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Steel for Armour

• satyendra
• February 19, 2020
• 1 Comment
• Armour steel, bainite, brinell hardness, DHA, Ductility, hardness, HHA, Martensite, microstructure, Quenching and tempering, RHA, Tempered martensite,

Steel for Armour

Various materials especially metals, ceramics, polymers, and composites have been used in light vehicle defence technology. With each material appearing significant in respective applications, metals are mostly used for ballistic protection due to its mechanical properties. The most common metallic material being used in the armoured vehicles is steel. Steels which are being used for the protection of armour are also known as armour steels. Steels of high tensile strength, hardness, and ductility are mostly used as a ballistic protection plate.

Of course, armour steels are not ordinary steels. These are high strength steels with combined hardness and fracture toughness. The main properties such as toughness, hardness, good fatigue strength, ease of fabrication and joining and having relative low cost make the steel a popular material for the hulls of the armoured vehicle. Steel is the best all round performing armour material in spite of its high density because of its properties like toughness, ready availability, low cost, castability, weldability etc.

Since the armour is used to protect a particular area, its practical weight is best described by its areal density (Ad), which is given by the expression ‘Weight of the armour system/area is being protected’. The unit of the areal density is kilograms per square meter (kg/sq m). Steels provide protection at realistic areal densities for many ballistic applications and also at an affordable price.

Armour steels have historically delivered optimized ballistic performance against a range of battlefield threats, including both armour piercing (AP) and fragmentation threats. For protecting battle tanks, high hardness steel armour has been in use for many years. Steel continue to be highly competitive armour material even today and the ballistic performance of steel continues to improve with incremental advances in steel metallurgy.

Armour steel is basically a high strength low alloy structural steel which has been treated to have property of very high resistance to penetration. This property to the steel is usually imparted by the thermo mechanical treatment. It is well known that the resistance to penetration of steel can be improved by increasing its texture intensity which can be obtained by thermo-mechanical treatment.The mass effectiveness of the armour increases with the hardness of the material. However, very hard armour tends to be brittle and to shatter when hit.

The conceptions such as hardness, strength and toughness are the main features for the ballistic performance of a given steel. Alloying and processing metallurgy are very important in the development of armour steels. Armour steel is required to have such properties as (i) high resistance to perforation and ballistic impacts, (ii) possible fabricability, and (iii) adequate fatigue and wear resistance under service conditions.

It has been reported that the high hardness of armour steel directly determines the ballistic performance and perforation mode. However, there is no basic correlation between hardness and resistance to perforation, as measured by the protection ballistic limit. In order to obtain high hardness values, studies on alloying design and heat treatment conditions are very popular to improve the ballistic performance of the steels.

Toughness is another critical property for armour steel under a dynamic attack of projectiles having high kinetic energies. It is generally considered that armour steels having high toughness are very useful to resist ballistic impacts without being fractured. As it is well known, alloying and also heat treatments affect the toughness of the materials.

Fabrication includes many operations such as cutting, welding machining and forming. It is known that the fabrication operations (thermal cutting, welding, and machining etc.) of armour steels need certain metallurgical properties in the armour steels. Low carbon equivalent (CE), limited segregation, low hydrogen content, low residual stress, and high ductility are the main property requirements for fabrication of armoured steels. Armour steels are required to be weldable. The fatigue behaviour of these steels is to be higher under cyclic stresses. Weld quality directly determines the mechanical properties of the armour steel.

Classification of armour steels

Steel armour can be classified into four main groups. These groups are (i) rolled homogeneous armour (RHA), (ii) high hardness armour (HHA), (iii) variable hardness steel armour, and (iv) perforated armour. Out of these four types, RHA steels are usually being considered as a benchmark material. RHA steel has been considered as the conventional armour for light armoured vehicles. It is a high quality alloy steel which is rolled out before being heat treated to give it an optimum combination of strength and toughness. The chemical composition as well as the most commonly used classification for RHA armour steels are given in Tab 1.

High hardness armour is the name given to a class of homogeneous steel armour which has hardness values exceeding 430 BHN (Brinell hardness number). Variable hardness steel plate introduces some advantages with varying through-thickness properties. By surface hardening one side of a thick low-carbon steel plate, it is possible to incorporate both hard disruptive and tough absorbing properties in a single material. Such steel is the dual hardness armour (DHA) steel and is much superior to all other metallic forms of armour. However, its use has been very limited because of its high cost. The main advantage is that, the more ductile backing layer is able to arrest crack propagation in the armour plate while the hard front layer is able deform or fracture the threat. The DHA steel is more efficient compared to HHA in defeating AP bullet with a steel core. In perforated armour, holes are introduced into the steel plates. These holes in high hardness steel plate have been shown to be an effective way of disrupting and fragmenting incoming projectiles. This mechanism can be regarded as edge effect.

It is known that the composition, processing and microstructure of the armour steel determine its mechanical properties which then can be correlated to and hence they critically determine its penetration resistance. However, there is a relationship between mechanical properties, specifically the mechanical metallurgy, of armour steels and ballistic performance.

Metallurgical properties of armour steel

The tensile strength and hardness of the armour steel relies on several factors but primarily the microstructure. The final microstructure of material is a result of series of heat treatment. Heat treatment can be categorized by austenizing, quenching and tempering.  Tempered martensitic matrix contributes higher hardness as the decreasing of grain size by carbon precipitation.

On studying the grain boundary effect, it is shown that smaller grain size increases tensile strength. Other than that, tempered bainite steel is reported to have superior hardness and toughness when contacting armour piercing 7.62 mm caliber projectile. Also, the percentages of alloying elements are very important for precipitating carbide particles. With proper heat treatment process, the precipitation of carbide particles can contribute to high strength in the steel structure. It is also noticed that boron, carbon, manganese, and nickel elements play a main role to improve ballistic properties of armour steel.

Microstructures

The ballistic performance of armour steel depends on the matrix having tempered martensitic or bainitic structure. This is achieved after application of austenization, quenching, and tempering on low alloyed steel. After the process of austenization and quenching, crystal structure of steel is changed from austenite ‘face centered cubic’ (FCC) structure into carbon supersaturated ‘body centered tetragonal’ (BCT) structure to form martensitic structure. Then heat treatment of tempering develops the strength and toughness of the matrix consisting of tempered martensite (1a in Fig 1). In case quenching is not proper then steel can have bainitic structure which can become tempered bainite after tempering (1b in Fig 1).

Bainite consists of lath type ferrite and precipitates within ferrite phase and at the boundaries of the laths. Apart from that, there can also be some quantity of retained austenite. Existence of retained austenite is caused by the imperfection during heat treatment process. The retained austenite presence is believed due to the non-uniformity of temperature and cooling rate, where the transformation of austenite phase to bainite phase is not complete uniformly. In addition, retained austenite is a softer phase compared to the martensite. Hence, the hardness of the material gets reduced. This retained austenite can decrease the strength to defend against the bullet.

However, RHA steel normally has a tempered martensitic phase (1a of Fig 1). Carbide precipitates is observed disperse in the matrix as well. Similar to RHA, other types of armour steels show fine structure in tempered martensitic phase (1c of Fig 1). Tempered martensite consists of recovery and recrystallization in the matrix to relieve the stress generated during the quenching process. Matrix phase is changed because the carbon atoms move out from the matrix in order to form carbide precipitates. Other than that, banding effect also appears. This banding effect is formed due to the rolling process. The process of rolling involves casting steel slabs of appropriate size and then rolling them into plates of required thickness. Hot rolling changes the coarse grain into the finer grain sizes and increases the mechanical properties.

Fig 1 Typical microstructures of armour steel as observed in optical microscope

Tensile properties

The typical stress-strain curve and the tensile properties are shown in Fig 2. According to the stress-strain curve, the armour steel shows a high tensile strength of 1750 MPa. The phases in the high strength low alloy steels are believed the main contributing factors for determining the tensile strength of the materials. The high tensile strength is due to the tempered martensitic phase in the microstructure of the armour steel. Nickel is believed to be the main alloying element responsible to increase the strength of steel besides carbon which also maintains its role as a strengthening mechanism. It is reported that during the tempering process, steel solution rejects carbon in the form of finely divided carbide phases, the high supersaturated solid solution of carbon in iron forming a martensitic microstructure. The final result from the tempering process is a fine dispersion of carbides in an alpha iron matrix. Precipitates of carbide particles are present in high strength steel, having the black particles as iron carbide. During the tempering process, martensite is decomposed to form carbide particles. This is due to the carbon atoms travelling out of the spaces between the iron atoms. The strain in the martensite is relieved as the carbon atoms leave the matrix. This behaviour contributes to higher strength and hardness. Hence, it is observed in the armour steel, the microstructure of tempered martensite with very small iron carbide islands results in its high tensile strength.

Fig 2 Typical stress-strain curve and tensile properties of armour steel

Hardness

There are several factors which contribute to hardness increment primarily its microstructure phase. Hardenability is to be adequate to achieve by high percentage of martensite. Armour steel normally consists of fully martensitic phase. Martensitic transformation process is generally enhanced by manganese due to decrement of critical quench speed. Apart from manganese, boron also controls martensitic transformation by preventing bainite and pearlite transformation.  Since armour steel microstructure is totally in martensite phase, further hardness of martensite is solely dependent on the carbon content of the steel.

Carbon is a very small interstitial atom which tends to fit into clusters of iron atoms.  It strengthens steel and gives it the ability to harden by heat treatment particularly if it exceeds 0.25 %. Carbon forms compounds with other elements called carbides, such as cementite, which exist as a precipitate and increase the hardness. Besides interstitial carbon which resists slippage, grain boundary also acts as a pinning point to resist dislocations. Fine microstructure consisting of high number of grain boundary in constant area increases resistance to slippage and this improves both tensile strength and hardness.

Strength and ballistic performance 

Based on mechanics of projectile impact, the penetration of bullets depends on many factors which occur over three phases namely the initial impact phase, stress propagation phase, and fracture initiation phase. During initial impact phase, the projectile kinetic energy is converted in impact energy on the surface or steel plate. The enormous force which acts on the steel plate can be reduced by increasing the hardness of the steel. Once the hardness of the plate is higher than projectile tips, the projectile gets shattered and the kinetic energy of bullet gets reduced proportionally to its mass.

A simple equation can be used to introduce the relationship between the most fundamental mechanical property of armour steel, i.e. its strength, and its resistance to penetration by AP projectiles. One of the most common and fundamental failure mechanisms experienced by homogenous steel armour is the ductile hole formation (Fig 3). This failure mechanism shows considerable plasticity and hence an estimate of the work performed in plastic deformation can provide a reasonable guide to the kinetic energy required to defeat a target.

Fig 3 Mechanism of ductile hole formation

The work of ductile hole formation (Wdhf) is equal to the work done in expanding a hole in a target to the projectile diameter. It is expressed by the equation Wdhf = (pi)* (D square)* ho* So/2 where value of pi is 3.14159, D is the diameter of a non-deforming projectile, ho the target thickness and So an appropriate compressive flow stress as the measure of material strength. The plastic strains required for the defeat of a metal target are usually large and hence a compressive flow stress at a high value of strain is appropriate. Estimates of the flow stress at large quasi-static strains are dependent on the actual rate of work hardening and here, a uni-axial quasi-static compressive flow stress at a true strain of 1.0 is used.

At this level of strain for steels, such a flow stress is generally insensitive to any further increases in strain. High strain rate steel properties at large strains can alternatively be used but may not necessarily offer significantly greater accuracy when making first-order estimates of ballistic performance with the equation given above.

The use of quasi-static yield stress rather than flow stress at high strain values provides a greater underestimate of the ballistic limit and the discrepancy is normally significant for steels which have high rates of work hardening. Hardness measurements are also to be used with caution as a measure of material strength since these measurements can only be used usually to estimate the material yield stress.

Development of hard protection plate goes back in the 1980s prior to its contribution to penetration resistance. However, hardness increment is only effective up to critical limit because it promotes brittleness in plate which leads to shattering effect. If a protection plate is not hard enough, a projectile tip made from high hardness material with sufficient kinetic energy can penetrate the plate. During impact, balance kinetic energy from projectile exerts large deformation and stress wave on plate over a short period of time. The ability of the plate material to resist bullet perforation depends on the tensile strength where good energy absorption capacity is needed in order to deform the projectiles tips.

Moreover, stress wave continues to propagate until the rear side and reflects the waves upon the back plate causing fragmentation of brittle metal (spalling effect). Hence, it is essential for protection plate to have sufficient ductility. This allows plate to bend so it can absorb the stress of impact at high velocity without shattering. If monolithic rolled homogenous armour (RHA) is used for the light vehicle armour as its protection panel then the thick monolithic panel is heavy and causes limitations on vehicle mobility. Hence, weight reduction approach is necessary towards protection panel design.

One of the most common methods for weight reduction while maintaining its strength is to use the composite protection panel. The design of laminate is required to retain the original strength, while reducing the weight of panel. Hence, the front material of protection panel is to consist of high hardness, while the back plate is required to have high ductility.

The response of the armour is primarily determined by the steel strength and toughness and projectile type at impact speeds below 2 kms/second. Plastic work is hence the key determinant of the ballistic performance of the armour with the penetration resistance of armour steels initially increasing with increasing flow stress. However, a complex relationship exists between the strength of an armour steel plate and its penetration resistance, shown schematically in Fig 4 where hardness is used to characterize material strength.

Fig 4 Relationship exists between the strength of an armour steel plate and its penetration resistance

The relationship between the hardness of a monolithic armour steel plate and its performance against armour piercing projectiles is shown in Fig 4. Changes in failure mechanisms result in a complex, discontinuous relationship between plate hardness and penetration resistance. This is expressed schematically in Fig 4a, and in terms of Brinell hardness values in Fig 4b, for an unspecified armour piercing projectile. The initial improvements which occur with increases in plate hardness in Fig 4a are a result of increased resistance to plastic flow in a ductile hole formation failure mechanism. Beyond a certain point, however, increased plate hardness results in decreased protection due to an increased susceptibility of the material to low-energy adiabatic shear failure. Further increases in plate hardness result in improved performance, but rather as a result of projectile fracture. At very high hardness levels, a lack of toughness can result in brittle fracture of the steel plate and thus erratic behaviour, depending on the specific steel impacted. Fig 4b suggests a similar relationship to the schematic of Fig 4a but with hardness values specified for the discontinuity in behaviour.

While ballistic performance can sometimes be correlated to the hardness, material hardness is simply a quasi-static measure of yield pressure for a specific indentor geometry which can be related to a compressive yield stress and thus the initiation of quasi-static plastic. Hardness is not a measure of a dynamic yield or flow stress which accounts for work hardening. Hence, strain rate hardening or thermal softening is generally required to fully define the armour steel resistance to plastic flow under projectile impact conditions.

The improved ballistic resistance of steel as a function of increasing hardness is well established and for this reason armour designers are more often incorporating higher hardness (higher strength) armour steels in their designs and structural armour solutions.

While increases in hardness increases resistance to projectile penetration (improved protection), this is not always linear and does not necessarily apply for fragmentation protection, as demonstrated by the ‘fragment simulating projectile’ (FSP) ballistic limits. Fragmentation protection decreases sharply with hardness, making the higher hardness armour steel grades such as HHA a poor choice for such applications. This reduced penetration resistance arises because impacts of blunt fragments cause high strength steels to fail by adiabatic shear plugging, a low energy failure mechanism. Adiabatic shear is responsible for the observed reduction in FSP performance.

It is seen that there is no difference between the ballistic performance of ‘ultra high toughness armour’ (UHTA) with hardness of 450 BHN and HHA (BHN 512). The UHTA grade has leaner alloying element content, providing improved toughness and weldability compared to HHA. UHTA is a better choice than HHA for structural applications and its more consistent ballistic performance allows a weight saving for some protection levels.

Around 2008, ‘ultra-high hardness armour’ (UHHA) steels with hardness values of greater than 570 BHN have been produced which have been assessed and applied as practical armour materials. UHHA steels can offer considerable performance improvements over HHA steels and also fulfill an equivalent ballistic role to dual hardness armour but as a homogenous plate. Ballistic performance increases at very high steel hardness values have been known for many years but it is only recently that armour steels have been produced which consistently meet ballistic requirements without shattering upon impact.

Overall, it is seen that the ballistic performance relates to steel armour hardness, though over specific hardness ranges there can be an increasing or decreasing relationship between ballistic performance and hardness, depending on the projectile and the observed armour failure mechanism. Another important influence of armour hardness is whether it is sufficiently high to deform or shatter a projectile, both of which strongly affect the ballistic performance. In practical terms, hardness is a measurement of strength that can be easily measured on a plate-by-plate basis, and it is particularly convenient as a quality assurance measurement.

Extensive historical studies found that the ballistic performance of structural and armour grade steels correlates to hardness and tensile strength but not yield strength. Interestingly, some studies have found a quite linear relationship with measured quasi-static tensile yield stress between values of 600 and 1700 MPa for quenched and tempered steels (Fig 5).

Fig 5 Linear relationship between quasi-static tensile yield stress and ballistic limit for a range of quenched and tempered steels

However, it has also been demonstrated in another study that a strong correlation exist between predicted and measured ballistic performance for a range of materials when the quasi-static compressive flow stress at high strains, i.e. flow stress at a true strain of 1.0, rather than compressive yield stress, which is used as a measure of material strength. The use of flow stress is reasonable when considering the large strains involved in a ballistic impact event, especially through ductile hole formation and many other failure mechanisms. The quasi static compressive true stress – true strain curve is almost flat at such large strains, thus this flow stress measure is also largely insensitive to the precise value of strain.

There is a reason for the enhancement in the flow stress of steel at high strain rates. In the vicinity of the initial flow stress, flow stress is affected by both temperature and strain rate, and strain rate enhancement can be explained by a ‘thermal activation model’ of dislocation movement. This model assumes that at temperatures lower than a critical temperature (dependent on strain rate), the flow stress depends on both a thermal component and a thermally activated component. The thermal component of flow stress is determined by the effect of long range dislocation obstacles (e.g. grain boundaries, precipitates, etc.) and is largely strain rate independent, but still dependent on temperature. The thermally activated component of flow stress is related to short-range obstacles (e.g. dislocations) which can be overcome by thermally activated glide of mobile slip dislocations due to thermal fluctuations and thus is more strongly affected by temperature and strain rate; and it is increased by either decreasing temperature or increasing strain rate. Decreasing temperature leads to reduced thermal energy while increasing strain rate reduces the time available for dislocation movement. Both circumstances result in a reduced ability of mobile dislocations to overcome short range obstacles and hence lead to strain rate hardening.

However, the ‘thermal activation model’ has been originally established for initial flow stresses. While this model can also be applied to larger strains, this is seldom done as there are no closed form solutions available that describe the stress-strain behaviour as a function of plastic strain, strain rate and temperature. At larger strains, empirical models or the semi-empirical models are often used to describe flow stress behaviour as a function of strain, strain rate and temperature.