Top Secret. 1942 report on research into new armor for the KV tank turret.
On the study of a substitute steel for the KV tank turret
The aim of the work was to test the possibility of reducing the content of the highly deficient alloying elements nickel and molybdenum in grade 6674 steel, which is currently used for cast towers. tank "KV".
Steel grade FD 6674 used in general production is characterized by the following established chemical composition:
For experimental work, the following steel grade FD 6648 was used:
From a comparison of both steel grades, it is clear that steel grade FD 6643 contains approximately 35% less nickel than steel grade FD 6674 - 1,7-2,0% instead of 2,75-3,25% and 30% less molybdenum - 0,20-0,3% instead of 0,3-0,4%.
To more quickly verify the correctness of the chosen direction, it was decided to fill one tower with molten iron from an electric furnace, process it according to the existing regime and subject it to shelling.
One tower was poured from an electric furnace. Tower No. 194, heat 61711. Wall thickness 110 mm.
At the same time as the steel grade was being tested on tower No. 194, a new forming technology was being tested.
According to the new technology, instead of six risers installed to feed the casting, four were installed (see Fig. No. 1), which made it possible to reduce the volume of fire-cutting work and metal consumption.
FORMING
The tower was formed in the soil using conventional filler and facing mixtures.
During molding, the model was installed at a slight angle. The aft section was inclined at approximately 5 degrees to the horizontal.
The inclined position of the poured part improved the feeding conditions of the rear part of the turret.
STEEL MELTING
Melt No. 61711 was carried out in the main electric furnace with a capacity of 15 tons.
The smelting process proceeded normally, without deviation from the accepted instructions.
The final deoxidation of steel was carried out with aluminum, which was introduced through a “chute” in the amount of 12 kg per 14 tons of metal.
The temperature of the steel in the furnace before tapping, as measured by Pirotto, is 15800C. Temperature at the trench is 15450From /po pirotto without correction/.
By measuring pirotto, the authors apparently mean contactless determination of the melt temperature using a pyrometer (editor's note).
The analysis of the molten steel is as follows:
Carbon - 0,25%; silicon - 0,26%; manganese - 0,50%; sulfur - 0,020%; phosphorus - 0,024%; chromium - 1,54%; nickel - 1,83%; molybdenum - 0,26%.
FILLING
Casting was done, as with all bulk production towers, using a dry mold. Metal was fed through a 90 mm diameter riser and 10 feeders, each 38 mm in diameter. The risers were topped up with hot metal.
In foundry production, a riser is the top, bottom, or side portion of a steel ingot (casting) whose dimensions extend beyond the required dimensions (editor's note).
The duration of pouring the tower (without topping up the risers), despite the high temperature of the metal, was 2 minutes 15 seconds, which significantly exceeds the average duration of pouring shaft towers (up to 1 minute 30 seconds).
The slow filling of the mold is explained by the small diameter of the cup and low ferrostatic pressure.
HEAT TREATMENT
The tower was processed according to the regime that was adopted at that time for shaft towers and included the operation of homogenization with isothermal annealing, hardening and high tempering.
High tempering is a type of heat treatment after hardening, carried out at temperatures of 500-700°C for 1-6 hours, depending on the dimensions of the product (editor's note).
Homogenization was carried out at a temperature of 1050-10600C for 30 hours and isothermal annealing at a temperature of 620-6500Within 15 hours.
Before being placed in homogenization, the tower was coated with a coating consisting of equal parts graphite and fire-resistant clay to protect it from oxidation and decarburization.
Quenching was carried out in water at a temperature of 8500C. The holding time during heating for hardening was 8,5 hours, i.e. 2,5 hours longer than specified in the instructions.
The water temperature in the quenching tank is 400C, holding in the tank for 15 minutes. The tower temperature after removal from the water is 65 – 700C.
High tempering was carried out at a temperature of 5600C for 11 hours. Cooling after tempering in water.
Fig. 2 shows a photograph of a fracture of a sample cut from the front part of the tower after heat treatment.
The fracture is not entirely satisfactory, as there is a large crystalline area in the center (Fig. 2).
The fracture in the remaining section is quite ductile with noticeable shrinkage. Hardness, measured across the sample cross-section at four points, and the Brinell indentation diameter was 3,6.
Due to the presence of a crystalline section in the middle of the sample, it was decided to subject the tower to re-quenching followed by tempering.
However, during the second reprocessing, the furnace time during tempering was significantly increased by 6 hours due to the inability to roll out the furnace carriage in a timely manner due to production problems. The hardness of the cross-section of the sample was 3,9 (indentation diameter), i.e., significantly below the technical specifications.
In connection with the above, it turned out to be necessary to carry out a third reprocessing according to the same regime as the first, but with a slightly lower tempering temperature.
After the third treatment, the fracture is fibrous with crystalline deposits /Fig. 3/. Brinell hardness, indentation diameter - 3,5-3,6.
TOWER TEST
Part No. 194, pl. No. 6171 was presented to the commission for testing according to the approved plan.
The turret was tested in the working position using a 76-mm regimental gun with armor-piercing shells (Fig. 2-06519).
A total of 9 shots were fired, 4 of which were fired at the frontal section; 3 at the left side; and 2 at the tail section.
The following results were obtained during the firing test:
1. The tested turret's armor resistance is not lower than the requirements stipulated by the temporary technical specifications for a 110 mm cast turret made of grade 6674 steel.
2. The armor resistance of the rear part of the turret, not supplied by the risers, is somewhat lower than the armor resistance of the front part of the turret, but is within the limits provided for by the temporary technical conditions.
3. The toughness of this experimental turret is not lower than that of previously tested cast turrets made of grade 6674 steel when tested with a 76 mm projectile, but at low temperatures the experimental turret cracked and broke from an 85 mm projectile at a speed of 800 m/sec over a distance of 330 m, which characterizes a sharp drop in toughness.
Turrets made from FD-6674 steel were not tested with an 85mm projectile.
Following field testing, the tower was crushed under a headframe to check for shrinkage porosity in the areas where risers were missing from the niche. The corresponding fractures are shown in Figs. 3, 4, and 5. As can be seen from their examination, no porosity was detected, but the fracture was somewhat dry.
In Fig. 6, an oxidized surface is visible in the fracture, indicating the presence of cracks prior to heat treatment. This crack originates from the riser trimming site and was caused by trimming the risers of an unheat-treated part (without prior tempering).
Three heat treatments, the presence of brittle damage, along with the changed wall thickness of the product (an order was received to switch to the production of rampart towers with reduced wall thickness), necessitated additional research, despite the satisfactory results of the shelling test.
METALLOGRAPHIC STUDY OF STEEL WITH REDUCED NICKEL CONTENT
Melt No. 61711 of the following chemical composition, in %, was subjected to laboratory testing:
The study was tested in laboratory conditions on samples cut from a 1200 x 1000 x 110 mm slab, poured together with the tower.
The slab was subjected to the following preliminary heat treatment.
Normalization 950 – 10000C, 15 hours exposure, air cooling.
High tempering at 660 – 6800C for 10 - 12 hours. Air cooling.
The study was conducted in the following sections:
1. Determination of critical points
2. to determine the influence of tempering temperature on mechanical properties
3. Determining the tendency of steel to temper brittleness
4. establishing the influence of low temperatures on impact toughness.
DETERMINATION OF CRITICAL POINTS
The determination of critical points was carried out using a Leitz device.
Critical points (or phase transformation temperatures) in metallurgy are temperatures at which structural changes occur in alloys (most often in steels): for example, the transformation of ferrite into austenite (Ac1, Ac3) when heated or the reverse processes when cooled (Ar1, Ar3). Their determination is important for optimizing heat treatment, alloying, and predicting metal properties. One of the classic methods is dilatometry, which measures the linear expansion (or shrinkage) of a sample with changing temperature. A Leitz dilatometer is used for this purpose—a device developed by the German company Ernst Leitz (Wetzlar, Germany) in the early 20th century. The Leitz dilatometer is a precision optomechanical device for recording expansion curves (dilatograms). It is based on comparing the thermal expansion of the test sample (a steel rod with a diameter of 3–5 mm and a length of 20–30 mm) with a reference sample (made of quartz or Invar, with minimal expansion). (Editor's note).
Heating temperature 950°C, heating rate 2°C/min, cooling rate 2,5°C/min.
The following results were obtained:
Ac1 - 740 ° C
Ac3 - 785 ° C
Ar1 - 705 ° C
Ar3 – 475°C.
EFFECT OF TEMPERATURE ON MECHANICAL PROPERTIES
Only impact specimens were made from heat plate #61711. Impact specimens were also made from FD 6674 steel, heat #21207, for comparative testing.
The manufactured samples were hardened from a temperature of 840 – 850°C.
Heating for hardening was carried out in a salt bath, holding for 50 minutes, cooling in oil.
Hardened specimens were tempered at temperatures of 150, 200, 250, 350, 700, 150, 550, 575, 500, 525, 650, and 700°C. The tempering period was 3 hours, followed by water cooling. All specimens were cut at a distance of 1/4 from the surface.
Table No. 1
Impact toughness values depending on tempering temperature /melt No. 61711/:
The data in the table show that, starting from a temperature of 250°C to 500°C, a sharp decrease in impact toughness is observed, reaching 1,2–1,5 kgm/cm2 at a tempering temperature of 350°C.
The decrease in this characteristic is explained by the fact that steel of this composition, like all chromium-nickel steels, has temper brittleness.
Table 2
Impact toughness values depending on tempering temperature (steel grade FD 6674):
The data in Table No. 2 show that the temper brittleness of grade 6674 steel is in the temperature range of 250°C – 500°C.
The presence of temper brittleness is especially noticeable in the fractures of impact samples (Figs. 8 and 9).
At tempering temperatures of 150–200°C, a fibrous fracture is observed; at a tempering temperature of 350°C, a crystalline fracture is observed. The most pronounced crystalline fracture is observed in samples made of FD 6674 steel.
Thus, at a tempering temperature of 450°C there are still large areas of crystalline rash, while at a tempering temperature of 550°C the fracture becomes fibrous again and with good shrinkage.
The decrease in impact toughness at a tempering temperature of 700°C is explained for both grades of steel by the fact that the samples were annealed in air.
MICROSTRUCTURE
There is no difference in the structure of steel grade FD 6674 and steel grade 6711.
At tempering temperatures of 150°C - 200°C, the structure consists of coarse acicular martensite. At tempering temperatures of 550°C, areas of troostite are observed in the microstructure, while the martensitic orientation is maintained.
At a tempering temperature of 550°C the structure is sorbitol, and at a higher tempering temperature coagulation of sorbitol is observed.
At a tempering temperature of 700°C, there are areas of martensite in the microstructure.
In order to test the effect of the cooling medium on impact toughness, the samples were subjected to the following heat treatment: Menage type samples made of 61711 steel and FD 6574 steel were quenched at a temperature of 840–850°C in water, then tempered at a temperature of 540°C, followed by cooling in water and in a furnace.
A Ménage specimen (Ménage specimen) is a standard prismatic notched bar used to test metals (including cast alloys) for impact toughness (impact bending) using the Ménage method. In foundries, impact toughness is one of the key mechanical quality indicators for castings (especially iron, steel, and non-ferrous alloys). Ménage specimens are cast separately or cut from the casting body/riser/special process sample poured simultaneously with the batch. This allows for the evaluation of how the melting mode, inoculation, cooling rate, and other casting factors impact the toughness and susceptibility of the metal to brittle fracture.
The test is carried out on a pendulum impact tester: the sample is placed on supports, the pendulum strikes opposite the notch, the expended work is measured (in J) and the impact toughness is calculated (editor's note).
The obtained data are summarized in Table No. 3.
According to the data in Table No. 3, a decrease in impact toughness is observed with slow cooling both in steel with a reduced nickel content and in steel grade FD 6674.
The fracture pattern of the samples confirms the obtained impact strength results. A decrease in impact strength during slow cooling is accompanied by a crystalline precipitate, while cooling in water produces a fibrous fracture.
Change in impact strength at low temperatures
In order to check the impact toughness values at low temperatures, Menager-type samples after heat treatment (set at a temperature of 850°C in oil, and tempering at a temperature of 550°C - cooling in water) were tested at low temperatures: zero degrees, minus 20°C, 40°C and 60°C.
Figure 10 shows the vessel used for sample cooling. Alcohol, which has a relatively low freezing point (-70°C), is poured into the inner compartment of the vessel, and the samples are then placed as shown in the figure. The inner compartment of the vessel is closed with an arc-shaped lid, through which an alcohol thermometer is inserted. Using a device passing through the center of the lid, the alcohol is stirred throughout the cooling process to ensure uniform and more intensive cooling of the samples.
Liquid oxygen is added in small portions to a ring-shaped vessel surrounding the inner compartment. Once the required temperature is reached, the required number of samples are placed in the inner compartment. The holding time is 25 minutes.
Temperature regulation was carried out by adding small portions of oxygen into the ring vessel or by adding a warm alcohol solution into the middle compartment.
After the holding time was completed, the samples were quickly removed and placed on metal supports cooled to this temperature.
The test results are summarized in Table No. 4.
The data in Table 4 show that the impact resistance of the steel grades studied remains virtually unchanged with decreasing temperature. The lower values for grade 6674 can be explained solely by the melting process; perhaps the steel in this melt was overheated and had a larger primary crystallization, which was not corrected by subsequent heat treatment.
Illustrating impact toughness with a large number of samples was not possible due to a lack of material. All subsequent melts with reduced nickel content will be tested for impact toughness at lower temperatures.
The obtained results indicated that the steel under study does not have reduced viscosity and, in addition, with a decrease in the tower thickness (from 110 to 92±8 mm), it became possible to expect more favorable conditions for the hardenability of the casting.
Based on the above, a decision was made to conduct new smelting.
The next melt, No. 21300, was smelted in the main open-hearth furnace.
The charge weight is 60000 kg. The melting yield was: 0,86% carbon, 0,24% manganese, 0,030% phosphorus, 1,85% nickel, 0,22% chromium, and 0,19% molybdenum.
Dephosphorization continued for an hour and 15 minutes. The phosphorus content at the end of dephosphorization was 0,018%, but the slag was not completely removed. Therefore, the final analysis yielded an elevated phosphorus content of 0,033–0,034%.
To create new slag, the first slag-forming mixture was added, consisting of lime and molding earth, with a carbon content in the bath of 0,50%.
To reduce the iron oxide content in the slag, 50 kg of chamotte was added before deoxidation; with a carbon content of 0,27%.
The average rate of carbon burnout during the dephosphorization period is 0,5% per hour; during the boiling period, 0,17% per hour.
Deoxidation was carried out using metallic deoxidizers, namely: 50 kg of ferromanganese, 250 kg of silicomanganese; 550 kg of blast-furnace ferrosilicon; 150 kg of 45% ferrosilicon. 32 kg of aluminum was added to the ladle.
The slag produced during smelting is good.
After deoxidation, the slag analysis is as follows:
Overall, the melting was carried out satisfactorily.
Chemical composition in % of melt No. 21300.
Due to the increased phosphorus content in melt No. 21300, which hinders the transition of steel to fiber, and the desire to not have an additional factor in the experiment, it was decided to cast another melt.
Melt No. 31452 was also smelted in the main open-hearth furnace.
The furnace operates on fuel oil and has increased oxidizing capacity, so melt No. 32452 was carried out using the diffusion deoxidation method.
The weight of the cage was 36 tons, with 25% of the cage weight consisting of chromium-nickel-molybdenum waste.
Duration of the melting period:
refueling - 20 min.
brewing - 1 hour 50 minutes
dephosphorization - 7 hours 50 minutes.
boiling time - 2 hours 05 minutes.
deoxidation - 1 hour 20 minutes.
total melting time: 12 hours 40 minutes.
When melted, the composition of the metal bath was as follows:
carbon 1,32%
manganese 0,20%
phosphorus 0,030%
The smelting process was well executed: boiling was intense throughout the oxidation period, as characterized by carbon burnout rates. During the dephosphorization period, the average carbon burnout rate was 0,36% per hour; during the pure boiling period, the average rate was 0,32% per hour; after the addition of the second deoxidizing mixture, it was 0,24% per hour.
During the dephosphorization period, slag was thoroughly removed. Phosphorus content in the metal was 0,015% after dephosphorization, 0,015% before deoxidation, and 0,022% in the finished metal.
The first slag-forming mixture consisted of 900 kg of lump lime and 50 kg of molding earth, added at a high carbon content in the bath (0,82%), which allowed the bath to boil intensively.
The second mixture (deoxidizing, making it possible to reduce the burnout rate before deoxidation) consisted of: 250 kg of lime, 120 kg of ground chamotte, 120 kg of ground coke.
The mixture was seeded at a carbon content of 0,33%.
Deoxidation was performed as follows: the metal was deoxidized with ferromanganese (30 kg) and 45% ferrosilicon (30 kg). The slag was deoxidized with a third mixture consisting of ferrosilicon and ground coke.
15 minutes before release, 300 kg of Vavilov cast iron was added to ensure the specified carbon analysis.
Vavilov cast iron is a colloquial or historical the name of a high-quality, high-strength spheroidal graphite cast iron (editor's note).
Chemical composition in %:
The melting process was carried out well. Three towers were filled during casting.
It was planned that after heat treatment, one of the towers of melt 31452 would be subjected to circular firing.
Towers of melts 21300 and 31452 were processed according to the regime adopted for gross production (normalization, hardening and tempering).
One tower No. 2236, square No. 31452, was subjected to double hardening, since in production it was allowed to process the towers in two modes:
1) normalization, hardening, tempering;
2) double hardening.
After heat treatment, the following results were obtained.
Table 5:
Tower No. 2266 was subjected to all-round fire.
The test results showed the following:
1) The speed at the rear strength limit (RSL) corresponds to the requirements of the technical specifications of 630–640 m/sec for projectile fig. 2-03545.
2) The wall thickness is uniform and is within 96–99 mm.
3) It is worth noting the good toughness when tested by shelling: no brittle damage was found, and there were absolutely no cracks on the inner side.
4) The obtained speeds (PTP and PSP) indicate that the armor resistance of the turret is practically the same in all its parts.
In addition to tower No. 2236, three more towers from these two castings were subjected to acceptance tests.
The test results are presented below.
Table No. 6:
Based on the results obtained, a decision was made to manufacture a pilot batch of 25 units, for which 4 melts were smelted and 23 towers were poured.
Table No. 7:
Some of the towers were heat treated and subjected to firing tests.
The results of heat treatment are summarized in Table 8, the data from field tests are presented in Table 9.
Table No. 8:
Characteristics of fracture hardness of parts of melts 31543, 31544, 21388, 21390
Table No. 9:
Due to the fact that the batch of turrets specified in Table 9 was tested not to determine the limits of rear strength and through penetration, but to determine armor resistance at control speeds, therefore the limits of rear strength and through penetration are not clearly defined.
Based on the table data, the following approximate assessment of armor resistance can be given:
Turret 2326 demonstrated somewhat reduced armor resistance during frontal testing; this can be explained by the fact that the front wall is 89 mm thick and, in addition, judging by the fracture, the heat treatment of the turret was not entirely satisfactory.
The data obtained allow us to draw the following conclusion:
1) Steel with reduced nickel and molybdenum content fully satisfies the requirements of technical specifications for cast armor with a thickness of 92 mm.
2) For cast turrets of the KV tank, steel of the following composition can be used:
Carbon 0,24 - 0,30
Manganese 0,30 - 0,60
Silicon 0,20 - 0,4
Sulfur up to 0,030
Phosphorus 0,00
Chrome 1,3 - 1,6
Nickel 1,7
Molybdenum 0,2 - 0,30
Note: The sum of sulfur and phosphorus should not exceed 0,055%.
3) The use of a new grade of steel for the KV tank turrets makes it possible to save nickel by 15% and molybdenum by 30%.
Appendix: Tower Test Report No. 94
The commission carried out field tests of an experimental cast turret of the KV tank, casting 61711 194, cast by the Izhora plant from steel with a reduced content of nickel (1,83%) and molybdenum (0,20%).
The tower was cast with a reduced number of risers (four instead of six) and one was treated to a hardness of 3,4 (imprint diameter).
Test conditions.
The turret was tested in its operational position by firing armor-piercing rounds from the 76-mm gun, model 2-06580. A total of nine shots were fired at the turret, including four into the front, three into the left side, and two into the rear.
Test results:
CONCLUSIONS
1. Tested experimental turret 194 to armor resistance not lower than the requirements stipulated by the temporary technical specifications for cast turrets with a thickness of 110 mm. Made of FD 6674 steel grade, adopted at UZTM.
2. The obtained velocities during testing with projectile 2-06529 indicate that the frontal part of the turret, the side and the rear exceed the standard velocity of the 76-mm gun, equal to 650 m/sec. of projectile drawing 2-03545 with a conversion factor of 0,89.
3. The armor resistance of the rear part of the turret in places located above the removed risers is somewhat lower than the armor resistance of the front and side parts of the turret, but is within the limits provided for by the temporary specifications.
4. The toughness of this experimental turret is not lower than that of previously tested cast turrets of FD 6674 steel grade, as determined by firing a 76-mm projectile.
But at low temperatures (down to -200C), the experimental turret cracked and was breached by an 85mm projectile with a velocity of 800 m/sec at a distance of 330 meters, which characterizes a sharp drop in viscosity.
5. The Commission considers it necessary to produce a batch of 25 cast turrets of the new grade of steel, on which more complete research and firing tests will be carried out, after which the issue of launching the new grade of steel into mass production should be finally decided.
6. The final decision on removing the profits must be made after examining tower No. 194 for the presence of shrinkage looseness (testing under a pile driver) and after checking the technology additionally on the next 5 towers.
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