APPLICATION OF WELDING FOR THE PRODUCTION OF BALLISTIC PROTECTIVE STRUCTURES

Abstract

In the common industry and especially in military industry there is a growing need for production of highly effective protective structures. For that purposes the most used materials are armor steels. They belong into a group of the fine-grained, increased strength steels, which are manufactured by intensive thermo-mechanical treatment at high temperatures and later quenching and low-tempering. Combination of the heat and mechanical treatments provides for the fine grains and exceptionally good properties of these steels, while the low-tempering enables relatively high hardness and good ballistic properties. However, sometimes there is a need to weld these steels in order to manufacture some specific assemblies. Since the way these steels are produced this is why the welding can negatively affect the material properties in specific zones of the welded joint, what could lead to worsening of the material's ballistic properties, as well. The aim of this paper was to determine influence of the welding procedure on that mechanical and ballistic properties. In that order the model plates were welded with the specially prescribed technology in three types of the joints: the butjoint, corner joint and the corner joint with the shielding plate. After the welding the test plates were subjected to the ballistic tests which consisted of shooting with three types of live ammunition at different types of the welded joints. At the end the comparative analysis of the results is given.

Headnote

In the common industry and especially in military industry there is a growing need for production of highly effective protective structures. For that purposes the most used materials are armor steels. They belong into a group of the fine-grained, increased strength steels, which are manufactured by intensive thermo-mechanical treatment at high temperatures and later quenching and low-tempering. Combination of the heat and mechanical treatments provides for the fine grains and exceptionally good properties of these steels, while the low-tempering enables relatively high hardness and good ballistic properties. However, sometimes there is a need to weld these steels in order to manufacture some specific assemblies. Since the way these steels are produced this is why the welding can negatively affect the material properties in specific zones of the welded joint, what could lead to worsening of the material's ballistic properties, as well. The aim of this paper was to determine influence of the welding procedure on that mechanical and ballistic properties. In that order the model plates were welded with the specially prescribed technology in three types of the joints: the butjoint, corner joint and the corner joint with the shielding plate. After the welding the test plates were subjected to the ballistic tests which consisted of shooting with three types of live ammunition at different types of the welded joints. At the end the comparative analysis of the results is given.

Keywords: protective structure, welding, armor steels; ballistic properties

Abstract

INTRODUCTION

The combat vehicles for the infantry were created from the tendency to increase the efficiency of the tanks and possibilities for their survival on the combat field. The problem that appeared was how to develop the armor, which would guarantee the safety to the personnel by preventing the penetration of the projectile from the antiarmor ammunition into the vehicle, while simultaneously realizing as good as possible its tactical-technical and combat-exploitation characteristics. Taking into account these requirements, it was inevitable to develop the special group of the high-strength steels, known as the armor steels that are being improved [1].

The Swedish company SSAB Oxelösund [2] has the highstrength steels in its production program, where the especially interesting is a group of armor steels, known under the commercial brand ARMOX, which are produced according to the strictly defined manufacturing procedures, [3]. Their excellent properties are resulting from the manufacturing process. They possess a very low content of carbon what positively affects their weldability, while the strength is being achieved by application of the thermo-mechanical processing (TMP) [1, 3]. However, despite their exceptional properties, when the armor is being welded, the worsening of those properties occurs, locally, due to the entered heat. Such spots represent the critical places on the structure and the objective of this paper is to show how those places (various types of the welded joints) behave in the conditions when being hit by the projectiles of different types [4, 5].

WELDING OF SAMPLES

The welded joints on combat vehicles, made of this or some other steel, represent the most vulnerable places of the whole structure. The reason for that is the fact that in welding of the armor steels the filler metals must be applied, which produce the weld metal of the significantly lower strength with respect to the base metal. Thus, the appearance of the cold cracks can happen, since the armor steels are very prone to hardening. Besides that, this steel belongs into a group of the conditionally weldable steels, which implies that adequate measures must be taken during the welding. One of the most important measures is to control the heat input, what is explicitly presented by SSAB in specifications of this steel. The heat input is limited to 200°C, since at the higher temperatures the excessive annealing occurs and thus the loss of all the positive properties induced by the TMP. In this paper are given recommendations that are mandatory to be followed in order to obtain as high quality welded joints as possible. The welding technologies are also proposed, all based on recommendations by the steel manufacturer, as well as the experts that have already dealt with this problem.

The experimental samples, needed for the ballistic tests of the basic zones of the welded joint, were made in the form of the butt, corner and corner-edge joints (Figure 1). The plates' dimensions of the ARMOX 500T steel were 200 x 200 x 8.6 mm and they were cut by the laser (Figure 2).

The welding was done using MMA welding procedure. The welding parameters are given in Table 1 while in Figure 3 are shown the plates' appearances after the welding, for all the three cases.

TESTING OF THE WELDED JOINT BASIC ZONES BALLISTIC RESISTANCE

Many countries have prescribed standards regarding the levels of the ballistic protection; the most used by the ARMOX manufacturers are STANAG 4569 (Table 2) prescribed by the NATO and EN 1522, prescribed by the UN, primarily due to the customers' requests [4]. The STANAG 4569 standard refers to degrees of the protection for logistic and light armored vehicles.

The standard includes threats by the ballistic projectiles, of the small and medium caliber, as well as the fragments simulating the penetrators, in order to simulate the artillery actions. It is aimed for the repeatable testing procedures for estimate of the ballistic protection of the armored vehicles' parts and for determination of the critical zones on those vehicles. The threats are divided into five levels, where the first level is related to civilian threats, while the other levels are related for various military threats.

RESULTS OF THE BALLISTIC TEST

Though the three samples were made for each type of joints (the butt, corner and corner-edge), the ballistic tests were done at one sample from each group, only. That was done primarily due to the complexity of the experiment and since the obtained data were sufficient to estimate the ballistic resistance. The objective of the experiment was to estimate the degree of damage, namely the type of penetration of the basic zones of the welded joint (the base metal - BM, the heat affected zone - HAZ, the joining zone - JZ and the weld metal - WM) by ammunition of the 7.62 x 39 type: M67 Ball, 7.62 x 51 NATO Ball (Ball M80) and armor bullet 7.62 x 54R B32 API (Dragoon's). The 7.62 x 39 M67 Ball bullet is not prescribed by the NATO standards, but by the Russian standards of the ballistic protection, which is not guaranteed by the SSAB.

The experiment was performed on the test field of the "Prvi Partizan DOO" company in Užice, Serbia, which has decades' long experience in producing the ammunition and the tests of this kind. The finishing, verification and homologation (approval) tests of ammunition are being conducted on this test field. The experiment was executed by the expert staff, according to adequate safety standards. The testing equipment included:

I Test barrel with the cover for measuring the velocity, of caliber 7.62 x 39 mm,

I Test barrel with the cover for measuring the velocity, of caliber 7.62 x 51 mm,

I Test barrel with the cover for measuring the velocity, of caliber 7.62 x 54 mm,

I Stand for the test barrel

I Ammunition 7.62x39 M67 Ball, velocity at v25=725 m/s,

I Ammunition 7.62 x 51 NATO Ball (Ball M80), velocity at v25 = 830 m/s,

I Ammunition 7.62x54R B32API, velocity at v25=790 m/s. The samples of the armor steel, prior to the commencing of the experiment, were firmly positioned in the wooden frames, to prevent the loss of energy due to motion of the plates when hit by the bullet. The distance from the exit hole of the test barrel to the sample was 10 m. According to the experimental plan, the welded joints were positioned in such a way that the weldment was perpendicular to the bullet motion direction, what at the corner and corner-edge joints should present the behavior of the base metal and the heat affected zone at the bullet impact at an angle.

Appearance of the butt joint after the bullet impacts is presented in Figure 4. Total of 10 projectiles were fired of the three calibers [4].

After the tests on the butt joint, the tests of the corner joint were performed, with the samples fixed as described earlier. The total of 9 bullets was fired of the three calibers. The entrance side of the corner joint is presented in Figure 5 [5].

Figure 5. Appearance of the tested sample of the corner joint from the entrance side. The penetrated spots - perforations were of the type characteristic for an impact by the sharp pointed projectiles into the armors of the small thickness. In some cases they also appear for the flat bullets' impacts at velocities that are close to the limiting velocities of penetration. Consequences of penetrations of this type are characteristic since the shape of the hole at the exit side resembles the flower petals.

At the end, the corner-edge joint was tested, which on the inside has little platelets made of the same material. The idea is that they should act as a protection in the case that the weld metal and its vicinity have been penetrated. The total of 8 bullets were fired of the three different calibers, into the characteristic zones of the welded joint. Results are presented in Figure 6 [4, 5].

CONCLUSIONS

In this paper the ballistic check the penetration resistance of three types of welded joints' zones were performed. Ammunition used was 7.62 x 39 M67 Ball, 7.62 x 51 NATO Ball, and 7.62 x 54R B32 API. Besides, the whole welding technology of the samples was presented.

Obtained results led to the following conclusions:

Щ The base metal, the heat affected zone and the weld metal are all bullet proof for the caliber 7.62 x 39.

Щ Test by the 7.62 x 51 caliber bullets showed that only the base metal is resistant to penetration.

Щ For the armor ammunition of the 7.62 x 54R caliber there are no obstacles, i.e. all the zones of the welded joint are threaten, even the protective plates in the corner-edge joint case.

Based on these results, one must recommend that vehicles constructions made of this steel must be so designed that all the zones of the welded joint should be well protected against penetration by any caliber projectiles. The weld metal should be hidden whenever possible, while the butt joints should be strictly avoided in any case. If these recommendations were not followed to the letter, the safety of the personnel in the vehicle, against the projectile penetrating the armour, cannot be guaranteed.

The reason that the welded joint is the weakest place at the structure can be the heat input during welding. Namely, the heat generated during welding leads to worsening of the properties of the material in the welded zone (softening of the steel). Although, the steel producer forbid the heating of the steel over 200°C in order to preserve the good mechanical properties, during the welding that cannot be achieved. Since the welding has to be used for joining that is the reason why we tested different types of joints in order to determine the most favourable option.

Acknowledgment

This research was supported by companies "DLS Special systems" Belgrade, branch Kragujevac and "Prvi Partizan Ammunition", Užice, Serbia.

Note: This paper was presented at IRMES 2022 - 10th International Conference on Research and Development of Mechanical Elements and Systems: "Machine design in the context of Industry 4.0 - Intelligent products", organized under the auspices of the Association for Design, Elements and Constructions (ADEKO) and University of Belgrade, Faculty of Mechanical Engineering, Department of General Machine Design, in 26 May 2022, Belgrade (SERBIA).

References

References

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[2] https://www.ssab.com/products/brands/armox, accessed on 12. 04. 2022.

[3] Lazić, V., Arsić, D., Nikolić, R. R., Djordjević, D., Prokić-Cvetković, R., Popović, O., Application of the high strength steel HARDOX 450 for manufacturing of assemblies in the military industry, Key Engineering Materials, ISSN 1013-9826, Vol. 755, pp. 96-105, 2017.

[4] Rakičević, M., 2018. Estimation of weldability and selection of optimal welding technology of bulletproof steel ARMOX 500T, MSc thesis, Faculty of Engineering, University of Kragujevac, Kragujevac, Serbia. (In Serbian)

[5] Lazić, V., Arsić, D., Rakičević, M., Aleksandrovih, S., Ðorðević, M., Marinković, P., Ratković, N., Uticaj vrste zavarenog spoja na balistička svojstva pancirnog čelika ARMOX 500T, Zbornik radova sa 37. Savetovanja proizvodnog masinstva Srbije, Kragujevac, 2018, 25-26 Oktobar, ISBN 978-86-6335-057-1, pp. 173-177.