On the shortcomings of the BMP-3 armor manufacturing technology
Aluminum armor, generally speaking, is quite capricious in terms of its production conditions. Any disruption to the manufacturing process or suboptimal choice of technology can lead to significant changes in its properties, which could impact the survivability and quality of armor protection made from aluminum-based alloys.
The BMP-3 also suffered from this to some extent, as discussed in an article by Russian researchers published in 1991. It described a new technology for manufacturing armor components for this vehicle (it's unclear whether it was subsequently adopted), but more importantly, it also highlighted the shortcomings of the existing technology. So, from a technical perspective, the material is quite interesting, so we recommend reading it.
Improved technology for heat treatment of BMP-3 hull parts
The first-generation BMP-1 and BMP-2 infantry fighting vehicles featured a welded steel hull with add-on components made of ATsM and D20 aluminum structural alloys. In contrast, the BMP-3's hull is constructed of aluminum armor with special armor. The use of aluminum allows for a 25% weight reduction compared to a steel hull with similar armor protection.
The first vehicle in domestic armored hull production with an aluminum hull (with a steel turret) was the BMD-1 airborne combat vehicle with bulletproof armor made of ABT-101 alloy with a thickness of 8...32 mm.
The new BMP-3 is the first Russian tank design to feature an all-aluminum hull and turret. The BMP-3's hull is equipped with ABT-102 aluminum bullet- and projectile-resistant armor. This armor alloy belongs to the Al-Zn-Mn system of deformable, heat-hardenable alloys (Author's Certificate No. 928271).
Armor properties are formed at all stages of production (casting, rolling, etc.), but the decisive stage is the heat treatment.
In the production of the BMD-1 hull, a heat treatment method is used, which consists of hardening the armor and subsequent two-stage aging of the plates and armor structures (Table 1).
Table 1
Unlike the BMD-1, the BMP-3 uses armor plates 10-60 mm thick. The armor hull and turret are welded from ABT-102 alloy, while the floor is made from stamped AMg6 blanks.
The armor differentiation is defined by the requirements specifications and imposes different requirements on the vehicle's front, side, and rear projections. Frontal projection components constitute a projectile-resistant group, rear components constitute a bullet-resistant group, and the sides constitute a mixed group. These conditions determine the mechanical properties (hardness) requirements for each of these component groups.
Thus, bulletproof resistance is directly dependent on the hardness H. As H increases, the speed of the limit of conditional damage Vпкп increases with a spread of up to 50 m/s (Fig. 1); to achieve this, it is necessary to ensure a hardness of ≥140 HB in the bulletproof group of components. Conversely, projectile resistance αпкп is inversely related to hardness (Fig. 2).
Fig. 1. Dependence of the speed limit of conditional damage Vпкп on the hardness and thickness b of the ABT-102 armor when firing a B32 bullet of 7,62 mm caliber at close range at normal: dark dots – hardness over 140 HB, light – not less than 140 HB
Fig. 2. Dependence of the projectile resistance αпкп when shelling the ABT-102 armor with 30-mm BT projectiles (distance 300 m) on the hardness H: dots – hardness > 140 HB, and triangles not less than 140 HB; the zone of the optimal value is shaded
In this case, the optimal value is a hardness below 140 HB. A mixed group of parts must balance both of these conflicting requirements and can be represented by a hardness close to 140 HB (Table 2).
Table 2. Optimum hardness of different groups of parts
An analysis of the existing heat treatment technology combining stage II aging with post-weld tempering reveals significant shortcomings. These include a discrepancy between the protective properties of components and their optimal values, as well as significant variation in properties between components of the same thickness but with different locations within the armor assembly.
The average hardness of various groups of parts obtained as a result of processing using existing technology, in comparison with the optimal hardness range, is presented in Table 3.
Table 3. Comparison of hardness by groups of parts (numerator – average values, denominator – spread)
The table is based on data from over 110 vehicles (220 measurements for each thickness). The dispersion was calculated as three times the variance. Therefore, the armor hardness is within the specified range with a 95% probability, and most components are outside the optimum range. The reduction in protective properties is explained by the combined heating of components of different purposes and different thicknesses (in a welded armor assembly) during the second stage of aging.
In order to eliminate the shortcomings of traditional technology, i.e. to obtain differentiated properties for groups of parts and to reduce the spread of mechanical properties, a technology for heat treatment of body parts was developed, which is protected by copyright certificate No. 236106 (Table 4).
Table 4. Heat treatment modes for body parts
The first stage of aging is performed to obtain the metal (ABT-102 alloy rolled product) in a state of maximum strength. The second stage is performed to re-age the metal and achieve differentiated armor properties, as well as corrosion resistance. Post-weld tempering of the assemblies is designed to relieve residual post-weld stresses of the first and second types, ensuring that the required strength and stress corrosion cracking resistance are met without altering the mechanical and armor properties achieved during the second stage of aging.
The technology research at the plant was carried out in three stages:
1. Post-weld “gentle” tempering of two hulls and towers to assess the reduction of residual stresses.
2. Heat treatment of ABT-102 cards in the II and III aging stages, simulating the mode of individual heat treatment, in order to clarify the temperature range and holding time based on the results of tests for armor resistance and hardness level.
3. Heat treatment using new technology.
Inspection of two hulls treated using the "gentle" tempering mode (stage 1 of testing), which passed running tests (after 10-12 months from the date of post-weld tempering), confirmed the effectiveness of post-weld stress relief and the absence of isolated and fatigue cracking.
The hardness of the sections processed using the sequential heat treatment technology (second inspection) was close to optimal, ensuring the required level of armor protection. Based on the results of the second stage, differentiated thickness-based aging modes were assigned for the second stage. Two hulls and turrets were processed using these modes. It turned out that the metal's properties are primarily determined during the second stage of aging.
The change in hardness resulting from post-weld tempering does not exceed the error limit of the Brinell method. Post-weld tempering ensured the required reduction in residual stress, preventing corrosion-induced cracking of the metal.
Testing of the cards with bullet and shell fire showed that the protective properties of all control cards meet the technical specifications. According to the αпкп criterion, a projectile resistance margin of approximately 2° was obtained. A certain margin of protective properties (Vпкп = 20–30 m/s) was also obtained under bullet fire.
Conclusion: Improved heat treatment technology for BMP-3 hull components improves their protective properties.
Source:
"Improved Technology of Heat Treatment of BMP-3 Hull Parts." A.A. Artsruni, V.R. Begichev, Yu.Z. Zasel'skiy, et al. "Bulletin of Armored Vehicles," No. 5, 1991.
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