Geometrical Parameters of 14G2 Steel Structure as a Material with High Ballistic Resistance

Introduction. Increasing the ballistic resistance of armor is a crucial issue in the design of modern military equipment [1]. At high impact rates, in addition to the basic mechanical characteristics, strict requirements are imposed on alloys regarding the specific geometric arrangement of their internal structure [2]. For homogeneous isotropic metal barriers, one of the key factors is the formation of a finely dispersed structure. The main challenge is that traditional homogeneous steels with an isotropic structure, such as tempered martensite, perform protective functions primarily through hardness and toughness. However, at high-energy, dynamic loads (about 10³ m/s), the effectiveness of such barriers is significantly reduced: either brittle destruction or through-penetration of the target occurs without significant dissipation of the kinetic energy of the indenter [3]. An alternative approach involves the use of heterogeneous, anisotropic materials with a more complex structural arrangement [4]. This article examines in detail the microstructural characteristics of steels with a natural ferrite-martensite composite structure (NFMC), which are considered a promising material for armor protection.

Over the past few decades, extensive research has been conducted on materials with NFMC structure, formed by heat treatment at temperatures in the intercritical range [5]. These materials are referred to as “composites” due to significant differences in the properties between the two phase components that make up the material. Within this composition, highly ductile and relatively soft ferritic layers alternate with high-strength martensitic plates, creating a structure with a distinct texture that is oriented along the direction of rolling. The process of producing these multilayer composites is described in detail in our patent [6]. These alloys exhibit the ability to effectively inhibit crack propagation by triggering interlayer delamination along interphase boundaries [7–9]. Earlier, the authors of publications [10][11] found that targeted control of the heating temperature for quenching directly changed the volume proportions of soft and hard components, determining the final level of operational properties. However, most of the tests were based on static stretching or bending methods [12–14]. In contrast, ballistic loading, characterized by extremely high strain rates, has different mechanisms of crack braking: inertial effects and wave processes begin to dominate [3][15]. In this case, the determining role is played not by the absolute strength of individual phases, but rather by the spatial parameters of the structure, such as orientation, length and thickness of the layers, their ratio, as well as the maximum length of the reinforcing layer. To date, the scientific literature has not provided comprehensive information on which structural parameters in the NFMC architecture are critical criteria for ensuring bullet resistance, nor have their optimal ranges been determined.

The aim of this research is to conduct a comprehensive analysis of geometric parameters of NFMC steel structure, including the grain-boundary angle, the dimensional proportions of cross-sectional layers, the critical length of martensitic layer, and other relevant metrics. The optimization of these parameters is expected to maximize the effectiveness of these steels in the design of armor protection systems. The relevance of the chosen factors is justified based on the following considerations: the amount of misorientation directly influences the level of shear stress that can cause the destruction of the high-strength phase; the ratio of the thicknesses of structural elements determines the predisposition to interfacial stratification; the critical length of martensitic layer makes it possible to evaluate the efficiency of load transfer from the matrix to the reinforcing phase. To successfully achieve this goal, it was necessary to consistently solve the following tasks:

− to assess the degree of orientation of NFMC alloy structure;

− to measure the volume fraction of the phases, the average cross-sectional width of the plates, as well as the average free path of the reinforcing phase in a soft ferritic base;

− to perform a theoretical calculation of the critical length of the martensitic layer;

− to analyze the ratio of the thicknesses of alternating ferritic and martensitic plates, which ensures the stable implementation of the mechanism for extinguishing the kinetic energy of a growing crack due to local interlayer stratification during intense high-speed impact.

Materials and Methods. The research was conducted on 14G2 steel samples with dimensions of 140×70×7 mm. The NFMC structure was formed by heat treatment — quenching of steel from the intercritical temperature range that initially had a banded ferrite-pearlite structure. The assessment of ballistic resistance was conducted using a comparative method. The characteristics of steel samples with NFMC structure and the same steel with the homogeneous structure of tempered martensite were compared.

Ballistic tests of targets made from these materials were performed from a distance of 50 m using a Dragunov sniper rifle (SVD). 7.62 mm caliber ammunition with a thermally reinforced core was used for firing. The flight speed of the damage agent was 103 m/s. Ten shots were fired at each type of obstacle.

As a result of the tests, we recorded a complete penetration of all targets with a homogeneous structure. However, when bullets hit plates with NFMC structure, we observed a ricochet effect, with only deformation depressions (dents) ranging from 1.5 to 2.0 millimeters remaining on the surface.

Geometric parameters of the structure were evaluated by a metallographic method using a Neophot 21 microscope and ToupView software to obtain quantitative data. At the same time, standard image preparation functions were used for subsequent analysis: framing, brightness adjustment, contrast adjustment, and other basic tools. In addition, built-in algorithms for applying a measuring grid and a scale were used, while the true size of the microstructures was determined by calibrating the micrometer object image. The automatic calculation of volume fractions of the structural components was conducted using the KOI-1 System software package, where stereological operations were performed based on setting brightness intervals. The study of the characteristics under consideration was performed on 15–20 fields of view for each individual sample. Micrographs were captured using a 10Mp ToupCam UA1000CA digital eyepiece camera with an optical resolution of 3584×2748 pixels. On the basis of the obtained microstructural data, the characteristic geometric parameters of the steel composite material were established according to standardized methods regulated by GOST R 54570–2011¹.

Research Results. According to the ballistic tests results, high bullet resistance of the material with the structure of natural ferrite-martensitic composite (NFMC) was established. Only the ricochet of the striking elements was recorded. This indicator compared favorably with the characteristics of metal with a similar chemical composition that had a homogeneous tempered martensite structure, the samples of which penetrated. In this regard, the results of the research are presented below (Fig. 1) on the microstructure of the specified NFMC composite material [7] reinforced with discrete-length reinforcing fibers [8].

Table 1 provides the results of determining the characteristic geometry parameters of the steel composite according to GOST R 54570–2011². Following the methodology of the mentioned standard, calculations were conducted based on the data obtained by superimposing the grid (Fig. 1) and then calculating the number of points of intersection of its lines with the studied phase.

where N_L⊥ — number of intersections of particles per unit length of the line perpendicular to the deformation axis; N_L|| — number of intersections of particles per unit length of the line parallel to the deformation axis; P_L⊥ — number of boundary crossings per unit length of the line perpendicular to the deformation axis; P_L|| — number of boundary crossings per unit length of the line parallel to the deformation axis; Al — anisotropy coefficient; Ω₁₂ — degree of alignment of the linear elements of the structure; V — volume fraction of martensite; SB_⊥ — average distance (from the center to the center) for the alignment phase; λ_⊥ — average free path (from edge to edge) for the alignment phase.

The angle of misorientation of the strengthening phase (Fig. 1), determined by the method of superimposing secant lines along the orientation axis of the martensitic strips, was 7–11°.

The volume fraction of martensite, calculated by stereological analysis of the microstructure using the KOI-1 System software, reached 28.37% after quenching at a temperature of 735℃.

In this work, we used the recommendations of GOST R 54 570-2011³, to evaluate parameters such as the average distance between phase centers SB_⊥ and the average free path λ_⊥ (the distance between the phase boundaries) in the structure shown in Figure 1. After calculating these values, we found that the average thickness of the hardening phase plates was h = 0.0051 mm, and the average free path λ_⊥, representing the width of ferritic plates, was c = 0.0131 mm. Thus, the thickness ratio of the ferritic and martensitic phases was c/h = 2.57.

The most important factor in any composite material reinforced with discrete reinforcing fibers is the longitudinal length of these fibers. This length must exceed a critical value l_кр in order to ensure full loading of the martensitic layers of the composition under study. The critical length of the martensitic layer in naturally composite ferrite-martensitic materials (NFMC) was calculated based on the equilibrium condition of normal and tangential stresses within the material [11]:

(1)

where h, b — thickness and width of the reinforcing layer; τ = σ_T / 2 — tangential stresses at the ferrite-martensite interface. It follows:

(2)

According to the calculation results using equation (2), considering the values of h and b measured in the NFMC with a total compression of 70% during rolling, the value of l_кр = 18.3 µm was obtained. A histogram of the distribution of value l in 14G2 steel with NFMC structure is shown in Figure 2.

Discussion. The obtained data on the geometric parameters of 14G2 steel structure, hardened from the intercritical temperature range (735°С) and having NFMC organization, met modern requirements for composite materials. The high degree of structure orientation, the length of the reinforcing plates was higher than their critical value, the ratio of the thickness of the matrix and the reinforcing layer were within the optimal values.

Coincidence of the values of orientation degree Ω₁₂ (Table 1), found by two different methods, confirmed the reliability of the results. A high degree of structure orientation was indicated by the condition Ω₁₂ > 0. When the impact was directed orthogonally to the rolling plane of steel composite, the misorientation of the reinforcing phase played a significant role. This fact was due to the dependence of the shear stress magnitude required to break the “fiber” on the misorientation angle. To achieve high impact resistance, it was important to minimize misorientation, which could be achieved by a significant degree of compression during hot rolling (at least 70%). The maximum allowable misorientation level [9] for these composites was 15° [10]. In the microstructure of the material studied, the misorientation angle ranged from 7° to 11°

In the process of varying the quenching temperature, the ratio of volume fractions in the martensite-ferrite system could change. In particular, during the heat treatment of steel heated to lower temperatures, the volume fraction of martensitic component of the alloy decreased, but the carbon concentration increased at the same time. This presented an opportunity to deliberately adjust the mechanical properties of the material.

In numerous scientific papers dedicated to the study of the reasons for the high strength of composite materials [11], the importance of the critical volume (V_кр) of the composite reinforcement component was convincingly demonstrated [12]. The calculations of this parameter were conducted taking into account the strength limits of the matrix and hardening phase. The absolute value of the critical volume was estimated at approximately 50% [13]. At the same time, similar estimations were made for the conditions of static loading of composites under tension (along the reinforcing fiber) or bending [14], as well as during dynamic bending during impact strength tests [16]. The ballistic stability of NFMC was evaluated under conditions of orthogonal high-power impact upon impact at a speed of ~10³ m/s. It was shown [15] that at such speeds, the mechanical properties of composite components could not affect the dynamic resistance at all. Instead, the role of inertial forces, paths and methods of shock wave propagation increased dramatically [17]. In this case, the concept of V_кр lost its original physical meaning and the proportion of the volume fractions of ferrite and martensite was assumed to be approximately 80/20. This unambiguously determined the thickness ratio of ferritic (c) and martensitic (h) plates at the level of c/h ≥ 3.

At this ratio, the growth of the crack was inhibited due to the stratification of the material within the ferritic matrix, significantly increasing the overall ballistic resistance [5]. The measured ratio for the structure under study was 2.57, which was slightly lower than the threshold value mentioned above. This naturally reduced the likelihood of delamination during high-speed crack propagation and reduced the chance of ricochet during high-speed impact. To achieve an optimal c/h ratio in this scenario, it would be advisable to adjust the quenching temperature from the intercritical range to below 735°C.

When conducting ballistic tests on plates with a thickness of 7 mm, according to the previously described methodology, a target sample with an isotropic tempered martensite structure broke through, whereas a projectile invariably ricocheted from a target with the NFMC structure. This was due to the fact that the layered composite, with optimal geometric characteristics in all parameters, was able to form local stratifications [5] as cracks propagated in it [12] sequentially from one layer to another along the interface and as the crack tip approached it. As shown in Figure 3, these stratifications created conditions for the initial crack to move to a new position, which did not contribute to its further development. Thus, the ferrite-martensite phase interface with a length of l > l_кр could serve as an effective barrier to crack propagation. Figure 3a illustrates how the trajectory of the crack in the NFMC sample changed when a ricochet occurred. In this case, a bulge formed on the back of the target, and a crack could occur (Fig. 3b). However, there was no material shift beyond the back side.

Conclusion. It has been established that the structure of NFMC steel has a high degree of orientation (Ω₁₂ = 0.352), and this circumstance plays an important role in the case of a powerful impact.

The ratio of the average widths of martensite and ferrite plates was 2.57. This suggests that it would be beneficial to reduce the quenching temperature from the intercritical range below 735°C to ensure an optimal structural balance. The data on the length distribution of martensitic plates showed a maximum that coincided with the calculated critical length of the reinforcing fiber (18.3 µm). This is a significant indicator, as the ferrite-martensite interfaces with lengths greater than the critical value can act as an effective barrier to crack propagation.

It has been proven that the excellent ballistic resistance of a natural ferrite-martensitic composite is achieved exclusively through the optimal combination of all geometric parameters in the microstructure of the composite material. At the same time, the successful achievement of the described NFMC configuration optimum (as a specialized armor protection) can only be guaranteed through the use of correct hot rolling techniques (total workpiece compression of 70% or more), as well as careful selection of optimal quenching temperatures within the intercritical range.