trials using the same 71-L cast armor and HHA specification of IS2 tank armor. UZTM hulls using RHA might be better, but they were only 90mm and also HHA.

The later changes in steel composition were dictated by economic reasons, and not that they realise that 15 years earlier the Germans were using a specific type of steel. Also, the armour thickness of tanks in 1937/38 had nothing to do with those used in the late 40s early 50s.

It´s an extraordinary claim You make and counterintuitive to everything I have read so far. Extraordinary claims require extraordinary proof. Do You have a primary source to confirm that 42SM, 43PSM and alike new armor steels developed in the late 1940´s early 1950´s were "dictated by economic reasons"? I ask specifically because it was (and still is) MUCH MORE DIFFICULT -and energy consuming- to produce RHA armor where the hardness exactly at meets with the -ballistically optimal (vs intact projectiles) 1st ductile-brittle transition point.
Are You -by inference- claiming that HHA as of ww2 vintage, was the better material but they knowingly choose an inferior hardness for their new post ww2 tanks because it was cheaper?
Maybe thats why they put the capped+ ballistic capped, reverse engeneered Pzgr39 designs into production, too? For the sake of economic reasons?

1943/1944 is relevant. The armor thickness of IS is comparable to T54. The ballistic protection specification is similar, too (resistence vs 75mm and 88mm cal). Yet, IS2 was HHA, while T54 armor was ordinary RHA similar to german WW2 tank armor in hardness specification. I may add, that the later Yugo tests confirm the ballistic soundness of the new 42SM grade steels. It could not be perforated by the 88mm KWK43 & Mod39 AP even at point blanc range. HHA 71-L was penetrable at in between 600 and 1000m from soviet tests.
Throughout ww2, the soviets followed their genuinely own, and distinct ideas about projectiles and armor in regard to hardness, and physical properties. The fact that the soviets, after end of ww2 studied, then reverse engeneered and put into service the supposedly "inferior" german projectile designs/hardness patterns and even german RHA armor hardness specifications to replace their distinct ww2 ideas of projectile and hardness belies the idea that they considered domestic ordnance items superior.
Once they were given enough time and material to study, they arrived at the same conclusions as the germans beforehand did already 1939, and as could be expected because physics are hard facts. It´s actually not only the soviets. The british in regard to armor also approached very close to the 1st ductile /brittle transition point by 1950, the US studied intensively german Pzgr39 and based their new AP-specifications on german AP hardness patterns as a result of these investigations.
Unwillingness to admit inferiority here by some revisionist today doesn´t concern me. It´s a problem historians have to deal with, when they ask the question How many lifes could have been safed had the soviets introduced a halfway decent projectile quality alone?

[Alejandro_]

Hello Critical Mass

I am struggling to read and understand your post because it has been edited several times. I went to look at my references and when I got back some stuff had changed. It is hard to follow the discussion in this manner.

Soviet armor during ww2 (up to and including 1945) was specified to resist an inferior AP quality (domestic AP and APBC). These inferior shells could be easily and more readily broken up by HHA, resulting in higher apparent levels of protection.

I am still puzzled by this. Let's take T-44, a WW2 design.

- Germans deploy Panzer IV with long 75mm guns, together with Panthers and Tigers, equipped with 75/88mm guns. Soviets upgrade T-34 and also decide to develop a new tank, as T-34 potential is exhausted. T-43 is dropped because is a dead end.

- T-44 is developed. Prototype is presented and tested. German PaK 40 gun, and 75/88mm guns. First can penetrate at less than 300 meters, while the latter at 700 meters. Sides are vulnerable from 600-900 meters.

Consequence: Armour is further reinforced, reaching 90mm at the front hull, 120mm in the front turret, 90mm at the turret sides, and 75mm at the hull sides.

Some other designs:

IS-2 obr 1944: IS-2 obr 1943 suffers serious losses in the battles of late 1943/early 1944. TsNII-48 looks into the problem and issues recommendations. With existing shape it is necessary to increase thickness to 145-150mm to survive hits from German 75/88mm guns. Recommendations on tempering regime and glacis shape issued.

Object 701 (IS-4). Designed during WW2 but not fielded. Designed for final phase of the war and post war. In August 1944 one prototype is tested againt Soviet 57, 76, 122 and 152mm guns. Also from captured PaK 43 (88mm) and KwK 42 (75mm).

Report on testing noted excellent resistance to 75-88mm with 1000 m/s velocity at an angle of 60 degrees in the hull and 30 degrees in the tower

Do You know how "adaequate" was quantified? 90-100mm could be defeated at 1000m when angled 60° as evidenced by IS3 turret trials using the same 71-L cast armor and HHA specification of IS2 tank armor. UZTM hulls using RHA might be better, but they were only 90mm and also HHA.

What is your source for 90mm (and 100mm) steel in IS-2 being HHA? And also for 71L being used in the turret.

In early 1945 the front hull of a IS-2 obr 44 was subjected to fire from a Nashorn in a test, and no penetrations were achieved. Uralmash design was more vulnerable -up to 600 meters IIRC-. As I said before, the problem was in the turret.

1943/1944 is relevant. The armor thickness of IS is comparable to T54. The ballistic protection specification is similar, too (resistence vs 75mm and 88mm cal). Yet, IS2 was HHA, while T54 armor was ordinary RHA similar to german WW2 tank armor in hardness specification. I may add, that the later Yugo tests confirm the ballistic soundness of the new 42SM grade steels.

Do you realise that 42SM armour was already used in Object 701 (hull), while turret used 66L, a cast type with medium hardness? Design of this tank started in Summer 1943, way before T-54.

Also, T-44 used a medium hardness steel, both in turret and hull. Prototype had been designed in early 1944, and was used as basis for T-54.

Even before that, KV-1 used steel with medium hardness.

Yes, IS-2 used HHA but tempering was changed. There are some data on US evaluations but it is not known if it was a 1943 or 1944 series vehicle.

In my opinion, you are jumping from the T-34 armour type to T-54 without taking into account all the research and design work done in-between.

To take the T34 example further, soviet MZ-2 armor was specified to resist domestic 45mm APBC almost at point blanc range even at perfect 90° angle and resist 76mm domestic AP in cracking / penetration at 500m/s under similar conditions (this specification was subsequently deleted from requirements.

You have made this point in several discussions, but if you read the website you linked you will see that there were economic/logistics reasons -again- for the decision. It was suggested to use KV-1 type armour (medium hardness), but nickel composition was too high (2.5%).

Yes, it could be reduced to 1.5% but would take time which the USSR did not have. Same reason for not producing T-34M.

Also, I am not sure how your first statement on "soviet MZ-2 armor was specified to resist domestic 45mm APBC" ties up with another one you made.

The T34´s MZ-2 was designed to resist 5cm which it did well (Pzgr Gg) but failed miserably against 5cm Pzgr 39 APC

I don't reall any testing in 1940 with this caliber in 1940. do you have any information?

Alexander Morozov highlighted these problems. The following excerpt can be found in a book about tank production at Factory #183 during the war. One of the activity areas was:

"The use of substitutes for rare materials. High production levels for the unfolding war required a large number of diverse material, especially high-quality steels containing large amounts of alloying elements. The problem was made worst by the fact that the Soviet economy was experiencing an acute shortage of these materials. Therefore, the designers have investigated to find opportunities to use other types of steel containing less alloying elements."

That´s why the thinner side armor of T62/T54 is moderately harder than the thicker glacis plates.

This does not tie up with my data. T-54 used 42SM in front and sides. The rear and roof was made of 49S, the floor used 43SM. Do you have a source to compare?

It´s an extraordinary claim You make and counterintuitive to everything I have read so far. Extraordinary claims require extraordinary proof. Do You have a primary source to confirm that 42SM, 43PSM and alike new armor steels developed in the late 1940´s early 1950´s were "dictated by economic reasons"? I ask specifically because it was (and still is) MUCH MORE DIFFICULT -and energy consuming- to produce RHA armor where the hardness exactly at meets with the -ballistically optimal (vs intact projectiles) 1st ductile-brittle transition point.

42SM was not developed in the late 1940s, as I said, it was introduced in mid 1944.

I was referring to changes in T-54 only. Factories were ordered to reduce the cost of vehicles by 10-15%. In 1954 MBL-1 cast steel was introduced with reduced nickel content and an increased content of manganese and chromium. In any case, I should have added technological requirements. The new casting used in T-54/55 turrets led to a reduction in cost of 3500 roubles and a reduction in molten steel of 2.5 tons.

Soviet evaluations saw similar patterns for cost reduction in US tank building when examining steels from captured vehicles in Korea. There was a widespread use of casting elements and use of armour without expensive alloys. US RHA was determined to be cheaper and simpler in composition to chromium-nickel armour used by T-54, but rather promising.

Regarding sources:

- Tank industry in the USSR during the Great Patriotic War, by Ermolov (2009)
- Stalin Armoured Fist (1943-1955), by M. Svirin
- M.V. Pavlov and I.V. Pavlov have published several books on Soviet tank design between 1945 and 1965. In the magazine Technology and Armament, 2008 number 10 there is an article about T-54 development.

[critical mass]

Soviet armor during ww2 (up to and including 1945) was specified to resist an inferior AP quality (domestic AP and APBC). These inferior shells could be easily and more readily broken up by HHA, resulting in higher apparent levels of protection.

I am still puzzled by this. Let's take T-44, a WW2 design.

- Germans deploy Panzer IV with long 75mm guns, together with Panthers and Tigers, equipped with 75/88mm guns. Soviets upgrade T-34 and also decide to develop a new tank, as T-34 potential is exhausted. T-43 is dropped because is a dead end.

- T-44 is developed. Prototype is presented and tested. German PaK 40 gun, and 75/88mm guns. First can penetrate at less than 300 meters, while the latter at 700 meters. Sides are vulnerable from 600-900 meters.

Consequence: Armour is further reinforced, reaching 90mm at the front hull, 120mm in the front turret, 90mm at the turret sides, and 75mm at the hull sides.

You might notice that there is a difference between specification and application of requirements. Requirements could be modified, even specifications were at times, as demonstrated by the T34. However, the T44, which in itselfe was a post ww2 tank which happened to be conceived and partially tested during ww2, was specified to resist domestic AP, and at one point had the requirements to include also german AP resistence. Another example was the IS2. This is a reasonable approach. The obj.701 is pretty much irrlevant as it was overprotected from the begin -against both, domestic and german AP.

Do You know how "adaequate" was quantified? 90-100mm could be defeated at 1000m when angled 60° as evidenced by IS3 turret trials using the same 71-L cast armor and HHA specification of IS2 tank armor. UZTM hulls using RHA might be better, but they were only 90mm and also HHA.

What is your source for 90mm (and 100mm) steel in IS-2 being HHA? And also for 71L being used in the turret.

In early 1945 the front hull of a IS-2 obr 44 was subjected to fire from a Nashorn in a test, and no penetrations were achieved. Uralmash design was more vulnerable -up to 600 meters IIRC-. As I said before, the problem was in the turret.

Yes, IS-2 used HHA but tempering was changed. There are some data on US evaluations but it is not known if it was a 1943 or 1944 series vehicle.

You will find the 71L used for turret casting in Postnikov 2004, I have mentioned it already in this thread, You migth whish to look it up. HHA is confirmed by various post ww2 metallurgic analysis (glacis 444BHN, turret casting 448BHN, under relatively ductile state. My own reading with XRF from a museum exhib gives 435, 442 and 441BHN).
Let´s talk about "tempering regime". Changes in the tempering do affect the actual BHN level discretely, but not substantially and never so much as to change HHA levels to medium hardness BHN levels (>80-200BHN change). The BHN level is correlated with the hardening and is predetermined by the crystall structure fixed during the quench from the first heating. Tempering regime refers to a second cycle consisting of both, a heating with subsequent quenching (typically below AC1, also called "temper" for this reasons). Steels are tempered in order to break up and rearrange the desired crystalline border structure and allow for a slight softening up (ca. 10-30BHN) in exchange for a more thorough increase in fibrous toughness of impact strength (as measured, for example by the Charpy V-notch test in combination with temperature charts to show the temperature where the material turns from ductile to brittle).
A change in tempering may indicate either that IS and IS-2 armor was suffering from excessive temper brittleness, undesired & inferior brittleness caused during the tempering of some alloys when allowed to cool slowly, -or alternatively, it was too hard for the threat it had to stand up against (intact projectile impact) and suffered from lower than expected resistence caused by adiabatic shear failure. The latter explenation would only be possible in case the soviets did not understand temper brittleness and had incorrect presumptions as to remedy the problem at hand (notice, this is possible in and during ww2, as the understanding of the phenomenon was contested till much later than ww2). There are cross over effects between both, such as increased or reduced spall resistence. But that´s related to ballistic resistence in some cases, too, making it difficult to handle (You need to find a compromise between mutually exclusive effects).

Btw, have You ever seen details of the Nashorn report? Have You ever seen and read it?

Do you realise that 42SM armour was already used in Object 701 (hull), while turret used 66L, a cast type with medium hardness? Design of this tank started in Summer 1943, way before T-54.
Also, T-44 used a medium hardness steel, both in turret and hull. Prototype had been designed in early 1944, and was used as basis for T-54.
Even before that, KV-1 used steel with medium hardness.
In my opinion, you are jumping from the T-34 armour type to T-54 without taking into account all the research and design work done in-between.

I will take the point that research was significant between T34 and T54, and I would appreciate to see more facts, I have not covered enough sources for T44 and other middle range designs, to be sure.
That the soviets had -like everybody else- RHA -what You call "medium hardness" in KV1 and many other pre-ww2 designs is really nothing suprising. The first time such armor was made dated back to early 1894, when Krupp made his famous Ni-Cr Plate no. 420 and other nations then started to aquire licenses still in the 19th century (I have the original firing trials of this plate). By 1940, this invention is almost half a century old and nothing new. The new perspective wasn´t RHA, it was HHA and how it was handled in relation to RHA:
The soviets adopted their new HHA ca. 1940 to replace the common RHA (i.e. armor with BHN ranging between 210 and 350 for most relevant section thicknesses). HHA beat their own shells much better up then common RHA could. However, initially, it could only be made in small section thicknesses succesful with the chemical composition developed, leaving KV1 with it´s thicker sections with normal RHA instead. Notice that I regard KV1 armor as better ballistically than HHA as employed in IS/T34, when attacked by Pzgr39. However, the latter had more fully sloped armor which may give somehow better all around levels of protection, except in the flat side plates.
I could explain why HHA couldn´t be employed in thicker sections -by reference to the chemical composition of 8S but I doubt many people here are able (or interested) in a rather tediuos lecture of chemical metallurgy.
Somewhen in mid ww2 (1942-1943) they adopted "their new" HHA, (which they believed to be "superior" owing to the effect it had on their own projectiles) also for thick sections and specified the new IS and IS2 armor -as well as T34/85 cast turret armor- as HHA. These designs were issued to the troops in the latter part of ww2 (1944/45) with armor, whiches hardness specification was based on wrong ideas ("HHA is superior, as demonstrated by the resistence to domestic shells"), which left these vehicles at a distinct disadvantage on the battlefield when they had to face shells with different properties than domestic shell in terms of projectile break up.
Now You state that for some prototypes (obj701 / T44) at the end of ww2, they reconsidered and employed RHA (42SM) again to replace HHA. I have no data and sources on these prototypes but if true, it´s interesting and entirely plausible. It would support a rational approach in decision making. However, these RHA are not like what was used later in T54 (see below), even though we might expect that 42SM was identic in chemical composition. However, the basic principle behind is sound: RHA is ballistically superior to HHA, unless the latter succeeds in breaking up the projectile. Once the armor is tested and judged by its resistence to Pzgr39 and Br-412D, it becomes quite apparant that there is something foul with HHA.

That´s why the thinner side armor of T62/T54 is moderately harder than the thicker glacis plates.

This does not tie up with my data. T-54 used 42SM in front and sides. The rear and roof was made of 49S, the floor used 43SM. Do you have a source to compare?

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It´s 42SM. The same type of steel but treated to different hardness levels. This was possible because (A) the soviets aquired new analysis instruments directly after ww2 (it would be interesting to know from where exactly) and (B) because systematic firing trials were executed between 1948 to 1951 in order to determine the best hardning range for ballistic protection. They only counted perfect projectiles (without damage) in these trials to get rid of the mudding in data caused by projectile damage effects on plate resistence (very clever if You want to understand plate resistence). Following this research, it was specified that the 42SM glaics plates needed to be made softer than the thinner side plates for otimal ballistic resistence. Again, both are 42SM steel, just quenched from different temperature levels, creating approx. 340BHN (80mm plates) and approx. 270BHN (100mm plates). This is the first time, the soviets adopted what the germans also realized during their armor research in the mid 1930´s: that there is no single "optimum hardness" for armor plate but an "optimum range", depending on section thickness, obliquity and threat expectation.
Of course, You may notice, that this is also why 42SM from older tanks is only chemically similar to 42SM while the same steel varies significantly in it´s physical properties from tanks made after the T54-3, particularely in terms of hardness, ductility and how hardness changed with section thickness.

[critical mass]

It´s an extraordinary claim You make and counterintuitive to everything I have read so far. Extraordinary claims require extraordinary proof. Do You have a primary source to confirm that 42SM, 43PSM and alike new armor steels developed in the late 1940´s early 1950´s were "dictated by economic reasons"? I ask specifically because it was (and still is) MUCH MORE DIFFICULT -and energy consuming- to produce RHA armor where the hardness exactly at meets with the -ballistically optimal (vs intact projectiles) 1st ductile-brittle transition point.

42SM was not developed in the late 1940s, as I said, it was introduced in mid 1944.

I was referring to changes in T-54 only. Factories were ordered to reduce the cost of vehicles by 10-15%. In 1954 MBL-1 cast steel was introduced with reduced nickel content and an increased content of manganese and chromium. In any case, I should have added technological requirements. The new casting used in T-54/55 turrets led to a reduction in cost of 3500 roubles and a reduction in molten steel of 2.5 tons.

Soviet evaluations saw similar patterns for cost reduction in US tank building when examining steels from captured vehicles in Korea. There was a widespread use of casting elements and use of armour without expensive alloys. US RHA was determined to be cheaper and simpler in composition to chromium-nickel armour used by T-54, but rather promising.

Regarding sources:

- Tank industry in the USSR during the Great Patriotic War, by Ermolov (2009)
- Stalin Armoured Fist (1943-1955), by M. Svirin
- M.V. Pavlov and I.V. Pavlov have published several books on Soviet tank design between 1945 and 1965. In the magazine Technology and Armament, 2008 number 10 there is an article about T-54 development.

I have singled out this part because it deals with a different question. I am aware of what MBL-1 means, but we were not talking about MBL-1 (introduced in T62 and T55 turret casting). Actually, my impression is that Your response has little to do with the question at hand. I stated that they introduced new armor hardness specs (including a section thickness dependent hardness shift -similar to german mid 1930´s through ww2 armor specs) exactly when the soviets also introduced german reverse engeneered AP shells and german AP-hardness patterns for their old APBC. This correlation cannot be put in doubt, it´s obvious by the timing of events, both were adopted officially 1951 for T54.
Your response that the change of armor was dictated by economic reasons doesn´t deal with this period but with events some years later. It´s a straw argument.